Blackheart BH5H Mod

[jmohr58]

Hello everyone,
I have a Blackheart BH5H 5W SE Cathode Biased head and would like to continue with some tone changes.
First of all I have already added an SB-84 Power Scaling Circuit from London Power and it works well. I can now Dime the Amp and not wake anyone up at night.
But I have always liked the sound of a good Marshall in Overdrive and would like some suggestions to make this BH5H more like a good Marshall in tone etc.
Any suggestions on the coupling and tone caps?
Would the Mallory 150 series give it the darker sound of a Marshall?
Or would I be better off using a Sozo Mustard cap or something else?
Also what about changing the resistors out for a lower noise floor in the preamp stage, including resistance values where needed?
Any help would be greatly appreciated.
I have enclosed the schematic for the amp for reference.
Thanks in advance,
Jerry

[Colossal]

Jerry,
I'm not sure which Marshall era you like, but to get a more Marshally sounding amp, you might consider changing:

1. R1 to 2-3W metal film (lowest noise at the input)
2. R3 to 820R (leave C1 as is...fully bypassed for reduced noise). You could also add a 0.1uF film cap in parallel with C1 (or not).
3. R5 to 33k (definitely do this one)
4. R7 to 820R
5. C5 to 0.68uF (use a polyester film cap...Mallory 150M or Sozo)
6. paralleling 250pF Silver Mica and 250pF Ceramic in place of C2
7. C3, C4, and C6 to Sozo Vintage Mustard caps (150Ms would be fine too)
8. Parallel C13 with 0.47uF 600V polypropylene film cap (shunt some of the rectifier ringing to ground) (e.g. Xicon MPP, Solen MKP, SBE 715P)
9. If you're bored, change D1-D4 to UF4007

I would make these changes incrementally, carefully evaluating the change in tone after each change. If you don't care for R3 and R7 as 820R, try 2k7 and 10k respectively, the latter combination being more the JCM800 vibe. But see how you like the amp with R3 and R7 as 820R. R9 could be changed out to 250kA (as a Master Volume) to regulate how much signal the power stage sees. You want to overdrive the output tube but not turn it into a garbled mess.

Definitely consider the tubes you are using for the amp as these will change the tone. In an amp that simple, I would use good quality components as you will be more likely to hear a difference...plus it's easy and fun to change. My preference would be to use Sozos as I think they have a little more clarity and musicality than a 150M but still retain the darker vintagey tone. YMMV of course. There are other things you can do to the amp, but the above changes would be a good start.

Let us know what you choose and how you do.

[jmohr58]

Thanks Colossal for the response.
I like the JTM45 style and the 59 Superlead and the Plexi's, but am not picky.
Also appreciate the JCM 800 series, but me at my age, the 70's era
Marshalls would be the ones for me. I had a Line6 Spider III 75W amp for my first amp, not really knowing much at the time in the way of amps. And for a short time I thought it was great, but quickly became dissullioned with this transistor based amp. Its not that it couldn't produce a kind of okay distorted sound and the hype over 400 presets is of course a key selling point for Line6. But, I found that out of the 400 plus tones, most were just duplicates and you really couldn't get that much variance.
I had also tried a Marshall MG 15FX because at the time I thought, hey it's a Marshall! That didn't last long either, but I started to research into the tube realm and haven't looked back. After deciding to trade in my transistor based amps and finding you really don't get much out of them on the way out of the door, as you pay to get them in the door. I couldn't afford alot in the way of a good tube amp. I tried a few hybrids but wasn't impressed, so I found a used Blackheart BH5H that worked good. I had already planned on modding it so, Warranty, who needs Warranty.

The first thing I did was to remove the 240VAC lines from the PT since the amp will probably never leave the states. Also from the factory Blackheart just budles up the wiring from the PT etc into kind of a birds nest so I straightened out the mess, twisted the seperate voltage lines together and resoldered them back to where they belonged and twisted and rearranged the other un-orgnaized wiring. This lowered the noise and buzz quite a bit.

Next was to change the tubes, so with a little research and asking questions, I went with a Tung Sol High Gain 12AX7 in the Preamp Stage and a Mullard EL84 set for early breakup in the Power Stage.

Next I chose to add a Standby switch by cutting the pcb trace at the junction of R14 and the + side of C13 (both points still connected) the switch breaks contact between the rectifier section and the first filter cap.

