AMPEX 350 Output Transformer bypass question

[jwhmca]

So I need to replace the output transformer on my 350. It pops and crackles. I have heard it does this because it has the B+ voltage going through the primary winding. Could I just route the B+ Voltage around the primary with a suitable resistor and then feed the primary the audio signal through a cap or something?

 

[pstamler]

I don't have a 350 schematic handy, but if it's anything like the 300 or 351 the output section is push-pull, two halves of a triode feeding the transformer. You'd need a pair of resistors and two capacitors, and you'd probably get a good deal less usable output. You might want to try replacing the output tube with a new one and see if the problem is really the tranny. If it is you might consider replacing it with something like a Jensen. Not cheap but decidedly good.

Peace,
Paul

 

[PRR]

I would suspect old resistors and dirty tubes before I'd be sniffing the transformer. If the paper is broke-down, it usually quits working at all very quickly. Resistors and tubes can crackle forever or until you fix them.

> has the B+ voltage going through the primary winding

ALL tube guitar amps have B+ in the primary. 99.44% of guitar amp crackles are bad tubes, bad resistors (especially carbon-comp), or bad capacitors.

 

[jwhmca]

Well, it has been totally recapped and I swapped the transformer and the problem went away

 

[emrr]

the first two commentators would have know what was up if this was a continuation of the original thread.  they can't be all that helpful when the info is scattered all over the place.  help yourself by making it easy for people to help you. 

http://www.groupdiy.com/index.php?topic=53140.0
http://www.groupdiy.com/index.php?topic=53140.msg678412#msg678412

your proposed solution is worth trying, as I had proposed it to you originally.  since it's a power tube used at line level, I think you can probably afford the headroom that will be lost.  try it.  I kept original iron alive in another piece with this method, though I used a choke rather than a resistor, as my example was a small power amp used at several watts output. 

 

[jwhmca]

Yeah, sometimes "original threads" can get sidetracked a little... and the core of the question really changed. It helps me in latter searching and others in "later searching" to have a specific question and answer chain.

Hence the new thread.

Sometime in the future, someone will do a search on "AMPEX 350 OUTPUT TRANSFORMER BYPASS" and they will hopefully find a clear chain answer... instead of digging through miles of various issues.

Anyway, IMHO- YMMV

 

[lassoharp]

If it is T402 you are referring to it's singled ended 6F6 biased for about 11-12ma.  If other tubes have been taken off the PS the B+ would go higher and raise the current.  And if 1.5K cathode resistor (R442) is dropping in value with age you can get big increases in the current the OT pri sees.  Check that resistor and measure the cathode voltage to see how much current you're pulling. 

I bought a set of Ampex MagSound transformers of ebay and the OT was bad - open primary.  MagSound preamp uses similar output stage as 350 - SE power pentode biased for low current.  Relatively small OT there and maybe using small gauge wire.  I have no idea of what the rated impedance specs are for that one or the 350 OT,    XXX-600ohm.  Maybe higher than typical for  power pentode loading?  If you have a meter that measrues AC Z, load the sec with 600r and measure pri Z.

If someone went in and changed R442 to something more typical for that tube, like 300-500r you'd probably have over double the current in the pri.

If the OT is going bad you may be able to rework the 6F6 as a CF output.  I think most 350s are point to point so you should have some latitude there.