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VR Tubes as Cathode Bias of 2A3 SRPP Loftin-White
VR Tubes as Cathode Bias of 2A3 SRPP Loftin-White
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Old 13th June 2006, 09:55 AM   #1
snoopyma is offline snoopyma  Hong Kong
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Default VR Tubes as Cathode Bias of 2A3 SRPP Loftin-White

See attached the circuit diagram of 6SL7 SRPP -> 2A3 S.E. Lotfin-White power amplifier.

Can I change the cathode bias of 2A3 from resistor chain to two 0D3 (each bypassed with 20 nf capacitor) connected in parallel? Can I delete the 100uf cathode bypass capacitor? Is the noise generated by the 0D3 a problem? What will be the pros and cons of the modified design?

Any advice is appreciated.
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Old 13th June 2006, 10:06 AM   #2
lndm is offline lndm  Australia
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I am using zener diodes successfully to bias my output stage. Why not VR's?
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Old 13th June 2006, 10:10 AM   #3
arnoldc is offline arnoldc  Philippines
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where do you take the grid voltage from now?
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Old 13th June 2006, 10:12 AM   #4
Zen Mod is online now Zen Mod  Serbia
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VR Tubes as Cathode Bias of 2A3 SRPP Loftin-White
Default Re: VR Tubes as Cathode Bias of 2A3 SRPP Loftin-White

Quote:
Originally posted by snoopyma
See attached the circuit diagram of 6SL7 SRPP -> 2A3 S.E. Lotfin-White power amplifier.

Can I change the cathode bias of 2A3 from resistor chain to two 0D3 (each bypassed with 20 nf capacitor) connected in parallel? Can I delete the 100uf cathode bypass capacitor? Is the noise generated by the 0D3 a problem? What will be the pros and cons of the modified design?

Any advice is appreciated.

- 20nF is too much ;always start in nano range,not dozens of nano;

- yes-you must delete 100uF cap

- you also need series resistor (value ? ) for each gas toob,for ensuring current sharing between toobz

- remember that you need at least 5mA for igniting and proper work of OD3; max current per tube is 40mA ; be sure that you know what's happening in every moment ,starting from powering up,to working conditions.....



after all that, let's hope that noise induced with gas toobz will be small enough ;I think that it will be
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Old 13th June 2006, 11:19 AM   #5
i_should_coco is offline i_should_coco  United Kingdom
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Running VR tubes in parallel is not recommended.

Due to differences in individual tubes, they will strike & run at different voltages meaning one tube is likely to get all the current and the second may not light.
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Old 13th June 2006, 12:06 PM   #6
martinab2 is offline martinab2  United Kingdom
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Have tried out this idea but on a much smaller scale see here Results were ok but nothing special although there were no problems with stability.

You would have problems scaling this up to work with a 2a3 since most VR tubes have a limit of about 40ma and don't like being run in parallel. If you can get your hands on a couple of these monsters then you might be in business.

good luck
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Old 14th June 2006, 12:02 AM   #7
arnoldc is offline arnoldc  Philippines
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wow! that's a big one!
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Old 15th June 2006, 08:31 AM   #8
Toropkin is offline Toropkin  Russian Federation
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Default Loftin-White without cathode bias

http://igdrassil.narod.ru/audio/pamps/l-w.html
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Old 16th June 2006, 05:00 AM   #9
snoopyma is offline snoopyma  Hong Kong
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Default Re: Loftin-White without cathode bias

Quote:
Originally posted by Toropkin
http://igdrassil.narod.ru/audio/pamps/l-w.html

Hi Toropkin,

Thank you very much for your reply. I read the circuit that you supplied with great interests. It appears that the circuit is simple and making sense. I have used web translation software to translate it to English, but are still difficult to read. Can you elaborate more as I cannot understand "the composition of "upper" BP, something less than 150mf is not recommended" as mentioned on the website.

Do I have to use two separate high voltage power supplies or a combined power supply with centre tapping?

Thanks.

Best Regards,
T.C. MA
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