On Jul 11, 10:37 am, "ian field" wrote:

Going white suggests that the getter deposit has been exposed to air
(cracked glass!) other than mechanical damage before or during fitting (bent
pins etc) - this would suggest a very severe thermal shock, such as might be
caused by a dead short on the HT rail.- Hide quoted text -

Certainly the tube lost its vacuum. I hate to harp, but Chinese 5AR4/
GZ34s tend to fail either that way, or through internal melt-down.

Again, unless the OP has a schematic or finds one, much of the
troubleshooting at this point will depend on some basic instruments
and some experience in what the readings mean. And here goes, climbing
swiftly out on a limb:

4 x EL84/6BQ5 = ~ 3.5A comes to ~22 watts
2 x 12AX7/ECC83 (run at 6.3V) = ~0.6A comes to ~38 watts
1 x 5AR4/GZ34 = ~2A comes to 10 watts

Altogether, comes to ~ 70 watts, then losses, figure your quiescent
power use will be something around 75 - 90 watts. I would expect about
80 watts, but that is a blind SWAG guess.

With a good current meter and a variac you should be able to run the
current up to the point where the rectifier *just* begins to pass DC,
and see what happens on the current meter. If it starts to climb
anything above 90 watts, or so, then you have a problem that requires
further attention before you go any further. And at 90 watts, be
exceedingly careful and look for hot-spots, transformer leakage and so
forth.

Note: when a strange piece of equipment crosses my bench for the first
time untested, usually I make the calculation above and then see the
actual current draw (vs. expected draw) as my very first pass at
diagnosis. It can be very helpful.

And, looking at the schematic that Ross offered, I see a couple of
places where a shorted small-value cap could create the precise dead-
short situation that Ian mentions. I know it is not the same as yours,
but it does appear pretty typical of the species. Check them all.

Note also that _VERY_ high voltages exist inside tube equipment. Do
not become a part of them. Be careful and treat the amp with great
respect at all times.

Peter Wieck
Wyncote, PA