Reasonable interstage PP-PP transformer

[Hugo145]

Hello,

I am looking for reasonably priced 1:1 interstage transformer.
It will couple push-pull 6SN7 driving push-pull 7236 tube, no DC unbalanced current, ungapped should work for me.
It's Class A so current is minimal but max Vrms is about 60V with primary C.T. being at B+ 250V.

My options are
1) Carnhill VTB2047 600R:600R - datasheet
Really like it but when I contacted Carnhill they suggested that High Level Output is considered to be 12.3Vrms. So now I'm not sure whether it can handle the voltage.
Also low nominal impedance means lower inductance.
2) Hammond 124F 15k:15k - datasheet
My only concern is primary DCR of 900 Ohm which means higher cutoff frequency.
3) Edcor XSM 15k:15k - datasheet
Specs are nice but it's much smaller then first two, again, probably means lower inductance.

Which one is my best bet?
Unfortunately Sowter and Lundall are out of my price range.

Thank you,
Igor.

 

[FullRangeMan]

If you live in the nice Ukraine forget North American brands due ship:
Try Ogonovski:
Ogonowski.eu - profesjonalne transformatry do wzmacniaczy lampowych
Or Hashimoto that are better than those:
Hashimoto Products

 

[Koonw]

Hammond Manufacturing 6W68HF
TRANSFORMER; INTERSTAGE AUDIO; IMPED : PRI 10K OHMS; SEC; 10K CT OHMS; MAX DC 30MA
Original Allied Electronics.


I use 6F12p triode section to drive it, reflected load 26k in Pri when Sec is has 13k load each winding. Additional feedback flatten the frequency response and cut ringing to minimal and it seems the sound quality is not affected much by the amount of this local feedback. The attached schematic as almost zero negative feedback. I have also completed PPP version, good sound especially Pentode mode although the Zout is still fairly high but because the front end does most of the job so output frequency response is good without further feedback. The 2nd attachment uses similar approach.

 

[gabdx]

Softone RC-20 Versatile Audio Interstage Transformer

 

[Wavebourn]

Hi Igor;

do you know Dorin Bodea from Romania?

 

[Hugo145]

Thanks everyone for a response!

Try Ogonovski:
Ogonowski.eu - profesjonalne transformatry do wzmacniaczy lampowych

Thanks, this looks promising, contacted Leszek.

Softone RC-20 Versatile Audio Interstage Transformer

Very good specs but with shipping that's 300+USD for a pair.

Hammond Manufacturing 6W68HF
TRANSFORMER; INTERSTAGE AUDIO; IMPED : PRI 10K OHMS; SEC; 10K CT OHMS; MAX DC 30MA

No C.T. on primary unfortunately, will not work for PP-to-PP.

do you know Dorin Bodea from Romania?

No, but I managed to google his contact details, will write to him, thanks for the tip!

 

[analog_sa]

My options are
1) Carnhill VTB2047 600R:600R

Just curious: how well do you expect the 6SN7s to drive a 600ohm transformer? I wouldn't be too optimistic.

 

[Hugo145]

Just curious: how well do you expect the 6SN7s to drive a 600ohm transformer? I wouldn't be too optimistic.

But isn't that just a nominal impedance?
Secondary is looking into the tube grid so reflected load on primary should be quite almost infinite, right?
My understanding was that nominal impedance gives you an idea of inductance at measured frequency, that's all.

 

[Wavebourn]

My understanding was that nominal impedance gives you an idea of inductance at measured frequency, that's all.

Yes, and a tube loaded on an inductance forms a high pass filter. The question is, a corner frequency. Also, on which frequency and voltage the core starts saturating.

 

[PRR]

But isn't that just a nominal impedance?...

It is the impedance target where the designer aimed for "good working".

A 600 can be run at 400 or 900, sure, even a little further if you are not real fussy.

But intended for 600, driven by the ~~12,000 of a pair of 6SN7 plates, probably will be very unhappy.

Shop for 10K or 15K. They are fairly available.

 

[analog_sa]

Secondary is looking into the tube grid so reflected load on primary should be quite almost infinite, right?

Yes, but the fact is a 600ohm transformer has very low primary inductance, perhaps just a Henry or two. Coupled with the high Rp of 6SN7 this means there will be issues at low frequencies, and this can be calculated or measured.

 

[Hugo145]

Thanks a lot everyone for the explanation!
There’s very sparse information available about insterstage transformers on the Internet. Perhaps they are not fancy these days as more modern and cheaper alternatives exist.

 

[gabdx]

Transformer drive is the best, final.

Capacitor drive is always a compromise, and resolved with feedback.

In the past 'golden age' professional audio quality was always full transformer coupling

I was actually shocked when I saw books of 500 pages one after the other with schematic describing transformer couplings.

 

[Wavebourn]

Capacitor drive is always a compromise, and resolved with feedback.

Quite opposite, feedback makes "capacitor drive" even worse.
But the "cpacitor drive" does not add such problems for feedback like interstage transformers.

 

[gabdx]

Hi wavebourn, I disagree, for my own sake I never built an amplifier with transformer drive because of cost. I have no doubt that they are the best way to achieve class B high drive for high power amplification, as demonstrated by technicians from G.E.

They remain and their variants the way to low distortion detailed sound by high quality tube amplifier builders.

I doubt this is relevant to domestic use under 150 watts, we can rest happy with our capacitor coupling.

Unless oscillation occurs (which is unlikely) the Transformer coupling (or balanced chokes) is the best.

Maybe one day I will build a class A amp just for fun which has a transformer coupling

 

[TheGimp]

I would expect as FET follower to provide better Class B drive capability .

 

[gabdx]

They are horrible A basic driver tube is nearly as good as a Mosfet. Fet drivers are for toy amps.

The transformer coupling is for a KT66, and you do use a capacitor too, however the transformer makes the tube operate to it's maximum voltage swing.