[Central-Electronics] 6MU8 and the 20a

[Jim candela]

I group,

   This is from Jim, WD5JKO, and I want to relate a recent experience with
the Central Electronics 20A. As some of you know I have went through a
number of 20A's with an goal of propping up the power to better drive a
grounded grid amplifier. I have gone over this before, but I have something
new to offer.

   One item that has bothered me about the 20a is that the 6U8 tube runs
pretty hot, and I have seen several get weak, and one flash over inside. The
Pentode section is for the 9 Mhz crystal oscillator, and the triode is the
audio driver tube driving the audio phase shifter. Over the years I have
tried several variations to the 6U8 that all share the same 9AE base
diagram. I have tried the 6EA8, and the 6GH8. No improvement here, and the
circuit seems best with the 6U8. Then I discovered by chance the 6MU8. It
has the same 9AE base, but is taller, and has more Gm and plate dissipation
on the pentode. The triode is about the same.

  Changing to the 6MU8 may not increase the output power on a stock 20a, but
it will increase the 9 Mhz level into the 6BA7 mixer. It seems that more
signal from the oscillator overcomes some of the non-linearity's from the
1N34 diodes in the balanced modulator. After the change, expect to get more
carrier insertion for a given knob setting. For example on two 20a's tested
here, I get the same power at the knob setting of 6.5 with the 6MU8 as I did
at 8.0 with the 6U8. The quality on AM seems improved, but that is pretty
subjective.

  I'd like to hear if the 6MU8 improves a stock 20A in any way. My guess is
it won't be too much diffrent, but it should last longer since it is a more
capable tube.


These tubes are on sale for $2.50 each (new) at http://www.tubesandmore.com/

Data Sheets:
http://www.pmillett.com/tubedata/HB-3/Receiving_Tubes_Part_2/6MU8.PDF

http://www.mif.pg.gda.pl/homepages/frank/sheets/049/6/6MU8.pdf


Happy DX with your 20A!
Jim
JKO
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