Don - The was the OT (Output) tranny center tap on the primary side that I was talking about. I haven't mentioned the PT wiring yet.
surfsup was talking about it I think. Now that you have mentioned it, you should move it away from the power supply cap ground. In the build guide layout plan you should note that the AC chassis ground, the PT center tap and the bias supply ground are all connected - and towards the rear of the chassis. This is better than what you have going on actually. FWIW, I think the AC ground should be on it's own ground and placed in the rear of the chassis (one of the OT tranny lugs), and the PT centertap and bias supply ground should on one of the PT's lugs.
I think the filament 100 ohm resistors should be on their own lug in an area right where your bias adjustment pot is, but more towards the power supply board. That's where I place mine, on a 3 lug point to point standoff, and basically I just carry the two filament wires from there in a twisted pair straight line to V5, pin 7 and 2. Which is to say that bias pot really takes up some crucial real estate, but if you did an overhead twisted pair like I do you would avoid that. BTW, I appreciate your desire to make it a pot that you can get to topside, considering your bias adjustment taps on V4 and V5, but there's probably a better way. Who else here is using a chassis mounted bias pot? Chime in with a pic maybe.
Too, Komet is an amp that you should take a look at for pics on where they placed this bias pot/bias measurement terminals. Since I don't have this installed in my amps I have never dealt with this. .......Mark probably has a word or two for you on this? Others?
OK, I am sorry this can't be whipped into shape in 5 minutes. And also note that when you say that you have another example of the amp - identical - that makes no hum, I am listening. That OT center tap hookup - if you did it wrong - would have explained this hum so easily. Damn, very sorry it wasn't this. Alright, tomorrow then.
Strange HUM.....
Moderators: pompeiisneaks, Colossal
Re: Strange HUM.....
Most people stall out when fixing a mistake that they've made. Why?
Re: Strange HUM.....
Woah, Don? I was looking one more time at your photos and I noticed the MV you installed. What version MV is this? I have not seen anything like it actually. I just assumed you had installed a LarMar type.
If you have a circuit diagram I could look at that would be helpful.
If you have a circuit diagram I could look at that would be helpful.
Most people stall out when fixing a mistake that they've made. Why?
Re: Strange HUM.....
Hi rooster....
We're getting off topic here now, but that is OK....
I am using the Trainwreck Express version A1a (Blue Guitar) with some modifications.
1. The first set of filter caps are connected ahead of the standby switch. (B+1 is connected after the standby switch)
2. I added a modified Lamar PPIMV. It resides between the .022uf caps and the Bias/220k resistor connection. It is isolated from the bias connection with another set of .022uf caps. This keeps the bias voltage from being affected.
If you look at the schematic, the circuit looks like this...
12ax7/82k----.022uf------MV----.022uf-----bias/220k---1.5k---EL34
The MV is a dual 1meg audio taper pot. 1 leg connects after the first .022uf and the other leg connected to ground. The wiper connects to the 2nd (added) .022uf and continues on to the bias/220k connection....
It does work very well-- it is smooth and allows good control of the volume HOWEVER (and you know this) the amp sounds best when the MV is maxed out.
We're getting off topic here now, but that is OK....
I am using the Trainwreck Express version A1a (Blue Guitar) with some modifications.
1. The first set of filter caps are connected ahead of the standby switch. (B+1 is connected after the standby switch)
2. I added a modified Lamar PPIMV. It resides between the .022uf caps and the Bias/220k resistor connection. It is isolated from the bias connection with another set of .022uf caps. This keeps the bias voltage from being affected.
If you look at the schematic, the circuit looks like this...
12ax7/82k----.022uf------MV----.022uf-----bias/220k---1.5k---EL34
The MV is a dual 1meg audio taper pot. 1 leg connects after the first .022uf and the other leg connected to ground. The wiper connects to the 2nd (added) .022uf and continues on to the bias/220k connection....
It does work very well-- it is smooth and allows good control of the volume HOWEVER (and you know this) the amp sounds best when the MV is maxed out.
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Re: Strange HUM.....
OK on the B+ routing.
As to the modified MV, I run a typical LarMar version in mine and still find it to be smooth, BTW.
However, in your modified version, you now have the four caps, two per side, connecting the pairs in series when the MV is bypassed - CORRECT? If this is the case, your caps change from .02 to .01 and this is a variable that would translate to less bass at the speaker when your MV is bypassed. If you got clever on me, you could increase these to .047 - but then face the result that there is more bass when the MV is engaged. Eh, and then there's the effect of the added parts: cap>resitor>cap. But OK, I respect your right to follow your own path. What you've got there will let the amp produce audio.
