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Old 26th August 2003, 01:57 AM   #1
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Question AC or DC???

Please help:

I am going to build a 2A3 power amp.

I use 12AX7 (connect the 2 triode in parallel) to push the 2A3.

Does the filament of 12AX7 (12.6V) and 2A3 (2.5V) is using DC or AC?

Thank you for your help.

Tuna Fish
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Old 26th August 2003, 06:11 AM   #2
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First of all ditch the 12AX7 and get a decent driver.

Heaters for the driver, assuming it's indirectly heated can be AC or DC, doesn't really matter.

For the 2A3, I'd recommend DC, fed from a current source (LT1085) with a common mode choke and a shubt cap right across the filament at the socket. I've tried this with 300B. 813 and GM70 and it's worked well, sounding better than AC to my ears, but without the hum.
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Old 26th August 2003, 06:41 AM   #3
SHiFTY is offline SHiFTY  New Zealand
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I like the SRPP 6SL7 as a driver, it sounds good to my ears.

Brett- do you have a schematic for your CCS idea?
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Old 26th August 2003, 06:56 AM   #4
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Shifty,

Brett and others have been helping me figure out a regulated DC supply for my 2A3 amp. Here's the 5 page thread on that:

PSUD modelling of a filament supply

I was planning on going current and voltage regulated, but now I might change my mind and go current regulated, and use the other pair of regulators I bought for my DAC's power supply (still deciding on which DAC to build).

Coincidentally, my amp has a 6SL7 SRPP driver too
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Old 26th August 2003, 07:02 AM   #5
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Hi Breet:

Thank you for your help.

Do you mean 12AX7 is not a good driver? Or 12AX7 is not suitable for 2A3 driver?
(I am a newbie, sorry for the silly question.)

Do you mean I need a second AC-DC rectifier and choke (or even a small transformer, cap etc) for the 2A3 filament 2.5V DC supply?
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Old 26th August 2003, 07:03 AM   #6
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Quote:
Originally posted by ShiFtY
I like the SRPP 6SL7 as a driver, it sounds good to my ears.

Brett- do you have a schematic for your CCS idea?
Hi ShifTy,

Do you have any schematic for 6SL7 pushing 2A3?

Any special feature for using 6SL7?

Thank you for your help.
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Old 26th August 2003, 09:58 AM   #7
SHiFTY is offline SHiFTY  New Zealand
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Try the schematic at http://www.angela.com/catalog/how-to/EZ.2A3.html

You could use a 12AX7 in place of a 6SL7 if you like. However it is probably not the best tube for this application.

Thanks for the info on the current source.
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Old 26th August 2003, 06:24 PM   #8
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Thank you for your schematic.

Any other 2A3 schematic can be shared?
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Old 26th August 2003, 10:16 PM   #9
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Quote:
Originally posted by TunaFish
Do you mean 12AX7 is not a good driver? Or 12AX7 is not suitable for 2A3 driver?
(I am a newbie, sorry for the silly question.)
The 12AX7 is a poor driver in most cases, and is definitely a very poor one for a 2A3. A 6SL7 IMO is not up to the task either, even in SRPP.

A 6SN7 would be the minimum I'd use, and a 6H6P, ECC99 or even a trided EL84 would be better. A 417A/5842/6C45 would be an option, as would about a dozen pentodes I can think of.

Quote:
Do you mean I need a second AC-DC rectifier and choke (or even a small transformer, cap etc) for the 2A3 filament 2.5V DC supply?
Yes, use a seperate winding or transformer for each 2A3 filament.

Circuit would be something like this;
transformer - Schottky diodes - cap - LT1085 voltage regulator - LT1085 current regulator - cap - common mode choke - 0.1u ceramic across filament.

There is another thread running that Saurav has linked in a post above that details this more.

Here is the schematic of thre Ronan reg which is similar to what I use(d) but I didn't use the first voltage reg, and added the CMC after.
Attached Images
File Type: jpg ronan reg small.jpg (54.5 KB, 844 views)
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Old 26th August 2003, 10:48 PM   #10
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Quote:
A 6SL7 IMO is not up to the task either, even in SRPP.

...

A 6SN7 would be the minimum I'd use
So what would it take to convert my driver stage from 6SL7 SRPP to 6SN7 SRPP? And is there any reason why the 6SN7 would not be a good choice for SRPP? Would you recommend a 6SN7 SRPP driver for a 2A3?

* Heater current 600mA vs. 300mA - I think I'm good there, my PT is rated for a few amps on its 6.3V tap

* Plate current 9mA vs. 2.3mA - again, I think my PT can handle this. I'll have to adjust the dropper resistor in the PS.

* Plate voltage is 250V for both, and my SRPP operates at around 300V (so 150 drop per tube). I'd have to check if this is a poor operating point for a 6SN7 though.

* Bias voltage -8V vs -2V - the cathode bias resistors might need to change (though with 2.3mA changing to 9mA, the bias voltage would probably get pretty close to 8V without changing resistors).

So... other than changing PS resistors to get 300V at the SRPP B+ with 9mA flowing through it, it doesn't look like much else will need to change. Or am I completely missing something here.

Is this something you would recommend?

Thanks,
Saurav

Edit: Never mind, I don't think that'll work. I'll need much more than 150V per tube to do a 6SN7 SRPP, 150V gives me a pretty strange operating point, I think.
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