200s

[Anonymous]

I just became the proud owner of a Sunn 200s(the inspection tag says
11-6-70).The current tube layout is: (2)6550,12AX7,6AN8A,5U4GB.
This thing basically sounds like a Blkfc Bassman but with a
tighter bottom end and a bit more sag when pushed.I have a couple
of questions: 6AN8A(driver tube?) differs from a 12AX7 in what
way? Common substitions? What rectifier tube substitions are
common? Thanks for reading.

[Bill Matthews]

The 6AN8A is a single triode tube whereas the 12AX7A is a twin triode.. DO NOT look for a replacement for the 6AN8A, it is a great tube, and readily available ( and cheap )  If I remember on my 200S it is supposed to have a GZ34/5AR4 rectifier tube not the 5U4 you have.. That is probably why you are having the sag you describe.  Try the 5AR4, I think you will appreciate the difference.. Good Luck, Bill

[Ryan Phelps]

Erik;
The 6AN8A is actually a twin tube.....a medium-mu triode and a sharp-cutoff pentode. In the Sunn amps the pentode portion was used as the second stage pre-amp and the triode as a phase inverter to feed the 6550 push-pull output tubes. This circuit was adapted from the venerable Dynaco Mark III power amp. Bill is right about the 5AR4/GZ34 rectifier......it will cause less sag than the 5U4 and will improve the bottom end for bass.

Good Luck!
Ryan