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Old 23rd February 2006, 07:16 AM   #1
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Default 6922 direct coupled to 2A3

hey vacuum gurus,

can anyone please give or point me to a schematic for this SE amp using the above-subject tubes? or is there a better circuit for this using same tubes? i have B+ of 330 V. thanks in advance.

gabriel
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Old 23rd February 2006, 07:26 AM   #2
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bring your B+ to 300V or so. at what current draw is your 330V measured with?

2A3 plate voltage = 250V + 50V bias + output transformer voltage drop. but if you're going to use Sovtek, your call

6922 in what topology? grounded cathode? SRPP? how much gain are you looking at?

you may want to see this - single ended 2a3 project
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Old 24th February 2006, 04:45 AM   #3
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Default 2a3 amp with 6922 driver

arnoldc,

i actually measured the 330v at the plate of the sovtek 2A3. is this too much or can the sovtek handle this?

i think i will go with srpp for the 6922. can you help me how to do this? i need the values for the resistors. my b+ could be in the 340v range i think. thanks

gabriel
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Old 24th February 2006, 05:53 AM   #4
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what is your 2A3 cathode voltage? cathode resistor value? (if cathode biased)

try the simple symmetrical totem pole:

200V, Ra = Rk = 330, 7mA, ~2V bias

ps.

don't forget to lift the 6922 heater voltage

pps.

i don't think your transformer B+ is enough. you will need something like ~ 450V
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Old 14th March 2006, 03:36 AM   #5
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anyone,

i'm measuring 355 volts at the plate of the 2A3 tube. the cathode resistor is 3k. i cant quite remember the cathode voltage but i computed the bias current at around 55mA and a plate dissipation of 9.6 watts. what i would like to know is that is it ok for the plate voltage to be 355 volts when anode voltage should be 250 Volts only for 2A3 tubes? or should i subtract the cathode voltage to get no more than 250 volts? another question is will the sovtek 2A3 not be able to handle plate voltage greater than 250 volts?

thanks
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Old 14th March 2006, 05:19 AM   #6
arnoldc is offline arnoldc  Philippines
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the Sovtek 2A3 is a tough tube. Some run it at 350 plate voltage.

With 355 volts at the plate, you need to subtract the voltage on the cathode.

You mentioned 55mA and 3K, so you must have around 165V across your cathode.

355-165 = 190V Vp. Below the 250V recommended limit.

Your plate dissipation also suggest you're not running your 2A3 beyond max.

cheers!

ps.

I also suggest you measure the 2A3 grid voltage, and subtract it from the cathode voltage. You should be looking at somewhere around 45V.
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Old 14th March 2006, 05:40 AM   #7
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arnoldc,

can u suggest what cathode resistor value be more appropriate to increase plate dissipation? i can only buy resistor values up to 10 watts so please suggest what values i can connect in parallel or series. thanks a lot.
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Old 14th March 2006, 06:05 AM   #8
arnoldc is offline arnoldc  Philippines
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it's not as simple as that. please post your schematic.
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Old 14th March 2006, 06:39 AM   #9
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arnoldc,

sorry but i dont know how to post schematics. but its very similar to the Black Art Audio direct coupled 2A3 with 6dj8 driver in srpp. the differnce is that i hav 355v b+, 3k cathode resistor for the 2A3. as for the driver tube, voltage dropping resistor of 4k then 10 k anode resistor. all the values of the rest is almost the same as the black art schem.

i measured the plate voltage of top 1/2 of the 6dj8 at 260 volts. i dont know if i need to drop this at srpp circuit. any help from you or any of the good guys here will be appreciated very much.
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Old 14th March 2006, 07:20 AM   #10
arnoldc is offline arnoldc  Philippines
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Your problem is that you have too low of a B+

At 260V the SRPP will produce about 130V to the 2A3 grid. 165-130 = 35V below the 45V you would want to drive the 2A3 with.

10K anode resistor? On The SRPP? The top tube does not have a plate load.
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