What would cause a valve amp to lose volume?

rico

This happened at rehearsal yesterday. We were pretty loud and and all of a sudden a massive loss in volume. It was as if 50w turned into 1w. 

Pre/power valve issue?

ICBM

Many things, but if it's a valve it will almost certainly be a preamp - a power valve failure won't take as much volume off as that and will produce an odd-sounding distortion.

What amp is it?

rico

ICBM said:

Many things, but if it's a valve it will almost certainly be a preamp - a power valve failure won't take as much volume off as that and will produce an odd-sounding distortion.

What amp is it?

Thanks - I will do the old preamp valve swapping trick. It’s a JCM800. 

Out of interest, what valve brands would you recommend for V1 V2 & V3? Is there a particular valve that works best for the PI? 

ICBM

rico said:

Out of interest, what valve brands would you recommend for V1 V2 & V3? Is there a particular valve that works best for the PI? 

RFT throughout.

rico

ICBM said:

rico said:

Out of interest, what valve brands would you recommend for V1 V2 & V3? Is there a particular valve that works best for the PI? 

RFT throughout.

Sorry - RFT?

ICBM

rico said:

Sorry - RFT?

RFT 12AX7s. The best-sounding in any late JMP or JCM800 2203/4.

It might not be a valve anyway though. One of the more common (though still rare) faults on these amps is the 100K plate resistor on V2A failing. Ideally it needs to be replaced with something a bit more robust than the original 1/2W carbon type.

rico

Ah - I had a google and they aren’t cheap! Next best option?

DJH83004

When a customer does not specify a preference, I tend to fit TADs in JCM800s, they are pretty reliable and you can swap the positions of the valves without having to worry if they are going to be ok in the cathode follower position, prior to the tone stack. You can get them in the UK through Hotrox.

rico

@DJH83004 I actually have TADs in 2 & 3 and I think a Sovtek in 1. Will replace them one by one to see if it is the cause of the issue. 

rico

@ICBM It's not a pre-amp valve. I just dropped it off to Guitar Aid and after some probing it seems like it may be the OT. Would a new one from Marshall be OK?



Thanks as always. 

ICBM

Ah... I remember now - this isn't an actual JCM800, it's a copy? It would be helpful to say so .

The transformer looks like a generic Chinese one, and that sort of very low output could certainly be explained by a shorted winding. While this is not unknown on Marshalls either, it's more common on this sort of thing.

A Marshall transformer (either Drake or Dagnall, depending on model) will be a better choice - if it fits. I notice the PT doesn't properly fit the chassis cut-out either...

rossyamaha

I've had this phenomenon alot over the years. It was only recently I realised it was the drummer turning me down. The drummer FFS!!

rico

ICBM said:

Ah... I remember now - this isn't an actual JCM800, it's a copy? It would be helpful to say so .

The transformer looks like a generic Chinese one, and that sort of very low output could certainly be explained by a shorted winding. While this is not unknown on Marshalls either, it's more common on this sort of thing.

A Marshall transformer (either Drake or Dagnall, depending on model) will be a better choice - if it fits. I notice the PT doesn't properly fit the chassis cut-out either...

Yes you are right, sorry! 

Is there a particular size for OT's or is it just finding one that is typically used on these models?

Pic mysteriously disappeared - reinstated above!

ICBM

rico said:

Is there a particular size for OT's or is it just finding one that is typically used on these models?

You will have to either assume that the transformer they use is the same mounting footprint as the proper one, or measure and check.

DJH83004

Why not drop Mike at Modulus a PM (he regularly contributes to the forum), he carries Dagnall TXs in stock and would measure one up for you. To be honest probably a lot better quality than the one fitted as well