SKYLINER build - inspired by ODS #124 and #060

[talbany]

The layout is correct 183 used the 4 ohm tap not 8 ohm. (Less GNFB)
On twin iron he used 8 ohm tap

Tony

Isn't the twin also 4 Ohm?

OOPS!! Yes Senior moment

Tony

[Stephen1966]

"Loosen it up," "sterile," - stiff... yeah. Regarding the second generation thread, I think you might be overestimating my intelligence on these matters, Tony! I mean, it's an interesting comparison but so much of Ben's sound is in his playing... I worked through the 3rd generation schematic which I understand is close to your 2nd gen layout, and there are a lot more design changes there, than I would care to attempt with my amp, GNFB, power section and so on... I think we all appreciate how you elaborated on this though.

The notes I'm finding are beautiful and complex, I'm sure a Jazz musician would love it. I'm just looking for something a bit more... unrestrained, and definitely more bloom. Marshall-esque? Maybe not, but... Because even if I hammer it with the volume at stadium levels the amp kind of just sits there like a forlorn Fender. The notes just don't reach out and grab you, the fundamental doesn't swell, at least not very much, and not in the effortless way the amps in the clips do.

I'm putting together a shopping list now, so thank you for the heads up. I'm not too interested in going all the way down that rabbit hole, I'm just being nosy: I want to know what's going on I don't have any reservations about using CFs in this part of the signal chain, but I'm ordering MFs as well. For both Bluesmaster and #183 style PI's. IIRC, the early incarnation of the #124 was more akin to the #183. The components are cheap enough and the swap on these boards is easy enough. Worth a try.

Steven
So I think there were a few things that were either mistranslated or misinterpreted!

The comment about the 2nd Generation/Bluesmaster in my 1st post was to simply illustrate some of the different sound characteristics of each of the PI's used in the ODS model amplifiers inherent to the design and not the parts!, since that's what I thought you were asking about (Phase inverters) So to avoid any misunderstandings here! When I talk about resistor types/brands I am talking about nuances or small changes in sound and not the things you described in the your last post to me (unrefined, Bloom). This is clearly something else going on with the amp and I don't think your cure is in your resistor types. Sorry for any confusion!
BTW..Your amp will be even more complex to diagnose here because it is a one of a kind and you didn't follow the layouts in the files section, hopefully you can get squared away!

Tony

Thank you for clarifying. I have some ideas about increasing the gain the first stages of the clean channel and I'm going to tinker with the load resistors in the power section, too (preamp voltages are all on the conservative side). I've been focusing on the PI for a while because I've been looking for a balanced output and I know that a slight imbalance there can boost the second harmonics which seem to be an essential element in the formation of bloom. First job is to replace the 5k pot with a 10k and then the reduce the plate resistors to give the pot a bit more of a say in proceedings, and then start tricking out the PI looking for any improvements it might bring. There's nothing essentially wrong with the sound of the amp [wish I had easy access to some recording gear - it's so hard to characterise!] but I "just know", this amp has more to offer. I see playing with the resistors as a fairly benign and easy thing to do and it seems unlikely to do any harm... Altering the load lines in the PI in this way might be all it takes, it seems to have limitless headroom, maybe it just needs a little less.. but with more gain. The question of NFB (local and global) wasn't something I thought of until we started thinking about the snubbing cap in the Bluesmaster circuit but this deserves further experimentation I believe.

I am likely to be confused about these things, its true - this is my first time round the block - but please don't think I don't appreciate your ideas. The devil is often in the detail and I'm into the nuances... as Richard said, "all the details matter."

More complex? Sure. I totally get it. But almost all complex tasks can be broken down into small steps, the PI is just one of the small steps. I haven't even gotten to the reverb or the FX loop yet

[Stephen1966]

It's actually the cleans which are bugging me more than the OD, I like the growl I get from the OD and as long as its not set too high, I get good note definition in power chords there.

The Bluesmaster - if we include the presence pot as well - has a NFB total resistance of close to 130k, the 183 and 124 is about 7k though in the 183 schematic I'm looking at the NFB is tapped off the 8ohm lead - I think that might be a mistake, Tony's layout shows it tapped off the 4ohm lead.
https://tubeamparchive.com/viewtopic.php?f=5&t=13095

I was lifting the NFB earlier last week to double check I had the OT wired up correctly - I didn't think to run a sound check then. Honestly, it's a bit of an ordeal to solder and unsolder but I could put a switch on the bottom of the chassis for it. I'd rather not dispense with it completely.

