Tube Question(s)

[rsm]

My "new-to-me" 1976 Fender Super Twin has the following preamp/EQ tubes:

2 x 7025
1 x 12AU7A
1 x 12AX7A
1 x 12AT7A

I'm only finding NOS tubes for the ones ending in A.

The A means it works in both parallel or series wired heater circuits; without the A, they only work with parallel wired heaters.

How can I determine if the Super Twin has Parallel or Series wired heater circuits?

I don't want to pay for NOS if they're not required.

 

[rsm]

schematic

 

[rsm]

anyone?

 

[NowYou'rePlayingWithPower]

Have no idea to the fender question, but I have a lot of tubes including those you listed if you want to pay shipping to your island.
Finally got a new Dynojet 707 tube tester the other day (well not new, but rebuilt/calibrated one from the 60s) so I can start going though my horde again.

 

[7704A]

You got a better schematic? The one you posted has almost illegible text, specifically on the parts that tell you the answer.

 

[glpg80]

Series is 12.6V, parallel is 6.3V. Both your power tubes and preamp tubes are wired in parallel.

 

[Aynirar27]

I don’t know this amp specifically, but I’ve also never heard of any common amps being wired in series

 

[7704A]

Series is 12.6V, parallel is 6.3V. Both your power tubes and preamp tubes are wired in parallel.

View attachment 294194

I was unsure if that was 61V or 6.1V on the rest of the heaters, but on reflection after reading your post 61V makes no sense for 6 heaters in a series string, so gotta be 6.1V. So add my vote to the parallel pile.

 

[rsm]

thank you all!

 

[rsm]

Here is the link to the schematic, may be more clear?

https://schematicheaven.net/fenderamps/super_twin_180w_schem.pdf

 

[shar-vell Dan]

I ran blackburn mullard 12AX7's & AT7's in my silverface twin w/o a problem.

 

[Racerxrated]

Series is 12.6V, parallel is 6.3V. Both your power tubes and preamp tubes are wired in parallel.

View attachment 294194

Based on this, you should be able to use any 12ax7 just fine.

 

[Racerxrated]

I ran blackburn mullard 12AX7's & AT7's in my silverface twin w/o a problem.

Me as well...I have a 76 Twin Reverb(bought it for the orange frame D120s ), I've rolled Pre tubes and not noticed any issues with what it came with (vintage RCAs) and any of my other 12ax7s.