Next I needed to be able to play at night without loud noise issues, so I added the SB-84 Power Scaling circuit. If you are unfamilar with this circuit, it power scales the output stage by interupting the 2 supply lines VA and VS and maintains the approx 30V diffenence between the screen and plate voltage with a trimpot for fine-tuning. But this kit also adds an additional pot to the mix that London Power calls a Drive compensation Pot but is basically a Master Volume control. This in addition to the Power Scaling pot. You still get to keep the Triode/Pentode switch and lets you drop from 3/5 Watts down to approx 1/10th Watt without a very noticeable change in tone.

Okay, now to your response,

I ordered various CMF60 Series 500V 1% metal film military grade resistors, they are rated as 1/2 Watt for military use but 1 Watt for commercial use. I planned on replacing the preamp stage resistors with these. I have included the datasheet on them.

1. Would you still recommend bumping up to a 2 to 3 watt metal film on R1?

2. In regard to C1 I have seen a post about replacing C1 with a 0.68uF and then replacing C5 with a 22 to 33uF to bump up the Bass response.
I see you suggest putting a 0.68uF at C5 and leaving C1 alone with the possiblility of adding a 0.1uF in parallel. In your opinion would your way keep the bass response at an optimal level?
(you see alot of things on the net so its hard to believe everyones posts on websites, but I see you have made alot of posts on this forum and seem to be up on the isssues so I think I would be more willing to follow what you say)

2., 4. Changing R3 and R7 to an 820R I am assuming is to increase the gain.

on point 3. R5 to 33K would this be more of a tonal change than a gain increase? Or is there another reason to do this change?

on 6. I had planned on changing to a 500pF 500V silver mica, but you indicate using both silver mica and ceramic to reach the 500pF value?

Doing some looking around on the forum last night I found an circuit for a bright switch on a Trainwreck schematic using a 100pF and 500pF silver micas to switch between, any advantage to this in my case?

When I ordered the CMF60 resistors I also ordered both Mallory 150's and Orange Drop 715P's in the 0.022uF 630V/600V size because I wasn't sure which would be the better choice, I see from doing some digging around the thought is that the Orange Drops are a Brighter sounding cap and would go better with a Fender Amp. So I have planned on using the Mallories in C3, C4 and C6.

I also got a 0.1uF mallory and orange drop so for your option of C1 would the Mallory be better there to?
I purchased one Sozo Mustard cap in the 0.68uF value so following your advise I guess it will go into C5's position.

Speaking with another guy that Mods and sells alot of SE amps he suggested changing R14 to a 470R to 500R 5 to 6 Watt Resistor to increase gain by about 1 watt.
He also suggested that you could change R4 up to 330K, Thoughts?

And one last thing, I see alot of schematics show what in my case is R2 being a 68K instead of a 33K, would there be any advantage in this?
I appreciate your help and advice.
Thanks,
Jerry

[Colossal]

The first thing I did was to remove the 240VAC lines from the PT since the amp will probably never leave the states. Also from the factory Blackheart just budles up the wiring from the PT etc into kind of a birds nest so I straightened out the mess, twisted the seperate voltage lines together and resoldered them back to where they belonged and twisted and rearranged the other un-orgnaized wiring. This lowered the noise and buzz quite a bit.

Good, anything you can do to reduce noise is a good thing. This is very important in single-ended designs where everything counts.

Next was to change the tubes, so with a little research and asking questions, I went with a Tung Sol High Gain 12AX7 in the Preamp Stage and a Mullard EL84 set for early breakup in the Power Stage.

Ok, sounds good. You can always try different 12AX7s for flavor if you make the circuit changes and don't feel it is quite "there". Tubes make a difference.

Next I chose to add a Standby switch by cutting the pcb trace at the junction of R14 and the + side of C13 (both points still connected) the switch breaks contact between the rectifier section and the first filter cap.

Cool.

Next I needed to be able to play at night without loud noise issues, so I added the SB-84 Power Scaling circuit. If you are unfamilar with this circuit, it power scales the output stage by interupting the 2 supply lines VA and VS and maintains the approx 30V diffenence between the screen and plate voltage with a trimpot for fine-tuning. But this kit also adds an additional pot to the mix that London Power calls a Drive compensation Pot but is basically a Master Volume control. This in addition to the Power Scaling pot. You still get to keep the Triode/Pentode switch and lets you drop from 3/5 Watts down to approx 1/10th Watt without a very noticeable change in tone.

Where did you locate the master volume pot and what value is it?

I ordered various CMF60 Series 500V 1% metal film military grade resistors, they are rated as 1/2 Watt for military use but 1 Watt for commercial use. I planned on replacing the preamp stage resistors with these. I have included the datasheet on them.

Those should be just fine.