How would you go about testing the NFB values?

I'm pretty sure the GNFB is on the 4 Ohm tap.

To test the GNFB, you can either replace the resistor with a 5K pot or just swap out values, should be pretty simple.

To test the LNFB, just lift one of the 10M resistor legs.. or put it on a switch.

The other thing I would highly recommend, is to let the amp settle and break-in a little. Give it 100 hours and see if it is still not to your liking.

Thanks! Great advice, I'll have a think about it... It would be easy to implement the way you suggest. Rather than reducing the resistance of the 4.7k resistor (assuming this is the one you mean), I'm curious what the effect of increasing it would be. Increasing high frequency distortion?

[ijedouglas]

Thanks! Great advice, I'll have a think about it... It would be easy to implement the way you suggest. Rather than reducing the resistance of the 4.7k resistor (assuming this is the one you mean), I'm curious what the effect of increasing it would be. Increasing high frequency distortion?

Yup, 4K7 resistor. Actually, I got it wrong, you would want a bigger pot there as you would want to try increasing the value. More resistance == less NFB == more lively. Maybe a 10K

I have an AN Wonderland I built a while back that I put both GNFB and LNFB on switches. It definitely livens up the amp when removing the GNFB

[Stephen1966]

Thanks! Great advice, I'll have a think about it... It would be easy to implement the way you suggest. Rather than reducing the resistance of the 4.7k resistor (assuming this is the one you mean), I'm curious what the effect of increasing it would be. Increasing high frequency distortion?

Yup, 4K7 resistor. Actually, I got it wrong, you would want a bigger pot there as you would want to try increasing the value. More resistance == less NFB == more lively. Maybe a 10K

I have an AN Wonderland I built a while back that I put both GNFB and LNFB on switches. It definitely livens up the amp when removing the GNFB

Appreciate the insight

[Stephen1966]

The Bluesmaster - if we include the presence pot as well - has a NFB total resistance of close to 130k, the 183 and 124 is about 7k though in the 183 schematic I'm looking at the NFB is tapped off the 8ohm lead - I think that might be a mistake, Tony's layout shows it tapped off the 4ohm lead.

The layout is correct 183 used the 4 ohm tap not 8 ohm. (Less GNFB)
On twin iron he used 8 ohm tap
BTW..An easy way to check GNFB is to take a 10k 15k pot wired as a variable resistor one lead to the NFB resistor and the other to the tap. You can then adjust the pot to the setting that works best then measure the pot value and replace the GNFB resistor to that value.Just make sure your presence control still works with that value.
Tony

Complementing what Ian said, this sounds like a good idea. I was thinking "what if I made a little PCB?" but no need. Your way is better. The pot could be substituted with the decade box. I would just need to watch for that whiff of burning resistors

[talbany]

"Loosen it up," "sterile," - stiff... yeah. Regarding the second generation thread, I think you might be overestimating my intelligence on these matters, Tony! I mean, it's an interesting comparison but so much of Ben's sound is in his playing... I worked through the 3rd generation schematic which I understand is close to your 2nd gen layout, and there are a lot more design changes there, than I would care to attempt with my amp, GNFB, power section and so on... I think we all appreciate how you elaborated on this though.

The notes I'm finding are beautiful and complex, I'm sure a Jazz musician would love it. I'm just looking for something a bit more... unrestrained, and definitely more bloom. Marshall-esque? Maybe not, but... Because even if I hammer it with the volume at stadium levels the amp kind of just sits there like a forlorn Fender. The notes just don't reach out and grab you, the fundamental doesn't swell, at least not very much, and not in the effortless way the amps in the clips do.

I'm putting together a shopping list now, so thank you for the heads up. I'm not too interested in going all the way down that rabbit hole, I'm just being nosy: I want to know what's going on I don't have any reservations about using CFs in this part of the signal chain, but I'm ordering MFs as well. For both Bluesmaster and #183 style PI's. IIRC, the early incarnation of the #124 was more akin to the #183. The components are cheap enough and the swap on these boards is easy enough. Worth a try.

Steven
So I think there were a few things that were either mistranslated or misinterpreted!

