Designing an OTL / Circlotron

[Dr Croubie]

Hi guys, i'm new to these forums but definitely not new to audio, been building my own amps since, forever?
In the past i've built myself a few amps, ss power amps: 120W/8r BJT, 320W/4r Mosfet (both with Toroids, diode bridge, and big caps), god knows how many guitar amps and effects, and a SS-moded version of Fred Nachbauer's phono preamp (http://www.platenspeler.com/diy/phon...og_phono-l.gif - his site is down but i found the schema elsewhere).
Basically, I replaced either r105 or 106/7 (so long ago i cant remember, probably r105) with a double-bjt constant-current source, the 3 neons with Zener diodes, unswitched magnetic riaa eq with whatever polystyrene capacitors than fell into my bag at Uni, and fudged a power supply with 2 back-to-back toroids (i think 240V->12V, then fed that into a 9V->240V to get 320ac, 450DC, and a 7812-driven heater supply).
It worked great, wish i had some photos of it all, or could at least listen to it, it's all on the wrong side of the world from me.

So now, I thought i'd try my hand at building my first tube power amp. I've been working with SMPS-repair for the last 6 months, and i've learnt a lot about Power Factor Correction and Switching circuits, so i can now build a very nice, clean, PFC-driven 400V rail, and switch it down to 12V for the heaters.

So i was thinking of trying an OTL circlotron, the main thing that's put me off from building a tube power amp in the past is the availability, expense, and designing of an output transformer.

So first, some theory questions. I've designed (very) high power RF (80kW-peak 10%-duty 50MHz) in the past, so i definitely know what happens when the source-transmissionline-load are not completely impedance-matched. But I was also taught that for any voltage source, output impedance should be as low as possible to minimise internal losses in the source and send all the power to the load. This is certainly true of the solid-state amps i've built, lowest possible output-impedance is best.
So which is right? Or is it that at low (audio) frequencies, the wavelength is so long that impedance mismatch doesn't matter so much?

So, to valves. I can certainly understand that a 400V swing at low current has to be transformed to a lower-voltage higher-current for a speaker-sized load. But for the schematics i've been browsing so far, many people just say 'add more parallel output tubes for lower output impedance', which i certainly understand. But (presuming i can get a spice program for linux and good models) how many is enough, and how low should the output impedance be for (eg) an 8ohm load? exactly 8ohms? lower? and what happens to the output power, distortion, frequency response, all of that, if i have a higher output impedance? (of course, i could just add x more tubes, but i'm on a limited budget here).


So, to the circuits. I've been googling OTL and circlotrons, and a fair few seem to favour EL509s, which i think i can source a few of. Are there any other suitable tubes with low output-impedance that can be used in OTL designs?

OK, it doesn't have to be a circlotron, but i certainly have the capability to make multiple isolated supplies with high Power Factor (and i'm obviously not one of those people who are simply allsolidstate=bad, anynegativefeedback=bad, addingharmonicdistortion=good, i believe in using the best of both worlds), and this amp is more an experiment than anything else.
Class A, AB, whatever sounds better and is cheaper and easier, i'm in Holland so i don't mind having a spae-heater in winter, i'll just turn down the central-heating whenever i listen to something if it's class A. Single/balanced inputs also don't matter, i can hack together a balanced-driver in an hour if i have to...

If anyone can help out with schematic suggestions from projects they've built, design tips, whatever, that would be much appreciated.

cheers
Doc C


ps, i also have the (dis?) advantage that I also don't have any speakers yet, on this side of the world. (my homemade dual 10" subs weigh a nice 44kg each, real 1"MDF in a 120L box, and 8" monitors are 15kg, so they're staying down under for now).
So i was looking for some very high efficiency fullrange, probably narrow-tower speakers, i'm in a small apartment so size and power handling can be fairly low, if i can get them 16 or 32 ohm that would certainly help. So if anyone can recommend any for <€200 a pair thanks for that too.

[bigwill]

Originally Posted by Dr Croubie

(presuming i can get a spice program for linux and good models)

LtSpice works nicely under WINE I use it often!

[Sch3mat1c]

Even the lowest impedance tubes will have to be paralleled in the ~50 quantity to get anywhere near ideal load impedance. Even for 16 ohm speakers.

