voltage rating for cathode bypass caps?

[jrebman]

Just curious if one can get by with a lower than 35v rating for the cathode bypass caps in the s.e.x.?

Not a big problem if I can't, but somewhere around here I have a couple of 1000uf BGs but I don't think they're 35v rated.

-- Jim

[JC]

Not sure which cap you are referring to; my SEX doesn't have any caps on the cathodes.

If you are referring to the cap designated as "Cbias", I imagine you could get by with a somewhat smaller Voltage rating, but not very much smaller.  IIRC, it has approximately 17 Volts across it for the 6DN7, a higher Voltage for the 6EA7.

As a practical matter, I don't think I would ever want to go below a 25 Vdc rating for Cbias in the 6DN7 version of the SEX, and I think you have to kick it up to a 50 Vdc rating if you want to re-bias for the 6EA7.

[jrebman]

Isn't this cap across cathode bias resistor for the power section of the 6dn7?  I'm blind and can't read the schematic so was just trying to make an educated guess as to the function of this cap.

Thanks,

Jim

[Grainger49]

JC is right the steady state voltage on that cap is listed in the voltage checks as 17V DC.  But as the amplifier turns on there will be some screwey voltage on it.  You can clip your meter leads there and see what happens.  This is kind of a logical way to see what it "floats" to during turn on.

Looking at the schematic that is the "Bias" cap which is attached to the driver triode's plate and the output triode's grid.

[jrebman]

Ok, I'll go back to the wiring instructions and see if I can sketch out enough of a schematic in my head to get a better idea of the circuit.

Is the driver stage a grounded cathode by any chance?  Never mind, I'll go figure it out for myself.

Thanks,

Jim

[JC]

Actually, the driver stage has a cathode resistor, but it is not bypassed.  The output stage has the cathode grounded.

In terms of tube pins, this means that Pin 6 has a resistor to ground, and Pin 3 goes straight to ground.

[Grainger49]

The capacitor you were interested in applies a negative bias to the grid of the output stage.

[jrebman]

Thanks both of you.  Grainger, I see you anticipated my next question :-).

I'm beginning to see what an interesting design this is.

Thanks again,

Jim

[Paul Joppa]

Hey Jim - sorry I'm late to this thread, I was at the party.  :^) 

The current SEX amp uses an old Western Electric topology, where the output tube cathode is grounded, and a negative voltage is generated across a resistor between ground and the negative terminal of the power supply. It's basically cathode bias, but the whole amp's current goes through the resistor, not just the output tube's current. That's less than 10% difference, of course since the driver current is only 2mA or so. The reason for this is so that the autoformer-wired output transformer can be connected to the output cathode and also be grounded for safety. (As you can imagine, that further requires that each channel have its own power supply.)

Eventually I want to give up the autoformer and go to a single power supply with a normal negative ground. That will reduce the complexity of assembly and save a few pennies. The current stock output transformer is not rated for DC on the primary, so it can't be used this way. I want to switch to a transformer that can take the voltage, and also has substantially better performance - you are not going to get those saved pennies, I am going to spend them on the better transformer!

[jrebman]

Thank you, Paul for the added background on the circuit.  It all helps and is appreciated very much.

Sounds like some interesting enhancements are in store.

-- Jim