# 6922 direct coupled to 2A3

#### gabriel611

##### Member
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

#### arnoldc

##### Member
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?

#### gabriel611

##### Member
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

#### arnoldc

##### Member
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

#### gabriel611

##### Member
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

#### arnoldc

##### Member
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.

#### gabriel611

##### Member
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.

#### gabriel611

##### Member
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.

#### arnoldc

##### Member
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.

#### gabriel611

##### Member
arnoldc,

could be why i'm reading only 260volts at the plate because of the 10k resistor. can i still get away with the needed voltage on the driver if i remove this resistor?

also, if i lower the cathode resistor on the 2A3, will i not be able to get the 45 volts to drive this output tube?

what are now my options with this mains transformer and said tubes? please suggest. thanks

#### arnoldc

##### Member
i guess not. you lower your cathode resistor, you pull more current, which means your B+ is going down even further.

Why are you so stuck with this power supply? What's the transformer secondaries? What's the filtering topology? (CRCRC, CLCLC, LCLC, etc)

ps.

If you're stuck with that power transformer *and* using tube rectification, ditch the tube rectifier and use SS. You will get a higher B+

pps.

it's just not enough... if the 2A3 grid is 130V from the SRPP, you want the cathode to be 45V above that, 175V. 175V / 50mA = 3K5 ohms cathode resistor. But you actually need 4V + 130V + 45V + 250V = 430V

#### gabriel611

##### Member
my filtering section after the 5u4g tube is composed of 20uf cap then 10H choke then 220uf cap which gives me a b+ of a little over 350volts. will changing the caps give me a higher b+? sorry but im just a newbie. and thanks for the reply.

#### gabriel611

##### Member
btw, i dont want to spend more as much as possible so im trying for this transformer to work.

#### arnoldc

##### Member
why sorry? we are all newbies here. Just wait when Sy, Thorsten, and Frank, and many others post in a thread

two options-

1. Use another power transformer

2. Ditch the 5U4, and go SS

#### gabriel611

##### Member
ok. please suggest what ss rectifier to use? can you help me with the power supply schematic? what will be its effect on the sound compared to tube rectifier? thanks

#### arnoldc

##### Member
what is the secondary of your transformer?