Unusual Transformer

jon · Post by **jon** » Mon Sep 07, 2015 3:18 pm

So I recently acquired a transformer that I would like to use in an upcoming build. The only issue is I'm not sure what it can really handle. Attached are the schematics and the specifications for the transformer. The original thought was something in the 120-150 watt range.

Leo_Gnardo · Post by **Leo_Gnardo** » Mon Sep 07, 2015 3:59 pm

Looks dandy as-is for a single ended hi fi amp with that 800V B+, just the thing for some radio transmitter tubes.

jon · Post by **jon** » Mon Sep 07, 2015 4:51 pm

I was kinda hoping to run a quad of EL34's or 6l6GC's like a music man

Leo_Gnardo · Post by **Leo_Gnardo** » Mon Sep 07, 2015 5:21 pm

jon wrote:I was kinda hoping to run a quad of EL34's or 6l6GC's like a music man

Well I s'pose you could run the screen grids from the lower-voltage hi voltage supply. Might consider 6CA7 or KT88 both less likely to break down at such a high B+. It's tricky working much beyond 600V, electrons like to arc, jump gaps they wouldn't dare at "normal" voltages. But as you said MusicMan does it. Also those rare huge 300 & 400-PS 1970's Fender bass amps and the venerable Ampeg SVT. As long as everything runs OK, no problem. But I have seen some spectacularly exploded screen grid R's in MusicMen. Rare but it happens. Heck with all that current available you could run 6 output tubes for a super power amp. Or experiment with a quad of KT120 or KT150.

'

Have a look at this for the 400-PS experience. My only complaint, what the heck does anybody need a reverb & vibrato for in a giant bass amp, one bell and one whistle too many IMHO.

http://timeelect.com/400-histy.htm

SilverFox · Post by **SilverFox** » Mon Sep 07, 2015 5:35 pm

You may want to check on the availability of some older TV deflection tubes. They are designed to handle high voltage of that nature. I've got, I think 6 in a 70's linear amp for CB radio. On that I may use a transformer out of a Peavey 160 Watt to power the linear.

So what is the advantage of running the tubes at a higher than typical voltage? And what's Head room about if that is the answer? Before clipping? Wow! Loud and clear. Over.

I just took a look at the spec sheet. That looks like the transformer I need for the Linear... I've got the Peavey and that is for an amp but thought I could use it for the linear....

silverfox.

jon · Post by **jon** » Mon Sep 07, 2015 5:39 pm

Sorry I should have indicated that I was planning on running high plates low screen into a high primary impedance Output transformer.

If I read the specs correctly I can run 700VDC @ 250mA a 100% duty cycle and 300mA @ 50% duty cycle. Factor in that I will not be drawing more than 100mA of 300VDC I THINK the PT will hold up.

Phil_S · Post by **Phil_S** » Mon Sep 07, 2015 5:57 pm

It looks to me like the spec sheet, taken in the context of the schematic, assumes you are using a doubler. The spec sheet says 700VDC @ 250mA under load.

If I'm doing the math right (always something you should double check with me doing it) backing out from the doubler to a simple FWB, I'm thinking you'll get B+ of about 320VDC AND that should raise the mA into the low-mid 400's. That should be enough to supply a quad of 6L6 or EL34 in a comparatively low(er) voltage deployment.

The math...the schematic says 268VAC is supplied. 268*1.4 for a FWB = 375VDC. Spec sheet says under load you get 700/820; that applied to 375 = 320, though I suspect it will be higher in real life, but not more than 375.

For the mA, working backwards from 250mA, the doubler has a multiplier of 2.8 on the voltage. I'm assuming there is an inverse effect on current (a leap?). So, 250*2.8 = 700mA. Pushing 700mA into a FWB the multiplier is 62% of 700 = 434mA. That is enough for a quad of either.

If you go with the higher voltage through the doubler, as spec'd, you can only run a pair.

The heater supply is over the top 6.3V @ 11A. After reserving 6A for a quad of EL34 or 3.6 fpr a quad of 6L6, you still have enough left for a whole lot of preamp tubes. You may have to figure out how to pair them up for a 12.6 supply or I guess you can try to balance the load on each half of the filament winding.

jon · Post by **jon** » Mon Sep 07, 2015 7:12 pm

What about combining the two supplies in a non voltage doubled arrangement, like a Hiwatt 200 or 400 does.

Think I could get 550vdc at 4-500mA?

Phil_S · Post by **Phil_S** » Mon Sep 07, 2015 8:56 pm

Combining the two secondaries can be done. I've done it. You are limited by the lower current rating of the two windings.

If my math assumptions are correct about current, then 100mA after the doubler might be 174mA through a FWB and this is a limiting factor. Since now you have no CT, FWB is what you will be doing with it. 174mA isn't enough for a quad and might be tight for a pair -- time to make your own calc and draw some load lines. You are more likely to get something less than 550VDC through a FWB with load. I'm thinking more like 520-530VDC.

If you don't already have it, you might want to d/l Duncan's PSU so you can simulate this.

jon · Post by **jon** » Tue Sep 08, 2015 1:50 pm

I have played around with the Duncan PS tool and have never had much luck using it.

Phil_S · Post by **Phil_S** » Tue Sep 08, 2015 2:49 pm

I might be able to help you with it. Tell us the ohm readings on the windings. I think we need the primary and the relevant secondaries. What rectifier will you be using?

jon · Post by **jon** » Fri Sep 11, 2015 1:24 pm

I think I am going to use this transformer to build an amp with a power stage like the Fender 160PS ( I already have the interstage transformer).

What I want to know now is weather I can simply build a BLK face Champ or Vibro Champ as the front end and hook up the interstage as my OT to drive the main output tubes?

Leo_Gnardo · Post by **Leo_Gnardo** » Fri Sep 11, 2015 5:52 pm

jon wrote:I think I am going to use this transformer to build an amp with a power stage like the Fender 160PS ( I already have the interstage transformer). - What I want to know now is weather I can simply build a BLK face Champ or Vibro Champ as the front end and hook up the interstage as my OT to drive the main output tubes?

Diabolical! (Fits your avatar, heh heh heh...) Oh my this is gonna be good. Easy enough to sling an a mid control too if you choose. Wish I could be there to hear the results in person.

jon · Post by **jon** » Tue Sep 15, 2015 12:53 am

Any thoughts?

ToneMerc · Post by **ToneMerc** » Fri Sep 18, 2015 12:13 pm

jon wrote:Sorry I should have indicated that I was planning on running high plates low screen into a high primary impedance Output transformer.

If I read the specs correctly I can run 700VDC @ 250mA a 100% duty cycle and 300mA @ 50% duty cycle. Factor in that I will not be drawing more than 100mA of 300VDC I THINK the PT will hold up.

If you want to chase the MM style supply, that transformer is rated far, far too small, the typical 4 output tube MM PT the HT winding is .9 to 1A. IIRC, the smallest MM (doubler) PT is around 690mA, .7A

TM

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