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Good tube amp designs for Biamping?!?

Good evening.

I´ve been thinking for some time about active crossovers and separate amps for the speaker drivers. I tried it a few years ago using a crappy DIY crossover and some equally crappy solid state amps, you can guess the results... I started building my first tube amp very shortly after that episode.

Now I feel that it might be the time to give biamping another chance, this time without any semiconductors involved.

I already own several different tube amps, but that is absolutely not an excuse for not building two more units, this time optimized for their mission.

I´m thinking PP class A bass amps (one 6AS7 per channel) and SE 12B4A for the highs. Should give about 10 resp. 1 watt per channel, which I think would be enough for my speakers.

As you can see I have this thing for low µ / low Rp triodes, which brings us to the real issue, the OPT´s.
Winding the OPT´s for the bass amp should be a breeze, just a lot of turns on a pair of big cores basically. Since the crossover frequency will be somewhere around 3000Hz there should be no need for any fancy winding techniques to get a good HF response, right?

The treble OPT´s on the other hand might be a problem.
I want flat response from say 1000 to 20 000+ Hz. Since the primary inductance won´t be much of an issue the number of turns could be smaller than for a fullrange unit, thus decreasing capacitance and leakage inductance, but what happens to the coupling from primary to secondary?
The prim/sec ratio needs to be 31:1 (6k/6R), which I believe is quite high.

Is there anyone out there who has done this before and is willing to share some experiences?

BTW: I though for a while about air core OPT´s, but I can´t see how that would make anything easier. Just a thought.

Na Zdorovje!


Paid Member
2002-11-01 9:53 am

Spot on. I ordered a custom 100% mumetal transformer from Sowter for treble duty only. Spec was 10 or 15W at 2kHz, Lpri 1-2H, Idc 135mA. Driven with a VV52 SE (300B on steroid kind of thing), sounded absolutely glorous.

Also, a crude measurement indicated that the -3db point was up around 100kHz+.

Sizing transformers to suit the bandwidth is *definitely* the way to go. Just so long as you don't decide to sell them later...

As for air cores, should be quite possible to get something done, but you'll be struggling to get more than about 1H primary inductance, and so are really limited to low Ri tubes. You'll just have to try them out and see if the coupling between cores is all ok... lots of sectioning required...

Mmmm, custom mumetal transformers and VV52s, yummy!!!
Sadly my hifi budget is a bit on the low side, so I´ll have to stick with IDHTs and homemade trannies.

For the tweeter transformer 1-2H primary inductance will be plenty and should not require too many turns. I guess my best option is to just wind a pair and try:D

I did some brainstorming last night about the bass amp and came up with the idea of using split load (say 75% on plates and 25% on cathodes) output stages to lower the distortion and output impedance. Am I on the right track here?
I´d like to avoid global feedback if possible.


Paid Member
2003-06-12 7:04 pm
Maine USA
> decreasing capacitance and leakage inductance, but what happens to the coupling from primary to secondary?

Leakage inductance and coupling factor are the same thing, seen differently.

If you have a 100H primary with 1H leakage inductance, you have a coupling factor of 0.99 (100-1/100).

This is a non-issue. Wind your primary for 500-1000Hz bass response, wind the secondary in any good way (even just secondary over primary), and you will be flat out to 50KHz or more without interleaving.

Core size is another matter. It can be smaller, but that increases the number of turns. If you like to compute that stuff, go for it. Otherwise the Sowter tweet-transformer is an obvious way to go, since Mr Sowter knows his iron.

> The prim/sec ratio needs to be 31:1 (6k/6R), which I believe is quite high.

The real issue is a large number of primary turns. Getting a low number of secondary turns is easy, though at some extreme point you may have to use copper straps instead of wire to get it to bend while keeping resistance low. A "half turn" winding is quite possible, though a full-turn winding will have better leakage loss. But for audio that tends to be a small fraction of an ohm; audio transformers always have "many" turns.

> air core OPT´s

Interesting but not real practical even at the top of the audio band. For estimation: go buy a "air core crossover choke" that is ample for your tweeter. Since distortion is a non-issue in air-core, you can size it for 2KHz or 3KHz. Weigh it, then get the same weight of a fine wire that is 5.5 or 6 times smaller in diameter (square root of turns ratio). For decent coupling with crummy air as your "core", you probably have to multi-filar the two windings. Unwind the choke, unspool the fine wire and cut it in 30 lengths. These should be the same length as the fat wire from the choke. Roll it all up again about the same size as the original choke. Of course it will end up bigger, perhaps significantly bigger because of all the insulation on the fine wire. So you have to re-check the inductance and do it over, until it comes out right. Long night at the slide-rule. And that insulation needs to be VERY good; most modern varnish is good enough to stand off 600V, but you have to wind carefully to avoid kinking or pinching through the varnish.

I'm not sure it has any practical advantage over an iron-core transformer designed for very-low distortion at 500-1000Hz, which will be "dead clean" at 3KHz. And the iron tranny will probably have far lower copper losses (I could picture the air-core coming out 60% efficient).
Ah, I´m enlightened!!!:D
Would a 25VA power transformer core be OK for this?
Core area is about 6cm^, just ordinary transformer iron AFAIK.
I think I´ll try something like 500 turns - 32 turns - 500 turns (prim - sec - prim), or would that be way too much?

Any advices on a good winding technique?
Just flat layers?
In investigating these budget transformers I tested the frequency response and found that the Hammond 125CSE and the Edcor XSE 15-8-5K both have 3 db points above 40 kHz. The Edcor unit is $18 USD although I don't know if it is available in Sweeden. Either of these may be easier than winding your own.

I have found that many mains (power) toroids will work good for your purpose, although you MUST keep DC out of them. I was using a CCS loaded, triode wired 6L6 for testing (parafeed without a choke).

The frequency response of the Edcor unit is plotted in the second thread.


Tubelab: I started this thread a couple of years ago and the my audio system has gone through major changes since then.
Still there might be some general interest in this topic, and I definitely believe customized output transformers would be a good idea in biamped tubed systems.

Nowadays I´m using FE167E fullrange speakers down to 80Hz and subs below that, so the OPT´s still has to cover almost the whole frequency range.
My 6B4G SET with Lundahl LL1664 OPT´s takes care of this in the best possible way, and a pair of LM1875-based chipamps will cover the bass range.

To get back to the topic a bit I can mention that I´ve made some DIY OPT´s lately and my skills has improved a bit since my first attempts. For example, I made a pair of 1,3k/8 SE transformers with 20% CFB (The secondary was a part of the CFB winding, think autoformer) that worked really well with triode wired PL504.
I´ve made some DIY OPT´s lately and my skills has improved a bit since my first attempts

OK, that is good. I based my statements on my few attempts at making OPT's. They were all ugly, and sounded worse. I figured out that I was a lot better at making amps, so I gave up on transformers. I saved all my stuff, so I may try again someday, but I have no time for it now.

I just now looked at the dates on this post. I guess that this is all old stuff. I guess I should check the date first.