[Boatanchors] James Millen 90800 exciter/transmitter

[Howard Holden]

Hello John, I am certainly no "expert", but I've been running a 90800 for 
nearly a year now, 80 and 40, with 40 either a straight crystal or 
multiplying an 80M crystal.

You didn't specify any operating parameters - band, straight or doubling, 
coil configuration, , currents, etc.

Just looked at mine, which at last run I was using an 80M crystal (3520), 
doubling. 80M cathode coil in place, coil switch in. 40M osc tank coil 
(43042) driving the PA straight through.

Oscillator cap is about 10-15% engaged. 600V supply, 25 ma on the 
oscillator, 75 ma on the amp, about 25W out. The trimmer next to the 6L6 is 
about fully closed. I never thought much about the actual settings, but mine 
seems to work well as it is.

The 40M osc tank coil measures about 12uH, so it's conceivable that with 
tube capacitances, and stray wiring capacitances etc., a fairly open setting 
of the osc tank cap is OK. Total capacitance should be about 40 pF with the 
12 uH coil. I didn't measure my 80M coils, or see what the 80M settings 
would be.

There are three (that I know of) schematics out there for the 90800,  but 
I've only looked at two. One (W8VR?) shows a fixed 20 pF, the other, a poor 
copy of of the actual Millen drawing dated 1947. The W8VR looks like a 
modern redrawn simplified version. The Millen drawing shows indeed a 20 pF 
trimmer between the osc plate and the amp grid. Perchance W8VR forgot to 
indicate that cap was a trimmer.

If you are running 80M, have you checked and made sure you aren't actually 
putting out most of the power on 40M by accident? Just a thought.....

73, Howie WB2AWQ

-----Original Message----- 
From: John Saxon via Boatanchors
Sent: Sunday, May 18, 2014 3:12 PM
To: boatanchors at mailman.qth.net
Subject: [Boatanchors] James Millen 90800 exciter/transmitter

A good day to all:

I am getting a Millen 90800 going.  I hope someone out there is an expert 
:-)

Got it putting out rf, but the oscillator plate tank cap is at minimum 
capacitance, and according to my materials should be about 80% of maximum.

On the transmitter there is a (looks like) mica trimmer from plate of 6L6 
oscillator to grid of 807 final.  It shows on the Millen schematic, but I 
don't see any word about adjusting it.  In the original QST article 
(November, 1940) it does not show a trimmer there, but a fixed 20 uufd cap.

Any words of wisdom?

Thanks & 73,
John
K5ENQ
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