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Opinions/Experience of Toroidal valve amps

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Hi to all,

I hope to build a high quality valve amp, I have looked into various designs and have sort of decided on a design by a chap called Menno Van der Veen from his book on valve amplifiers with toroidal output transformers.

I have in my possession a good number of el34 tubes and was intending to build the design that uses 8 of these tubes utilised as triodes to produce 70watts per channel.

I really cant afford to make any mistakes with my choice of design as i can just about afford to build this amplifier!!!

Therefore please could you share your experiences of these designs, good or bad or suggest any other high quality designs of the highest quality and with a minimum of about 50watt output.


many thanks
 
There's no need to go into an expensive toroid design to get good quality results. The standard E&I design from Sowter transformers, Majestic (UK) and so many other sim vendors are capable of very good performance.. without any need for DC current balancing.

If you can afford to build a toroid o/p stage amp,with extra current balancing circuitry trown in then I don't think financing becomes an issue.

I am not the only engineer (on technical grounds) against using toroids in o/p stages. About a year ago there was such a heated thread on this issue but needs searching.

richj
 
I looked on the search but couldn't come up with anything that looked 'heated' (judging just from title, post count, views, and optionally if it was locked, ahem)....

I thought (from what I've read, at least) that toroids were closest to being 'perfect' transformers (or at least, better than EI or C cores)?
Aside from the (and this is from memory) fact that they can be - is it magnetised or polarised - by dc, and that they're rather unsightly unless hidden in a bean-tin, how come then they seem to be frowned upon?
Admittedly this seems to be output rather than power toroids that take the brunt of the bashing, but surely if they're the 'perfect components' they'd be the choice of the professional™?
Or do their foibles present a greater technical challenge than EI/C cores so they're not worth the effort - cost vs gain or whatever?
 
Norman Crowhurst did an amp with an o/p toroid with flux balancing thrown in. My resource ability is very slow speed up here.
The advantage with E&I core is lower permeability due to micro lamination gaps.....this is a natural advantage in subsonic LF rolloff as it actually improves the LF transient response and limits out of balance currrent/ core saturation to allow mismatched tube sets with 10% current misbalance.....This would be too high for a closed iron band toroid as the BH curve essentially goes to zero whereas a gapped core with laminations has a marked sloped hysteresis loop. Getting into 1st guadrant magnetics behaviour is unavoidable and is further complicated as nfb introduced in whatever form tends to extend b/w. So a toroid o/p tranny as it stands without a core gap will saturate easier at a lower frequency for a given excitation than an E&I with sim core area. One sees many inflated specs with toroids i.e b/w -3dB at 5Hz at 1W which indicates nothing apart from not being able to hear it and the amplifer o/p section cannot handle Bmax without running into overload and severe thd.
Having painted a diffcult picture with toroid designs, one can see the E&I laminations create an automatic design solution in LF design. If you follow the Crowhurst design, it is complicated but I follow the adage "more of less is better" esp in audio amp design.
A tube failure in a toroid o/p amp will often be dramatic, whereas with an E&I one can often just keep playing til the interval.

richj
 
My experience of using toroidals as output is entirely different to Rich's. Mine are power toroidals so not up to the spec of Piltrons.
A few points though;
Piltrons are gapped, or so I have been lead to believe. Go check. This means they are capable of tolerating tube imbalance. They even make SE toroidals.
My power torodials happily go down to 10hz before they start to complain.
High frequency bandwidth will always far outstrip an EI. Infact ringing is the real thing to design out.
My outputs are good to about 70Khz without recourse to negative feedback.
Current balancing can easily be achieved by placing Constant Current Sinks in the cathodes. These can be as simple as LM317's. The desire to get perfect current matching brings its own sonic benefits and can relatively easily be turned into a fully differential output stage. EI transformers suffer from current imbalance even though they can tolerate it.
Once or twice th pin connection of one of the cathodes has gone dicky - the result is nothing more than distorted sound out of the speaker.

Remember in all this - mine are cheap mains transformers. I assume that you will be going for good quality Piltrons, so you can expect performance to be orders of magnitude better than mine.
Take all this as you may - but remember I am talking from experience and not prejudice.

A last though - I always thought that C-cores offered the best performance.

Shoog
 
Ex-Moderator
Joined 2003
In theory, a toroid is a better design, but its virtue is also its downfall. The core of a toroid is a spiral-wound continuous strip rather than a series of laminations. As other posters have suggested, this means it doesn't have a gap (not quite strictly true, but pretty close), and the lack of gap means you get enormous primary inductance with very few turns (good thing). The magnetic coupling from primary to secondary of a toroid with even windings is also very good, giving good HF response (even a power transformer can pass a good 10kHz square wave).

The problem is that the gap has a linearising effect and losing it makes primary inductance very unstable. Toroids need very tightly matched currents to avoid core saturation - it's no surprise that Menno also has a bias servo to enforce balance.

I would suggest that an amplifier with a toroidal output transformer is not ideal as a first build, a more traditional design will be more forgiving and require less post-construction fettling. I would also point out that a 70W amplifier is quite an onerous build. Unless you happen to be an electronics engineer, you would probably do better to start with something a little smaller and guaranteed to work. Something based on a Mullard 5-20 has a very good chance of success, even if it is (only) 20W.
 
Firstly, a big thanks for all the responses, I think i am now in a position to make a more reasoned decision on which design to go for.