1. Would you still recommend bumping up to a 2 to 3 watt metal film on R1?

Personally I like higher wattage metal films in that position as it's very important to reduce noise at the input to as low as possible. If you have noise there, it gets amplified and sent on, so getting a very quiet resistor for a large value (1M) is important to getting that small, incremental reduction in the noise floor of the amp. I think you will probably be fine with your choice however. I wouldn't obsess over it.

2. In regard to C1 I have seen a post about replacing C1 with a 0.68uF and then replacing C5 with a 22 to 33uF to bump up the Bass response.
I see you suggest putting a 0.68uF at C5 and leaving C1 alone with the possiblility of adding a 0.1uF in parallel. In your opinion would your way keep the bass response at an optimal level?

First we have to look at the layout of the BH5. You want to make it Marshally sounding but there are limits to this. You are only working with two gain stages and the tone stack is plate fed off the first stage. So we are really talking about voicing changes to get more of the Marshall sound. In regards to replacing C1 with an 0.68uF, this is usually done when the cathode resistor there is 2k7. 2k7/0.68uF is the traditional values on the JCM800 so that's why I was asking what flavor of Marshall you prefer.

Personally, I would leave C1 at 22uF for now. The reason for this is with that value, all frequencies down to quite low are bypassed increasing the gain. Bumping the cathode resistor up to 820R (with 22uF) is similar to the input stage of the Normal channel on a plexi where 820R/330uF was used. I would not use 330uF, it will be muddy for sure. Another important aspect of keeping V1a fully bypassed (22uF) is that 120Hz noise from your heater filaments (yours are DC heated) will be shunted to ground. The BH5 uses brute force DC for the heaters but I will be willing to bet that it is not perfectly quiet without the proper regulation circuit and low ESR filtering. Since the tone stack immediately follows V1a there will be plenty of low end and you will have sufficient bass to dial back even. It's very important not to overdo it with the low frequencies in a preamp as you can set up conditions for blocking distortion.

(you see alot of things on the net so its hard to believe everyones posts on websites, but I see you have made alot of posts on this forum and seem to be up on the isssues so I think I would be more willing to follow what you say)

That's kind, but I'm just another opinion. Do some experimentation and see what works well for your tonal tastes.

2., 4. Changing R3 and R7 to an 820R I am assuming is to increase the gain.

It does not change the gain, it changes the operating point of the stage to be "warm biased". 1k5 where V1a and b are now is considered center biased with typical voltages (280V). 820R will give you a more Marshall vibe and will produce more asymmetrical clipping with a large input signal. If you want more of the JCM800 vibe, go with 2k7 and maybe even change C1 to 0.68uF - 2.2uF or so. This will be closer to the bright channel on a plexi and a JCM. But keep in mind that the EL84 is a bright sounding tube so you might not want more bright. Got to keep the bigger picture in mind.

on point 3. R5 to 33K would this be more of a tonal change than a gain increase? Or is there another reason to do this change?

R5 shifts the midrange frequencies. 33k is classic plexi. You could bump it up to 47k if you want more of modern type tone (hot rodded Marshalls, Slawn, Friedman, Cameron, etc). 33k sounds really good though and is classic Marshall. Check out the enclosed tonestack simulation.
100k = Green trace
47k = Red trace
33k = Yellow trace
Notice where the "trough" is. Decreasing the R value shifts the mids up.

on 6. I had planned on changing to a 500pF 500V silver mica, but you indicate using both silver mica and ceramic to reach the 500pF value?

You could use a silver mica exclusively. Paralleling the caps gives you a little tonal complexity. Some find mica caps a bit too shrill. Ceramic will give a more grainy tone. Classic Marshalls use ceramic in that position. You can try mica or ceramic or blend them. You gotta watch your ceramics however as some poor quality ones can be a bit microphonic.

Doing some looking around on the forum last night I found an circuit for a bright switch on a Trainwreck schematic using a 100pF and 500pF silver micas to switch between, any advantage to this in my case?

I would not bother with a bright switch. If you need more brightness, you can add a cap across the volume pot but my guess is that the amp is plenty bright already.

When I ordered the CMF60 resistors I also ordered both Mallory 150's and Orange Drop 715P's in the 0.022uF 630V/600V size because I wasn't sure which would be the better choice, I see from doing some digging around the thought is that the Orange Drops are a Brighter sounding cap and would go better with a Fender Amp. So I have planned on using the Mallories in C3, C4 and C6.

Stick with the Mallories (polyester) for C3, C4, and C6. I have nothing against Orange Drops, they are polypropylene. For a more Marshall tone however, I would stick with polyester for C3 and C4. You might try a 715P for C6 and see what you think. You might actually like it (give the amp some clarity).

I also got a 0.1uF mallory and orange drop so for your option of C1 would the Mallory be better there to?

