Alrighty. I did order some Sylvania 6DX8 just to test against the russian ECL84 equivalents, but didn't manage to wait them to get through the customs and redid the amp for 6AQ5 (actually for the russian equivalents 6p1p:) ) with a separate PI tube of'course.
10thTx many thanks for the suggestions on the changes - I used most of these with a few changes. I left the 120pF across the coupling caps at the end. I ended up using 25uF cathode bypass cap (and made it switchable from 2.2 uF).
I ended up using 300 Ohm for the cathode bias resistor on the power tubes, since a quick simulation in LTSpice showed a bit safer plate current (given the PT is rated for 75mA only). Now the plate voltage is 243V and cathode is 13.22V on the power tubes.
I am very happy with result. The amp has more clean headroom now; bass response is fuller, highs are a bit more raw and open.
Few things left to be done, though:
- will have to play with the tone stack to dial out a little bit of 300-350 Hz-ish frequency (or build a separate notch filter)
- there's some sort of crackle/scratchiness at the notes tail (you can hear it 0:58-0:59 from the sample below and further on) which I have to find the reason for and will appreciate input. Couple of thoughts are - I got as a gift a bunch of russian military PIO 0.01 caps that I used as coupling caps to the power stage. They usually have some small DC leakage; Second is that I put 12AT7 as a phase inverter since I liked the sound quite a bit (I have some 12AV7 on order and will try it as PI as well), so probably will have to re-calc the plate resistors. Didn't have the time to replace it back to 12AX7 and find out whether crackle disappears or not, but will check today.
Other thoughts will be much appreciated
Here's the sound bit (just hit record and didn't play anything in particular, mostly different notes/chords to check the amp's response). It is a strat.
http://soundcloud.com/synchu/scrappy-61p1-12av7-12at7pi