...or maybe it is more common and I don't know how to search for it properly...
so...long time lurker, blah, blah...
Anyway, I just finished building an 18w amp with my own design for the 2 preamps. Channel 1 is simply a single gain stage 12ax7 and Channel 2 is a 3 gain stage trainwreck style with the eq right after stage 1.
The amp sounds amazing but the issue i am having is when the gain for either channel is turned all the way down, the guitar signal still comes through the amp fairly loud. It is really dark and has very little distortion, and when I turn up the gain for either channel it is like the signal engages and acts/sounds normal. I have literally torn the thing apart to find the problem and have traced it to the second or third gain stage on the trainwreck style pre (does not do it when only the first gainstage is running), but no matter what I do I cannot get rid of the blasted signal when the gain is down.
I mean, it is kind of cool in that it can be used as an effect, and I can kill all of the signal by rolling off the guitar volume pot, but this is freaking annoying.
Any ideas?
so...long time lurker, blah, blah...
Anyway, I just finished building an 18w amp with my own design for the 2 preamps. Channel 1 is simply a single gain stage 12ax7 and Channel 2 is a 3 gain stage trainwreck style with the eq right after stage 1.
The amp sounds amazing but the issue i am having is when the gain for either channel is turned all the way down, the guitar signal still comes through the amp fairly loud. It is really dark and has very little distortion, and when I turn up the gain for either channel it is like the signal engages and acts/sounds normal. I have literally torn the thing apart to find the problem and have traced it to the second or third gain stage on the trainwreck style pre (does not do it when only the first gainstage is running), but no matter what I do I cannot get rid of the blasted signal when the gain is down.
I mean, it is kind of cool in that it can be used as an effect, and I can kill all of the signal by rolling off the guitar volume pot, but this is freaking annoying.
Any ideas?
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Hi, Most likely you have the volume / gain pot wired incorrectly or the pot is faulty in that it does not go to "0"; there is some residual resistance left when turned all the way down.
Looking at the pot with the shaft pointed towards you, left most lug is ground, center lug goes to the next stage, and right most lug is the "hot" signal.
If you could post a schematic, that would help.
Daniel
Looking at the pot with the shaft pointed towards you, left most lug is ground, center lug goes to the next stage, and right most lug is the "hot" signal.
If you could post a schematic, that would help.
Daniel
It could be crosstalk.
When you say "Turn the Gain down", do you mean turn the input volume potentiometer to the GND end or literally turn down the gain. If you mean the latter then the amplifier will still have some gain even in the minimum gain position.
When you say "Turn the Gain down", do you mean turn the input volume potentiometer to the GND end or literally turn down the gain. If you mean the latter then the amplifier will still have some gain even in the minimum gain position.
the pots are all wired correctly...and when I simply use the first gainstage in the trainwreck prethe issue goes away. The gain pot is right after the first gain stage and there are no pots after the first gain stage.
i don't have a schematic on my computer (it is handwritten) but maybe it will help to at least show you a schematic it is similar to.
i don't have a schematic on my computer (it is handwritten) but maybe it will help to at least show you a schematic it is similar to.
Attachments
Andy...I have the pot wired to ground...so I am shorting the whole signal. Signal still comes through.
Yea - sounds like either a faulty pot or there is another path for the signal other than through the pot. Get access to the pot with no connections at all to it. Measure from both of the outside terminals to the center connection while adjusting the pot to its extreams. You might find a "bump or dead spot".
i tested the pot through only the first gain stage and it worked perfectly...i don't think the pot is the problem
It's also likely that with that much gain, the signal could be capacitively coupling between various circuits in the wiring, so that some of the signal is literally bypassing the volume control. Try removing (shorting out) the 820 ohm grid resistor and see if that doesn't help the matter. That will make sure that grid is truly grounded when the control is turned to minimum.
Dave
Dave
In the actual preamp I do not have an 820R there, it is straight through to the second grid. I do however have a 680k resistor right before the grid of the third stage along with a 220k resistor to ground right before the 680k resistor...if that helps.
I went ahead and bypassed the 680k resistor with no change, and I then shorted right before the 680k to ground and that cut the volume in half. In fact there is a lowering of the volume when I short out the signal right before gain stages 2 and 3. When I do it right before 1 or after the coupling cap of 3 then it kills the signal completely. So I'm guessing somewhere between stage 2 and 3 I have a leak?
I went ahead and bypassed the 680k resistor with no change, and I then shorted right before the 680k to ground and that cut the volume in half. In fact there is a lowering of the volume when I short out the signal right before gain stages 2 and 3. When I do it right before 1 or after the coupling cap of 3 then it kills the signal completely. So I'm guessing somewhere between stage 2 and 3 I have a leak?
