Extra Sunn 190L Tubes?

[Cheeto333]

Hi all,

I recently got my hands on an old (late 60s, early 70s) Sunn 190L head in unknown working condition. Schematics / parts lists attached.

There seem to be two extra tubes + sockets installed: a 12AX7 and a 7739. Note the difference between my amp and the pictures shown on this listing.

Any guesses on what these mods may be?

The normal 12ax7, 12au7, 6an8, and 6550s all test fine on my tester, I haven't tried powering up to see if it works.

Thanks for any help.

 

[PRR]

7739 is a 5,000MHz funny-tube, not a mini?
http://www.mif.pg.gda.pl/homepages/frank/sheets/201/7/7739.pdf

Looking at the 1972 plan, my guess is that early production used tubes in place of those transistors. 12AX7 could be the reverb recovery or trem oscillator. If your trem is the same light-bulb plan, a larger tube would be needed for that. But what "7739" it would be, I do not know.

 

[Cheeto333]

Hmm, that doesn't look right. There's another set of lettering, looks like it says 7025? Looks like these tubes here.

My game plan is to power up the amp slowly using a variac, following procedure here. I should probably change out all those old caps...

 

[Loudthud]

Looks like some kind of after market mod. Sunn never used an eyelet board. Maybe a replacement for the phase inverter.