Fender EVH 5150 III Combo Problem

[stevenrb718]

All the channels produce sound, but channels 1 and 2 are exceptionally low output while channel 3 is loud. When messing around with the EQ for ch 1 and 2, it seems like low and mid don't really work, while increasing the high adds a piercing tone, almost like an old transistor radio sound. Also, when rolling up the high, the overall volume plummets and distorts. When the high is on 0, the channel is a bit louder and the mid control now might work a bit, but the channels are still a bit reserved. Any ideas?

[52 Bill]

Any ideas?

It sounds like you have a parasitic oscillation happening here. That would explain the high piercing tone and the loss of power and distortion. When the amp oscillates, the amp spends all of it's energy reproducing the high frequency tone, which causes the overall volume to reduce and to distort.

There may be a bad tube, or a broken component or even a wire. What sorts of skills do you have and what sorts of tools are at your disposal?

[stevenrb718]

High piercing tone might've been the wrong way to describe that. As the "high" knob of the channel 1 eq increases, the tone becomes more hissy-distorted and the volume drops. The rest of the eq I don't believe works but doesn't cause this problem. Obviously channel 2 does the same thing as the controls are the same. Channel 3 seems fine but to my ears is extremely loud at the lowest levels, yet I have nothing to compare this to so maybe it's just me. I'm not amp-savvy by any means but I am tech-capable. I already checked the tubes with a tester I have and they all read properly. Upon taking it apart, nothing was noticeable, but there was a soldered wire running between a couple of caps, which I found a bit awkward.

[stevenrb718]

No more ideas?

[nevetslab]

High piercing tone might've been the wrong way to describe that. As the "high" knob of the channel 1 eq increases, the tone becomes more hissy-distorted and the volume drops. The rest of the eq I don't believe works but doesn't cause this problem. Obviously channel 2 does the same thing as the controls are the same. Channel 3 seems fine but to my ears is extremely loud at the lowest levels, yet I have nothing to compare this to so maybe it's just me. I'm not amp-savvy by any means but I am tech-capable. I already checked the tubes with a tester I have and they all read properly. Upon taking it apart, nothing was noticeable, but there was a soldered wire running between a couple of caps, which I found a bit awkward.

Tube V3 is the one that's common to Ch 1 & Ch 2's Tone circuits. I'd swap that tube out for starters. There are relays involved in the channel select at the Tone Circuits, directly following the Ch 1 & Ch 2 Volume pot wipers. I've attached the Schematics file for your amp in case you don't already have it. It provides component placement drawings in addition to the schematics.

Fender EVH 5150-III head.pdf

There's also a relay following the channel switching that selects the output from Ch 3 Volume /Ch 1/2 Volumes, which then feeds the effects loop, so that too is a possible culprit. The heaters of V1 & V4 tubes are in series, fed DCV. 1/2 of V1 tube is the front end for Ch 1 & 2. I have had preamp tubes where one side stopped working, or barely worked. Obviously if either V4 or V1 tube's heaters failed, you wouldn't have Ch 3 working.