#36 mod first mod to 2203/2204
Stage 1 input 33k shield on anode. Cathode 2k7/0.68uf. 470k/500pf 1meg a pot/ 1000pf bright cap silver mica
Stage 2 Shield on anode. Cathode 10k/.1uf 470k/500pf/470k. 500pf over the 100k plate resistor
Stage 3 Shield on anode. Cathode 10k/.1uf 470k/500pf/470k. 500pf over the 100k plate resistor
Stage 4 Cathode 820r/0.47uf. Tone stack 33k/250pf the rest stock
100pf fizz cap
NFB 100k/0.1uf
Make sure that the shield is on anode or it will not work.
It will be just one big hiss oscillation machine
Hi Nigel,
Many thanks for summarizing the #36 mod - these are the specs as I understood them from the various posts in the Metroamp forum. I have not tried them personally but that should definitely be much more gainy and have more midrange than the #39 version I tried. And indeed - I did not have any plate caps or plate shields and it was very noisy when I added cathode bypass caps to the 10k resistors.
BTW I moved on from my #39 conversion and changed some of the parts.
Stage 1: input 33k, shielded cable, Rk 2k7, Ck 0.68uf, Ra 100k, Ca 220pf, 22nf coupling cap, 1 Meg II 680pf into 1MA gain w/ 1000pf bright cap
Stage 2: Rk 1k5, Ck 1uf, Ra 100k, voltage divider 470k into 470k II 220pf,
Stage 3: Rk 10k, no Ck, Ra 100k, 470k into 220nf then 470k to ground
Stage 4: Rk 1k, no Ck, Ra 100k
Stage 5: standard Marshall cathode follower
Tone Stack: slope 56k, treble 250pf/220kB, bass 47nf/1MA, mid 22nf, 25kB
Power amp: standard Marshall Bass amp, 100pf fizz cap, 47k/8 ohms NFB, 0.68uf presence cap. I may change the NFB to 100k/8 ohms and the presence cap to 0.1uf.
Cheers Stephan