[TMC] This darn amp

[Meir Ben-Dror WF2U]

Chris,

I own and use a PAL-350A, (and a PAL-350) but I never encountered any problems with them, except a bum (cracked and falling apart) plate circuit breaker in the -350 power supply.
A steady screen current means that something is acting like a fixed load resistor, instead of just the screens drawing the current.
Provided the 4CX250's are known good, I'd suspect the screen bypass caps which are built in the tube sockets. They (or one of them) could act as load resistors - the're known to go bad occasionally.

73, Meir WF2U
Landrum, SC



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On Oct 19, 2021, 11:31 AM, at 11:31 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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