If you parallel C1 with an 0.1uF I would use the 715P. Also, just to be sure, we are talking about adding 0.1uF to C1 in parallel, not removing C1 and replacing it with 0.1uF.

I purchased one Sozo Mustard cap in the 0.68uF value so following your advise I guess it will go into C5's position.

Sure, you can use it there. Sozos (being pricey) are usually best used "inside" the signal path, but they are also used for cathode bypass.

Speaking with another guy that Mods and sells alot of SE amps he suggested changing R14 to a 470R to 500R 5 to 6 Watt Resistor to increase gain by about 1 watt. He also suggested that you could change R4 up to 330K, Thoughts?

Sure you could change R14 if you wanted to, you would be increasing the screen voltages doing so. I don't have a voltage chart for your amp or know how that might affect your power scaling circuit (if at all) so can't comment. I would focus more on tone than trying to get more power out of the amp. You could add a choke in place of R14 too. Personally I would stick with R14 as 1k as it's going to give the amp a little power supply compression and this is a good thing. The amp is very simple. You want to make a little amp seem as much like a big amp in both tone and feel as possible.

And one last thing, I see alot of schematics show what in my case is R2 being a 68K instead of a 33K, would there be any advantage in this?

68k is the typical low input value for a each channel (bright and normal) on a plexi or most amps with Hi and Lo inputs. 33k (or 34k actually) is the high input value. You might notice a slight rolling off of highs using 68k which could be an advantage to getting a slightly darker tone. Most people use the high input value for single input amps. If you voice your preamp to the bright channel of a plexi (2k7/0.68uF for V1a) I would probably use 68k.

[jmohr58]

Thanks Colossal,
I appreciate the info on the changes, but I really appreciate the fact that you are explaining the reasons for the changes. I studied and got an Associates in Electronics in 94, but it was purely transistor based theory.
We may have spent a few hours total on tubes but nothing of substance.
Wound up going straight into the computer field, so as they say if you don't use it you loose it.
I look at old notebooks of the formulas and therems I did back in Tech school and have a hard time believing that I actually did these and that I understood what I was doing - At that point in time. Such extreme high levels of math and its all gone now!
So now I am trying to learn as much as I can on Tubes/Valves.
My Blackheart is my entry point, but in the future when finances are better I would like to build a good amp from scratch. So getting a good understanding on what I am doing with this unit gives me a foundation to build on. So thank you once again for taking the time to explain the reasons behind the changes.

I have been looking around the forum and really wasn't familar with the Trainwreck amps, but they seem like a good amp and worthy of an attempt to build one when the time and finances are right! Any suggestion on a certain model that you like?

I have included a couple of photos, one of the way the AC wiring is when the amp is purchased. At least when I got mine. The next is how it is now.

There is some metal sheilding tape under the SB-84 board because the Power Transformer is right above it and I thought it might be a good idea.

You asked about the Master Volume, the SB-84 kit comes with a dual gang 1M, but when I emailed London Power on some questions I had about installling the circuit. I will include the response I received regarding the Drive Comp Pot/ Master Volume:

"DriveComp replaces the 100K grid-leak for the output tube. The two sections of the pot are wired in parallel. You can sub a 100K instead, if you have one or find that easier. X terminals tie to the coupling cap from the preceding stage. O terminals tie to ground. Lift the end of the 1K5 grid-stop for the output tube out of its hole and wire to the wipers fo the DriveComp pot." (This was from Kevin O'Connor at London Power)

X and O are the 2 outer terminals on the pot. (Facing from the rear with the terminals facing up O on the left X on the right)
So at the other end of the wires, R10 has been lifted out of its hole on the side farthest from the tube and the red wire in the picture has been attached to it's free end and heatshrinked. A white wire is now in the hole R10 was originally in and goes to X, the black wire is attached to the ground plane and goes to O.

So I am using a 100K single gang audio taper pot. And it is located in the hole on the front panel where the P/T switch was originally. I have the Power Scaling Pot next to it where the Power Switch was. Those switches are now on the back panel with the Standby switch.
I also removed the board for the speaker jacks, installed a 3 position switch and put 2 Switchcraft 1/4" jacks in place.

I will put the CMF60 1M resistor in R1 and see how that goes, but will order a 2Watt Metal Film just in case. It presently is a 1 Watt Metal film. The company I order from regularly doens't carry a 3Watt in a 1M size. If need be I will source elsewhere.
But we shall see how it goes with the CMF60.

You are correct that the amp is very bright. So making it a warmer and darker amp is the goal.

