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Re: [Amps] 2 x 1625 amp?

Subject: Re: [Amps] 2 x 1625 amp?
Date: Sun, 5 Jun 2005 22:39:04 EDT
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In a message dated 6/5/2005 8:34:21 P.M. Central Standard Time, writes:

Hi  Guys:

Appreciate your generous help on my previous  matters.

Permit me kindly to ask for help on this one.

I have a  1948 homebrew using 2 each 1625's in RF deck.

Its modulated by 2 each  807's.

It works.

But - I'm not sure that I have the grid bias at  the 'proper value'...and the 
output is lower than I'd expect for these  tubes..unless they're weak...which 
could be since I've been hard on the  rig in my attempt to get it operating.

Presently, the grid bias is at  -56 volts.

The 'Book' says -90 for telephony @ 600 volts on the plate (  I've got 700 ).

The gent who built the rig - W2PLY - apparently used  -35 volts or a C 

The two 1625's are in push-pull and link  coupled to the output coax.

Sooo..the big question do I know  if I've got them at max  ouptut?

Amps  mailing  list

Back in those days the grid bias was derived from the grid current flowing  
through a grid leak resistor. The -35v you mentioned is termed "protective 
bias"  and keeps plate current in check in case rf drive is lost. To reach 90v. 
 additional 55v is needed. If the two tubes are running 5ma grid current 
each,  for a total of 10ma, the resistor value needed for the grid leak is 
or  5500 ohms. Enough drive is power is required to make all this happen. If 
drive  is higher, then the bias will be higher which will tend to make the 
circuit self  regulating to a degree. The power output will only increase 
with higher  drive because of the added grid bias voltage If the drive is too 
low to reach  rated bias voltage then the output power will be lower. You 
might then have to  experiment with the grid leak resistance to achieve best 
output power with the  available drive level.
The power output from a pair of 807's should be on the order of 80 to 100w  
at the plate voltage you mentioned. The grid leak bias voltage will give an  
excellent indication of adequate drive level. 
The recommended maximum plate voltage for class C, plate modulation is  
600vdc as the peak voltage will approach 1200v on 100% modulated peaks. You may 
get away with 700v if your tubes can handle the extra voltage during 
 There should be no problem as far as cw or carrier operation is concerned.
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