[Amps] TL-922 question

[R L Measures]

On Aug 28, 2006, at 8:39 PM, Georgens, Tom wrote:

> Hello -
>
> I have a question about my TL922.
>
> When I tune on 80 and 160, the LOAD control bottoms out before I can
> peak the power.

If the Load-C is at max, the bandswitch probably has some padder- 
section contacts that evaporated.

> The output power is about what I would expect so the
> peak must be close but I am uncomfortable that the tubes are not  
> seeing
> as good of a match as they could.
>
> The amp has always been real close to the 0 position but I don't
> remember it being quite like this.  To be complete, I have made the
> following mods to the amp.
>
> 1. Inserted a HV fuse resistor

It is more important to limit peak-current with a glitch-R than to  
fuse the HV. In the event of overload, the stock xfmr-primary fuses  
blow quite reliably.

> 2. Directly grounded the 3-500Z grids

Had you checked the before and after grid-resonance with a dipmeter,  
you would have found that grid-resonance frequency Decreases slightly  
with the grids directly grounded -- precisely the opposite of what is  
advertised by amplifier "experts".

> 3. Replaced a blown blocking cap (1000pf).  The service manual says to
> use 2000pf but the operating manual says 1000pf

Even 200pF is enough for the DC blocking-C because it's a high Z  
circuit.

> 4. Replaced the tubes (unmatched)
>
> The seems to work otherwise and has seen extended operating on the  
> high
> bands.

Bingo.  It's gotta be the bandswitch contacts,  If you occasionally  
hear the amplifier spit, it's from intermittent bursts of approx.  
120MHz energy on the bandswitch.  This problem can be fixed for under  
$16.
>
> I would like to get the tuning back in range.  Could any mods have had
> this effect?  Is it worth trying to add to the padder caps?

The padders are there.  You need a bandswitch wafer or maybe just  
some fixed contacts if the rotary contact is not toasted.

cheerz, Tom
>
>
> Thanks and 73
>
> Tom W2SC
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R L MEASURES, AG6K. 805-386-3734
r at somis.org