Problem with Victory V40 amp - one output tube cutting out

Philtre

Help needed!

The V40 has an output tube switch which allows to switch between each output tube (effectively half power) or both. As I use it at home, I normally have it switched to the left (as I'm facing the amp) so only using one output tube.

Today I switched it to the right and...no sound. So I switched tubes around and, for a moment both sides worked. Then there was a loud "click" sound and the output on the suspect tube died. So I switched the tubes again and the same thing happened but on the other side.

So the suspect tube works for a few seconds, clicks and then cuts out. After that there is hum from the amp.

Is this a tube problem or something else?

Is it safe to remove one output tube at a time and then power up the amp to test?

Philtre

I'm guessing it must the tube rather than a socket. Is it possible for an EL34 to work for a minute or so, then make a loud "pop" or "click" through the speakers and then stop working? Perhaps it craps out when it warms up?

ICBM

If the fault moves with the valve then it is the valve and not the amp.

It is possible for there to be bad joints inside the valve pins, but it’s more commonly on the filament pins which will make the sound fade out slowly - for it to pop and cut out suddenly but without the valve then almost immediately blowing, it would have to be on the cathode (pin 8) or screen (pin 4).

Sometimes these can be resoldered by heating the pin and adding a little fresh solder to the end so it wicks down inside. Otherwise, the valve is scrap.

Philtre

@ICBM Thanks for the reply.

Further testing shows it moves with the valve. From cold it functions briefly (from a few seconds to a few minutes) then pops and cuts out (leaving just an increased hum). If I let it cool down it does it again.

I'm going to just suck it up and buy a new matched pair of JJ EL34s.

Philtre

I wonder if using only one of the EL34s by means of the Victory's low-power mode switch has somehow weakened the other one?

ICBM

Philtre said:

I wonder if using only one of the EL34s by means of the Victory's low-power mode switch has somehow weakened the other one?

Almost certainly not. The fault sounds like a bad connection in the valve, which can't have been caused by running with no signal.

Is the faulty one also a JJ? I've come across more pin connection problems with JJ EL34s than any other valve type. Small sample though...

Philtre

Yes, JJ.

Philtre

A quick update for completion's sake...

Bunged in a brand new matched pair of JJ EL34s, biased them, and, so far, everything is good. Overall amp hum is down, too. I reckon that rogue valve has been dodgy for some time.

Funkfingers

Philtre said:

I wonder if using only one of the EL34s by means of the Victory's low-power mode switch has somehow weakened the other one?

I understand the thinking behind the either or both output valve switching. It could keep the amp usable in the event of a single output stage valve failure.

The rest of the time, it might have been wiser to adopt either the pentode/triode mode switching seen in some Marshall designs or the Cornell output attenuator. 

Philtre

@Funkfingers I guess that's an added bonus, to operating the amp at lower levels. The Victory manual says about the "Single Ended Mode Switch":

"This switch, located through a hole in the rear grill, is used to switch off either of the 2 output valves. This results in the amplifier running ‘single-ended’ which is a very pure Class A mode. If the front panel power switch is on LOW, in single-ended mode the V40 produces around 0.5Watts rms. This should allow playing at home without getting lynched by the neighbours.The switch throws both ways to alternate between each output valve for even wear if this mode is used extensively."

Funkfingers

Philtre said:

"This switch, located through a hole in the rear grill, is used to switch off either of the 2 output valves. This results in the amplifier running ‘single-ended’ which is a very pure Class A mode."

Class A Mode is an expression which has been used and abused a great deal by marketing departments. I am not qualified to properly explain the difference between real Class A Mode and some of the things that get marketed as Class A but, strictly speaking, are a hybrid class.

[Insert ICBM lecture here.]

I would expect Class A operation to shorten the life of most output stage valves. 

ICBM

Funkfingers said:

[Insert ICBM lecture here.]

I would expect Class A operation to shorten the life of most output stage valves. 

Possibly. Or it might not make any difference.

It may adjust the bias as well, otherwise the waveform wouldn't be symmetrical - in which case the valve will be running hotter - or they might just not bother about that, since a lot of single-ended amps don't!

In fact, to be strictly correct single-ended is not Class A anyway - the class definitions were intended to describe what happens specifically in push-pull circuits. In proper audio design *any* single-ended stage should be biased to amplify the full waveform at maximum output. Single-ended is simply single-ended - Class A is push-pull where all of the output devices conduct throughout the waveform at full output.

Probably the most misused term in all guitar-amp-speak...

Philtre

So, what do they mean by "off" when one of the output valves is not used (even though the heater is still on). Maybe it's just safer to run them both?

ICBM

Philtre said:

So, what do they mean by "off" when one of the output valves is not used (even though the heater is still on). Maybe it's just safer to run them both?

Without seeing the schematic or measuring the amp it's hard to be sure.

There are three ways of turning one valve off -

Filament off. Rarely used for some reason, but is the best for the valve. Clearly not what's done here if you can see the filament glow.

Forward current off - disconnect screen or cathode. Most commonly used, but could theoretically cause 'cathode poisoning' (if that applies to audio valves).

Input signal off. Mesa have patented this, and have been willing to threaten to sue other companies to stop them copying it. This will prevent any possibility of cathode poisoning, and maintain power supply hum cancellation in the OT (which the other two won't) but will essentially be running the valve at idle the whole time so it will still slowly wear out.

Any of them will be less hard on the valve in the short term than running both fully, but in the long term might cause unequal wear, which is why Victory give you the option of one or the other.