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Old 6th December 2011, 07:58 PM   #531
cnpope is offline cnpope  United States
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Quote:
Originally Posted by Alastair E View Post
Well, Fixed-Bias wouldnt be that difficult.
[...]

Doing so, and we are back to the more usual Inv. Futterman design, we lose the 'fiddle-free' self-biassing on the O/P pair--But we do gain higher power possibilities.
It means also, an additional PSU rail referenced to -B of say, Minus 75V, adjustable for correct current through the tube....
The output stage would then be very like the Hans Beijner design, and that is giving me almost exactly 25W rms into 8 ohms at clipping.

Quote:
Originally Posted by Alastair E
[...]
Maths was never my strong-point! seems to me rather an odd way to reference power though its the accepted 'norm'
First, take the P-p value, divide by two then take approx a little less than 2/3 (the rms of whats left) and Thats the RMS Watts in Audio...(Which seems to be approx half the RMS Watts in electrical engineering...)
[...]
V_{rms} = V_{peak}/sqrt(2) for a sinewave, and V_{peak}=V_{peak-to-peak}/2. I would think electrical engineers must use this too. The heat dumped into the resistor R is precisely V_{rms}**2/R. Reading off V_{peak-to-peak} is convenient when looking at a trace on a scope, but the voltage between the two terminals of the load never gets greater than one half of V_{peak-to-peak} at any time. And the extra sqrt(2) denominator takes care of the time averaging of the oscillatory voltage. (Literally, the square root of the time average of the voltage-squared.)

Chris
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Old 6th December 2011, 08:13 PM   #532
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Hi Chris,
Yup--Quite right.

My original idea for this OTL was not the persuit of Power, but for ease of build and a fair performance.
--For reliable High Power, We DONT build OTL's!

The power being only 3.5W does however surprise me greatly, Certainly Sounds like at least 20W!

--Be that as it may, the O/P stage is easily adaptable to Fixed-Bias, the rest of the amp should drive it without issues.
Hans Beijner's design is nice and isn't Hugely different from this one....
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Old 6th December 2011, 08:42 PM   #533
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What attracted me first with this design was the lack of adjusting and fiddling.

Alistair lets try to get the most out of it without fixed bias.
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Old 6th December 2011, 08:48 PM   #534
M Gregg is offline M Gregg  United Kingdom
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Originally Posted by flatheadmurre View Post
What attracted me first with this design was the lack of adjusting and fiddling.

Alistair lets try to get the most out of it without fixed bias.


Looking forward to your progress.

My components should be here by the weekend...

Regards
M. Gregg
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Old 6th December 2011, 09:24 PM   #535
M Gregg is offline M Gregg  United Kingdom
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Just some thoughts..

Regards capacitor types and this is not audiophile types just observations from experiments.

If you want bass drive on the auto bias amp then use PET "yes polyester" caps as the output coupling caps Vishay 1813 type 400V work well..
I mean you have to turn it down to protect the speaker cones..then again its only 3 watt..LOL

Orange drop 716 give a better sound stage ..the trade off is less power in the bass you could say easy listening..values are the same 470nF
The orange drop 715 is OK but it has steel leadouts so less treble a bit more veiled..

All just for fun...

Regards
M. Gregg
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Last edited by M Gregg; 6th December 2011 at 09:28 PM.
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Old 6th December 2011, 09:29 PM   #536
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Quote:
Originally Posted by flatheadmurre View Post
What attracted me first with this design was the lack of adjusting and fiddling.

Alistair lets try to get the most out of it without fixed bias.
Excellent Plan!

Some sort of clamp say, a high power zener on the cathodes of O/P pair, set to conduct just above the normal bias voltage would help maybe...

Any other ideas..?
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Old 6th December 2011, 10:24 PM   #537
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Looking at Alastair's circuit in post #10 , I see that there is a big waste of power over the 150 Ω cathode resistors , because of the voltage drop in there , and when the volume goes on , this loses is getting bigger . And if we want to get more power from the output tubes we have to remove those resistors and use fixed bias . And after that we have to specify the capabilities of the PI stage , this is my opinion .
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Old 6th December 2011, 10:30 PM   #538
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Yup Very true...

But, what about a MOSFET in the cathode-circuit replacing the resistor and cap, with a zener setting the bias volts.

Yes, Still power lossy, but no additional losses as output increases,--effectively a fixed bias of sorts....
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Old 6th December 2011, 10:38 PM   #539
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Yes it would be much better than a resistor , and will act like current source for the output tubes, and you can bypass it with capacitor too .
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Old 7th December 2011, 02:48 AM   #540
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Amp slowly coming together.

Small signal up and running.

Ht 350v regulated can go up to 380 with maintained regulation.

Heater regulated and biased at 50v.

PI stage modded as suggested by Alistair.

+/- 150 bolted together ready for prying into the amp.

Its VERY tight
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