Amp issue: Ever increasing Bias on 2 EL34 valves only

lukedlb

Ok, this amp issue is beating my local amp tech, and he's one of the best.
The bias on one pair of the 4 EL34 power tubes slowly increases. The amp is a nicholls from the late 60s / early 70s. 100 watts with four inputs. Similar to my old (and sold) Sound City 100 watt Mk3.
I will get a full explanation and the failed solutions soon. And there's me thinking it was down to the fuse blowing every so often (imagine my embarrassment when I left the amp with the tech saying there's something wrong with the amp and he saw that two valves were blown white).
In the meantime, I'll post some gutshots to give you all an idea.

lukedlb

https://imgur.com/a/nWQODqP

ICBM

Probably the coupling cap from the phase inverter on that side leaking. I would replace both of them.

RiftAmps

ICBM said:

Probably the coupling cap from the phase inverter on that side leaking. I would replace both of them.

My first thought too, but you'd have to assume that the tech is competent enough to have checked that already, no?

@lukedlb when you say the bias is increasing, do you mean that the -ve voltage on pin 5 is climbing towards zero, or that the current running through the valve is mysteriously increasing whilst the bias supply remains stable?

ICBM

RiftAmps said:

My first thought too, but you'd have to assume that the tech is competent enough to have checked that already, no?

I would have thought so too, but I think it’s the most likely explanation. If it does it even with new valves it’s not one faulty one in the pair pulling down both of them either… not many other options if the other side is remaining stable.

(Or not unless it’s a DSL/TSL100, which was my first thought .)

lukedlb

Yes, tech has tried that. And these:

Replaced grid resistors

Replaced capacitors

Bias circuit redone

Reversed the inputs

Inverted output transformer

Replaced the valve sockets...and many other tests and various measures

lukedlb

RiftAmps said:

ICBM said:

Probably the coupling cap from the phase inverter on that side leaking. I would replace both of them.

My first thought too, but you'd have to assume that the tech is competent enough to have checked that already, no?

@lukedlb when you say the bias is increasing, do you mean that the -ve voltage on pin 5 is climbing towards zero, or that the current running through the valve is mysteriously increasing whilst the bias supply remains stable?

I don’t know the answer to that, sorry. 

ICBM

Which pair is it that runs away - the ones nearest the PT, or nearest the preamp valves?

jca74

Love the 7.5 ohm/15 ohm outputs...

lukedlb

ICBM said:

Which pair is it that runs away - the ones nearest the PT, or nearest the preamp valves?

It’s the valves near the preamp

lukedlb

RiftAmps said:

ICBM said:

Probably the coupling cap from the phase inverter on that side leaking. I would replace both of them.

My first thought too, but you'd have to assume that the tech is competent enough to have checked that already, no?

@lukedlb when you say the bias is increasing, do you mean that the -ve voltage on pin 5 is climbing towards zero, or that the current running through the valve is mysteriously increasing whilst the bias supply remains stable?

The current running through the valve is increasing. The tension on the bias on that pin is slightly lowering, even if all the components around the valve have been replaced.

lukedlb

The amp tech is the most experienced I know and the only person my friends and I trust with vintage amps. But this issue is perplexing even him so I wonder if any of the amp techs have ever faced a similar issue. 

ICBM

lukedlb said:

The amp tech is the most experienced I know and the only person my friends and I trust with vintage amps. But this issue is perplexing even him so I wonder if any of the amp techs have ever faced a similar issue. 

Not on such a simple vintage amp as far as I can remember.

If he hasn't done it already - although I assume he has, if he's experienced - I would disconnect the PI coupling caps at the power valve end first and make absolutely certain it's definitely a power stage problem. If that fixes it, and they've already been replaced, try reconnecting them but disconnecting the NFB loop.