Metallic pops when on full volume even heard quietly when no speaker is plugged in!

[nedhogan]

Im pretty sure that having your amp on full volume when theres no speaker plugged in is very very bad for it but it was an accident the first time when I heard this quiet noise. It sounds like a vibration or maybe tiny arc's of electricity when the guitar is strummed when its on full whack with no speaker in. When I plugged the speaker in I found that this noise overides the sound of the guitar, so it is very loud, then, but not always, the volume of the amp starts changing. If the amp is turned back down the noise disappears but the effect on the output of the amp takes a while to wear off.

It is very unusual, Im not sure what it could be. Id have said the output transformer is on its way out, it is 50 years old, but the effect on the output of the amp and the way it changes just baffles me. Unless its an effect brought on by the OT arcing.

I would really apreciate anyones input on the matter.

The amp is around 22-30 watts it could be slightly higher.

This is its schematic although it is slightly different now, as it now has a master volume on it.

[nedhogan]

Does anyone have a clue what could be going on?

[nedhogan]

The two outputs were wired backwards to each other so when loud a tiny spark would jump through the paint fron the jack nut. Changed it round so earth is earth on both of em. Hopefully problem solved.......

[Steve Conner]

That's an interesting fault! :-O

The good news is that it probably saved your output transformer when you cranked it up without a speaker load.

[nedhogan]

Man the noise is still there, with a speaker in this time! Probabley screwed the ot now. : (

[frus]

check the (first) filter cap
and don't strum at full volume with no speaker, are you crazy?

[nedhogan]

Yes I am crazy Frus.
But you know, I bet its one of the el34's I always think the worst has happened when a tube goes dodgy. One of them is the most rattly tube I've ever encountered and it wasnt when it was new. Which was only last month by the way! Wish I had a spare to try.

P.S the first filter cap is new.

[nedhogan]

This is a little audio recording of the sound, Im turning the volume up and down while the guitar is ringing... Voice_011.mp3

[nedhogan]

It doesn't follow the tube, but I noticed that in one socket the power tubes' blue glow (from the grid?) goes on and off with the crackles. The problem is definatly somewhere around the power tube/ot area and its driving me mad.

[Jazz P Bass]

The 'blue glow' is the outcome of the tube working.
If the glow goes away when the crackling occurs, you are on the right path.
If you have verified that all connections are good To the socket, then it may very well Be the socket that is bad.

[nedhogan]

Im ordering a new output transformer at the end of the month. I've checked everything and decided that it must be a bad turn on one side of the primary winding of the tranny that breaks down under load or something.
Id take the tranny apart and try rewinding it if the laminations werent so totally stuck together with rust and varnish.

[nedhogan]

That was close! Im glad I didnt buy that transformer. Found the problem....
It was the flippin' master volume that I'd put in. For some reason, when the power is on the resistance through the dual 500k pot paralleled with 1M resistors is still around 500k? But when its off its around where it should be, 330K. This was putting too much resistance between the grid and ground and doing something strange to the output.

Well trying to measure the resistance while the amps on is very difficult as it squeels when you put the probes on the phase inverter output. But thats what I found.
Removed and the amp is happy again

Does anyone have a clue what the heck is going one with this? Id like a master volume. Is it because theres to much current for the pot or something. If I install a dual 250K pot on its own will that do the same?

Its annoying because I installed the mv exactly how the LarMar thread described and checked and checked the circuit, it was deffinatly right. I just used a resistance of 330K rather than 220K because thats what my amp had.

[pdf64]

The noise may have been oscillation, caused by the wiring to the master vol coupling back to an earlier circuit.
Pete.

[nedhogan]

didnt sound like any osillation Ive ever heard, too instant and reactive to tube output.
Im just going to put a 220k dual pot in and be done with it. The noise was definatly a result of there being too much resistance from the grid input to ground, EL34s dont like more than 330K on a cathode bias at least. It sends them insane! (by the looks of things)

Just confuzed about how two 500k pot with 1M resistors between lugs 1 and 2, where 1 is ground and 2 is grid and 3 is input, still manages to mess the grid to ground resistance up. Must just be me, I must have missed something.
If it does the same with a 220k dual pot then it must be aliens. (or maybe oscillation)

[52 Bill]

Just confuzed about how two 500k pot with 1M resistors between lugs 1 and 2, where 1 is ground and 2 is input from PI and 3 is grid input, still manages to mess the grid to ground resistance up. Must just be me, I must have missed something.
If it does the same with a 220k dual pot then it must be aliens. (or maybe oscillation)

Can you post the link to the schematic that you used? I don't understand your number references to the pot lugs.

Are you using the pot to replace the 220K grid bias resistors?