[AMPS] PL-172 as GG Triode?

[km1h@juno.com]

On Wed, 23 Sep 98 10:27:00 -0800 Rich Measures <measures@vc.net> writes:
>
>>
>>I have a homebrewed PL-172 amp deck I'm working on (someone else's I 
>bought
>>cheap...don't throw rocks at me!).  In tracing thru the circuit, it's 
>a
>>cathode-driven amp with provision for grid bias, but both the screen 
>and
>>supressor are DC grounded.  
>>0V on the supressor causes me no grief (it's even rated there), but 
>0V on
>>screen?  



That sounds like it was setup up for Super Cathode drive Scott. See
various Bill Orr articles for some info.
The screen is at ground but the cathode is now at some negative
potential. Looking at my original Penta specs I would say it would be
-500V.

 This circuit was developed to absorb excessive drive from the exciter
and before swamped grid circuits became popular.


 
>
>The suppressor is usually operated at the same potential as the 
>cathode.  
>Grounding the suppressor and grounding the cathode is normal, however, 
>
>grounding the suppressor and driving the cathode may cause IMD probs.  
>
>
>> I haven't ever seen a Triode-connected PL-172 in the literature
>>(not that it's not there of course).



It is not triode connected, the screen still is functional.



>>Has anyone on the list seen a successful triode-connected PL-172 
>circuit?
>
>not I
>
>I doubt that the tube has been damaged.  The grid is way more rugged 
>than 
>the grid in a 4cx1000A. 


Correct, but a 5W control grid is still reason for caution.  The 30W
screen rating is pretty healthy for a 1000W disapation tube.
In this modern time of enlightenment I would opt for a 50 Ohm swamped
grid, a real screen supply with regulation as per G3SEK's QEX circuit
and/or WWW page, and a 10 Ohm or so cathode resistor to improve IMD and
eat up some drive if necessary.  The original -27dB 3rd order  and -41+dB
 5th  order specs of the 172 never made sense to me but perhaps the Brits
on the reflector can help there...they seem to be the recognized experts
in multi-grid IMD performance.



 .  I would be tempted to operate the tube in 
>Class AB1, grid-driven, Scott.  Even if some way could be devised to 
>operate the PL-172 as a high Mu triode, cathode-driven tuned inputs 
>are 
>not a piece of cake.  Grid-driven tuned inputs are.  .  


IMO, that is asking for disaster plus Penta states neutralization is
required in such a circuit. A conventional tuned grid driven 4-1000A is
fine and forgiving....a scarce and expensive PL-172 is not.

The Hallicrafters HT33A and some other amp ( I forgot the name) used the
172 in swamped grid service....which BTW does not require neutralization.
I can mail you a copy of the original specs and curves if you give me an
address.

73  Carl   KM1H




>
>
>Rich...
>
>R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  
>
>
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