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TheGimp 28th February 2013 02:46 AM

Designing an Interstage Transformer
I wish to design and construct an interstage transformer for an SET amplifier using a 6A5 or 6B4G.

Driving tube will be a 6N7 with both plates in parallel for an effective RP of around 5500R.

Max bias current will be 10mA.

I just finished reading chapter 5 of RDH4, and plan on re-reading it. I've also been reading other works about transformers in general.

I'm probably going to have a lot of questions, but will try to go through the design process.

The cores I have are stripped from a set of24W line matching transformers. The laminates weigh 556gr per set.

The cores are loss-less EI cores with an I measurement of 1.122cm X 6.692cm by .45mm thick.

Tongue width is 2.23cm
Stack height is 2.57cm
Window is 1.05cm by 3.0cm (with the bobbin I made installed)

The cores are probably marginal for the project, however I have them so I plan on using them for a first iteration design.

estimated MLP 15.35cm
estimated MTL 14.021cm

Step 1. Calculate required inductance based on minimum frequency response and the driving tube plate resistance and drive voltage to the grid of the output tube (35Vrms).

F1=10Hz (yea, aggressive)

Use Xl=5* Rp = 5* 5500R =27500

Xl=2* PI * 20 * L
L = XL/(2 * PI * 10)
L = 27500/(2 * PI * 10) = 218 H

This is the first problem. It may be necessary to abandon the 6N7 for the 6SN7 with an Rp of 3850 for parallel plates, which would cut the inductance requirement to 152H.

So, is this correct so far?

TheGimp 28th February 2013 03:35 AM


That is 6A3, not 6A5.

jcdrisc 28th February 2013 04:45 AM

To get reasonable bandwidth and phase response you will have to segment the windings.
Homegrown audio transformers are usually low quality and are rarely worth the effort.
Better to go to Jensen or Sowter and buy a guaranteed product.

SemperFi 28th February 2013 12:29 PM

It is VERY difficult to make an IT with that high inductance that isn't anything than a muddler. (It will have horrible high frequency response and muddle your sound).
However I don't believe winding ITs and OTs is such a mytical black art as so many claim. The trick is having the proper tools, such as a winder of some sort that lets you wind with proper tension and that lets you lay the wire nicely along the bobbin. Doing a few transformers you can probably get decent tension simply by hand feeding, but for how many turns?...

Interleaving and winding layers to be with as low capacitance as possible isn't easy, but doable. You'll need patience to run some test windings if you only want the best. I don't think the cheaper manufacturers like EDCOR does much fancy interleaving and other techniques in their transformers.

I think for one you really need to relax your specs. Why 5xRp? 10Hz as low end goal is admireable, but is there really that much to gain in happiness? Better to aim for 20 or even 30Hz and get better wide band response IMHO.

Anyway, hope you give it a try. I've just bought some left over amorphous cores for same idea, interstage trannies, but will use lower Rp tubes like 6922 and trioded 6W6s. I think you core size is plenty for this useage. As you know you need some air gap since it'll be SE, and that'll require even more turns to make the Henrys. Good luck.

Oh, what size wire are you using?

JMFahey 28th February 2013 01:18 PM

Buy it.
Expensive? ..... Who cares?
SE systems are for people with fat wallets anyway.

SemperFi 28th February 2013 01:20 PM

But this is diy isn't it? If any one here does diy to save money I think they picked the wrong hobby.

the_manta 28th February 2013 01:30 PM


Originally Posted by SemperFi (
The trick is having the proper tools, such as a winder of some sort that lets you wind with proper tension and that lets you lay the wire nicely along the bobbin. Doing a few transformers you can probably get decent tension simply by hand feeding, but for how many turns?...

IMHO the trick is experience and knowledge. Winding machine, proper laminations and wire can be bought. But the technique to get the lowest stray capacitance is based on experience and skill.

sjs 28th February 2013 01:50 PM

I have to say you are starting with probably the most difficult audio trans to wind imho.

For good performance from interstage driver transformers, bi-filar windings are by far and away the best method to get good bandwidth, but you will need high quality double insulated wire, and v good winding technique to reduce risk of primary to secondary breakdown.

I would suggest your best bet is to buy commercial from Hammond, Audio Note or Monolith Magnetics.
Best of luck

tinitus 28th February 2013 02:00 PM

he, a few months ago I began to construct my own amp box handles
basicly because I thougt the were way too expencive
and sometimes too much work to find the right one

scrapped some, but have had my share of fun doing it
and I can now make one in no time

but learning how to wind a trafo probably takes a bit longer

6L6 28th February 2013 02:51 PM

Remember that you need to calculate the windings to be just a kiss undersize, because of you can's fit the bobbin into the lamination window you are sunk.

Please take lots of photos of your progress, I know most of us will like to see the process!!! :) :) :)

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