EVH 5150 iii EL34 50w volume keeps changing slowly while playing.

[sdn25]

Picked up a used 5150iii el34 head recently and it has an issue where as you keep playing, the volume slowly but surely swells up and down. Happens on all channels. As the volume changes, the timbre of the sound does not, purely volume.

I have checked all pre amp tubes with good ones, the issue persists.

I have attached a picture of the waveform of the recorded audio from the amp. The dips you see are the volume changes I was talking about.

 

[twothemax]

First bypass the fx loop. Insert a cable from the send jack and plug into the return jack. See what happens.
If that don't work I would check or replace the power tubes. Change both and see what happens. If it solves your issue make sure you rebias the power tubes.
Let us know

 

[TheKman76]

Someone else will say it if I don't... The most likely point of failure in a valve amp, is the valves. Preamp valve can last a very long time, power valves not so much.

If you don't know how much work these have done, this would be a good starting point.

 

[FleshOnGear]

Yeah, could be a tube. If all the channels do it, it’s probably a tube later in the signal chain. Start by swapping the phase inverter tube for a new tube (the small tube that’s closest to the big power tubes). If that doesn’t help, sequentially swap out each preamp tube individually with the new tube, working backwards toward the preamp tube closest to the amp’s input. It could also be a bad power tube, which might require swapping out the whole set of power tubes. But try the small tubes first.

If this doesn’t help, might be time to visit a tech. Good luck!

 

[PelliX]

In addition to the suggestions made, give all the pots and switches some good twists/pushes. Wiggle the jacks. Does the volume also change when you go straight into the FX return? If so, we can rule out just about anything in the preamp. If not, the power section is seemingly OK.

 

[XTRXTR]

Check the bias circuit verify you get consistent negative voltage when you hear the volume dip and rise. Heater circuit as well test for consistent 6.3V.

 

[sdn25]

Thanks for the suggestions guys, after trying all the methods listed, turns out there’s nothing wrong with the amp. I moved it to another wall socket and the issue doesn’t occur anymore - not sure why but that’s the case for now.

I even took it to a tech who said he measured a constant 50w output when testing over 2 hours. This finding would corroborate the faulty / inadequate outlet solution. Let me know what you guys think.

 

[PelliX]

How dodgy is the mains voltage over there? Any heavy duty gear on that circuit (air conditioning units, HVAC stuff, etc)?

 

[sdn25]

How dodgy is the mains voltage over there? Any heavy duty gear on that circuit (air conditioning units, HVAC stuff, etc)?

Honestly I have no clue, just moved recently and don’t have a meter on hand. It’s a fairly new (2017 built) dorm room.

Nope there’s only 2 chargers and a pedalboard supply connected. The new socket I tried is on a different wall.

 

[PelliX]

Then, unless plugging it into the previous socket causes the issue to rear its head again, I'd say it's likely unrelated. If it does, have someone check it out.

 

[sdn25]

Then, unless plugging it into the previous socket causes the issue to rear its head again, I'd say it's likely unrelated. If it does, have someone check it out.

Update: it’s 5am and I turned the amp on, the issue came back (on the new socket).

I had it checked out by a tech who said he found no fault with it. He play tested and did a measure of the amp output for 2 hours. He said nothing out of the ordinary.

Quite puzzling I must say. Is it possible the amp is drawing too much power from the wall, causing it to brown out? Kinda like how a PT in an old Marshall Sags as you play hard.

 

[PelliX]

Update: it’s 5am and I turned the amp on, the issue came back (on the new socket).

I had it checked out by a tech who said he found no fault with it. He play tested and did a measure of the amp output for 2 hours. He said nothing out of the ordinary.

So it's intermittent... I'm guessing output valve, capacitor..

Quite puzzling I must say. Is it possible the amp is drawing too much power from the wall, causing it to brown out? Kinda like how a PT in an old Marshall Sags as you play hard.

No, not really. The thing is that if such amounts of energy were drawn it would have to be dissipated. Presumably as heat, so it would be getting hot as hell.

I'd swap the output valves - failing that it's probably time to start probing, chopsticking and shotgunning parts...

 

[FleshOnGear]

@sdn25, I can’t remember, did you say you tried plugging a different source into the effects return of the amp? If not, I’d try that. Take a preamp out or effects send from another amp and plug it into the return of your EVH, or use a pedal that has high output.

Based on your description, I’d suspect the effects circuitry. Maybe J11 is acting up, causing a varying voltage divider before the effects send? Maybe the driver circuitry that controls J11 is faulty?
http://support.evhgear.com/schematics/EVH_5150III_AmpHead_Schematics.pdf

 

[XTRXTR]

Intermittent is the worst. How is a tech supposed to find the fault if it doesn't show up while he has it? I mean if you chopstick and all, swap around preamp tubes, an old known good set of power tubes, all with a DMM and a scope, a function generator and you can't find anything...

I mean it needs a baby sitter watching it until it coughs and then hope it keeps coughing while you jump into action to find the trouble.

 

[Ken Underwood]

I have never seen so many stabs in the dark by unqualified people, trying to help but only making matters worse.
Take it back to this Tech to get him to give it the once over, as there obviously is something going on.

 

[sdn25]

@sdn25, I can’t remember, did you say you tried plugging a different source into the effects return of the amp? If not, I’d try that. Take a preamp out or effects send from another amp and plug it into the return of your EVH, or use a pedal that has high output.

Based on your description, I’d suspect the effects circuitry. Maybe J11 is acting up, causing a varying voltage divider before the effects send? Maybe the driver circuitry that controls J11 is faulty?
http://support.evhgear.com/schematics/EVH_5150III_AmpHead_Schematics.pdf

Hmm, haven’t yet tried that. I’ve tried jumping the loop using a patch cable so far to no avail. Is the test gonna determine if the loop circuitry is at fault? Or is it to determine if the pre amp section is to blame?

 

[sdn25]

Ok I’ve bit the bullet and ordered some new power tubes to test with. I can only hope that’s the issue. They arrive either tomorrow or the day after - I will update you guys once they’re in.

 

[Pete Farrington]

Ok I’ve bit the bullet and ordered some new power tubes to test with

That’s not biting a bullet, but rather fundamental cost. You’ve got a valve amp, so you need spare, known good valves for it.
One way or another, they’ll all fail eventually. A spare is needed, on hand, because it’s sure to happen at an inconvenient time.

 

[sdn25]

Plugged the amp in again, the problem is gone now . Really taking me for a ride here, this amp.

 

[sdn25]

That’s not biting a bullet, but rather fundamental cost. You’ve got a valve amp, so you need spare, known good valves for it.
One way or another, they’ll all fail eventually. A spare is needed, on hand, because it’s sure to happen at an inconvenient time.

From my symptoms, would you too reckon that it’s the power tubes causing the issue?