Thanks for the explanation for using a combination of silver mica and ceramic on C2. It sounds resonable, the amp presently has a 470pF ceramic as stock. I will probably just leave it alone until I source both 250pF silver and ceramic., But as a thought what about loweing it to maybe a 390pF silver or ceramic, what effect would this have?

On the C1 matter to clarify, yes, I am going to take your advise and leave the 22uF in place and as you suggested try the 0.1uF Orange Drop 715P in parallel with it. Then replace C5 with the 0.68uF Sozo.

I will also try the 68K at R2.

One other thing I had planned was to put an extra 4700uF 35V Cap in parallel with C18 to see if it would help knock down some more heater buzz.

Thanks again Colossal for your insight, and the time you have taken to help me with this project.
Jerry

[Colossal]

So now I am trying to learn as much as I can on Tubes/Valves. My Blackheart is my entry point, but in the future when finances are better I would like to build a good amp from scratch. So getting a good understanding on what I am doing with this unit gives me a foundation to build on. So thank you once again for taking the time to explain the reasons behind the changes.

Modding this amp will get you in good shape for a scratch build. You'll really enjoy it and it will sound great.

I have been looking around the forum and really wasn't familar with the Trainwreck amps, but they seem like a good amp and worthy of an attempt to build one when the time and finances are right! Any suggestion on a certain model that you like?

Well, they are all fantastic amps. It depends on what you are after tonally. You'd be best served to spend some time searching the forum and definitely check out:
http://www.youtube.com/results?search_q ... ndall&aq=0
Watch everything from Glen Kuykendall. He has an original Express and is considered the benchmark in Express playing and tone. Be forewarned however that an Express is not an entry level build. It can be done but will take some skill and patience and tubes to get right.

I have included a couple of photos, one of the way the AC wiring is when the amp is purchased. At least when I got mine. The next is how it is now.

Great job. Much better. You definitely want to keep AC away from clean, filtered DC wherever you can in the amp.

You asked about the Master Volume, the SB-84 kit comes with a dual gang 1M, but when I emailed London Power on some questions I had about installling the circuit. I will include the response I received regarding the Drive Comp Pot/ Master Volume:

"DriveComp replaces the 100K grid-leak for the output tube. The two sections of the pot are wired in parallel. You can sub a 100K instead, if you have one or find that easier. X terminals tie to the coupling cap from the preceding stage. O terminals tie to ground. Lift the end of the 1K5 grid-stop for the output tube out of its hole and wire to the wipers fo the DriveComp pot." (This was from Kevin O'Connor at London Power)

I'm a bit confused by this because the grid leak in your amp (R9) is 220k. You would be using the master volume pot to replace R9 and it should be a 250kA pot (with a 2M2 in parallel to make the pot's max value about 220k). You can certainly use 100k but this will be reducing the amount of signal driving the power tube (into distortion). This may or may not be a good thing.

I will put the CMF60 1M resistor in R1 and see how that goes, but will order a 2Watt Metal Film just in case. It presently is a 1 Watt Metal film. The company I order from regularly doens't carry a 3Watt in a 1M size. If need be I will source elsewhere. But we shall see how it goes with the CMF60.

Like I said, I wouldn't obsess over R2. I just use 2-3W metal films and good ones to help reduce any noise at the grid.

You are correct that the amp is very bright. So making it a warmer and darker amp is the goal.

Consider changing R10 to 8k2 to 10k, or even higher. This will dump some brightness and maybe smooth it out a bit. There are other things to do to trim the brightness but add the R10 change to your To Do list. Do this last after you do the Marshallizing steps. The first thing is to get the amp's voice in the vicinity of where you want it, then dump some brightness if it's still needed.

Thanks for the explanation for using a combination of silver mica and ceramic on C2. It sounds resonable, the amp presently has a 470pF ceramic as stock. I will probably just leave it alone until I source both 250pF silver and ceramic., But as a thought what about loweing it to maybe a 390pF silver or ceramic, what effect would this have?

The goal here is Marshall tone. Some of the Plexis used a 56k mid slope resistor (R5) with 250pF treble cap, but most used 33k with 500pF (sometimes 560pF). I would stay with the higher values as you will hear the mids more and the amp will be slightly gainier. There will be slightly less attenuation with the higher value letting more mids/low-mids through. This is desirable from the Marshally standpoint. But keep the idea of lowering it in mind. It is a way to tailor the high end response of the amp too. A ceramic cap is going to give it a bit more of a grainy edge. You might like it, then again, you might not.

On the C1 matter to clarify, yes, I am going to take your advise and leave the 22uF in place and as you suggested try the 0.1uF Orange Drop 715P in parallel with it. Then replace C5 with the 0.68uF Sozo.

