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Old 12th Nov 2007, 1:09 pm   #21
flyingtech55
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Default Re: Output Transformerless Audio Amplifiers (OTL)

Hello Darius

Very novel way of using a valve. It goes to show what versatile devices they are. However, I too am surprised that the control grid is able to survive such a large current without collapsing (although I don't in any question the accuracy and perspicacity of what you say). One would imagine that the grid would at least gas and 'poison' the device over time.

TimR
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Old 12th Nov 2007, 1:42 pm   #22
kalee20
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Default Re: Output Transformerless Audio Amplifiers (OTL)

Hi Darius,

I'll have to disagree with you! In 'normal' mode, yes the anode will dissipate power, but then, it's usually designed to, by having fins, a dark outer surface to radiate heat, and chunky supports to conduct heat away from the hottest parts and spread the thermal load. Plus, it's the outermost electrode so it stands the best chance of getting rid of its heat.

The grid on the other hand, is fine wire (so bad heat conduction); it is close to the cathode which is already hot; it is surrounded by a metal jacket (the normal anode) which will reflect heat back in; and, the normal grid supports are no longer than absolutely necessary so as not to collect any stray ions which could cause grid current. So, heat dissipation capabilities are minimal!

By all means, try this configuration as an OTL amplifier, and after all the results will speak for themselves. But, I can forsee a big tradeoff between output power and valve life.
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Old 12th Nov 2007, 4:28 pm   #23
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Exclamation Re: Output Transformerless Audio Amplifiers (OTL)

Quote:
Originally Posted by kalee20 View Post
Hi Darius,

I'll have to disagree with you! In 'normal' mode, yes the anode will dissipate power, but then, it's usually designed to, by having fins, a dark outer surface to radiate heat, and chunky supports to conduct heat away from the hottest parts and spread the thermal load. Plus, it's the outermost electrode so it stands the best chance of getting rid of its heat.

The grid on the other hand, is fine wire (so bad heat conduction); it is close to the cathode which is already hot; it is surrounded by a metal jacket (the normal anode) which will reflect heat back in; and, the normal grid supports are no longer than absolutely necessary so as not to collect any stray ions which could cause grid current. So, heat dissipation capabilities are minimal!

By all means, try this configuration as an OTL amplifier, and after all the results will speak for themselves. But, I can forsee a big tradeoff between output power and valve life.
Hi kalee20,
the text is written in english!!! http://members.aol.com/sbench102/inverted.html
It gives answers all your questions and considerations. Please read it!

Kind regards,
Darius
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Old 14th Nov 2007, 2:01 pm   #24
kalee20
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Default Re: Output Transformerless Audio Amplifiers (OTL)

Hi Darius,

As a matter of fact I had read the text when I wrote my reply, although I've since read it more carefully.

'Steve' does make the valid point that in inverted operation, the dissipating grid is surrounded by a non-dissipating, cool, anode. This is something I'd not taken on board, and it will help things.

Having said that, I'd still be cautious if I expected long life. I'd experiment, maybe finding an example of a valve where I could see enough of the grid to find out at what dissipation it glowed red, and I'd test periodically in 'normal' mode for grid current, to get a feel for any gas liberation inside the valve.

Bringing back on topic, it would still be interesting and fun to build an OTL amplifier like this. Even if the valves are shortlived for a reasonable output power, who knows, if results are worthwhile, an enterprising valve manufacturer might turn out a valve specifically designed and specified for this mode!
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Old 14th Nov 2007, 6:19 pm   #25
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Smile Re: Output Transformerless Audio Amplifiers (OTL)

Quote:
Originally Posted by kalee20 View Post
Hi Darius,

As a matter of fact I had read the text when I wrote my reply, although I've since read it more carefully.

'Steve' does make the valid point that in inverted operation, the dissipating grid is surrounded by a non-dissipating, cool, anode. This is something I'd not taken on board, and it will help things.

Having said that, I'd still be cautious if I expected long life. I'd experiment, maybe finding an example of a valve where I could see enough of the grid to find out at what dissipation it glowed red, and I'd test periodically in 'normal' mode for grid current, to get a feel for any gas liberation inside the valve.

