Diezel VHX suddenly darker/muddier, volume shift? Preamp tubes?

[C0UNT3RP01NT]

Okay, so I've been playing around with this amp through headphones. Finally got a cab last week. Everything sounded like I would imagine up until a few days ago.

Turned it on, and it sounded slightly off. Darker, maybe slightly muddier. It still sounded pretty Diezel-y. A pretty subtle change, but it just sounded off to me. Like a downtuned sound without the downtuning. I noticed mostly when I play single notes, but it's on the chords too. Restarted it. Sound was back to normal. But then I tried to play it today, and the darker slightly muddy sound returned. Switched out power cables, messed with guitar cables, reset the amp. No change. I messed with the EQ, and I could cut out some of the darkness, but then it almost sounded anemic. Again, it's a subtle change. Small enough that I think I might just be crazy.

But then! In the process of playing around on it, there was maybe a 5-15 second period where it gradually quieter. Slightly, but genuinely quieter. Then the sound gradually came back to the original volume. This was on the clean channel. But the darkness is definitely present on the other channels. That or I'm crazy.

After doing some research, this sounds like an issue to do with pre-amp tubes? I wouldn't know honestly. I've never messed around on the inside of an amp, but I'm decent enough with electronics. If it wouldn't be that, what else could it be? All the power amp tubes are glowing. Can't see the preamp tubes due to the shields.

I've switched guitar cables. Ran it without my wah or volume pedal in front. I had it on a power strip, but I moved it to a heavy duty extension cord. Muddiness has remained.

The only other thing I could think of, is that I ran an HX stomp into both the front end and the FX loop, but I had that set to instrument level on the ouputs. I also borrowed a looper from a buddy, and ran that in the FX loop. Turns out it was one of those jam stations, with the drumbeats and that didn't sound good on the speakers, so I removed it instantly. I've also turned it on and off too fast a few times on accident where I got the nasty pop, but the speakers look fine.

I've never changed tubes or ever really opened up an amp, so I don't really have a reference for going from muddy to clear. I'm pretty sure it shouldn't be too hard, I just don't want to replace them if it's something else.

 

[Philhouse]

It does sound like a possible preamp issue to me due to it's variable nature and volume inconsistencies.

@leodiezel should be able to diagnose. (I can't seem to do that link/tag thingy properly sorry) Perhaps just pm him if he doesn't see this thread.
Leo Diezel

 

[Leo Diezel]

It looks like you did all the debugging already, since the volume is dropping I would say it may be one of the tube getting bad somehow.

Hard to tell if it's one of the preamp of the Poweramp.

One way to check would be to run the guitar to FX Return and check for volume drop (it will sound very dark but that is normal), if no volume drop occur I would try to replace one of the first 4 tubes!

Make sure the speaker cable is fully inserted in (it would make 2 clicks).

Thank you!

 

[C0UNT3RP01NT]

Okay so after further testing on a separate amp, I think it might actually be a microphonic pickup. I went over and jammed on my amp at the practice space and I was occasionally getting odd harsh harmonics (I hear it pretty often on the F note at the 6th fret of the B string). I only have one electric guitar, so I can’t say for sure, but that does appear to be the common denominator.

I’ve been doing my research on what it could be, but I’m new to troubleshooting amps and guitars. Are there any specific ways to tell the difference between a microphonic tube and a microphonic pickup? One thing I did read was that you can physical motion through the guitar if it’s a microphonic pickup. I can hear my clip-on tuner when I wiggle it at high volumes, along with other noises like my arm bumping the guitar.

I’m playing on a Gibson Les Paul Studio with coil tapping. Either way, I’m probably going to wax pot the pickups, and since I already ordered a preamp tube for V1, I might just go ahead and swap that out.

@Leo Diezel I tried the FX return suggestion with no volume drop. Unplugged and securely replugged in all the cables. However, the odd harmonic does show up when I play through the return.

It’s an annoying issue, but it doesn’t seem like an expensive fix.

 

[Leo Diezel]

Okay so after further testing on a separate amp, I think it might actually be a microphonic pickup. I went over and jammed on my amp at the practice space and I was occasionally getting odd harsh harmonics (I hear it pretty often on the F note at the 6th fret of the B string). I only have one electric guitar, so I can’t say for sure, but that does appear to be the common denominator.

I’ve been doing my research on what it could be, but I’m new to troubleshooting amps and guitars. Are there any specific ways to tell the difference between a microphonic tube and a microphonic pickup? One thing I did read was that you can physical motion through the guitar if it’s a microphonic pickup. I can hear my clip-on tuner when I wiggle it at high volumes, along with other noises like my arm bumping the guitar.

I’m playing on a Gibson Les Paul Studio with coil tapping. Either way, I’m probably going to wax pot the pickups, and since I already ordered a preamp tube for V1, I might just go ahead and swap that out.

@Leo Diezel I tried the FX return suggestion with no volume drop. Unplugged and securely replugged in all the cables. However, the odd harmonic does show up when I play through the return.

It’s an annoying issue, but it doesn’t seem like an expensive fix.

Odd harmonic or choked sound with multiple amp can also means pickup may be too close to the strings

 

[C0UNT3RP01NT]

Odd harmonic or choked sound with multiple amp can also means pickup may be too close to the strings

I lower the pickups. It helped a bit. It's not perfect but it will do for now. Thank you for the suggestion!

New pickups are on the G.A.S. list, but there are bills to be paid first.

First class customer service, you guys are wonderful!