Wacky submini tube preamp problem

[Ripthorn]

So my great little submini tube amp decided to go weird on me the other day.  All of a suddent the clean channel became very quiet and more distorted than usual.  I also noticed that if I used the (switchable) tonestack (with gain recovery stage afterward), the volume was not only louder (it used to just be the same level as the clean channel) but the tone was no longer thin and distorted.  I swapped the preamp tubes and nothing.  So I started probing around and found that at the output of the clean channel, the tone changed depending on whether I sent the signal to the tonestack or straight to the volume pot.  With the signal going straight to the volume pot, it was still thin and distorted, but if I switched on the tonestack (which is down the signal chain) the sound was again full and clean, like it used to be.

So this has me thinking that something had to change so that something down the signal chain is loading the clean channel (since the gain recovery stage has a really low output impedance being an opamp).  I just don't know what it could be.  Any suggestions?  This is a really weird problem and has me scratching my head pretty good.  Also, I don't think it is the power amp since I am taking the signal from the line out jack that cuts the signal to the power amp.  The only other thing it could really be (it seems) is the relay or the volume pot.  Help me out on this one.

[Pablo1234]

Wow thats a bummer problem you have. First thing I always do when makeing a device is draw the schematic and at each stage I would put a note as to what voltage I expect from the stage with 3 difrent signals inserted. I use 800 hz 1Vp-p, 1100 hz 1Vp-p, and 3Khz .7Vp-p. Then I will test thies calculations and get the real values at each point. With the complexity of most devices and some oversights I make about the circuit design I am typicaly withing 20% of the calculated value. But once I have the values I can then always troubleshoot my circuits later on with the values.

I know that dont help you much now but its something to keep in mind.

So lets clarify your circuit, you have a buffer stage - switch to slect the No Tone stack / Tone stack with componsating gain for the load of the tone stack on the clean channel?
I would take a realy close look at the switch you are useing, thier my be a short somewhere. if you are louder on the tone stack and quieter on the no tonestack settings with distortion I would say the signal is bleading to the other channel. Isolating the circuits from the amp all together may help you find the problem.

[frequencycentral]

Starved heater somehow?

[Ripthorn]

You know Rick, that is an interesting comment, because one of my preamp tubes was lighting up like a Christmas light, but the voltage drop across the heater was only reading about 6V.  It is running in series with a second tube on a 12V supply and both tubes read right about 6V drop across them.  I have no idea how that could be seeing as how one of them is so bright while the other is about the brightness I am used to seeing.  Any thoughts here?

Also, to clarify my initial post, I listened to the output of the clean channel right after the decoupling cap.  The tone changed dramatically depending on whether the signal then went to the volume pot or to the tonestack/gain recovery.  I think it is so weird that the signal was changing so dramatically depending on where it went AFTER I listened to it.  This seems to suggest loading, but I can't think of what would be causing it.

[frequencycentral]

A really bright heater sounds like a problem. Is it the same if you replace the bright tube? Sure there's not a short around where the two heaters meet in series? I've never had a submini go bad, but I wonder if the heaters glow brighter when they do. The only way I've ever got a really bright heater was once on the breadboard when I had two submini heaters in series but accidentally grounded the conjuction. That heater got a full 12v and glowed so bright. But it survived.

[Ripthorn]

I checked for a short, but like I said, each tube was dropping ~6V.  I will swap that tube yet again, I suppose.  This is really starting to get strange.

[Ripthorn]

Okay, so I'm pretty sure this isn't a bad tube, because the I have put in a new tube for the third time and everything is still the same.  I have checked to make sure the bias is the same with the tonestack engaged and disengaged (don't know why it would be different, but the clean channel with tonestack bypassed sounds like it is biased poorly).  The heaters are all in order.  It seems like it has to be a loading issue, but I can't figure it out for the life of me.  When I put the audio probe after the DC blocking cap at the end of the clean stage, the tone and volume changes dramatically when I turn the tonestack on and off.

I hope this doesn't sound redundant.  I am just stumped to no end.  I know I could just make it so that the tone stack is always on, but the thing is that it was working perfectly for a good couple or few months before this happened.  Anyone have any insight?

[bancika]

what kind of preamp is it? do you care to share the schematics?
tnx

[Ripthorn]

I don't mind sharing the schematics, as it is a minor variation on a circuit drawn up by Jered in this thread http://www.diystompboxes.com/smfforum/index.php?topic=73648.0.  In his drawing, I have omitted the tone section and the output of the preamp goes to a relay that sends the clean signal either to the tone stack toggle switch and on to the volume pot or to the next two gain stages then to their tone stack toggle and on to its volume control.  The lack of volume and tone also affects the dirty channel, so if the problem is loading, then the input impedance of a triode stage is not sufficient, but the input stage of an opamp is.