26-01-2018, 10:06 AM
Whilst saving up for an OPT for the big amp I built, I was after another project. I rumaged amongst my valves and tfmr's and settled on trying to use the 2C34 triode. After reading Tricomp's thread - https://www.vintage-radio.net/forum/...ad.php?t=67511 , I thought I'd give it a bash.
His amp is in class AB2 and he uses some non standard valves on the front end and PS/driver, I only have your bog standard valves namely the ECC triodes and octal equivalents as well as a few pentodes. I have quite a few 6H8C Russian 6SN7's, so thought I'd use them. The only problem there might be is that we need to drive the 2C34's into positive grid current, this really needs a beefy valve like the 12BH7 but because I like to make life harder for no other reason than trying to use only octal valves, I'm stuck with the 6H8C, which should be ok, maybe. There are hefty octal triodes, but I don't have any at present.
Anyhoo, I knocked up a simple circuit using one half of a 6H8C as voltage amp with a 150k anode resistor, 2k2 cathode R, no bypass cap, followed by a direct coupled cathodyne using 22k R's top and bottom. With a HT of 300v for both, I get an OP of 60v P-P for a IP of 1v RMS ish, so not brilliant, but should be just enough as I need 100v g1 - g1 on the 2C34's.
The OP of the cathodyne is de-coupled to two cathode followers, 20k cathode resistors, biased at around -7v, so around the midpoint. The OP is then de-coupled to the OP stage; the 2C34 has two 47K grid resistors connected to a -15v DC fixed bias/bench PSU.
I'm getting a 15v P-P OP, around 3.5w, so could do better, but I threw this circuit together mainly to test the OPT's I have.
Here lies part of the problem I think. I have two OPT's. the one I'm using at present is a Wharfdale with a secondary that can wired to get 15/22/45:1 Z ratio. I'm using the 22:1, which gives a Zpri of 3K8 for 8ohm load. The 2C34 datasheet specs a Zpri of 10k. I tried the 45:1 tap (16k ish Zpri), this being a bit too much, OP was reduced.
Tricomp uses a direct coupled CF to his OP stage with constant current sinks fed by a negative voltage -82v. I've experimented with this, but found I have to bias the valves nearly into cutoff to get the cathodes to sit at -15v.
He also uses an odd PS which I can't quite get my head round, it looks like a floating cathodyne or maybe a paraphase. I've never used the cathodyne PS before, as it has little gain, prefering to use an LTP. The problem with the LTP is that it needs an extra triode, so you end up with using only one triode of a double triode valve as the input/gain stage - wastefull.
I'm off to try the other OPT I have which has two separate secondary's and a higher Z ratio, but the circuit as is can drive the 2C34's but needs a pentode on the front end I think, maybe a EF37 triode strapped to give more headroom. I'm just winging it present, prefering to get a circuit working then refine it, rather than work out everything in advance.
Any comments/ideas welcome as always, Andy.
His amp is in class AB2 and he uses some non standard valves on the front end and PS/driver, I only have your bog standard valves namely the ECC triodes and octal equivalents as well as a few pentodes. I have quite a few 6H8C Russian 6SN7's, so thought I'd use them. The only problem there might be is that we need to drive the 2C34's into positive grid current, this really needs a beefy valve like the 12BH7 but because I like to make life harder for no other reason than trying to use only octal valves, I'm stuck with the 6H8C, which should be ok, maybe. There are hefty octal triodes, but I don't have any at present.
Anyhoo, I knocked up a simple circuit using one half of a 6H8C as voltage amp with a 150k anode resistor, 2k2 cathode R, no bypass cap, followed by a direct coupled cathodyne using 22k R's top and bottom. With a HT of 300v for both, I get an OP of 60v P-P for a IP of 1v RMS ish, so not brilliant, but should be just enough as I need 100v g1 - g1 on the 2C34's.
The OP of the cathodyne is de-coupled to two cathode followers, 20k cathode resistors, biased at around -7v, so around the midpoint. The OP is then de-coupled to the OP stage; the 2C34 has two 47K grid resistors connected to a -15v DC fixed bias/bench PSU.
I'm getting a 15v P-P OP, around 3.5w, so could do better, but I threw this circuit together mainly to test the OPT's I have.
Here lies part of the problem I think. I have two OPT's. the one I'm using at present is a Wharfdale with a secondary that can wired to get 15/22/45:1 Z ratio. I'm using the 22:1, which gives a Zpri of 3K8 for 8ohm load. The 2C34 datasheet specs a Zpri of 10k. I tried the 45:1 tap (16k ish Zpri), this being a bit too much, OP was reduced.
Tricomp uses a direct coupled CF to his OP stage with constant current sinks fed by a negative voltage -82v. I've experimented with this, but found I have to bias the valves nearly into cutoff to get the cathodes to sit at -15v.
He also uses an odd PS which I can't quite get my head round, it looks like a floating cathodyne or maybe a paraphase. I've never used the cathodyne PS before, as it has little gain, prefering to use an LTP. The problem with the LTP is that it needs an extra triode, so you end up with using only one triode of a double triode valve as the input/gain stage - wastefull.
I'm off to try the other OPT I have which has two separate secondary's and a higher Z ratio, but the circuit as is can drive the 2C34's but needs a pentode on the front end I think, maybe a EF37 triode strapped to give more headroom. I'm just winging it present, prefering to get a circuit working then refine it, rather than work out everything in advance.
Any comments/ideas welcome as always, Andy.
