Telefunken ECC83/12AX7

[bluebird]

Quite why they thought it was necessary to dc couple three tube stages I do not know but, since there is no feedback at dc, the dc conditions of the second and third tubes could vary over quite a wide range

They could have easily put a cap from the last cathode to the phase inverter grid, it wouldn't have had to be that big. They are doing a few things that I've never seen in most guitar amps of that era. Deriving the presence control  with the little feedback network preceding the phase inverter is odd. Usually its feedback from the output transformer secondary to the phase inverter. The more I look at it the weirder it gets!

The amp sound great by the way, if not too loud for me to use now. I haven't plugged it in for years.

I remember being a teenager at my local music store and hearing a 100W Hiwatt through a matching 4X12 cab and it was the chunkiest, cleanest, loudest, bell like tone I ever heard.

 

[ruffrecords]

My apologies, I should have been clearer. It is the stage preceding the tone control that seems to eat new production tubes (V2 slot). The other cathode follower at V3 is (and always has been) a new production Tung-Sol.

I thought I'd best take the chassis out of its cabinet and measure the voltage at V2 pin 3: ~170V.

Ah, so the cathode is very close to the Vhk limit. Add 10V of signal and it is exceeded. I am surprised the cathode dc voltage is so high. No need really; 100V or so would have been plenty. Perhaps the NOS Mullards have a greater production margin on this parameter.

Cheers

Ian