I assume that auto bias in your amp means they have cathode resistors
in place, no negative voltage supply for the grids of the output tubes,
and no adjustment for bias.

(I have seen old wirewound cathode resistors develope breaks from the
weld points to their leads, and all four of them developing this. It
was a very old amp, however, and not likely here.)

When you say the tubes glow on and off, do you mean the filaments in
the center of the tube, or the plate structure or one of the grid
wires are glowing?

Do all the output tubes glow at once, or are a couple of them glowing?
Do the signal tubes do this as well?

If it's not the filament, but the plates/wires on both channels which
are glowing at an interval, then current through the tubes is greatly
changing, and is common to both channels.

If bias is not a common point to both channels, maybe its a low freq
feedback problem through one of the power supply caps, such as internal
arcing, cold solder joint, dried electrolytic causing a
charge/discharge or ground/unground at low frequency. You could get a
bench cap somewhat close to the cap values, and substitue each ps cap
one at a time to see if the problem stops.

Also, maybe a power supply resistor is arcing internally. I use a
plastic cheap stethescope to listen for this as it usually is audible,
but make sure it is NON CONDUCTIVE. I would imaging a drinking straw
would also work for listening for arcing in resistor bodies.

Also, you can power the amp up on it's side, turn out the lights and
watch it from a safe distance, and watch the underside for any arcing
to ground, etc.

Hope this helps
Bob H.



Dave xxxx wrote:
Bob H. wrote:
Does this amp have fixed bias, coming off of a single bias supply?

Bob H.

its got "auto bias"

I reread the original post regarding the popping, and now remember
having a similar sitution, ending up being a component lead being close
enough to the chassis to sometimes arc, and sometimes not. When it did
arc, it played hell with the entire circuit. Also, sometimes a wire's
insulation can degrade with high voltage over time if it's against the
chassis, and eventually will arc through the insulation.

Try to visually localize the popping noise if you can. Be careful of
the voltage present in the amp.

Bob H.

" Dave xxxx" wrote in message
k...
Keith G wrote:
Absolutely not, Dave's an 'old friend'....

ta old chap I am piggy in the middle

more information

Primaluna Prologue Two amp has gone mental again, i.e. broken down

It's the same problem as before where one or more output valves start
glowing on and off like buggery, loudspeakers drivers being thrown fully
out and back again, loud pops. this is happening with or without music
playing


Dave, Peter's come up with a pretty through investigation procedure and I'm
sure he'd get to the bottom of it, but if I were you I'd just run it in to a
repair tech unless you're confident about fettling it. Contact your seller
first? Also, is there a warranty running on it?

(I've got Phil's Ming Da here and these Chinese amps are pretty well
stuffed - not like a bit of homebrew with plenty of space to play about in!)

Keith G wrote:

Dave, Peter's come up with a pretty through investigation procedure and I'm
sure he'd get to the bottom of it, but if I were you I'd just run it in to a
repair tech unless you're confident about fettling it. Contact your seller
first? Also, is there a warranty running on it?

Thanks for the vote of confidence. Like most basic diagnostics, it
takes longer to write it out than to actually do it. I get VERY
squirrelly about visiting suggestions on other's equipment remotely
until I have a very good understanding of both the nature of the beast
and its proper behavior. So it drives me nuts when I am presented with:
"I have this amp, it doesn't work.... Any ideas?

Apologies to Dave are in order and herewith tendered.

Peter Wieck
Wyncote, PA

wrote in message
oups.com...

Keith G wrote:

Dave, Peter's come up with a pretty through investigation procedure and
I'm
sure he'd get to the bottom of it, but if I were you I'd just run it in
to a
repair tech unless you're confident about fettling it. Contact your
seller
first? Also, is there a warranty running on it?

Thanks for the vote of confidence. Like most basic diagnostics, it
takes longer to write it out than to actually do it. I get VERY
squirrelly about visiting suggestions on other's equipment remotely
until I have a very good understanding of both the nature of the beast
and its proper behavior. So it drives me nuts when I am presented with:
"I have this amp, it doesn't work.... Any ideas?



That will be the 'completist' in you, but no bad thing!

I believe Dave's amp is quite new and almost certainly still in warranty (?)
which is why I suggested he might follow the 'take it back to shop' route.
(Stuffed, it was an eBay Chinese amp, of course!! ;-)

As we are crossposted, let's bring the RATs up to speed on my 2 Chinese 300B
SETs:

Amp No. 2 - little used, perfect behaviour and now gone - snatched away by
an EL34 PP user after he heard it!! (??)

Amp No. 1 - thrashed mercilessly, volume pot went on the fritz (replacement
was promised but never arrived), WW2 Chinese military driver valve fizzing
and had to be replaced, fragile/spindly lead on a (Boer War) German
capacitor broke and has had to be resoldered. Otherwise, still goes like a
train - OK, possibly a bit more Western Pacific than Western Electric, but I
love it and use it all the time!!

:-)