Hi Stephen,
So you have -161 V at pins 3 of the output tubes, but hardly an output signal. When there is no nearly short circuit somewhere in the output wiring this means that the output tubes draw just a very small current during the most positive voltage level at the grids, pins 2.
Ideally you measure the waveform of Vg1 versus Vk differentially with e.g. a 7A13 or a 7A22. You could also first obtain the waveform of Vg1 in AC. The average is (should be) zero. Now measure the average of Vg1 with a DMM in DC mode. Substract the DC Vk from this DC level and then add the observed AC waveform of Vg1. What are the two levels of Vg1k?
With Vk = -161 V I expect some Ia = 50 mA per tube. In triode configuration of the EL84 that nominally means Vg1k = -2.5 V (Philips graphs). In our configuration even a more positive Vg1k.
R59 is really needed as pull-up resistor. Not open circuit?
Albert
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On Tue, Mar 22, 2022 at 05:32 PM, Stephen wrote:
On Mon, Mar 21, 2022 at 06:02 PM, Albert Otten wrote:
Stephen, some time ago there was a problem with S242B. The switch seemed to
be
closed all the time. I assume your reading now is still in High Ampl mode.
Then it looks like the opposite failure of S242B: open all the time. That
results in pin 3 sitting at the most extreme negative voltage. The wiper of
R246 is disconnected and Amplitude has no effect at all. Perhaps you can
check
the behavior of the switch with a resistance meter.
Albert
Hi Albert,
I removed the switch, tested it, cleaned it, but it¡¯s actually fine. Put it
back, but nothing has changed.
I do have a perfect square wave on the High Amplitude position, it¡¯s just
not more than 140mV into 50R.
This have me wonder¡. Could this be related to the bias circuit¡? Like in
a tube amp?