Are you applying the bias voltage to the MV via a single wire from the bias pot? And bypassing any circuit board connection of the bias supply?
Are the wires that connect to the MV from the caps on the board sheilded in any way? Are they sheilded to ground? Are they sheilded by the bias supply?
As to the modified MV, I run a typical LarMar version in mine and still find it to be smooth, BTW.
However, in your modified version, you now have the four caps, two per side, connecting the pairs in series when the MV is bypassed - CORRECT? If this is the case, your caps change from .02 to .01 and this is a variable that would translate to less bass at the speaker when your MV is bypassed. If you got clever on me, you could increase these to .047 - but then face the result that there is more bass when the MV is engaged. Eh, and then there's the effect of the added parts: cap>resitor>cap. But OK, I respect your right to follow your own path. What you've got there will let the amp produce audio.Are you applying the bias voltage to the MV via a single wire from the bias pot? And bypassing any circuit board connection of the bias supply?
Are the wires that connect to the MV from the caps on the board sheilded in any way? Are they sheilded to ground? Are they sheilded by the bias supply?
Most people stall out when fixing a mistake that they've made. Why?
Re: Strange HUM.....
rooster....
Single wire Bias is connected as shown in the drawing (through the 220k, connected between the .022uf & the 1.5k resistor.
Bias is variable from 28-35volts. I am running 30-31 volts at pin 5 on the EL34s, which If I remember, is about 45ma at the bias test jacks (pin 8+1 to ground through 1ohm resistor) on the rear panel.
MV is shielded wire (not sure if it needed to be), with the shield connected to the little ground buss that runs across the back of all the pots, which is also chassis grounded.
EL34's are JJ's
Still playing with an assortment of 12ax7's (GT, Radio Shack, Ruby, Sun, etc) to find that 'magic' combination.
I was talking to a guy last week-end (who works on amps) and I asked him if he had any tubes....He says "yes, but they are kinda cheap". He showed them to me....they were RUBY......hmmmm..........
Overall, this amp is quiet, except for a little hiss when turned up loud.....
oh, and, a little hum which is only heard if you sit real close to it....(are we back on that topic again?...LOL)
Single wire Bias is connected as shown in the drawing (through the 220k, connected between the .022uf & the 1.5k resistor.
Bias is variable from 28-35volts. I am running 30-31 volts at pin 5 on the EL34s, which If I remember, is about 45ma at the bias test jacks (pin 8+1 to ground through 1ohm resistor) on the rear panel.
MV is shielded wire (not sure if it needed to be), with the shield connected to the little ground buss that runs across the back of all the pots, which is also chassis grounded.
EL34's are JJ's
Still playing with an assortment of 12ax7's (GT, Radio Shack, Ruby, Sun, etc) to find that 'magic' combination.
I was talking to a guy last week-end (who works on amps) and I asked him if he had any tubes....He says "yes, but they are kinda cheap". He showed them to me....they were RUBY......hmmmm..........
Overall, this amp is quiet, except for a little hiss when turned up loud.....
oh, and, a little hum which is only heard if you sit real close to it....(are we back on that topic again?...LOL)
Re: Strange HUM.....
Er, almost, Don. 
The LarMar has all it's leads sheilded by the bias voltage. The reason being that once you fly these leads off the board and pass the resistance (220K) they are suseptible to stray fields. Shielding them with a ground strap doesn't really cut it. But I will let this go in one minute....
I have one last thought on this subject? Would you lift/disconnect the bias voltage connection to the MV at the pot, and turn the amp on, leaving it in Standby mode, and check the hum? Thanks.
The LarMar has all it's leads sheilded by the bias voltage. The reason being that once you fly these leads off the board and pass the resistance (220K) they are suseptible to stray fields. Shielding them with a ground strap doesn't really cut it. But I will let this go in one minute....
I have one last thought on this subject? Would you lift/disconnect the bias voltage connection to the MV at the pot, and turn the amp on, leaving it in Standby mode, and check the hum? Thanks.
Most people stall out when fixing a mistake that they've made. Why?
Re: Strange HUM.....
rooster...
1. You are saying that the MV wire shielding should be connected to bias voltage as opposed to chassis ground?
2. I don't understand your 'remove bias from MV' comment....The bias is not connected to the MV....it is isolated from the MV by the 2nd set of .022uf caps.