Hi nogood,
The exact schematic is probably the best way to solve this conclusively. Questions, wild stabs and try-this, try-that could go on for ages without it. Just sayin.
Scans of napkin drawings (or whatever you can do) are perfectly acceptable.
..Todd
The exact schematic is probably the best way to solve this conclusively. Questions, wild stabs and try-this, try-that could go on for ages without it. Just sayin.
Scans of napkin drawings (or whatever you can do) are perfectly acceptable.
..Todd
This may seem like an obvious question, but does your build match the schematic? I only ask because I've built a few guitar amps using similar 12AX7 building blocks for the gain stages and the "first switch on" faults I've had have ALL been down to me missing out a connection somewhere. It's usually taken quite a while to spot the problem, even when going through the schematic with a red pen and checking everything... 😱
My gut feeling is still telling me that you've got a dodgy potentiometer, but you say that you've tested this. Can I ask how you tested it?
Paul.
My gut feeling is still telling me that you've got a dodgy potentiometer, but you say that you've tested this. Can I ask how you tested it?
Paul.
Dragondreams - I tested the pot by disconnecting everything after the pot and routing the pot to the PI. When I did that everything worked like a charm. When I wired everything back in the same problem emerged. My gut keeps telling me it is the pot too, but i even wired it to another 1MA pot and the same problem persisted.
As for the build being like the schematic...i'll run through everything again. It wouldn't be beyond me to wire something incorrectly.
I'm freaking losing my mind trying to figure this out...lol
As for the build being like the schematic...i'll run through everything again. It wouldn't be beyond me to wire something incorrectly.
I'm freaking losing my mind trying to figure this out...lol
Welcome to the wonderful world of valve amps! 😀
I drive my lady friend up the wall and across the ceiling when I start ranting because my latest creation fails to work properly the first time... 😱
It sounds very much like you've eliminated the pot as the problem.
Have you tried "chopsticking" the amp? I've had some odd faults that have turned out to be caused by cables being in the "wrong" place when it comes to internal layout, though admittedly these have usually been oscillation issues rather than anything like your particular problem.
Is there any chance you could post a pic of the layout? I may be clutching at straws, but you never know. Someone may spot something. 😉
Boy you know if you really have all the plate resistors connected together to B+ it could be climbing over the top so to speak. This is where another source of B+ would come in handy like a lab supply. Try a nice big electrolydic from B+ to ground on the side of the 18k resistor where the plates come together .
It looks like you've build it Fender style where 2 input channels come in as the 2 different inputs of an LTP. You are turning down the pots of BOTH channels, right?
EDIT: Oops, I think that's actually Marshall type, but you get my point.
EDIT: Oops, I think that's actually Marshall type, but you get my point.
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Good point (I've only just looked at the schematic 😱 ).
You could try filtering between each pair of plates to decouple them. A 1K resistor and a cap around 47uF would probably do it.
very good point on the plate supply...i went ahead and added a 47uf cap with no luck (maybe a touch less 120hz huh...haha). I then replaced the 18k resistor with a 200R one (i noticed that i was not getting 350v from the supply but 300v) and I put stage 1 and stage 3 plate supplies before the 200R resistor and stage 2 and the other preamp after the 200R Resistor and put the 47uf cap after the 200R Resistor. Still no dice.
I do think there is a touch less ripple so big thank you there, though my initial problem is still there.
I do think there is a touch less ripple so big thank you there, though my initial problem is still there.
I suspect that some part of your wiring is working like an antenna and picking up signal from the input later down the line (even in the power amp section). I had a build that would still play through the power amp section even if the FX send (switched) was in use. Only way I could kill it was with a shorting plug in the return when not using PA. Never had time to work out the cause.
You might try shorting the grid of each section in sequence and see if you can get an idea of where it is being injected. The chopstick test is a good idea too.
You might try shorting the grid of each section in sequence and see if you can get an idea of where it is being injected. The chopstick test is a good idea too.
well i have narrowed it down to gain stage 2. When I test the signal after stage 1, and tonestack, and gain pot it works fine. When I test the signal right after stage 2 the problem arises. Stage 3 is fine when I bypass stages 1 and 2.
I have connected and reconnected everything multiple times, switched tubes, and switched around everything i can think of. Could the actual tube socket somehow be bad??
I have connected and reconnected everything multiple times, switched tubes, and switched around everything i can think of. Could the actual tube socket somehow be bad??
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