Could someone explain how this amp was working?

[Wes]

I recently was working on a friends amp (Ampeg Super Jet sj-12r) who
complained about random volume loss. When I first fired it up the amp
sounded pretty good, but the left 6L6 had a stronger than normal blue
glow. I swapped the left tube for the right and the glow followed the first
tube and it took about 15 seconds for the tube from the right to red plate
in the left socket.

I put them back the way they were and put a bias probe on and checked
cathode current. The blue glower was pulling about 70mA in its left socket
while the red plater wasn't pulling any current in its socket. After more
swapping, and no current draw from that socket, I found that that socket
had a busted 470@5W screen resistor. Changing that out and putting the
red plater back in, of course, it started red plating. No problem, got new
tubes for this amp on the way....I just have a couple of questions about
how this amp was working:

First, how the hell was this thing working with only one tube drawing current?

How did the screen of the red plater (having no voltage) kill the cathode and ultimately the tube??

[MWJB]

It wasn't, if "working" means sounding like it was supposed to. Amps will still produce "noise" in quite bizarre circumstances (best not explored, but for example I saw a Princeton that had lost a wire from the preamp coupling cap...that still made sound). In a fixed bias PP amp, the amp will still run with just one power tube (as long as the voltage hike on that tube isn't too much for it), it won't sound the same as 2 properly functioning tubes though. Likewise with a blown screen grid resistor, the amp will produce sound, but will be heavily distorted with no headroom. Pulling a tube in a cathode biased amp will hugely increase plate current on remaining tube(s), possibly killing it/them.

The tube probably shorted, killing the screen grid as a byproduct, rather than the other way around.

If he amp is cathode biased I'd double check condition of cathode resistor & bypass cap before installing more tubes.

[Wes]

The tube probably shorted, killing the screen grid as a byproduct, rather than the other way around.

Good point. That's what I figured.

If the amp is cathode biased I'd double check condition of cathode resistor & bypass cap before installing more tubes.

Fortunately it is fixed biased....saves me and my buddy a bit of trouble.

I thought that something would be audible or either the OT would get the
crap kicked out of it by running one tube in a PP pair...

heh....

Thanks for your input, MWJB!

[bluesfreak]

Hi Wes,

fixed bias wouldn't hold me away from measuring current with new tubes anyway. Usually the vendors do bias their Amps pretty cold to avoid warranty issues so having this amp measured and either tweaked (change bias resistor) or modded (install a pot for getting the bias easily adjustable) would be the correct way to work...

just my 2c

[52 Bill]

I thought that something would be audible

When one side of the push pull pair dies the signal will distort. At low levels the signal will actually sound ok until you drive it.

It's an unusual distortion. Try pulling one output tube and listen to the amp.

[Wes]

bluesfreak-

Pretty sure this amp has a bias trimpot. 'Least I saw a trimpot down
on the board.... I checked the grid voltage and it was sitting at about
-36V.

When the new tubes come in I plan on setting it all up....



Bill-

I'll check that out. Is it possible to damage the OT that way?