Ok, sounds good. You can always lower C1 to 0.68 to 3.3uF or so if you find the amp too "woofy" or thuddy instead of punchy. Also, it will depend on the value(s) you choose for R3 and R7. 0.68uF is going to give it more mid focus. But for noise reduction it's probably not a bad thing to have C1 22uF. Try changing R3 and R7 and C5 first before changing C1.

I will also try the 68K at R2.

That will help darken the amp up a bit.

One other thing I had planned was to put an extra 4700uF 35V Cap in parallel with C18 to see if it would help knock down some more heater buzz.

You might want to be careful with that. The heaters are powered with brute filtered DC. I would check the current rating of the heater filament winding on the power transformer. When you use huge caps to filter like that, the power factor is very poor and a lot more current will be drawn to keep them topped off than if the heaters were heated with AC. You certainly could try it though. Worst case the heater filament winding is fused and it will let you know if it is too much.

Thanks again Colossal for your insight, and the time you have taken to help me with this project.
Jerry

No problem Jerry. I'm interested to see/hear what you arrive at with the mods. Start with changing over R1, R2, R3, R7 and C5 and the tone stack caps. Then listen to it and see if you are closer to the Marshally sound you want. If the distortion is flabby or not enough, we might want to change the master volume pot but that's for later as is changing R10, etc. Be *sure* to change only a small number of variables (best even ONE) at a time and evaluate. But you should be ok changing out the R and Cs I just mentioned in one shot. After that, if it's not there, then characterize and reevaluate.

[jmohr58]

Hello again Colossal
In your last relpy:

jmohr58 wrote:
You asked about the Master Volume, the SB-84 kit comes with a dual gang 1M, but when I emailed London Power on some questions I had about installling the circuit. I will include the response I received regarding the Drive Comp Pot/ Master Volume:

"DriveComp replaces the 100K grid-leak for the output tube. The two sections of the pot are wired in parallel. You can sub a 100K instead, if you have one or find that easier. X terminals tie to the coupling cap from the preceding stage. O terminals tie to ground. Lift the end of the 1K5 grid-stop for the output tube out of its hole and wire to the wipers fo the DriveComp pot." (This was from Kevin O'Connor at London Power)

I'm a bit confused by this because the grid leak in your amp (R9) is 220k. You would be using the master volume pot to replace R9 and it should be a 250kA pot (with a 2M2 in parallel to make the pot's max value about 220k). You can certainly use 100k but this will be reducing the amount of signal driving the power tube (into distortion). This may or may not be a good thing.

When Kevin emailed me about this I was confused to, because I had been trying to study some printouts on tube circuitry and had came to the conclusion that R9 should be the grid leak resistor in the amp. But then I got his email, and since the SB-84 is his design and he does have a series of tube books etc. I thought I must be wrong. Which is why I modded as explained.
One thing with Kevin though, he is very intellegent and does seem to know his stuff, it is really hard to get an answer from him at a level that a beginner in the field can grasp. Plus he always say's reference TuT XX about whatever subject you are asking about and I don't have the money to buy his books on the subject. So I have been trying to find help from other sources when needed for these reasons.

The way you have explained the circuits and reasons for doing things has been clear and logical. And the key point is: I can follow along with my circiut diagram or schematic and I don't feel like my head is gonig to spin off.
In the SB-84 circuit the DriveComp/Master voume does not connect to the SB-84 in any way, so if I am following you correctly, I would be better to put R10 back the way it was and remove R9 and attach it across the outer lugs of a 250KA pot, then using the same X O orientaion, hook the wiper to the pad where R9 was closest to R15, the X terminal to R9's other pad and ground the O terminal.
I am including my circuit layout I have been using, please excuse the scribbles on it. But maybe this might help you with the orientation of the board in general.
I will work on changing over R1, R2, R3, R7 and C5 and the tone stack caps and see where that gets me first. I can agree, like flying a plane - Small corrections!
Thanks Colossal
Jerry

[M Fowler]

Holy crap you two love to type look at those long posts.

Mark

[Colossal]

"DriveComp replaces the 100K grid-leak for the output tube. The two sections of the pot are wired in parallel. You can sub a 100K instead, if you have one or find that easier. X terminals tie to the coupling cap from the preceding stage. O terminals tie to ground. Lift the end of the 1K5 grid-stop for the output tube out of its hole and wire to the wipers fo the DriveComp pot." (This was from Kevin O'Connor at London Power)

In the SB-84 circuit the DriveComp/Master voume does not connect to the SB-84 in any way, so if I am following you correctly, I would be better to put R10 back the way it was and remove R9 and attach it across the outer lugs of a 250KA pot, then using the same X O orientaion, hook the wiper to the pad where R9 was closest to R15, the X terminal to R9's other pad and ground the O terminal.

