The AM Forum

T-103, the screen and bias low voltage transformer, in Johnson Thunderbolt that I was trying to get going let out the smoke last night, with the 5U4 pulled!  Can anyone send me the specifications on this transformer, i.e. secondary winding voltages and currents?  Thanks Jay WA3ONG

This is what the old P. Dahl catalog shows:

JOHNSON THUNDERBOLT LV T-103
SCREEN & BIAS TRANSFORMER, P/N 22.1421
CORE) EI-l25 X 1.75 STYLE #) 04 E-I LAMINATED CORE
PR) 0-110-120 VAC 60 HZ 1 PH
S1) 783-219-0-219-783 VAC @ 0.1A CCS
S2) 5.0 VAC @ 3.0A CCS
S3) 6.3 VAC @ 1.6A CCS

Rodger WQ9E

never mind, slow on the trigger.

Hammond has em in the catalog.

pwdp13250

klc

Heres what I have.

Note that they were made back when AC line voltage was 110v,
and the primary is usually tied into the 110 winding. There is
another tap for 115V.

When I moved the tap to 115 on my Tbolt, the 400A filaments
seem to glow whiter color than the orange they were before.
I thought this was due to the swap of the wire, but now I see
T103 is not responsible for the 400A's filament. All I can figure
is the 110V tap loads the voltage down enough to effect T102.

That primary tap on the screen/bias transformer definitely should NOT be changing your filament voltage on the finals.  I would measure the filament voltage at the final sockets to see what is actually being provided.

How is the ground/neutral lead connected to the branch circuit for your Thunderbolt?  With the parallel connected primaries of the two lower power transformers, the primary tap on one shouldn't impact the voltage distribution across the other.

The Thunderbolt (along with the Desk KW) were designed in the days when tying ground and neutral together at individual outlets and appliances was considered acceptable.  It is simple with both of them to change to a proper isolated neutral setup which is advisable to avoid issues and comply with current code. 

Rodger WQ9E

I don’t know how much it’ll make people cringe…but dealing with old gear and modern AC line voltages, I’ve taken to inserting resistance in the filament primary as needed. At some point I was gifted about a hundred 2 and 16 ohm 10w resistors, so I just make up networks until the filament voltage is correct at the tube socket. I don’t much care if the B+ runs high, but big expensive out of production tube filaments are another story…. A side benefit is reducing inrush, which is a concern with some tube types.

Ed

This is the last updated schematic of mine:
https://www.mediafire.com/view/r5vsj666nir2kki/TBOLT_KA2PTE_.jpg/file

Since its using an external 3KV supply, they broke the ground that the filaments were originally tied to
and inserted the "plate" meter inline to register current - actually a cathode meter. I guess for dipping purposes,
cathode current almost equals plate with the difference being grid.

Yes, the one side of the AC coming in is tied common to chassis gnd. T102 and 103 both have one
side of their primary connected to that and I suppose you could say the 2 filament windngs on T102
are "centertapped" now because they share the common gnd.