Bringing back on topic, it would still be interesting and fun to build an OTL amplifier like this. Even if the valves are shortlived for a reasonable output power, who knows, if results are worthwhile, an enterprising valve manufacturer might turn out a valve specifically designed and specified for this mode!
Hi kalee20, (don't know your name,sorry.)
The most importand point is you need much lower plate voltages for the same kathode current! Thus the power dissipation is much lower than in a conventional topology.
This gives longer live for the valve than in an conventional circuit.
The operating voltages are "transistor like".
This inverted operation of a triode is the only OTL topology that makes sense to me.
Let's hope that some people who are able to make valves are reading this thread
and that they will design us a special triode for this mode.

Kind regards,
Darius

PS: In the 6AS7 (=ECC230) you can see the grid. and 5W is enough.
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Old 17th Nov 2007, 8:59 pm   #26
spaceshuttle69
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Default Re: Output Transformerless Audio Amplifiers (OTL)

Hi,
I'm an newbie here. Sorry for my poor english.

I tried the inverted mode operation some years ago.
The first tube that I tried was the E236L because I had a lot.
Later I used 4 PL509 pentodes in push pull mode.
The PL509 could serve very high cathode / and grid / current.
Once during the experiment I had forgotten turn on the negative plate voltage.
The wire from the cathode to the pin melted immediately.
The current was more than 2.5A!!!

About the circuit.
I tested two different configuration. First I wired a totem pole output stage and later a circlotron. There was not much difference.
I got 4W effective output power on 8ohms load. Unfortunately I lost my measuring notices but I remember the supply voltage was 32V (2 undependent circuit for the circlotron)
I used two stages of differential amplifiers builded around an ECC83 in the first stage and an ECC82 in the driver stage.
The frequency range was 12-100kHz in 2dB. The output resistance ~0,5ohm!
with 3-6dB feedback. unfortunately I did not measure distortion, but the sound was great.

The main problem was to make high AF signal to drive the plates.
I found that the anode voltage not efficient enough without the second grid.
Later I found that the second grid is more efficient so unnecessary connecting the anode. That was very funny, a working amplifier with unconnected anodes!

The PL509 has very strong cathode and grid, but the anode and the second grid are too far so high AF and negative bias voltages needed.

I purchased some 6S33S /6C33C/ because I thought that the less distances between the electrodes will solve the problem and it has very impressive cathode. I did not put this tube into an amplifier just I measured the inverted characteristics.
I turned very sad because its grid did not like the high current.
When I increased the current the linearity between the grid voltage and current lost at higher currents than 300mA /two halves together/.
I think the reason is the secunder emission.

Then I found the nearest flea market some cheap 6N5S /6H5C/ russian power triodes /6AS7G equivalent/ and I measured them.
The maximum grid current without any problem was higher.
My hopes raised again. The 6N5S and his sibling the 6N13S is more cheaper than 6S33S.

I have bought some 6N13S this week and I going to build a new experimental circuit.
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Old 20th Nov 2007, 8:57 am   #27
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Thumbs up Re: Output Transformerless Audio Amplifiers (OTL)

Welcome spaceshuttle69,

this is good and very (!) usefull information, thanks for sharing it with us.
How much volts peak to peak do you need at the grid 2 of the PL?

Kind regards,
Darius

Last edited by arjoll; 20th Nov 2007 at 10:55 am. Reason: removed excessive quoting
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Old 22nd Nov 2007, 9:25 pm   #28
spaceshuttle69
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Default Re: Output Transformerless Audio Amplifiers (OTL)

The PL509 needs 400V negativ G2 voltage to close the cathode current.
The max. current flows at 0V so it need 800V peak to peak AF voltage.
I could not present that just 400-500V p-p.
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Old 25th Nov 2007, 10:33 am   #29
oldeurope
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Post Re: Output Transformerless Audio Amplifiers (OTL)

Quote:
Originally Posted by spaceshuttle69 View Post
The PL509 needs 400V negativ G2 voltage to close the cathode current.
The max. current flows at 0V so it need 800V peak to peak AF voltage.
I could not present that just 400-500V p-p.
This is too much. Thus I'll wait for a better valve.
Kind regards,
Darius
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