[TMC] This darn amp

[Chris Bolkan]

Hi Gary!

Great questions. I'll do my best to answer.

1) Check that your screen voltage at the socket is holding steady as you
drive it. Could be a high resistance in the supply circuit.
*I have not checked this. I did find a power supply wiring error which I
corrected and that brought the screen voltage up to spec. I then measured
proper screen voltage at the socket. I have not however verified that the
screen supply is stiff at the socket. I will apply some resistive loading
and verify that the voltage remains the same. *
*I have begun checking the capacitor on the screen pin in the sockets. I
have only started this effort and found right before bed last night that
one of the sockets (haven't checked the other yet) has only 4pf of
capacitance. The pin is not leaky to ground. It's like the cap in the
socket just opened up. I do not know what the effect of this bypass cap
being open can be. If it is degenerative feedback this might be my problem
but I have to check the other socket first and learn what the effect of a
high impedance screen to ground is.*

2) What is your plate idle and screen idle current?
*Plate idle current is set to 90ma on V203 and then to 160ma with V204.
Screen current reads 0*

3) Also check the band switch, may be the TANK is way off frequency.
*The tank tunes nicely dipping plate current with dune and peaking output
with load*

4) Do you get any dip in plate current when you turn the plate tune cap?
*Yessiree a nice dip*

5) Shorted output padder cap?
*What is a padder cap?*

6) Do you get a peak in output power when tuning the plate tune cap?
*Yes, I think so but I can't positively remember. I have been looking at
the plate current when dipping and the scope in parallel with the dummy
load when loading for peak. I **dip the plate current with the tune and **load
for peak output and dip the plate and load for peak etc while bringing up
the drive. I can't actually say if the output was peaking at the dip. I
should know but I don't. Sorry.*

Obviously I need to sort out the tube socket screen bypass cap thing, and I
can easily check for screen supply stiffness at the screen pin on the
socket. Can you tell me the effect of high impedance on the screens?  There
are inductors in series with the screens so without the bypass capacitance
the screen(s) would be high impedance.

Thank you for the great questions Gary. I appreciate everyone's help.

Chris
KI7NXY

On Tue, Oct 19, 2021 at 11:04 AM Gary Schafer <garyschafer at largeriver.net>
wrote:

> Check that your screen voltage at the socket is holding steady as you
> drive it. Could be a high resistance in the supply circuit.
>
> What is your plate idle and screen idle current?
>
> Also check the band switch, may be the TANK is way off frequency.
> Do you get any dip in plate current when you turn the plate tune cap?
>
> Shorted output padder cap?
> Do you get a peak in output power when tuning the plate tune cap?
>
> 73
> Gary. K4FMX
>
>
> Sent from my iPhone
>
> > On Oct 19, 2021, at 10:31 AM, Chris Bolkan <bolkyboats at gmail.com> wrote:
> >
> > Hi TMC lovers!
> >
> >
> > Got the power supply corrected. Now puts out a nice solid 360V to the
> final
> > screens and all other voltages are correct.
> >
> > The good: All voltages correct, finals now bias nicely just as I think
> they
> > should and it tunes up nicely.
> >
> > The bad: At 240ma plate current, single tone with a drive of 20, it puts
> > out 120V p-p into a 50 ohm load. That calculates out to 35 watts RMS and
> is
> > almost exactly what the inline wattmeter shows. Even when I pump it up to
> > over 300ma it only puts out just 50-70 watts. I don't like doing that for
> > long as occasionally the breaker trips because I have it set
> conservatively
> > until I figure out what is wrong.
> >
> > So correcting the screen voltage and making it all "right" still did not
> > change the output power. That's what it was before even with what I
> > consider a profound change.
> >
> > This is also puzzling: There is no change in screen current (on the
> meter)
> > no matter what I am doing tuning this thing up. The meter reads very
> close
> > to 0 (about a third of the way up the scale as it should, but nothing I
> do
> > changes screen current. I verified the proper screen voltage on the final
> > tube socket pins so I know the sockets are not shorted. Since the screens
> > get their voltage off the measurement divider, if the current is changing
> > it should register.
> >
> > I still have not checked the capacitance on the screen pins of the
> sockets
> > yet. If those were somehow low would that have a degenerative feedback
> > effect? Even so it seems highly unlikely that both sockets have failed in
> > exactly the same way. This is so confusing.
> >
> > I've been posting a bit lately about my PAL 350. I've not seen a single
> > comment so I guess I don't know if the posts are even going out. Well,
> > here's the latest. Any ideas would be greatly appreciated!
> >
> > Chris
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