Master Volume trouble

[hesamadman]

On my 30 watt AC30 with a 2204 preamp, I have realized a problem with my MV. I haven't turned this thing on in a while. I turned it on today and noticed that if the MV is turned the way down, signal still passes to the power amp. The MV follows the tone stack identical to a 2204. This may have always been like this I'm not sure. Generally I will play my other 30 watt which is very similar but setup for less gain/reverb/and has a PPIMV. I'm wondering if it's something wired wrong from the get go or maybe just need to install a resistor on the MV? The amp works and sounds 100% fine of the knob is turned up just a tad. It's just that when turned all the way down, some signal still passes. High frequency signal. Very trebly.

[sluckey]

Connect a gator jumper lead between chassis and wiper of the MV. Does that kill the signal?

[hesamadman]

It kills the main signal. The unwanted signal when MV is all way down, still present. I have that signal entering somewhere don't I

[sluckey]

yep

[hesamadman]

Tapping with a chop stick. MV the way down. (Pre gain seems to affect the signal. Obviously higher it's turned the more signal I'm getting.) But tapping tone stack components, that gain stages filter cap, and the first half of the PI comes through on the speaker pretty well. Any suggestions for trouble shooting?

[TIMBO]

Great Topic, I have three amps that are doing the same thing.
The interesting thing about one amp is it has a twin. This is the one I built http://www.guitargear.net.au/discussion/index.php?topic=46089.0
A mate built one off the same schematic. There is some differences, mainly in the PSU and components.
I added a voltage divider prior to the PI and the other had a pot.(master vol)
Mine does not suffer this problem??????

[hesamadman]

I have no idea. Ive tried so much. Completely bypassed tone stack. That means cathode follower direct to PI CC. Replaced pot (obviously). 


I think the PPIMV in my other one masks this issue.

[sluckey]

There are several ways for signal to sneak around that MV. Wiring error, or crosstalk due to sloppy layout and lead dress, insufficient decoupling between power supply nodes, not enough power supply nodes, or even sub par filter caps.

[PRR]

> if the MV is turned the way down, signal still passes to the power amp

How much?

Disturb-the-preacher loud? Or wake-the-neighborhood LOUD?

You never get 100.00% cut of a signal. 99.9% is doing good. Nothing wrong. Just the cussedness of high-gain AMPlifiers built of ohmic conductors that radiate into other conductors.

[hesamadman]

Disturb the sermon kind. Gets louder as I turn up the gain. AND its present if I disconnect the master volume from the PI input. I've spent a long time on my layout. I don't think it's that or bad lead dress. But it could definitely be a wiring error. However the amp works just fine otherwise. I really believe it's between V2B and the PI. All of those components are very noisy when you tap on them. Shoot, since Sluckey mentioned the possibility of a filter cap, I'm wondering if it's the filter cap for those stages I just mentioned.

[hesamadman]

Mine does not suffer this problem??????

Do you have any suspicion as to what your problem is?


I am still having this issue. in a completely different amp than originally posted but its the same schematic though. It is using hoffman AC30 power amp. This amp does not do this with a PPIMV.

[hesamadman]

Ok so I found a solution sort of. Here is my schematic. The 1M resistor just after the MV. That was put there when I had a PPIMV. When I changed to a pre PI MV that was still there. Removing the 1m resistor remedied my problem. However, I was told that the double caps for my reverb should actually be a resistor first then a cap. I am worried that when I go to add my reverb, it may require me to change this back with a resistor and bring the problem again.