Grid leak biasing

[Tomislaw]

Regarding the Supro: Those old amps like Supro and Gibson et al often had multiple inputs for different instruments, microphones, accordians, phono players with crystal cartridges, intending for the amp to be a portable PA system for the whole band. So, lots of tubes. Then add tremolo or reverb and the tube count rises. 6973 you can deal with more or less like a 6BQ5-EL84 as far as OTs go. Speakers were often quite generic low-watt "full-range" types.

I was referring to The Coronado which originally had 3 12AX7's - channel one had just one triode, channel two used two triodes and tremolo was another. The new version is part of the 1964 Reissue Series but the circuit was changed as 4 preamp tubes are listed in the specs.  According to the info, plugging into channel one links it in parallel with channel two, so they probably use another tube to keep the channel one signal in phase with the other one. This not true to the original, despite their producer's claims the amp is a faithful re-creation of the old Coronado. There are also single jacks for each channel which suggest a significant but probably inevitable change in the input voicing compared with the original circuit.

I too came up with a similar topology for the one I have built, adding to channel one a switchable gain stage with an attenuator in front of it, but decided to use just one input and add a three way channel A/A+B/B selector. I really like the dirty tone I'm getting out of it and it's still far from the hard distortion even though each stage shows gain of ~50.