[TMC] TMC PAL 350/500

[Chris Bolkan]

Hi TMC lovers!

I recently posted a question regarding neutralization and asking how I
could tell if I had soft finals.

I noticed several issues tha made me wonder if my finals were soft
which were 1) the bias controls did not have enough range to obtain proper
bias current. It was close but never got high enough even with pots turned
all the way to max. 2) Low power output. I could only get about 35 watts
output. 3) The screen grid current never changed and it is one of the key
measurements for proper operation.

So while I was looking more closely into the grid bias thing I pulled the
finals and checked the bias range of the pots. All good and changed voltage
as it should to -150 when unkeyed. So while I had the tubes out I looked at
the screen voltage metering circuit and checked the divider resistors. All
good. Then while I had the finals out of the sockets I measured screen
voltage.  300V, not 360V!

I followed it back to the power supply and it was 300 straight out of the
regulator! I found out that someone had mis-wired one of the TMC changes.
There are supposed to be three regulator tubes in my PS. One 0A2 and two
0B2s in series to create the 360V. My PS was wired with 3 0A2s! Two 0A2s
were in series creating 300V screen bias. The third 0A2 was used to create
the -150V in the LV circuit! That is supposed to be a Zener in my PS!

The correct reverse polarity Zener is on order and will properly wire the
three correct tubes in series for the screen. All of this circuitry takes
place on a little bracket on the back of the PS and all of the wires are
already there to do it right. I should have the Zener in a couple of days.

I am thinking that proper screen voltage will make a big difference!

I will update the group when I get results!

Chris