Bad Cat Hot Cat "Gain" channel static noise?

[chuckb]

I built a Bad Cat Hot Cat last year and I have a problem going on with the gain channel. The problem is that when using the gain ch I get a lot of interminent static from the volume pot on my strat. Every time I turn the vol pot up or down I get this scratchiness. It's like when you turn a dirty pot up and down. This isn't the pot on my guitar because it doesn't happen with any of my other amps. The clean ch on the Bad Cat is fine also with no problems with the vol pot on my guitar. So I've conclude that it's the gain ch but can't seem to find where it is. I had a friend of mine bring over his guitar and it does the same thing with his as well. Anyone got any idea where I can start to find where the problem is coming from. Thanks in advance.

Chuck.

[chuckb]

Anyone?

[chuckb]

Hey guys, do you think it might be agrounding issue?

[madkatb]

Try a cap in line with your input, say .02. If the scratchiness goes away then it's DC on your guitar. Which means your gain channel is leaking DC somehow. I guess you can check for DC just by using a multimeter across the guitar end of your patch cord. The cap would then be the solution to the problem.

[Chuck H]

Try a cap in line with your input, say .02. If the scratchiness goes away then it's DC on your guitar. Which means your gain channel is leaking DC somehow. I guess you can check for DC just by using a multimeter across the guitar end of your patch cord. The cap would then be the solution to the problem.

That cap wouldn't be a solution. It would be a bandaid. If there is DC on the input jack that's a problem that needs to be addressed, not covered up.

IME this can often be a tube problem. If the amp didn't do it before, and nothing has been changed, try a different tube in the V1 socket. If the amp has always done it you may have a grounding issue or a wiring fault or error.

[guitarmike2107]

Have you fitted the 100nf cap, which keeps the DC off your guitar pot.

[madkatb]

My apologies. I know they use a cap on the input of amps with VVR power scaling to stop DC from appearing on the guitar volume control and creating scratchy sounds. If that's just a bandaid there's gonna be a lot of pissed off people.....

[Chuck H]

Not being familiar with the VVR implementation I can only say that sometimes there are what is considered acceptible consequences to circuits that are then designed around. If implemented correctly I don't see why VVR would do this anyway. At any rate, on any amp WITHOUT superfluous circuits that need to be compensated for there shouldn't be any DC at the input jack, and so, there is a problem and the cap would not fix that problem. DC on the input jack means there is DC on the first stage grid. This will change the bias of that stage (usually for worse instead of better) and this needs addressing.

[chuckb]

Hey chuck, the amp has always done this. At first I thought it was my guitars vol pot but I installed new pots with a treble bleed kit and it was still there. I have several other amps that I use and there's no problem with the guitars vol pot with ant of these. I think your right in that it may be DC on the first stage grid. The amp doesn't sound to bad when played through the gain ch but it's real muddy though. Any suggestions on how I can go about aproaching this problem?

Chuck.

[Chuck H]

Has it always had the same preamp tube in V1???

If there are any feedback circuits to the V1 grid check them for leakage. Example, on one of my designs I use a 4.7pf cap from plate to grid a triode stage. If that cap leaked it would ruin the bias for that triode. Also, if your using a shielded input lead make sure no single strand of the braid is stray and touching something it shouldn't be.

It's tricky because typically there is nothing on the V1 grid lead. Definitely check for DC potential on the chassis wuth the black probe on your house AC ground. I'm betting on the preamp tube.

[Chuck H]

OOOooo... Well, this is a little embarrassing, but...

I just looked at a schem for the Hot Cat (or "a" schem for the Hot Cat) and it shows the first triode for the gain channel is grid leak biased. That means the cathode is grounded and a small DC charge builds on the grid. This particular schem also showed (grunt) an input capacitor. Obviously this is needed to stabilize the bias. So... If the first triode of your gain channel is grid leak biased you'll want to install an input capacitor or change it to the much more common cathode bias arrangement.