The comment about the 2nd Generation/Bluesmaster in my 1st post was to simply illustrate some of the different sound characteristics of each of the PI's used in the ODS model amplifiers inherent to the design and not the parts!, since that's what I thought you were asking about (Phase inverters) So to avoid any misunderstandings here! When I talk about resistor types/brands I am talking about nuances or small changes in sound and not the things you described in the your last post to me (unrefined, Bloom). This is clearly something else going on with the amp and I don't think your cure is in your resistor types. Sorry for any confusion!
BTW..Your amp will be even more complex to diagnose here because it is a one of a kind and you didn't follow the layouts in the files section, hopefully you can get squared away!

Tony

Thank you for clarifying. I have some ideas about increasing the gain the first stages of the clean channel and I'm going to tinker with the load resistors in the power section, too (preamp voltages are all on the conservative side). I've been focusing on the PI for a while because I've been looking for a balanced output and I know that a slight imbalance there can boost the second harmonics which seem to be an essential element in the formation of bloom. First job is to replace the 5k pot with a 10k and then the reduce the plate resistors to give the pot a bit more of a say in proceedings, and then start tricking out the PI looking for any improvements it might bring. There's nothing essentially wrong with the sound of the amp [wish I had easy access to some recording gear - it's so hard to characterise!] but I "just know", this amp has more to offer. I see playing with the resistors as a fairly benign and easy thing to do and it seems unlikely to do any harm... Altering the load lines in the PI in this way might be all it takes, it seems to have limitless headroom, maybe it just needs a little less.. but with more gain. The question of NFB (local and global) wasn't something I thought of until we started thinking about the snubbing cap in the Bluesmaster circuit but this deserves further experimentation I believe.

I am likely to be confused about these things, its true - this is my first time round the block - but please don't think I don't appreciate your ideas. The devil is often in the detail and I'm into the nuances... as Richard said, "all the details matter."

More complex? Sure. I totally get it. But almost all complex tasks can be broken down into small steps, the PI is just one of the small steps. I haven't even gotten to the reverb or the FX loop yet

Steven
Thanks for the rundown makes perfect sense now I wish we would of had this discussion earlier. After being here forever you are not alone as I have seen many posts here where some builders have asked the same thing? where is the crazy note bloom this thing is famous for. I can just tell you that I've built some amps where the notes just don't want to bloom as well as others do! and there is this one... https://www.youtube.com/watch?v=qyB4HedEOvs
https://ampgarage.com/forum/viewtopic.p ... dback+city

that bloomed into feedback on just about every note,so don't beat yourself too much about it. Do your best to get it balanced, voltages set and get the GNFB and break the thing in..
PM me if you have any other questions!
Out of here
Tony

[Stephen1966]

... Thanks for the rundown makes perfect sense now I wish we would of had this discussion earlier. After being here forever you are not alone as I have seen many posts here where some builders have asked the same thing? where is the crazy note bloom this thing is famous for. I can just tell you that I've built some amps where the notes just don't want to bloom as well as others do! and there is this one... https://www.youtube.com/watch?v=qyB4HedEOvs
https://ampgarage.com/forum/viewtopic.p ... dback+city

that bloomed into feedback on just about every note,so don't beat yourself too much about it. Do your best to get it balanced, voltages set and get the GNFB and break the thing in..
PM me if you have any other questions!
Out of here
Tony

Wow! This is like an episode of the Twilight Zone... I'm going to enjoy this.

[Stephen1966]

Ok, I had a listen to the feedback monster you built Tony. That thing is wild! I scanned the whole 18 pages of the post, real quick as well. There was no single explanation why it would feedback so readily, I think as you concluded, yourself, it was probably a combination of factors. As one person commented, the whole thing is a system, and I tend to agree with that as well. Following your PM, I did as you suggested...

Richard, pointed this out as well, that the snubbers on the PT plates to ground were odd. I think I screwed up when I interpreted bepone's advice, as in "1000pf across the whole primary would be ok". I think I must have read it as across the primary, with a 1000pf on each leg, like you might say, you can find McDonald's across the country - I don't know now, can't remember what I was thinking. In any case, they were never a necessary component so, I followed your advice, Tony.

