Volume Control problems

[EETStudent]

Ive begun building a Fender Bassman 5F6 preamp, and i encountered a couple problems. The volume control is actin funny. With it at ''0'' i still get decent sound, not good tone at all, but some volume. Same volume level continues til about level 3, where it then shoots up REALLY loud and toneful. As for troubleshooting, i inserted my guiter into the volume control, which is between the first and second stage, and from here it was responding correctly, but not very toneful. So i figure its something between the first stage and the volume, correct? I notice on the schematic that the bypass cap for the first stage is 250-6. 250uf, 6v?? The other ones read 25-25, which to my understanding is the norm. maybe this is affecting the volume, but i just dont know. any help?

[ricach]

Should have nice tone at that point. Which volume control (of the two) are you talking about? Exactly where did you insert your guitar - at the junction of the .02 and .0001 caps? The bias cap you're referring to is 250uF-6V. Try inserting at the input to the grid on the 12AX7 (at the junction of the two 270k resistors). You should have decent (but reduced) volume, and wonderful (but not euphoric) tone.

You said you only begun building the preamp - what are you using for the PI and power amp side?

[Don Moose]

To me that sounds like a linear-taper pot in a log-taper spot.
Or <i>maybe</i> a reverse-log (those are pretty hard to find, though).

Measure the resistance from wiper to ground with the wiper at 50% rotation - if the value is 50%, then you have a linear pot. If it's 10 or 20%, then it's the right pot. If it's 80 or 90%, you have a reverse-audio taper.

Hope this helps!

[ricach]

Don Moose, why would you suspect that? That's the second time I've heard that an audio vs. linear pot can give unexpected results. Isn't a linear pot going to ramp up quicker, but still go through the same values of resistance as an audio taper? Isn't zero'd out on linear also zero'd out on audio = zero ohms? And although a '3' on linear might correspond to a '7' on audio, isn't the impedance identical at that point? What am I missing here?

[Enzo]

Impedance? A 10k pot is a 10k pot. Zero is zero with either taper, and all the way up is 10k on either taper. But what is in the middle is where it differs. On a linear taper, half way up is half the resistance. But on an audio taper pot, half way up is a point only about 20% of the resistance. Your ear and the circuit doesn't increase volume in a linear fashion in response to linear resistance increases.

So if the thing won't go to zero, I would have to say the pot is not right. Zero should be zero. But a lot of pots don't make it all the way to zero. On a 470k pot, having a zero point of 20 ohms or something is usually not an issue. An ohm meter and some clip leads will let you chart the resistance of your pot.

BUT... The taper can also be part of it. SInce ausio taper means about 20% of the resistance at halfway, putting a linear pot there means now you have 50% of the resistance at that point - a lot louder then. So your volume tends to just right up fast with a linear pot and then less and less variance as you turn past half.

So EET here could have both things going on - a bad pot and the wrong pot. At least from the description.

[EETStudent]

I changed the type of pot and that HELPED, but didnt solve the nonexistent zero thing. Heres a few things I meant to mention earlier, and that may have an effect on the problem. As for the different channels, Ive only installed one input and its a Bright channel input, so no normal ins. That cathode bypass cap that i questioned, well instead of using 250-6 im using a 25-25. I need to buy a 250uf cap to check this, but that may be causing a problem. The coupling cap to the phase inverter is acting weird as well. It neatly soldered into the PI section, but when i turn the amp on I get very, very low volume unless I just tap the node and then it immediately gets loud.

[Don Moose]

Originally Posted by EETStudent

It neatly soldered into the PI section, but when i turn the amp on I get very, very low volume unless I just tap the node and then it immediately gets loud.

That does sound like some iffy soldering - it probably wouldn't hurt to retouch all the joints - and you may want to give special attention to the wires to the pots (both ends). I think your ground run from the volume pot may be higher-than-acceptable impedance. ... you do have actual ground wires from the pots, right? you're haven't soldered on lug to the pot body, and rely on the pot's mechanical connection to the chassis for ground? (popular practice, but not really reliable).

Hope this helps!

[EETStudent]

Hey Don, I do have wires running from the pot to ground. Regular volume hookup. In, Out, then ground. Could that loose coupling cap be a result of a loose terminal on the tube socket? I remember it being kinda loose and maybe pulled up a bit. Not sure but I will touch up the joints

[Enzo]

A cathode bypass cap of 25 instead of 250 will not cause problems. it would only affect the bottom end response a little .

