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I have same problem. Even more. I can't change patches and control parameters with MIDI messages using Ableton or other MIDI host.
You can't put ax7 in place of au7 because au7 is conducting more current and has lower output impedance (drives tone stack and other stuff...
1M is ok, 5.6M is too high for 12ax7. 5.6M came from 6sl7 ampeg circuits.
c13 - 22pF only shunts RF to the ground and can't affect audible HF.
Usually bad resistor can make such noise especially when too hot
Wrong tube usage leads to wrong conclusions
for sine waves
That is what I mean. Yes, amp will play right after standby off with no harm for tubes but better to "cook" it a little before playing.
that toroidal inductor mounted inside the preamp board, it is not looking to weight as much.
Those intelligent techs also say that standby switch in guitar amps is to prolong tube life
Why there is recommendation to warm up tubes(with all voltages applied) for 15-20 minutes before adjusting the bias?
Here is my assumption. Ported cab has higher impedance spike in low end than sealed one. It is transformed in to higher than usual load to output...
Plates need to be warmed up too with idle current.
Maybe because SVT cab is sealed and other cabs were ported?
The only true way is to draw a load line for that particular output section when 8 ohm load connected to 4 ohm tap and see is it safe or not.
it is not tubes. May be bad cable or wrong connection inside the amp.