1. You are saying that the MV wire shielding should be connected to bias voltage as opposed to chassis ground?
2. I don't understand your 'remove bias from MV' comment....The bias is not connected to the MV....it is isolated from the MV by the 2nd set of .022uf caps.
Re: Strange HUM.....
Don - Look at the pic of the LarMar MV first. It is controlling the the drive of the PI in a very clever way and doing it by using the bias supply.
Obviously you have something different. How in the world is the bias voltage applied to your MV? Where exactly does it join the party?
Oh. I looked at your drawing again and see what you have done. So your flying leads are inbetween the two caps in series, and one pair of them grounds via a 1M resistor, the other controlled by the pot and then directly to ground. Hm, this is very foreign to me so I can't comment.
Understand that I brought the MV up because I was looking at any voltage that was 'on' when you turned your amp on, but still left it in standby mode. That would be the filament supply and your bias supply. All in hopes of disconnecting them to see if your hum issue was reduced.
Obviously you have something different. How in the world is the bias voltage applied to your MV? Where exactly does it join the party?
Oh. I looked at your drawing again and see what you have done. So your flying leads are inbetween the two caps in series, and one pair of them grounds via a 1M resistor, the other controlled by the pot and then directly to ground. Hm, this is very foreign to me so I can't comment.
Understand that I brought the MV up because I was looking at any voltage that was 'on' when you turned your amp on, but still left it in standby mode. That would be the filament supply and your bias supply. All in hopes of disconnecting them to see if your hum issue was reduced.
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Most people stall out when fixing a mistake that they've made. Why?
Re: Strange HUM.....
Oh, yeah, in extolling the virtues of the LarMar MV, yes, all flying leads are sheilded by the bias supply. Very clever feature.
Most people stall out when fixing a mistake that they've made. Why?
Re: Strange HUM.....
Both of my 1meg resistors are variable. It is a dual-stacked pot.
I see what you are hinting at now.... You think the bias voltage might be leaking into the circuit somewhere?
I see what you are hinting at now.... You think the bias voltage might be leaking into the circuit somewhere?
Re: Strange HUM.....
Heh, well, maybe. Right now your trannies are placed in the wrong position - and yet you have another Express with trannies positioned like this one WAS and it does not hum. (I assume you used identical iron?) So, all things being equal, the hum speaks to something else. It is possible either the filament supply or the bias supply is contributing to the hum.
So the iron has to be corrected ultimately, but in the meantime it's possible your layout is contributing. To me, since layout is King to reducing anything non-musical. I see room for improvement in your build, and that's speaking as someone who has built a a few of these differently and not had the issue you are experiencing. No, not a self-declared expert on TW circuitry. FWIW, I don't waste much wire in mine - or components - while your build doesn't seem too concerned about this. Which is fine, BTW, if yours is quiet. Quiet is the goal in a build like the Express because everyone who knows the amp knows it has everything musical figured out. It's just a matter of getting the circuit on the table, so to speak.
Alright, signing off, Don. I think you have enough to work with here. If the hum doesn't bother you at this point, you're done. If it does, keep at it.
So the iron has to be corrected ultimately, but in the meantime it's possible your layout is contributing. To me, since layout is King to reducing anything non-musical. I see room for improvement in your build, and that's speaking as someone who has built a a few of these differently and not had the issue you are experiencing. No, not a self-declared expert on TW circuitry. FWIW, I don't waste much wire in mine - or components - while your build doesn't seem too concerned about this. Which is fine, BTW, if yours is quiet. Quiet is the goal in a build like the Express because everyone who knows the amp knows it has everything musical figured out. It's just a matter of getting the circuit on the table, so to speak.
Alright, signing off, Don. I think you have enough to work with here. If the hum doesn't bother you at this point, you're done. If it does, keep at it.
Most people stall out when fixing a mistake that they've made. Why?
Re: Strange HUM.....
rooster....Thank you for all the info you have given me. I checked my other TW last night and it is dead quiet. There is obviously problems with my current build.
I'm gonna go to work and move the transformer(s) and the MV "Lamar Version" parts are on order.
Again, thanks to you and everybody else who has commented on this thread.
I have lots of work to do and am excited to see the results....
I'm gonna go to work and move the transformer(s) and the MV "Lamar Version" parts are on order.
Again, thanks to you and everybody else who has commented on this thread.
I have lots of work to do and am excited to see the results....