Re: Modifying W Plugin to use 6DJ8 Tubes


Dave Wise
 

6DJ8 CONVERSION SUCCESS

CAUTION: I prefer a constant-current supply, even though you can't
interchange 6DJ8/7DJ8/PCC88/E88CC/6922 due to heater current.
They all have to be the same and the regulator set for that type.
You can also do a constant-voltage supply, but a Nuvistor Type W
will see more drift with mains voltage. (FET Type W doesn't care
either way.)

1) Change R290 from 150 ohm to 250 ohm.
2) Add isolated current source across V124-V214-V114.
It's just an LM317 with a resistor between OUT and ADJUST,
and take output from ADJUST. Two 15 ohm 1/4W resistors in
parallel gives 170mA, close enough. This gives the 6DJ8's
350mA so they drop about 18V.
3) Annotate new tube types.

You have to handle 100V backfeed if a tube burns out. Just put
a diode in series.

I've tried a few sets of 6DJ8, and everything makes 26-27MHz
vs 27-28MHz on the 8416's. I thought they were identical.
FWIW, Sylvania made a single datasheet for 6DJ8/7DJ8/12DJ8.

OTHER 6V TUBES

6BQ7 and 6BS8 fail with Q154 starved.
6ES8 gives Q154 2-3V but only makes 17MHz, same as 12AU7. What's up with that?
Looks like 6DJ8 is *the* way to meet spec.

TRANSFORMER POWER SUPPLY FOR 6DJ8's

6V-24V WORKS

I fitted S/N 7465 with 120-0252-00, which is used in Type 3T77 and 3T77A.
I have several. The best position is the rear bulkhead upper right
corner with the long axis of the core vertical. I can't see any hum
at 1mV and line triggering. I put the rectifier/filter/regulator
high on the rear bulkhead to the left of the transformer, securing
the rectifier and cap with hot glue to facilitate short leads. I
strung the diode between the cap and the LM317 to avoid a tie point.
With 200uF/50V (two 100's in parallel), I see 4V peak to peak ripple
at IN, with the trough 8V above OUT. It's a symmetrical triangle wave.

OTHER TRANSFORMERS

TEK

Maybe can use 120-0481-00, used in Type 1A4. It has a 20V winding
that puts out 135mA, should be okay at 180mA since the other
winding won't be used.

OTHER

Nobody sells a 6V-24V transformer AFAIK.

Maybe a 110V-36V transformer on pins 13-14 with
primary and secondary reversed, rectifier, filter, and LM317.
We need at least a 20VA rating to stay within spec for copper loss.
That means a big transformer and hum shielding.
Specify 110V|110V primary and 15+15 secondary.
Convenient available space in the plugin is 65x50x50mm.
Here are YHDC 20VA transformers.

PE5424E-M is PCB mount, 57x48x44mm. Not a toroid.
The following don't fit:
PE5424K-M PCB mount plus holes, 75x49x44mm.
LKB5424-L bolt-down with wire leads, 75x43x50mm.
PU3917B low-profile bolt-down, 68x55x33mm.
PTC25 25VA PCB with hole toroid 60x60x38mm.

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