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

[FleshOnGear]

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.

Well, if the amp still wavers in volume with another preamp plugged into the return, the problem is isolated to the effects return recovery gain stage and the power amp. If the problem ceases, that isolates the preamp, including the effect send circuitry. But you said the volume variation occurs through all channels of the amp, so that would indicate that the problem is in the effect send circuit, as that is common after all the channels.

 

[Pete Farrington]

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

An intermittent dodgyness in the heater circuit of V7b or V8 seems most likely to me.
As it can cause slow smooth volume fade outs/ins. And the issue is common to all channels.
It could be within the valve, at the pin to socket contact, or a failed solder joint, eg at the valve socket.
Output valves seem less likely, as it’d be unusual for such an issue to affect both, yet the fade out seems to retain symmetry of upper snd lower peaks of the waveform.

 

[aikiguy]

If it were my amp, and I had a second guitar handy, I would see if it acted the same way. Also, I would be inclined to look at something as simple as a guitar chord. It may not be your guitar or chord but at least you could rule it out by trying. It could be something stupid….

 

[sdn25]

Update: swapped the power tubes, problem persists.

I’ve made an observation, however. As the amp heats up (I presume), the issue slowly goes away I
i.e. the problem is most severe when the amp has just been turned on. I could guess this is indicative of a bad solder joint off the top of my head. Any other possible cause?

I also now have access to a multimeter, so i can do basic troubleshooting steps you guys suggest.

 

[sdn25]

@Pete Farrington any tips?

 

[Pete Farrington]

I suggest to reseat the valves in V7&8 several times, maybe apply a tiny bit of contact cleaner to the valve pins beforehand. If using lubricated cleaner, be even more careful not to use too much, and use kitchen paper to soak up / clean any excess and to screen off the nearby chassis area.
Same for any cable headers carrying heater power.
Check for the issue.

If still occurring, try your known good spare 12AX7 in V7.
Check for the issue.


If still occurring, replace the previous valve back into V7 and try your known good spare 12AX7 in V8.
Check for the issue.

If still occurring, replace the previous valve back into V8 and, using strong bright light and visual magnification, closely examine all heater circuit solder joints (on pcb and anywhere else). If any look less than perfect, remove existing solder and reflow.
Check for the issue.

Hope that makes sense, ask if unsure

 

[sdn25]

Some photos of the tubes sockets of the power tubes and phase inverter, they’re all mounted to the same daughter board. Let me know if my eyes missed anything as they don’t look too bad to me. The screen resistors look a bit toasty but that seems to be a common occurrence on the EL34 version of this head from what I've seen.

 

[Pete Farrington]

It’s hard for me to assess lead free stuff.
Certainly R139 looks worse for wear, I guess it’s the screen grid resistor?
If so then I suggest to replace both.

 

[sdn25]

It’s hard for me to assess lead free stuff.
Certainly R139 looks worse for wear, I guess it’s the screen grid resistor?
If so then I suggest to replace both.

would those resistors have caused the volume issue?

 

[Pete Farrington]

would those resistors have caused the volume issue?

Hmm, I’m not sure. How loud was the amp being played when the clip in post 1 was recorded?

 

[sdn25]

Hmm, I’m not sure. How loud was the amp being played when the clip in post 1 was recorded?

Volume was set at about 10:30. It’s set just at the transition point there’s a small bit of power tube distortion mixed in the tone. But do note that this volume issue happens at all volume levels.

 

[sdn25]

I found that 2 ceramic disk caps for v1 were microphonic when tapped with a stick. No other part in the amp was microphonic. Could this be the failed part causing these drops? Specifically C8 and C37. @Pete Farrington

 

[Pete Farrington]

Schematic link?
All I can find is the 6L6 version which doesn’t seem to match http://support.evhgear.com/schematics/EVH_5150III_AmpHead_Schematics.pdf

 

[sdn25]

Schematic link?
All I can find is the 6L6 version which doesn’t seem to match http://support.evhgear.com/schematics/EVH_5150III_AmpHead_Schematics.pdf

I can’t seem to find it online either. I have emailed fender to request for the el34 schematic.

But here’s a link to an updated 6L6 schematic that looks much more similar to the el34. C37 can be found on V1 but C8 doesn’t seem to exist.

http://support.evhgear.com/schematics/EVH_50W.pdf

Let me know if this works in the meantime

 

[sdn25]

Fender says since the amp contains lethal voltages, only fender authorized service centers are allowed to have the amp schematics and they won’t send direct to consumers. Bummer. What do you suggest I do?

 

[Pete Farrington]

It’s kinda to be expected that ceramic caps are a bit microphonic. It’ll be more apparent the earlier in the signal path that they are. So unless it was obviously causing a problem, I wouldn’t be concerned about it.

 

[sdn25]

It’s kinda to be expected that ceramic caps are a bit microphonic. It’ll be more apparent the earlier in the signal path that they are. So unless it was obviously causing a problem, I wouldn’t be concerned about it.

Occasionally, when the drops occur, I get a sort of crashing distortion sound at the lower peaks. Could this suggest that it may be an issue within the pre amp? This is to help narrow it down.

 

[sdn25]

Okay guys, I threw in the towel and returned the amp back to the seller. I was persistent in trying to fix it since the amp was at an unbeatable price. Luckily I found another one near me for about the same price in seemingly better condition overall.

 

[sdn25]

Also if anyone reads this and if you own an el34 5150, I highly recommend putting in 6CA7s. They make the amp sound much better imho. Tighter low end, clearer mid range, less muddy at higher volumes, and much more!

 

[Pete Farrington]

if anyone reads this and if you own an el34 5150, I highly recommend putting in 6CA7s

Did you eliminate the possibility that the EL34 in the amp were knackered, eg try your spare, known good fresh set of EL34 in it, to ensure that the comparison to your 6CA7 was a fair one?