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?
Comments
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.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
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...
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
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.
[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...
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
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.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein