Bypass tonestack

[goldenGeek]

I have this amp that I'm trying out different stuff with. It was built as a "mini" JCM800, 2xEL84 and 3x12ax7, 20W output trafo.

Anyway, I was trying to bypass the tonestack and replacing the pre amp with a Matchless Spitfire-ish pre amp with a simple tone control. I tried to just bypass V2 and insert the output of the pre amp stage (volume pot) into the .022uf that goes into the phase inverter. There was sound but extremely low volume. Is that supposed to be like that when you leave out the V2 and TMB-controls/circuit?

Edit: It looks like I've done pretty much like in the 1974, I guess I should check all pot/resistor values and connections...?

[Littlewyan]

Did you completely disconnect the rest of the circuit? So did you remove the connection to the grid of V2a from the volume control and remove the connection from V2b's cathode to the 0.022uf Cap before doing this?

[goldenGeek]

Did you completely disconnect the rest of the circuit? So did you remove the connection to the grid of V2a from the volume control and remove the connection from V2b's cathode to the 0.022uf Cap before doing this?

Yes, removed all connections (and the tube) from V2

[Reeltarded]

So it's one stage pushing the PI?

I think I found your problem! I bet you get some great cleans though.

[Littlewyan]

One stage should be enough to give you a good volume though. The Marshall 18W is only 1 stage and that overdrives a pair of EL84s.

Does the volume become resonable when you turn it up?

I think we could really do with a circuit diagram or some photos at least to see whats what.

[goldenGeek]

One stage should be enough to give you a good volume though. The Marshall 18W is only 1 stage and that overdrives a pair of EL84s.

Does the volume become resonable when you turn it up?

I think we could really do with a circuit diagram or some photos at least to see whats what.

I can post pictures later today, but it's the pi and power amp from 2203/4 but with lower voltages and EL84, starting at the .022uf that goes to V3. Pre-amp is the spitfire pre-amp.

It was working fine before when the pre amp and tonestack was a 2203/4, it was not extremely loud, but it had plenty of volume compared to now. The volume is REALLY low now, so there has to be a problem. I will check everything - I used some new 1m pots that I haven't tried before, I guess that's where I'm gonna start the troubleshooting.

[Littlewyan]

Measure the pots and make sure they measure close to 1Meg.

Have you got a scope you can use?

[goldenGeek]

Measure the pots and make sure they measure close to 1Meg.

Have you got a scope you can use?

Yes, I have a scope, but I'm not very good at using it. Do you have any tips on where in the amp to look and what to look for?

[Littlewyan]

Well basically I was going to suggest hooking up a signal generator on the front end and then going through the amp with the scope seeing where the signal drops.

One quick thing, have you tried replacing or swapping the 12AX7s around in case one has gone bad?

[goldenGeek]

Well basically I was going to suggest hooking up a signal generator on the front end and then going through the amp with the scope seeing where the signal drops.

One quick thing, have you tried replacing or swapping the 12AX7s around in case one has gone bad?

I will try the signal generator/scope approach, thanks.

I pulled the tubes from another amp that I used a couple of days ago so I think they should be okay, but I can always try another pair.

Edit: hey, the EL84's is not "fresh" from another amp. I'll try to replace those as well.

[Littlewyan]

Just make sure you don't touch any high voltage leads with your probe. I once slipped, touched the screen supply and blew my scope. Luckily its just a USB scope so didn't cost much.

For the signal generator I normally use a 100mV 1Khz sine wave. I've heard of guys using a 10mV signal but that seems a bit small to me.

And don't forget, one hand behind your back whilst doing this

[goldenGeek]

So this turned out to be an idiotic error like always... there was a wire that was not connected Anyway, I got some training in using the scope so it was good for something.

[Firestorm]

So this turned out to be an idiotic error like always... there was a wire that was not connected Anyway, I got some training in using the scope so it was good for something.

Funny how often it's something stupid. I only ever made one mistake that produced fireworks. And that was just a simple rebias being done right in the music store. Lead slipped, put B+ on the grid. Fire shot out of the bias pot. No serious damage except to my ego. After that I quadruple and quintuple check everything.