[Boatanchors] EF Johnson thunderbolt

[D C _Mac_ Macdonald]

VR octal tubes were:

VR-75 = 0A3
VR-90 = 0B3
VR-105 = 0C3
VR-150 = 0D3

7-pin miniatures were:

0A2
0B2
0C2
0D2

IIRC

73 - Mac, K2GKK/5
Since 30 Nov 1953
Oklahoma City, OK
USAF (Retired) 61-81
FAA (Retired) 94-10
________________________________
0C3From: boatanchors-bounces at mailman.qth.net <boatanchors-bounces at mailman.qth.net> on behalf of KA2PTE <ka0CD2pte at 3950.net>
Sent: Thursday, July 8, 2021 18:51
To: Boat Anchors List <boatanchors at mailman.qth.net>
Subject: Re: [Boatanchors] EF Johnson thunderbolt

Ok got a line on some used chokes and will be ordering soon.

Meanwhile I am trying to remember where I left off on this as far as the VR
bias tubes go.

The original setup was a VR75 neon type for the operating bias tube, and (2)
VR150 plus (2) VR100
in series totalling 550Vdc for the screen supply.

At one point another guy who restored a Tbolt said you can try a VR100 for
the bias tube, which I did, but did not get good results.
I am not sure if I had left the screen tubes in the original config for that
though.

I did seem to get somewhat better results with all the screen tubes as VR150
types, and also I think with the operating bias tube
as a 150 as well, but at some point L8 gave out as its done 3 times so far.

I have the tube sheet, and I think for AB1 which the amps been modded for,
using (2) 1K dropping resistors in series with the operating bias tube,
I am told thats good, but I also read the more screen volts the better. the
only reason I can think that the VR100 tube was not a good one for the bias
are those dropping resistors? I am guessing there is a region I need to hit
for the "sweet spot" on the bias for AB2 and 75 is the closest?

--
Steve Hearns
KA2PTE

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