First, just playing the amp to get a sense of the baseline, melodic stuff and chords, and then, detaching one of the caps from the one of the power tubes (one side of the OT). This did indeed improve the tone. Not by much, but I could hear the notes were fuller with the tiniest hint of feedback when the volume was pushed way up. So (in for a penny, in for a pound) I detached the other one as well. The best results were gained with the Rock/Jazz switch in the middle position - effectively PAB - and the additional gain played a part I think, which I will come back to in a minute. There's reasonable sustain, and playing with the Volume and Gain, I can get a nice note without too harsh an attack. In some positions, it's really percussive, in others not so much. Meanwhile, I was making very small adjustments to the PI trimmer, the effect being barely discernible... it's still not "giving it up" though. You have to be way beyond comfortable listening levels to get any sense of feedback and then, it's only like a quiet layer inside the note.

I decided to really push it in OD. Again, this was with R/J switch in the PAB position and here, I get feedback more readily. I also get the "sizzle" on the bass notes particularly, less well defined on the treble side. And, in the higher positions I get that kind of hollow tubescreamer kind of effect (don't know what else to call it) on the melodic notes. If I put it in Rock mode, the effect is muted and in Jazz, it disappears altogether. This is really loud though, I have a concrete floor in the basement and I can feel the sound vibrations in the soles of my feet. Stuff was rattling all over the shop. Fortunately, none of that rattling seems to coming from the cab. After about an hour of this, I switched it back into clean mode and instantly felt the volume and sound pressure drop away. Like half volume, even though I didn't touch the Volume or Gain. The Level and Ratio of the OD was about 45% and 55% respectively, at other positions, I lost the tubescreamer effect. The purest high notes though have a slight, unpleasant distorted sound at the edge

So it makes me think that the clean channel is not providing enough gain. I understand that for an equivalent volume from clean and OD the OD has to be set lower as the clean affects the OD, and Volume affects the Level similarly. I've been thinking of the different ways I might increase the gain in the clean channel. First, I could up the voltage leading to the PI, as I mentioned before, all the voltages are on the conservative side and I'm going to try a smaller resistor at that node - instead of 2.2k/5W, a 2k/5W or smaller, a 1.8k. I expect this will increase the voltages by 5-10V which is all I need. But this would increase the voltages across the board, downstream - PI, reverb, OD and clean channel. Next, I could try increasing the voltages of B+8 which feeds the clean and goes on to the FET. The FET is hovering around 16V which seems sort of normal listening to the feedback of others in that corner of the forum but I know it could probably take 20V without turning to a smoking pile of ash.

The other way of increasing the gain, or increase the signal strength if that is not exactly the same thing, is to change the grid resistor to the first stage of the clean channel. At the moment, it has a 33k in that position, which I took from the schematics. But both the 183 schematic and Tony's layout of the 124 show a 22k grid resistor. The picture of the original #124 is hard to make out, it looks like a 5 band resistor (orange, orange, orange, red, gold? = 33.3k/5%). Anyway, I never really questioned this in the execution of the schematic but I think I got a 22k at the same time as the 33k so I can try and see if it improves the performance of the clean channel. I have a theory, that by running the clean channel a little hotter, I can improve the balance between the clean and the OD and make that "bloom" a little easier to access. maybe even get it to generate without bypassing the tone stack with PAB.

I had greater expectations of the effect of removing the snubbing resistors but I'm not entirely disappointed, I feel it's getting there. But adjusting the clean channel gain, running all the voltages downstream of the power section higher, trimming the GNFB, and adjusting the PI resistors to run hotter would all appear to be needed as well. I am interested in what you think. Please don't hold back if you think I am overlooking something.

In the meantime, Tony, did you ever consider putting snubbing resistors on the plates to ground with your screaming monster...? I can attest to the fact that it works to kill feedback if not perhaps entirely with a large enough cap, then at least a little, particularly if it proves unmanageable as some others found with their amps.

[ijedouglas]

Before changing dropping resistors, have you tried rolling through different tubes? In my experience, different tubes had a far greater impact on plate voltages than tweaking dropping resistors. Looking at various gut-shots HAD was pretty consistent with the values of his dropping string. I may be way off on this one but it has worked for me.

Regarding the V1a grid resistor. I'm not sure you do much for the gain by going from 33K to 22K. I read few posts from Charlie Wilson where he points out that 33K was used on low-plates and 22K on high-plates (#0102 being the exception 33K => high-plate). I think this may change the tone more than the gain

What is the taper of your pre-amp vol pot, 10% or 30%? Sometimes the taper may result in the pre-amp seeming to be weak when in actual fact it just needs to be turned up to 6-7

I would concentrate on V1 tube and pre-amp vol pot initially (maybe your TMB pots as well if the PAB has a huge bump in vol).

