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Old 14th December 2010, 07:02 PM   #21
StoneT is offline StoneT  United Kingdom
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That's funny
I've just been reading that thread for a couple of days and came back to this one to say 'balls to this! I'm building an M60! (or the like)"
Most gracious thanks Atmasphere for the schemos you've posted.
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Old 15th December 2010, 08:13 AM   #22
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Thanks Atmas for the circuit, very interesting. It is, of course, basically a redrawing of the totem pole circuit, with the need for two fully floating indepedent supplies for the output valves. It does, however, seem to simplify the drive requirements a bit, so that helps.
I've seen this somewhere in a solid state design but don't remember where.
The output impedance for totem pole and cyclotron will be the same, and in the case of valves output power will be entirely dependent on the valve peak current ability.
However as power is I squared times ohms, then doubling the valve count will quadruple the power output (to a point).
The 2 valve version will just manage 5 W peak, 2.5 W RMS.
The 4 valve will do almost 4 times that, ie nearly 20 W peak, 10 W RMS
And so the 8 valve version 4 times that? Well no actually. As the output voltage swing increases, this removes the available voltage across the valves themselves so they are capable of delivering less current, so for the 8 valve we are dowm to 60 Watts peak, which for us is 30 Watts RMS.
Now we have a stereo amp delivering 30 W per channel running 16 6AS7G's on full tilt.
You'll want to turn that radiator off, even in this weather.
Interesting to note that as the output falls with increasing output volts, this is negative feedback at work so the output signal won't be quite so dependent on varying speaker load impedance.
It still will be, but not as bad as if from a true current source.
If you want to find out in advance what it will sound like then that's quite easy.
Use your normal transistor amp setup, and add a resistor (say 80 to 100 ohms, 10 watt) is series with the speaker.
Of course the output will be much reduced, but you will hear the effect of the varying load impedance has on the performance.
Perhaps you don't have to bother to built the valve OTL after all.
Right, must now go and get some work done.......
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Old 15th December 2010, 10:19 AM   #23
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Quote:
Originally Posted by toprepairman View Post
The 6AS7G (6080) is rated to run at 125mA, that means it's happy with peaks up to 250mA. All well made valves have a fair bit of surplus built in to cope with ageing, so with the sails up and a good following wind you might just get 400mA peak. With both sections strapped together that gives you 800mA peak. Across an 8 ohm load that translates to 5 Watts peak. It's still only 2.5 Watts RMS.
Perhaps Atmasphere you'd like to share you circuit with the rest of us.
Regards
Henry
Using only 125 plate volts, I get about 500mA peak per triode out of these tubes. When driving the grids positive with a MOSFET follower I get just under 1amp!
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Old 15th December 2010, 11:49 AM   #24
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Well done Semper. 400 to 500 mA peak is very much the same ball park, and these peak outputs will be very much dependent on the manufacturer and date of manufacture etc.
All of the discussions so far have been with normal drive levels, ie high impedance source, and not driving positively and drawing grid current.
Adding all these things is rather going away from the original thread of not getting too complicated.
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Old 15th December 2010, 12:28 PM   #25
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Why thank you. To be honest I've pretty much skipped this thread so not surprised I'm going off topic, but noticed your post on output current from the 6AS7 and figured I'd mention my findings.
I do use normal drive levels tho and normally I don't use a follower to drive the grids positive.
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Old 15th December 2010, 03:56 PM   #26
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Quote:
Originally Posted by toprepairman View Post
Thanks Atmas for the circuit, very interesting. It is, of course, basically a redrawing of the totem pole circuit, with the need for two fully floating indepedent supplies for the output valves. It does, however, seem to simplify the drive requirements a bit, so that helps.
I've seen this somewhere in a solid state design but don't remember where.
The GAS amplifier used a similar setup. It is worthy of note that the Circlotron is **not in any way** a redrawing of a totem-pole. This can be seen by the fact that the cathodes of both push and pull are driving the load while the plates of each are tied to their power supplies.

Quote:
Originally Posted by toprepairman View Post
The output impedance for totem pole and cyclotron will be the same, and in the case of valves output power will be entirely dependent on the valve peak current ability.
The output impedance of a Circlotron tends to be 1/2 that of a totem-pole circuit.

