Troubleshoot my amp Vol 8: Case of the Quiet Diezel VH4

[narad]

So I don't often think of the VH4 as my favorite amp, but it's the one I plug into almost every time I play if I don't have anything specific in mind. The tone is just always saturated enough and never gives me ear fatigue, can with one knob go from classic stuff to metal, just overall straightforward amp. Never had any issues with it and it sounds good at low volume so never had to crank or attenuate it.

So my disappointment when I turn it on the other day and the volume is super low. As I turn the channel and master volume, the volume increases, but not by very much. With the master and channel at about 4, I can basically achieve the same loudness I'm used to with both set around 1. It sounds as it should though -- maybe identically to usual, just maybe 1/5-1/10th the volume. So since nothing crazy to my knowledge happened with this amp, I thought maybe it's tubes, but the weird thing is is that the VH4 has lights indicated the tube health, and none are lit up. All the power tubes and preamp tubes appear to have normally glowing filament. I'm still inclined to swap some power tubes but annoyingly these have kt77s and I don't think I have any extras. I could swap in some other tubes, but this doesn't have external bias points, and then things are going to get complicated.

I checked the speaker cable and cab, tried one of teh 8 ohm outs instead of the 16th, hit every switch on the front. Still seems the same.

Thought I'd open it up to hear some candidate ideas -- does this sound like tube stuff still? Am I okay throwing in some EL-34s and not rebiasing, just to troubleshoot?

 

[Edika]

Not sure what the issue would be but you can try swaping the preamp tubes first and see. If that doesn't help, try some contact electrical cleaner on the volume knobs and the tube amp sockets, as well as all the jacks.

If that doesn't help you can see about the power tubes. Not sure if EL34 are lower current than KT77's. If that's the case, it'll at least show you if it's the power amp tubes or not.

Then it would be something a tech should have a look at, as it might be something more complex in the circuit.

 

[tedtan]

Could be a preamp tube, could be dirty contacts on the tubes or sockets. I’d try swapping the tubes around and, while you have them out, cleaning the tube pins and the socket connectors with DeOxit or similar before anything else.

 

[narad]

if a preamp tube went, wouldn't the tone suffer while the volume controls remained roughly intact? The tone is almost identical on CH4 where I am most of the time.

 

[bracky]

Bypass the preamp by running into the effect return. If the problem persists then it’s in the power section.

 

[technomancer]

^ yep

It does sound like a tube issue... could be pre or power but if there isn't any crackling / popping I would start with the preamp tubes depending on the above.

Also tube glow isn't really an indicator of health... the shot tubes in my Friedman all looked fine.

 

[Jon Pearson]

A quick thing to do would be to see if plugging into the return jack with another preamp or amp send yields a similar low volume or if it sounds loud again. Might help to identify if it's a preamp failure or power amp failure.

 

[narad]

Yea, good call trying to isolate the pre/power amp -- seems so obvious!

 

[Kosthrash]

Sounds silly, but if you have also sound interruptions, it might be a guitar cable failure? Or bad contacts...

 

[narad]

Sounds silly, but if you have also sound interruptions, it might be a guitar cable failure? Or bad contacts...

Checked all that.

Hmm, so straight into the return, the master volume seems to work as expected. Guess I'm on to the preamp tubes, which fortunately I probably have some appropriate ones around.

 

[Jon Pearson]

Checked all that.

Hmm, so straight into the return, the master volume seems to work as expected. Guess I'm on to the preamp tubes, which fortunately I probably have some appropriate ones around.

Right on, roll a good tube through the positions and see if you get any improvement. If it is happening on all the channels, look to see what stages are shared across channels.

 

[narad]

I've got V1, V4, and PI targeted, but unfortunately they're deeply recessed and the outer two are set back behind other things, so I'll have to pull the whole chassis before I can simply try them out. So sadly not something I'm going to get done tonight.

 

[Asdrael]

Something similar happened to me (on another amp). Pulled my hair out for a bit. It turns out a pedal I barely ever use, in my FX loop, just decided die and was simply passing 20% of it's usual signal. Removed it, all was good again. Maybe it's that stupid?

 

[crushingpetal]

Bypass the preamp by running into the effect return. If the problem persists then it’s in the power section.

Nice!

(I had a weird tube issue a few months ago: why didn't I think of this? Come to think of it, why don't I hear more people giving this advice?)

 

[technomancer]

Something similar happened to me (on another amp). Pulled my hair out for a bit. It turns out a pedal I barely ever use, in my FX loop, just decided die and was simply passing 20% of it's usual signal. Removed it, all was good again. Maybe it's that stupid?

First thing you do when trouble shooting is cut it down to guitar -> cable -> amp -> cable -> cabinet. Rules a LOT of things out.

 

[budda]

First thing you do when trouble shooting is cut it down to guitar -> cable -> amp -> cable -> cabinet. Rules a LOT of things out.

Then shit bricks when the problem persists

 

[technomancer]

Then shit bricks when the problem persists

Nah then swap cables, swap guitar, swap tubes, THEN shit bricks when the problem persists

(or in my case curse and swear and crack the chassis...)

 

[narad]

Heeyyyy, it's fixed! It was the V4 preamp tube -- the first one I tried and I could actually remove without taking the chassis out, but didn't trust how the new preamp tube felt going in at the angle I had it, when I couldn't see the socket, etc. So pulled the whole chassis to have it fixed 20 secs later lol

Anyway, thanks for all the advice! I would have probably ordered some replacement power tubes otherwise thinking it was maybe a bad power tube (but not so bad as to blow the tube fuse and trip the indicator light), but that loop suggestion really made it clear where the fault lied.

Now anyone know how to fix amp feet marks in tolex? Seems I had my amp sitting on the cab a bit too long
j/k, but man, still kind of annoys me that that happens.

 

[budda]

Nah then swap cables, swap guitar, swap tubes, THEN shit bricks when the problem persists

(or in my case curse and swear and crack the chassis...)

I probably did all but the tubes . It failing mid set on a showcase slot was not a good time.

 

[Jon Pearson]

Heeyyyy, it's fixed! It was the V4 preamp tube -- the first one I tried and I could actually remove without taking the chassis out, but didn't trust how the new preamp tube felt going in at the angle I had it, when I couldn't see the socket, etc. So pulled the whole chassis to have it fixed 20 secs later lol

Anyway, thanks for all the advice! I would have probably ordered some replacement power tubes otherwise thinking it was maybe a bad power tube (but not so bad as to blow the tube fuse and trip the indicator light), but that loop suggestion really made it clear where the fault lied.

Now anyone know how to fix amp feet marks in tolex? Seems I had my amp sitting on the cab a bit too long
j/k, but man, still kind of annoys me that that happens.

Sweet! Good news. The tolex thing is a bit annoying, I wonder if heat might take it out... not sure if that might also make the glue brittle though.