[martin manning]

Looking at various gut-shots HAD was pretty consistent with the values of his dropping string. I may be way off on this one but it has worked for me.

Indeed, very consistent.

Regarding the V1a grid resistor. I'm not sure you do much for the gain by going from 33K to 22K. ... I think this may change the tone more than the gain.

Correct. It’s making a LPF with the tube’s grid capacitance. 22k will admit more HF than 33k, but the corner point for 33k is already about 48 kHz.

[Stephen1966]

Looking at various gut-shots HAD was pretty consistent with the values of his dropping string. I may be way off on this one but it has worked for me.

Indeed, very consistent.

Regarding the V1a grid resistor. I'm not sure you do much for the gain by going from 33K to 22K. ... I think this may change the tone more than the gain.

Correct. It’s making a LPF with the tube’s grid capacitance. 22k will admit more HF than 33k, but the corner point for 33k is already about 48 kHz.

Ian - rolling tubes, I like that idea. For a start, I could swap the Cl and OD tubes and see if that works to bring them into a better balance, but I have some other 7025s and 12AX7s knocking around. I could try those... any solution that doesn't require soldering has to be good

Regarding the dropping string, I've been careful to calibrate the variac to 240V each time I've measured the voltages off it. You would need to look at my PSU to see how it is different to a standard ODS.

https://ampgarage.com/forum/download/file.php?id=63806

Mine has 8 stages of filtering to accommodate the reverb, 9 if you include the FET. It's more like the #060 in that way. The topology of the #060s PSU isn't very clear but rootz was able to put together a sequence in Spice which proved to be almost spot on. The thing no one could account for, was the secondary HT voltage coming off the actual power transformer. That was off the calculated value, down about 5 volts. That carries through the whole string so that by the time I get to V1 I am down about 10V. It was a remarkable calculation by rootz, to put all the voltages in such close proximity to the original measured voltages of the #124. Single figure percentage... I think the biggest deviation was under 5%. So for a while now, I've just been sitting on it as all things considered, it was fine to be getting along with. My plan is really for a very fine adjustment. To leave the OT and screen nodes as they are and reduce the next stage's resistor which feeds the PI and downstream, by a couple of hundred ohms. If this were a regular ODS, as you both seem to suggest, I agree that I would be better off looking elsewhere. This is a part of the circuit where as Tony mentioned it doesn't really follow any previous layout - loosely the #060 perhaps but it was all devised by rootz and me, with rootz deserving the most credit. There's a little of Martin's precision power supply for the ODS in there too. They look very similar, but they aren't the same.

Regarding pots, they are all standard CTS 450s I believe, hard to tell - the serial numbers on CTS pots can be cryptic - but the supplier told me they are "normal"! And so, they should be 10% taper for the audio types. The clean channel starts to wake up at around 50% rotation. If I had numbers on my amp, I'd want it to go up to 11 (of course) don't know where you have 6-7. Imagine it's 60-70%. so maybe the pots aren't a big issue. Anything less than about 50-60% though, is bedroom level. And I need the Master around that level as well. PAB... huge bump? No, I wouldn't call it huge. It's what I imagined it could be when you drop the tone stack. It doesn't seem abnormal.

Increasing the HF might not be such a bad thing by reducing Rg, but I am probably overestimating the effect on gain and you are probably right that I'm likely to get more meaningful results from a bit of tube rolling. I really don't want to mess with the 100k/1.5k, Rp/Rk magic... I still want that rich tone the low plates are renowned for and now, I understand the grid resistor is a part of that. I will defer to your better judgement on this, leave it alone and look elsewhere.

[ijedouglas]

Got it on the power supply

As for the tapers, I think 124 has CTS 2-35 pots which I believe are 30% tapers. I have built with both 10% and 30% but prefer the 30%.

[talbany]

Steven

Glad your making progress PM sent

Good Luck
Tony

[martin manning]

By consistency on the dropping string, I believe Ian and I are referring to the 22k and 2k2 from PI to OD and OD to clean, which produces a more or less standard ~300V on the PI plates, ~195V on the OD, and ~190 on the clean.