Sunn Sonic I (EL34 version)

[paka]

Greetings, this is my first post here.

I bought a Sunn Sonic I amp which is an EL34 version (7199 rectifier). Caps and pots say it was made in '67. I have no experience with Sunn amps but in my opinion it sounds weak, muddy and it distorts in unpleasant way. I switched tubes with no improvement so I took it to my bench.

It has original Dynco A470 OT and aftermarket Hammond 372FX PT. PT is rated for 300V on a secondary winding but actually throws 324V.

The amp has got an original Mallory cap can from 1967. I measured voltages on specific points and they differ a lot from Sonic I-40 schematic (the only Sonic schematic I found, take a look at the attachment). I understand that is a different amp but these values are very low for a class AB push-pull amp based on EL34. To make things worse, there are bigger resistors between caps.

Can someone help me to verify those values? Is a PT suitable for this amp? If yes, should I start to play with different caps and resistors to achieve proper voltages?

[loudthud]

I think this schematic is closer to what you have: http://el34world.com/charts/Schematics/files/sunn/sunn_sonic_i40.pdf

The Hammond 372FX has a secondary of 300-0-50-300. That's too low unless you use a solid state rectifier. The 374BX at 375-0-50-375 would be closer to the factory spec transformer. Triode ELectronics makes a replacement for the Dynaco PT with a 370-0-55-370 secondary. Link: http://triodeelectronics.com/pamaivpotr.html

If you need a 240V primary, use the Hammond or maybe Triode has a unit with that primary.

[paka]

Thank you! This is very helpful.