driver Cf stage in SSS

[grtamp]

Hi everyone,
I have questions regarding the driver stage Cathode follower - dc coupling of the SSS amps.

I have a doubt , and I hope someone here can explain me!

The tube with configuration cathode follower (V8 for #001 and V6 for #002) presents the anodes to a potential of about 440V B +,how it's possible?
Can the 12AX7 works on these voltages? it seems really strange!

Here in the forum there are many people that have built SSS amps , no one has had problems with this tube?

Furthermore , if we measure between the cathode and anode we have a voltage arch of about 480 / 490V ; with about -50V bias !.

Thanks all
R.

[martin manning]

The tube with configuration cathode follower (V8 for #001 and V6 for #002) presents the anodes to a potential of about 440V B +, how it's possible? Can the 12AX7 works on these voltages? it seems really strange!

Apparently yes. They are tougher than the spec sheet would suggest with respect to Va-k. Vh-k is reasonable at 50V. There are better ways to execute a CF driver stage, IMO.

[lovetone]

I had an sss based around 002 in for repair made in Italy can’t remember the make, it had a Spades symbol from a pack of cards for the logo.

In that amplifier the cathode follower lasted around a month and gave up on one side. I tried different makes of tube but all failed.

I changed the dropper string to give 350 / 360 volts and the anode which fixed the problem. I remember there where lots of issues with that amplifier came in more than a handful of times after that each time with a different fault.

[martin manning]

Interesting... Va-k reduced to ~400V. Anyone else had issues and/or modified the circuit?

[grtamp]

From the pics of SSS , HAD seems use Cf dc coupled , is not a better way go with ac coupled?

[David Root]

I used 350VDC anode and direct coupled in my SSS, no problems. Driver tube is a 12BZ7, which is a little different. h-k is 180V max which helps I think, although my cathodes are at about 52V IIRC. So Va-k ~300V.

[martin manning]

I used 350VDC anode and direct coupled in my SSS, no problems. Driver tube is a 12BZ7, which is a little different. h-k is 180V max which helps I think, although my cathodes are at about 52V IIRC. So Va-k ~300V.

You must mean -52V, so Va-k is ~400V?

[David Root]

oops, sorry, that's right Martin. Duh!

[martin manning]

Another vote for keeping DC Va-k at 400V or less, then. Thanks, David.