Quote:
Originally Posted by toprepairman View Post
However as power is I squared times ohms, then doubling the valve count will quadruple the power output (to a point).
The 2 valve version will just manage 5 W peak, 2.5 W RMS.
The 4 valve will do almost 4 times that, ie nearly 20 W peak, 10 W RMS
And so the 8 valve version 4 times that? Well no actually. As the output voltage swing increases, this removes the available voltage across the valves themselves so they are capable of delivering less current, so for the 8 valve we are dowm to 60 Watts peak, which for us is 30 Watts RMS.
Now we have a stereo amp delivering 30 W per channel running 16 6AS7G's on full tilt.
You'll want to turn that radiator off, even in this weather.
Interesting to note that as the output falls with increasing output volts, this is negative feedback at work so the output signal won't be quite so dependent on varying speaker load impedance.
It still will be, but not as bad as if from a true current source.
If you want to find out in advance what it will sound like then that's quite easy.
Use your normal transistor amp setup, and add a resistor (say 80 to 100 ohms, 10 watt) is series with the speaker.
Of course the output will be much reduced, but you will hear the effect of the varying load impedance has on the performance.
Perhaps you don't have to bother to built the valve OTL after all.
Right, must now go and get some work done.......
I'm guessing that you are using some assumptions above that are incorrect. With eight 6AS7Gs (which is 16 sections) the amplifier can make 60 WRMS into 8 ohms. It will make about 80 WRMS into 16 (and will also run cooler as the output section becomes more efficient), but as impedance is increased above that the power will start to drop off slowly.

The sounds is *quite* different from putting a resistor in series with a solid stage amplifier. It is quite lively and transparent, yet also very relaxed as open loop distortion is rather low.
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Old 15th December 2010, 06:37 PM   #27
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Given that the circuit drawing is incomplete I did have to make one assumption. That where is written Bridge1(+) etc. these are the power supplies for the output valves, namely a secondary winding of a transformer, a bridge rectifier and a largish reservoir capacitor.
This capacitor is thus connected from the anode of one valve to the cathode of the other, and from the ac standpoint it doesn't matter whether the load is connected from cathode to cathode or anode to anode or indeed from anode to cathode (given a DC blocker).
If you now only draw the valves and the caps you will see the ring (hence cyclotron) of valve/cap/valve/cap.
This is easily redrawn to be valve/valve/cap/cap, and so we are back to the totem pole, with the same output impedance.
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Old 15th December 2010, 06:51 PM   #28
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There is no current through the 'ring'. It is well-know that Circlotrons are quite different from totem poles. There are two secondaries on the power transformer, each supplying identical supplies.

There is a discussion about the capacitors in the power supplies that starts on this page of this thread:

Hybrid Circlotron Amplifier with only 3 components on the signal path
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Old 16th December 2010, 12:11 AM   #29
cnpope is offline cnpope  United States
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There is quite a nice discussion by John Broskie, on his website at John Broskie's Guide to Tube Circuit Analysis & Design, in an article called "Cars, planes and circlotrons." Viewing the power supplies as being dead shorts as far as AC is concerned, he makes the point that a circlotron and a totem pole configuration are not so very different. What is different, as far as I understand it, is how the driving signals for the output tube grids are referenced. In a circlotron, there is a complete symmetry between the way the tubes in the two banks are driven. In a conventional totem pole configuration, by contrast, the upper tube(s) is working like a cathode follower, and the lower like an anode follower. This is because the signal to the upper tube is effectively, as far as AC is concerned, between grid and anode, whereas for the lower tube, the signal is effectively between grid and cathode.

John Broskie has a schematic (labelled (c) in the set of 6 schematics on page 16 of his article) which is a totem-pole that seems to be essentially symmetric, as far as AC is concerned, and which I think he would claim to be equivalent to the circlotron. As he says, a tube doesn't "know" whether it is a cathode follower or whatever, it just knows about the voltages between its various electrodes. It seems to be true that even though his schematic (c) doesn't look manifestly symmetric, as far as the AC signals are concerned it would behave the same as the circlotron.

Of course it would be somewhat perverse actually to build his schematic (c), since it would still need the separate power supplies for the right and left channels, and one might as well in that case go for the true circlotron.

And I think it is still the case that if the grounding is done in the "conventional" way for a totem pole (i.e. one speaker terminal grounded), then it does operate asymmetrically, with one tube a cathode follower and the other an anode follower.

Chris
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Old 16th December 2010, 08:38 AM   #30
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Many thanks to Chris for his contribution. John Broskie's article is brilliant, couldn't have written it better myself.
So now over to StoneT to build one of the amps, no output transformers but a whole stack of power transformers. And those capacitors for the output stages, what to go for?, no mention anywhere of what they should be.
Lets start with a minimum of 4700uF at 160 Volts, oh and for good measure they should be LoZ types and 105degree with alll that heat floating about.
Comments welcome

Greetings to all
Henry
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