[TMC] This darn amp

[Chris Bolkan]

Thank you for that, Meir!

Now I know the posts are getting out! :-)

I will measure the capacitance today and report back. The caps in the
sockets must be C-241 and C-242, 1000pf on the schematic, because they do
not show up in the BOM. John Poulton also suggested I check the sockets but
I did not after I discovered the screen bias voltage was incorrect. I
figured that just HAD to be the problem but apparently not!

I'm learning a lot from this even though it's a bit frustrating.

Chris

On Tue, Oct 19, 2021 at 8:45 AM Meir Ben-Dror WF2U <wf2u at ws19ops.com> wrote:

> 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
>
>
>
> Get BlueMail for Android <https://bluemail.me>
> On Oct 19, 2021, 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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