Jerry,

Do you have a schematic for the SB-84? You can PM it to me if you do not wish to make it public. We have a general gentlemen's rule here not to publish the schematics of amps in current production. If it is technology or art in the public domain, then it is ok to. If I understood you correctly, it is being used to vary the voltage on the screens as a means to regulate output power. It seems logical that the DriveComp/Master is independent but since I don't have any of the TuT books and have not used O'Connor's methods I can't say for sure. I hesitate to advise you to change the pot from 100k to 250kA since I am unsure how it was intended to be used with the SB-84 circuit. Intuition tells me that you should have 250kA in the circuit. If the pot is truly independent of the SB-84, then it seems like it would be ok. This will be important to determine because part of the Marshall sound is the dominance of power amp distortion and you want to ensure sufficient drive to the power tube.

[Colossal]

Jerry,

Thanks for the information. Very helpful. I can't see any reason why you can't or shouldn't be using a 250kA pot for your master volume. If you get one to replace the 100kA, get a 2M2 resistor to go along with it. In parallel with the pot (soldered on pins 1 and 3 of the pot, wiper being 2), it will bring the total resistance to around 225k which is very close to the desired grid leak value of 220k for that design.

Have your parts started to roll in yet?

[jmohr58]

Colossal,
Thanks,
I will switch out the Master Volume as you describe. It does seem
to make more sense to adjust the signal at that point.

Should I solder the unused outer lug to ground on the 250KA/2M2 pot that won't be used with the wiper to control the signal flow, just leave it open or tie it to the wiper? Which method will keep any unwanted noise down the best?

All ready have the one Sozo and the Mallory and Orange Drop Caps plus the CMF60 resistors But will have to place another order for a good 250kA pot and the 2M2 resistor. I usually go thru TubeDepot and they have a 1%1/2 Watt Metal Film in the 2M2 size with a good CTS pot. That should do nicely.

Would have already started repacing parts but the wife complains about solder fumes, so I have to do work on it when she isn't around. Even complains if I work on it in the basement. Say's she can still smell it and complains of headaches.

Did you get a chance to look around Kevin's website and products?

And after reading the Q and A on the LP power scaling,
Have you be able to form any opinons regarding Kevin's products.
(other than they are a bit expensive)

And for future reference, regarding Trainwreck amps, are there websites which you can order kits from or do you have to source the chassis , and other parts yourself then just follow the schematics and layouts in the public domain and/or in the Trainwreck section of the forum?

Thanks again for the help.
Jerry

[jmohr58]

Colossal,
Before I submit my order on the 2M2 resistor and the 205KA pot,
Would changing the value of C2 above 500pF make any improvement for the better, or am I just better to stay with the value I have on C2. You seemed to indicate in an earlier post that a bright switch would be un-necessary with the values of 100pF and 500pF to switch between so, I was curious if going to say a value of 680pF or 1000pF on C2 make it darker or just cause problems.
Thanks,
Jerry

[Colossal]

Colossal,
Thanks,
I will switch out the Master Volume as you describe.
Should I solder the unused outer lug to ground on the 250KA/2M2 pot that won't be used with the wiper to control the signal flow, just leave it open or tie it to the wiper? Which method will keep any unwanted noise down the best?

I've attached a schematic. Also on the drawing I showed a change in the grid resistor (R10) which we will save for AFTER you have modded the tone stack and cathode resistors. I also added a switch which you might like to tighten the feel of the amp. Finally, there is a mod that would allow to dial in the output tube's bias. If you go with the 1,000uF cap it will stiffen the feel of the amp and tighten the bass a bit to be more like a fixed bias amp (which a Marshall is of course). If you do use the 1,000uF cap, it would be a good idea to cool the bias slightly. If you notice, I changed R11 to 100R 5W so that you can use VR6 (I would use a 1k 2W pot) to make the fine adjustment. TP1 and TP2 are bias test points around Rbias, a 1R 1% precision metal film. This allows you to measure the current and set your bias accordingly. This is OPTIONAL however, but food for thought. Right now though, I would just focus on the changes we talked about initially: cathode resistors going to 820R, tonestack cap changes, input resistor changes, etc. If you like the voice of the amp after that, great. The Master Volume change of course would be one you'd want to do as well.

Already have the one Sozo and the Mallory and Orange Drop Caps plus the CMF60 resistors But will have to place another order for a good 250kA pot and the 2M2 resistor. I usually go thru TubeDepot and they have a 1%1/2 Watt Metal Film in the 2M2 size with a good CTS pot. That should do nicely.

