Any thoughts on output transformers?

[ALL212]

I've been gifted a pair of boards and some parts for a dual EL84 + 12AT7 tube amp (1 amp, dual channel, per board so two amps total).  Part of the gifting was a pair of Edcor GXSE10-8-5k output transformers.  I ordered a pair of 6.3v tranny's last November from Edcor and just got them yesterday.  I'm in no hurry but Edcor makes these on demand per batch so it takes some time.  Edcor tranny - about $30 but wait 8 weeks.  Hammond has some available but looks like either $60 each for their cheap ones and $150 each for the "good" ones.

Any recommendations for a source of fairly inexpensive output tranny's that might arrive before next Christmas?  10w, 8 ohm, 5k ohm?

I've attached a picture of the current status.  The big blue tranny has 12.6v outputs for filaments and I know I can reduce that but for testing I used an old Sansui tranny that had 6.3v on it.  Then I forgot the big blue had 12.6 that could be brought down to 6.3 and I ordered new 6.3v tranny's just for filament.  oops... 

 

[Paul Birkeland]

Well, these are triode EL34s, so you sure don't need 10W output transformers!

You could see if Magnequest can make you a pair of Robin Hood transformers for this project.  The last pair that I bought showed up in about a month.  Really though, the 10W Edcor transformers that you have don't have the best bandwidth specs, but you also have to keep in mind that these numbers  will improve as power drops, so they are likely just fine for what you're up to.  (Which is operating under 2W)

On the power transformer, I'm guessing the 12AT7 is heated with the 12V winding and the EL84 heaters are stacked in series to also work off the 12V feed?  If so, why not just leave it as is?

 

[ALL212]

I'll try Magnaquest.  As far as the filament goes I've attached the PCB layout which calls for 6.3 and the schematic.  If it's not appropriate for those to be here just kill 'em.

I assumed that if I ran 12.6 through the board I'd release some magic smoke so I didn't go there.

And I lied - it's a 12AX..not AT

 

[Paul Birkeland]

Well, I'd recommend ignoring the schematic.  The amp won't work if it's built like that (the EL34 plate is not connected where it should be).

This is probably supposed to be setup as a pentode wired output stage with the screen tied to B+.  Also, the comments about the feedback resistor are a little misleading.  You want to use enough feedback to provide adequate damping and distortion performance but without making your amplifier into an oscillator.  With the amp setup like this, the Magnequest transformers should support more feedback than the Edcors.

 

[ALL212]

LOL!!

It's been very confusing trying to figure this thing out.  Not surprised about your comment on ignoring the schematic.

The board appears to work.  Whether I'll get quality sound out of it really isn't important.  I'm sorting out transformers, designing a plate for this... the fun stuff.

And so far no smoke and I've not electrocuted anyone...yet.

Thank you

 

[Chris65]

Well, these are triode EL34s,..

The amp won't work if it's built like that (the EL34 plate is not connected where it should be).

Am I missing something?..shouldn't the tubes be EL84's 6BQ5's

Ordered some Electra-Print OPT's recently, took 2 weeks to reach the other side of the Pacific.

 

[braubeat]

I'll second Electra-Print, good service good quality. I have used them several times.

Michael

 

[Paul Birkeland]

Am I missing something?..shouldn't the tubes be EL84's 6BQ5's

Oops, yes, EL84, not 34.