EVH 5150IIIS 100W EL34 Noise/hum problem

[chriss962]

Hi all,

I'm dealing with a persistent noise/hum issue on my EVH 5150III Stealth 100W EL34. I've done extensive troubleshooting and need your input.

Setup:

• Amp: EVH 5150III Stealth 100W EL34 (brand new, second unit – first had same issues)
• Pedalboard: CIOKS DC7 + CIOKS 8 Expander (fully isolated), Furman AC-20E power conditioner
• Cables: Mogami with SquarePlug connectors, shortest possible
• Tested with/without: ISP Decimator X G String (new), TC Sentry, Source Audio Ventris/Nemesis, MXR EQ

Symptoms:

1. Constant hum/brum – audible even in standby (power on, HV off). Gets significantly louder when standby engaged.
2. FX loop hiss – any pedal in the loop (digital or analog) raises hiss dramatically. Ventris/Nemesis make it unbearable.
3. Channel switching pops – loud electrical pops through speaker when changing channels.

What I've ruled out:

• Not ground loop – Palmer PLI-01 isolator changed nothing.
• Not pedalboard power – CIOKS isolated outputs, Furman conditioner, same board dead quiet with EVH Stealth 6L6, Mesa Badlander, Victory Kraken.
• Not cables – high quality, short, re-routed away from PSU.
• Not preamp tubes – swapped V1, FX loop driver (V7), phase inverter (V8) with known good Tung-Sol Gold – no change.
• Not effects order – tested with only ISP in loop, only Ventris, only EQ. Hiss/Hum always present.

What I've tried:

• ISP Decimator X G String in 4CM – helps with hiss but hum remains.
• Moving digital effects before amp input – reduces hiss but hum unchanged.
• Different power outlets, with/without Furman – no difference.
• Two different amp units (used and brand new) – identical problems.

Questions:

1. Is this hum/hiss level considered normal for this specific EL34 model?
2. Any fix besides modding the loop or selling the amp?
3. If mod is the only way – which tech in Europe (Poland) can do it properly?

Thanks for any help.

 

[Alpha Stryke]

First : take a known good guitar, take a known good instrument cable, plug straight in amp w/ nothing else no loop no pedals and
see what that sounds like.

 

[chriss962]

First : take a known good guitar, take a known good instrument cable, plug straight in amp w/ nothing else no loop no pedals and
see what that sounds like.

I've already done that.
I tested with two different guitars (Gibson Les Paul Custom Black Beauty and Ibanez RG5320C Prestige) and with different high‑quality instrument cables.

The hum is still present even with no guitar connected at all – just the amp and a speaker cable.
The FX loop hiss appears the moment I plug anything into the loop (even a single patch cable with no pedal).

 

[Alpha Stryke]

80%+ of tube amp problems are tube related.
If you ran a "mismatched" speaker load you may have damaged the output transformer.