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6922/ECC82 Preamp
6922/ECC82 Preamp
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Old 13th February 2020, 10:17 AM   #21
6A3sUMMER is offline 6A3sUMMER  United States
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Some original 6DJ8 data sheets had two different filament to cathode maximum voltage ratings.

That was because with the tube wired in series for Cascode mode, the top triode needed a higher maximum filament to cathode voltage rating.

You have to pay attention to which triode is wired for the top half of that cascode stage.

As a cascode RF amplifier:
Of course if you cap couple the plate of the common cathode triode stage, to the cathode of the grounded grid stage, it does not matter.
But then you need a separate load for the common cathode stages plate, and a separate inductor (current sink) from ground to the cathode of the grounded grid stage (usually a bias resistor was in series with the inductor).

The same attention to the separate filament to cathode voltage ratings of the two 6DJ8 triodes, needs to be accounted for in SRPP stages too.
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Old 14th February 2020, 04:48 AM   #22
PRR is offline PRR  United States
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Originally Posted by ontariomaximus View Post
couldn't the circuit linked in post #1 be improved by direct coupling the first stage to the second?
How is that better? Save a few parts?

The AC-coupled form does not slam the second grid to B+ at turn-on, like a DC-coupled form would.

Yes, thousands of DC-coupled amps lived millions of hours just fine. I'm indifferent. But not inclined to change a known-good(?) plan on a thin whim.
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Old 14th February 2020, 08:23 AM   #23
6A3sUMMER is offline 6A3sUMMER  United States
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Slamming the grid of a tube that is RC coupled is a matter of timing and circuit particulars.

Direct Heated B+ rectifiers; Cathodic B+ rectifiers; Solid State rectifiers
DHT output tubes, DHT driver tubes, cathodic output tubes, cathodic driver tubes.
And the combinations thereof.

If B+ comes up before a tube is warm, the plate will be at B+.
The Cap of the RC coupling may cause the grid of the next tube to rise all the way to B+ (only temporarily, until the cap charges).

If you look at some of the amps on this forum, you will see 4uF coupling caps and 1 Meg Ohm grid resistors. That takes a long time for the cap to charge.

Each circuit has a different combination; many of them that have RC coupling Do Cause Grid Current at power-up.

And then there is what happens during a Hot-Start (the amp is warm, then the power goes off and comes on again before the tubes cool down).

I know of instances of cooked grids on 12AX7 tubes in some Audio circuits.
That wire is extremely fine.
The 12AX7 grid is Not designed for RF Oscillator service.

Try a 6J6 instead. The maximum grid current is rated in the data sheet.

Oh . . . and did I forget to say that I love to use RC coupling between the driver(s) and the output tube(s).

Post #1 has an RC coupling of 0.1uF and 1 Meg Ohm. With B+ applied and Cold 6DJ8 cathodes, the cap will charge up 63.7% of the 250 B+ in one time constant (0.1 Second). That is OK.
A Hot- Start instance with a collapsing B+ and then B+ coming on again quickly is probably OK too.
. .. But a 1uF cap there might be kind of unfriendly to the grid.

Last edited by 6A3sUMMER; 14th February 2020 at 08:34 AM.
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