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Vacuum Tube OTL power amp!!
Vacuum Tube OTL power amp!!
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Old 6th December 2011, 09:48 PM   #531
M Gregg is offline M Gregg  England
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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.


Looking forward to your progress.

My components should be here by the weekend...

Regards
M. Gregg
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Old 6th December 2011, 10:24 PM   #532
M Gregg is offline M Gregg  England
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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

Last edited by M Gregg; 6th December 2011 at 10:28 PM.
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Old 6th December 2011, 10:29 PM   #533
Alastair E is offline Alastair E  Wales
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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, 11:24 PM   #534
Dimitris AR is offline Dimitris AR  Greece
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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, 11:30 PM   #535
Alastair E is offline Alastair E  Wales
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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, 11:38 PM   #536
Dimitris AR is offline Dimitris AR  Greece
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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, 03:48 AM   #537
flatheadmurre is offline flatheadmurre  Sweden
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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.

Itīs VERY tight
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Old 7th December 2011, 09:03 PM   #538
M Gregg is offline M Gregg  England
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Components arrived today..


I have fitted the 270 ohm gate stoppers on first stage ccs..

47K now on each side of the PI...

I have used a 10K as grid stopper on PI..

Took about half an hour..

Sound has changed....more detail in the highs...

just breaking in at the moment ..then some scope readings..

Next step to look at the 6c33c cathode resistors...

Regards
M. Gregg
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Old 7th December 2011, 10:01 PM   #539
Alastair E is offline Alastair E  Wales
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Ive made some Mosfet CVS for the cathodes of O/P bottles.

Still running the PL509 and nothing else changed, the 11V P-P Maximum before clip has increased to 13V P-P. I carefully set this up for a Perfectly clean and nice sine-wave, before measuring.

--The MOSFETS got Nice and Hot, during the 2-3 mins I had it at full-chunter!

Not a Vast improvement in power.

Checked the PI, and looks like I'm running out of steam in that stage, Currently it manages about 65V P-P on each of its outputs, further increasing drive exhibits that flat-top and bottom nipple-distortion as illustrated on 'The Valve Wizard' website the Grid-Resistor is now 100K instead of the 470K I originally had there to prevent this.

For the 6SN7, we need more +B to get much more outta it I think, My +B to the PI is only 320V.
Any suggestions welcome for this issue, but we would prefer to keep the 6SN7....

So, Partial success. For further power increase with my current PL509 pair I need to get more drive from the PI as well as probably more +-B rails to O/P stage.

The drive and cathode-resistor/mosfet mods would carry over to any O/P triode used, and particularly the Cathode resistor/mosfet mod should show benefits for higher current triodes like the 6C33C
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Old 7th December 2011, 10:54 PM   #540
tubemax is offline tubemax  Netherlands
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Hi Alastair E, some time ago I read on Tube CAD website about EL509 OTL version but with different driver tubes...I saved a picture of tubecad design of EL509 driven by 6N1P - maybe helps to try out...

Max
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