In my own defense grid leak bias is almost never used on any tube guitar amp built since the fifties. So my suggestion that there was a problem was for the right reasons, though I was wrong. And madkatb was right, but for the wrong reasons. Sorry, I know that's a left handed apology.

[chuckb]

The unput cap on the schematic calls for a 0.1uf which I have on there now.I can change it for another value,? The cathode for the gain ch is just a wire going to ground. what would you suggest for common cathode bias arrangement,22uf/50v and a 1.5k resistor?

chuck.

[chuckb]

Hey guys,You were all right, the problem, was the input cap.I changed out the old cap and replaced it with another of the same value(0.1uf) and problem solved. The old cap had a crack in it so I guess that's what was causing the DC to leak through. It's fine now. Thanks alot for your help,I appreciate it.

Chuck.

[chuckb]

Hey chuck, I'm finding the gain channel to muddy sounding. Would a cathode bias arrangement on the gain ch clean up some of the muddiness?

Chuck.

[Chuck H]

I can't remember the "drawbacks" of grid leak bias. IIRC it presents certain fidelity issues but I can't say what they are. It's worth searching around here for more info. I'll probably do that myself but I'm entertaining company for another couple of hours.

[chuckb]

Hey chuck,No problem,thanks for all your help. I'll do a search and see what I can find.

Chuck.

[Chuck H]

It looks to me that grid leak tends to be more sensitive to proper circuit wiring from a noise perspective. The leak resistor should be right on the socket otherwise it can be noisy. At least thats what I read. And it's also bad about being overdriven producing an ugly distortion when preamp based pedals are used in front of it. Though I guess that's not an issue with a "gain" channel. Still, I did notice that a lot of builders choose the more common cathode bias arrangement when building these amps. It wouldn't be hard to try it if your inclined. Not much difference in tone reported from guys who have tried it both ways. You'll want to use a fully bypassed cathode resistor (1k or 1.5k resistor with a 10uf or larger cap across it) and a 1M 0V reference as seen od nearly all guitar amps.

[chuckb]

Hey chuck, I went ahead and put a 22uf/50v cap with a 1.5 resistor on the cathode and it works pretty good. I found it to be an improvement from the way it was. Thanks for your help,much appreciated.

Chuck.

[Chuck H]

K, good then. I'll assume that's a 1.5k resistor and not a 1.5 ohm resistor, right? Also, be sure your grid to ground resistor is 1M. Also be sure the 1M grid to ground resistor is moved to the input jack BEHIND the 68k series resistor (if it has one) otherwise it forms a voltage divider that will reduce gain. Did you try it with the input cap removed?
Now that you have a cathode bias you can reduce bass frequencies by reducing the size of the bypass cap. You can drop about 6dB from a chosen knee frequency by using lower values. Some will argue the specific frequencies and tout the inhearant frequency limitations of guitar amps but I hear a clear difference between different values up to 3.3uf. So maybe try a .68 (typical Marshall "bright" channel value), 1uf, 2.2uf and 3.3uf. Choose the one that de-muddifies (technical term) the tone to your liking.

[chuckb]

Hey chuck, Yes,I used a 1.5k resistor for the cathode bias arrangement.I have the 1M grid resistoron the input jack as you mentioned. I didn't remove the input cap,but I'll try it and see what difference it makes. I'll also try different caps as well in the cathode arrangement a and see what kind of difference it makes to the tone,(de-muddifies). I'll let you know how things work out when I've tried some of your suggestion.

Chuck.

[guitarmike2107]

HA me too chuck, I heard a difference going from 22uf to 47uf, it was subtle but helped my 1974 clone allot. I cant explain it, maybe the bass just gets less smeared or maybe it was the ESR of the different caps?

chuckB if you use a higher value resistor, 2.7k to 4.7k on the cathode you will get slightly more compression from that stage too, which will also help the tightness.

[chuckb]

Hey guitarmike21o7, thanks for the info. I'll give the higher value resistors a try as well.

chuck.