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]

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.