[TMC] PRogress report No. 3?

[Roy Morgan]

At 10:45 PM 11/3/2005, telegrapher at att.net wrote:
>...i was fretting about the PA overload circuit breaker popping when i 
>turned on the HV through the Transmit voltages switch.... under the 
>troubleshooting of the HV supply section, to replace one or both of the 
>872 rectifiers. ...
>I don't know if there is any break in period or particular processes for 
>getting the 872's up and on line properly or not

Larry,

Yes, there certainly is.

Be it well recognized that these are mercury vapor rectifiers, and as such 
are subject to... "having the vapors", as they used to say a century or 
more ago, usually of females.

872A's, and their more common cousin, the 866A, can be unpredictable. In my 
early experiences with a TMC GPT-750, the HV breaker would pop often and 
more or less predictably until the 872's had "cooked" for quite a long time.

Running these things regularly seems to be good for them.  The RCA Handbook 
3 pages on these tubes tells about needed warm-up times (of 3 to 20 minutes 
?) but that may not be enough.  I have a little plank with a filament 
transformer, line cord and fuse, and two sockets. This lets me "cook" a 
pair of 872's at will for what ever time I think they need.

>One thing i'm not sure about is the fact that when i adjust the mid 
>voltage to 500v (per the manual) the resting plate current also increases.

Resting plate current of the PL-172?

>This voltage is for the 6146 driver and not the PL-172 PA tube.  Resting 
>Ip should be 220 ma.  Evidently i've got more to learn about this beast.

Did you look to see if there is a coupling cap between the 6146 and the 
PL-172 grid circuit?  If so, I'd suspect leakage.

>Maybe tomorrow i'll take the HP sig gen out and use it for drive to the PA 
>and see how things are while continuing through the tune-up process.

Note that the transmitter accepts input at the 100 milliwatt or less level. 
A normal signal generator may be able to drive it to at least modest output.

>At least most of the problems have been overcome.  Now i can hope that the 
>final is ok and not flat.

Is it a thoriated tungsten filament? I suspect not, bit if it is, search 
for the lore on treatment of old thoriated filaments and don't believe what 
you first run into.

>With the unskilled types like some of us owners are, we live and learn the 
>hard way.

But it's FUN, yes?

Roy

- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
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