I have come to a similar conclusion in that the design i was considering may be a bit OTT. Therefore does anyone have any suggestion on a well proven design, with the an idea of where to get hold of the design, specs etc???
 
If you are careful with your choices you can buy oversized toroidals and build a small amp. When you feel a bit more confident you can expand the amp to use the extra capacity.

Using 6080's in a push pull design is a cheap way to learn all the principles of working with toroids, and the ratio will then lend itself to upgrading with more conventional parallelled valves.

Shoog
 
Therefore does anyone have any suggestion on a well proven design, with the an idea of where to get hold of the design, specs etc???

I'ld start by elimination. Unless you have ESL-pannels or such you'll never need 70watts. el34 pentode push-pull class AB with around 30watts will be sufficient for almost any other kind of speaker. Remember, tube-watts are louder than transistor-watts; the ca. 15watts of triode-strapped el34 PP will probably also do and have distinct advantages. http://audiotropic.netfirms.com/Projects/ampEL34.html

There are alot of circuits based on the Classic Williamson; look e.g. for the PP design by Claus Byrith (lundahl website) or the Mapletree audio Stealth, or http://www.diyparadiso.com/rootel341.htm
Those have the well-regarded Long-tailed pair phase-splitters. Several improvements can be made: CCS in the LTP, regulated screen supplies etc.

If you can afford a good phase-splitter transformer I'ld definitely go that way, possibly the best sounding solution overall and elegant in it's simplicity. See e.g. here: http://www.audiodesignguide.com/my/pp3.html

Simon
 
If Plitrons are gapped, if so where in the construction- Or is amorphous material with distributed gap being used ?
The position of the gap is crucial to the leakage inductance behaviour as it then spews out of the doughnut, one is then back to square 1.
Amorphous material is expensive but more volume is required for the same L.
Anyone know who uses exotic core materials for toroids ?

rich
 
Hi,
WAD (world audio design = the design team in the magazine HiFi World) designed and released a valve power amp with toroidal power and output transformers.
After a while they took it off the market and brought out mk2 with a pair of EI output transformers and a very full technical explanation of why the toroids did not suit the output duty. It kept the toroid power. I think it was Kel80 using parallel push pull el34 in ClassAB.
They sold mk2 for a long time. I believe it was well liked.
 
Piltron do indeed build SE toroids, so it is part of their design approach to introduce gaps.
I could find no direct reference to a gap in their PP toroids, but I would imagine since they understand the technique they probably have a nominal gap. I base this on the fact that all of the designs which I have seen, which use the Piltron toroids, use nothing more sophisticated than a current balancing pot for the output stage - and talk about its use for eliminating hum - nothing more.
Very highly regarded designers have used Piltron transformers so they obviously have something good going for them. I notice that all the detractors are speaking from a theoretical standpoint and can not point to personal experience.


I will be showing my amps at a local DIY meet this weekend. I am hoping for a great reception and I will report back with the impartial impressins of those present.

Shoog
 
could find no direct reference to a gap in their PP toroids, but I would imagine since they understand the technique they probably have a nominal gap. I base this on the fact that all of the designs which I have seen, which use the Piltron toroids, use nothing more sophisticated than a current balancing pot for the output stage - and talk about its use for eliminating hum - nothing more.

I have AMPLIMOs (= Plitrons) running in a 2x mono pp amp, and experimented some with it during the design. The current has to be balanced within a few mA to avoid distortion - that's why I assume there is no gap in their pp transformers. I picked tubes with little drift for the output stage - 2x2 EL34 triode strapped for each mono - stays within 1-2mA difference once balanced, no servo required :) .
Marcus
 
Well that has cleared up that missunderstanding.
I personally see the requirement of achieving current balance to be a good thing with plenty of benefit as a result.

Have you compared your finished amp to a similar EI based design. If so did it come out better or worse ?


Shoog
 
I don't like the idea of trimming o/p tubes within a few mA....o/p stages are notorious to drift over time especially even with slight heater volt variations. The 6550B (now defunct- clear glass button getter) was well known specimen for taking ages to light up and settle down.....and the slightest heater volts variation changed all.
Using power pentodes/beam tetrodes config as triodes does settle the characteristics down to some consistency. Lower gain etc but even this I'm very sceptical running an o/p stage without being able to take a 10% misbalance.
However there are some "wildcat" toroid o/p stage designs around which have worked (mysteriously) but have to be regulary re-tweaked and nothing more annoying than to up-end a heavy chassis.

Long live E&I
richj
 
Shoog# presuming one has some high time constant caps in the 317 circuit, Doesn't that add another pole in the LF response ? I can see for class A that works fine but in AB1/2 where peak currents are x2 quies ?
My tubes in AB are set at 80mA quies per tube and full power AC drive is around 110-130mA per tube. Noway can one get'em matched so tight in AC drive.

I can see that class A drive using a toroid core and constant current drivers, one is on to something but can one be quite sure that the tubes are behaving with matched gm with AC signal drive ? That's where the troubles start.
richj
 
I have mine setup to run in Class A triode. I'm certain that it must stray into class AB at times - but it doesn't seem to be an issue. I also have it setup as a differential pair but I don't know how that effects things.

Surprisingly the bias point stays reasonably constant over the year or so that the valves last (running all day most days). Eventually the tubes go into oscillation when they stray to far out of step.

Shoog
 
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