[EETStudent]

Touched up the joints and replaced the pot, and still the problem continues. Circuit seems fairyl simple and it doesnt seem like there are many places to go wrong. Any other troubleshoot routes I can take?

[Don Moose]

There's always R.G.Keen's Tube Amp Debug Page

Two ideas before you go trying to tighten socket pins - is the coupling cap (or any other component) cracked or showing signs of heat (soldering) or mechanical stress?

Have you checked to resistance from the bottom of the volume pot to GND and compared it with other ground runs?

It's usually something simple that you will kick yourself for taking so long to find (long experience speaking).

You will find it. You will fix it. For now, I think I suggest you mess with something else for a few days and come back to this with a fresh (and open) mind.

Hope this helps!

[EETStudent]

Great advice Don. thanks a lot. My wiring is fairly amateur, so the problem probably therein.

[EETStudent]

what does it mean when your volume is completely cut...at level ''2''? and at level "0" there is volume present?

[tboy]

That would suggest a leakage path that doesn't go through the volume control. The leakage would mix with the normal signal at a point where the two signals have opposite polarity.

[Enzo]

Classic crosstalk.

[EETStudent]

Im not exactly what you mean by leakage path, nor crosstalk. could you explain?

[Enzo]

OK, the signal comes in the front and steps stage by stage throught the amp. Now lets say somewhere in the middle a coupling cap fails open, so the signal path from the plate of one stage is broken on the way to the next grid. The signal in the earlier stages - those before the break - are enough to radiate some signal around inside the chassis. The rest of the circuit after the break has enough sensitivity and gain to pick up some of this scattered signal. SO you hear a much weaker signal coming out the speaker. This is what he called leakage, I call it crosstalk.

Crosstalk is where two channels interact. Imagine on a stage where you sing into a mic. When you are not singing, your vocal mic still picks up some of the drums from behind you. That would be a form of crosstalk, though not electronic. or now and then your local TV news show has the weather guy start his forecast, but they don't turn his mic on. But you can still hear him in the background coming through someone else's mic. That is also acoustic crosstalk.

In poorly designed audio mixers or multitrack tape decks, sometimes a hot signal on one channel will bleed into the channel on either side a little. That is electronic crosstalk in the board.

In the guitar amp this is happening all the time, but the signal itselg is WAAAY stronger than any crosstalk, so you never hear it. it might have some small contribution to the sound of an amp, but that is another story for another time. But when the main signal cannot make it through the amp, all that is left is this crosstalk.

Now you have an example where it does matter some. Your volume control all the way down has cut off the main signal, but there is still audible crosstalk coming through. it isn't coming through the volume control, it is just leaking around things. That is what you hear when the control is down. As it turns out in your amp, the main signal and the crosstalk are out of phase at that point.

So as you turn the volume up from zero, the main signal starts to increase. But it is adding itself to the crosstalk already there. Since they are out of phase, they start to cancel. Full cancellation happens when the main signal is up at the same level as the crosstalk. That is the nothing that comes out at 2 or whatever. Then as you continue up with the control the main signal gets louder and louder, and crosstalk is swamped and forgotten.

[pdf64]

Try a different 12ax7. I've got a NOS Mazda ECC83 from Watford Valves (with the cryo treatment, though I don't think that's the cause) which does the exact same thing. Somehow the input from the jack is getting onto the grids of both sections, though I can't see how. As the amplified signal from the first section is inverted compared to the unamplified signal that's getting on to the grid of the second section, the 2 signals cancel out at about 2 or 3 on the volume control which is between them. If the scope is triggered from the sig gen, the signal on the second plate can be seen to fall to zero, then invert, and then get full amplification, as the volume is turned from min to max.
Other identical Mazda ECC83s don't do this, the other cryo 12ax7 (EH) I've got doesn't do it.
So try a different tube and see if the problem goes away.
Which is always the golden rule for troubleshooting a tube amp anyway.
Hope that helps - Peter

[EETStudent]

yeh man you guys are incredible. So basically ive got a leakage somewhere. thats another step in the right direction. lol Enzo thanks for all your little analogies. Ill have to let you guys know when I fine tune this amp and get it cooperating. I cant even begin to explain to anybody the amount of knowledge Ive gained over the past month or so. Schematics and concepts used to terrify me, but now i got more confidence and understanding than i thought i would ever have. thanks for everyones help