Those parts should be fine for the MV.

Would have already started repacing parts but the wife complains about solder fumes, so I have to do work on it when she isn't around. Even complains if I work on it in the basement. Say's she can still smell it and complains of headaches.

Did you get a chance to look around Kevin's website and products?

Yeah, I have checked out his stuff before. I wish he wouldn't beat around the bush and say: this product scales the voltages on the screens.

And after reading the Q and A on the LP power scaling,
Have you be able to form any opinons regarding Kevin's products.
(other than they are a bit expensive)

I think it's good quality stuff and does what it is designed to do but other than that, I don't have any opinion. There is a member here, Dana Hall, who makes and sells a similar device that is quite flexible for voltage regulation. He calls it the VVR (up to rev3 now), Variable Voltage Regulation. Do a search here and you'll find information on it. They are great, work very well for this application and are very reasonable.

And for future reference, regarding Trainwreck amps, are there websites which you can order kits from or do you have to source the chassis , and other parts yourself then just follow the schematics and layouts in the public domain and/or in the Trainwreck section of the forum?

You are in the absolutely best place anywhere for information on Trainwreck amps, hands down...end of discussion. Start by going through the Trainwreck Files board on this site. All of the schematics you need, bills of materials, reference materials, builder's reference, all of it is here. I would spend some time doing searches. Any possible question you will have has been answered many times before. It's best that you do some research as you will be better responded to by the group if you don't appear to be asking to be spoonfed Everyone here is very nice though.

Trainwreck Files:
https://tubeamparchive.com/viewforum.php?f=3

[Colossal]

Colossal,
Before I submit my order on the 2M2 resistor and the 205KA pot,
Would changing the value of C2 above 500pF make any improvement for the better, or am I just better to stay with the value I have on C2. You seemed to indicate in an earlier post that a bright switch would be un-necessary with the values of 100pF and 500pF to switch between so, I was curious if going to say a value of 680pF or 1000pF on C2 make it darker or just cause problems.
Thanks,
Jerry

I've attached a snapshot of the effects of changing C2 (treble cap). This will increase the gain somewhat, allowing more through in the middle frequencies. Personally, I would stay with 500pF on C2 for now. You need to see how the amp responds to your voicing mods first. Bumping C2 up to 680 or 1000pF will not make the amp darker (see how the trough shifts to the left in the graph?). This is bypassing treble as well. Try going with 68k for R2 to start. You can also increase R6 or even add some parallel capacitance to ground on V1b increasing the Miller capacitance to shunt highs to ground (but might want to wait on that for now).

[jmohr58]

Thanks for the advice Colossal.
You've been a great help.
I will get my order placed for the Master volume and resistor.
Do the initial tone stack mods, swap out the cathode and input resistors and see how that works.
On C2 increase, since I am placing an order was just wondering on this for the future if it was justifiable and if my line of thinking was correct that if lowering C2 made it brighter than higer values would make it darker.
Your response allowed me to understand that C2 changes aren't that simple and more is going on behind the scenes.
So it looks like I will be ordering parts again in the future possilby so I will hold off on C2 for now.
I like the looks of the Mod you have drawn up and will be something to add a bit more complexity to the overall tone of this amp. In the future of course.
I hope the info on the Power Scaling answered some questions for you as well regarding the voltage on the screens in the operation of his circuit design.
Kevin doesn't seem to want to give anything up, unless you spend a fortune on his books and products etc. Even on his Q and A page I feel he only gives part of the story in his answers.

I had seen posts on this forum regarding the VVR circuit and thought it was a simular device. Since there are quite a bit of posts on people using it I can probably find out most of what is required to add it into an amp by researching the posts of the past.

Definately need to get my old O'scope fixed. It's an old Conar model 251 single trace, and googleing for schematics etc. doesn't get to far. But it was free and it does power on, displays, and feeding a 120vac input does give a quasi sinewave - rounded on the top and v shaped on the bottom. I suspect it's problem is the Sprauge multi Can cap marked 175 - 300uF @ 250v well times 2.
Or possibly one of the other electrolytics in it getting old and starting to fail. Dead just doesn't know it yet! Will have to source those caps from Just Radios or another Antiques Ectronics parts website that deals in high voltage higher value caps. If it is the multi sectioned can, will probably just get singles and disconnect the can and leave it in place for looks. Usually cheaper than replacing those cans.

Well have a good weekend, the wife is dragging me out to an SCA event.
It's the Ceilidh weekend, (pronounced Kaylee's or so I think) so much for a quiet weekend at home.

Anyway, Thanks again Colossal for your help.
Jerry