Re: Updated info on the problem

"Mark in Philly" <mrysak@xxxxxxxxxxx> said:


Updated from the original post:
I measured both amps. The amp without a problem reads nominal in all
respects. The following readings are for the problem amp.

The AC voltage from the power transformer is nominal at 400VAC. I read
resistance of the diodes in circuit and they are as they should be,
about 4 ohms in the plus direction, about 30K in the negative


Hold it right there.
Something tells me you didn't use the diode test position of your
meter to measure the diodes.
If they're silicon types (which I assume), they should measure about
0.7V in forward direction, and infinity in reverse direction.
To be absolutely sure, solder them out of the circuit, or at least one
leg.

Measuring diodes on an ohms range won't tell you anything meaningful.


direction. All the power capacitors look fine. The voltage at the
power capacitors, which should be 500v is actually, 400v. The choke
running between the power capacitors looks fine.

The voltages at pin 3 and pin 4 on all the 8417s is 400v instead of
500v. The Bias readings are correct at -26V.


OK, so let's assume the bias rectifier, caps and pot are OK.
There may be several reasons for the lower voltage.
#1. A rectifier diode, supply capacitor, choke or even the power
transformer may have a problem.
#2. The circuit draws too much current.

Personally, I think it is # 2., because the resistors at the anodes
were fried.
Be very careful when doing the following:
Remove the output tubes, keep your voltmeter clipped with crocodile
clips to the B+ and ground, and switch your amp on for a very short
period of time only, just long enough to read the voltage when it has
stabilized.
You should now read the nominal B+ voltage or higher (apparently in
this case it is 500 volts).
If that is correct, the problem lies in the amp circuit.
Try then swapping the power tubes from the other amp and check if the
problem persists. If so, the circuit is at fault, and most likely the
coupling caps are leaky.


If the readings are still too low after pulling the power tubes, one
or more of the components in the power supply is/are defective.
As other posters suggested, the most likely culprit is the
electrolytic capacitor (or several if there are more in the circuit)
in the power supply.

Further testing could be to measure the divided voltages over each
individual capacitor in the power supply, before and after the choke.
They must be roughly divided in two, meaning about 250 V each.
Be very careful, because both "upper" caps carry high voltages at both
terminals!

You can then try to unsolder one leg of the supply choke (remember to
isolate it!) and measure again (remember, keep the "on" time as short
as possible).
This way, you can determine whether the first series string of caps
before the choke is at fault, or the second string after the choke.

This advice is actually not without danger, as without current drawn
from the power supply, the voltage over the supply caps may rise too
high.
OTOH, when the tubes are still cold, there's a surge as well, so this
test should be possible without blowing caps.
However, I suggest you wear safety goggles and keep your hand on the
"on" switch. 500 V into several hundreds of uF is no joke!

Best would be to use a variac and turn up the mains voltage slowly,
but judging from your other posts, you don't have one at your
disposal.


The voltages for the 7199: Pin 1 is 220v instead of 290, pin 2: 65
instead of 80, pin 3: 20 instead of 30, pin 6: 3.3 instead of .7, pin
7: 0.7 instead of 0, pin 8: 70 instead of 90, pin 9: 65 instead of 80.


The voltages on the 7199 are likely to be lower as well, since they're
further "upstream" in the power supply.
The voltage reading on pin 7 and 6 do worry me a bit, however..
Pin 7 is the grid of the penthode section, which should be at ground
potential.
Any DC voltage at that point is likely to disturb the tube's operating
point.
Try disconnecting the source and measure again the DC voltage.
If it is still there, the tube draws grid current and should be
replaced.
Try swapping with the one from the other monobloc.


The voltages at the time delay relay 6N030T are 400 instead of 500.

Switching out 7199 or 6N030T has no effect on the readings.


OK, so you already swapped them. Strange.
Try cleaning the tube sockets, as the voltage at pin 7 should be 0 V
DC.


Thanks again for any input. I'll post results of Sander's suggested
tests on the 5th.


In any case, I'd replace the coupling caps to the grids of the power
tubes in both channels, just as a precaution.
Also, if the bias rectifiers are of the selenium type, replace them as
well.

Good luck!

--
"All amps sound alike, but some sound more alike than others".
.

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