That said, if you don't mind that you are always operating in the current limited region of the amplifier's performance, and that the only way you will EVER get a usefully low output impedance is with lots of NFB, then go ahead, you can do a fine job (given that restriction). Use high current tubes (6AS7, sweeps, 6S33S, et al.), use grid current if necessary.

Tim

[burgunder]

If you want to design such an amp perhaps it should be a headphone amp, then it would be OK with an output impedance around 120 Ohms.

[quinnling]

Checkout this thread:
What tubes for a tube amp?

This long thread has the Schematic and Parts for what you may be looking for: Atmasphere M60 Circlotron OTL

[Jeb-D.]

6as7g or 6c33c are your best choice as far as output tubes go.

In regards to output impedance, the 6c33c is the bigger of the two and has an internal impedance of about 80R, so figure it would take 10 in parallel to match an 8R load. Adding NFB will lower the distortion and output impedance (stabilize the output voltage in relation to load impedance changes) but it doesn't increase your power output or current delivering capability. That's because the internal I2R losses of the tube remain the same regardless of the amount of feedback. Most OTL designs which are intended to drive real world speakers use NFB, but if paired with the right speakers, you can get away without it.


I suggest class AB unless your planning to build high impedance or high sensitivity speakers. 1pair of 6c33c's per ch will output 20-25Wrms into 8R class AB. 4 6c33c's per channel can give 80-100Wrms into 8R class AB. If you notice, doubling the output tubes quadruples the potential output power. That's because current is primarily the limiting factor, not power.

[jfitz57]

Hi.

Tubes like 13GB5, 21KQ6/29KQ6 are similar to EL509/PL509 and are $1.00 each from thetubecenter.com . You can series the heaters and run it directly from the power line/mains. The problem with a circlotron (besides the drive circuitry) is that you need two isolated power supplys (iron transformers).
If yer gonna build an amp, built your own speakers too. Towers with 8 8-ohm drivers wired in series should work.

Jim

[jfitz57]

Hi again

You said you worked with SMPS's so forget the iron.
And, RF and audio are two different worlds.

Jim

[kavermei]

Hi,

Low-power speakers and OTL is the worst mismatch one could imagine. Think about it, you'll probably only swing 5% of the supply voltage into the load and still need a huge amount of tubes to get a reasonable damping factor. There is no valid reason for doing this (apart from novelty I suppose).

Since you're thinking of high efficiency speakers may I suggest something like EL 84 or 2A3 tube single-ended *with* output transformer. OPTs for these tubes can be found all over the place in all price categories, so your concern over availability and expense is unfounded. As a bonus such an amp would be far more reliable and consume far less heater power than any OTL.

If you build, say, a BIB with small fostex drivers for speakers and build a modest EL84 SE amp, you'll be all set for perhaps half the price of the OTL amp alone.

Kenneth

[atmasphere]

Originally Posted by kavermei

Hi,

Low-power speakers and OTL is the worst mismatch one could imagine. Think about it, you'll probably only swing 5% of the supply voltage into the load and still need a huge amount of tubes to get a reasonable damping factor. There is no valid reason for doing this (apart from novelty I suppose).

Since you're thinking of high efficiency speakers may I suggest something like EL 84 or 2A3 tube single-ended *with* output transformer. OPTs for these tubes can be found all over the place in all price categories, so your concern over availability and expense is unfounded. As a bonus such an amp would be far more reliable and consume far less heater power than any OTL.

If you build, say, a BIB with small fostex drivers for speakers and build a modest EL84 SE amp, you'll be all set for perhaps half the price of the OTL amp alone.

Kenneth

This is pretty well untrue. OTLs **shine** when driving high efficiency speakers! You don't need many tubes to do it either. Its easy to build a small OTL that will outshine any 2A3 amplifier made, for about the same cost.

originally posted by Sch3matic: Even the lowest impedance tubes will have to be paralleled in the ~50 quantity to get anywhere near ideal load impedance. Even for 16 ohm speakers.

That said, if you don't mind that you are always operating in the current limited region of the amplifier's performance, and that the only way you will EVER get a usefully low output impedance is with lots of NFB....

None of this is true either! 50 power tubes would get over 500 watts, with maximum power into less than 3 ohms. If you are driving 8 ohms you only need 8 6AS7Gs to make reasonable power -60 watts- and that is class A with zero feedback.

Take a look at the link to 'what tubes for a tube amp?' posted above.