[AMPS] Shorted Pi-Network Turns

Phil Clements philk5pc@tyler.net
Mon, 11 Sep 2000 11:08:59 -0500



> "Parasitic suppressors should, if possible be kept out of the plate tank
> circuit in order to keep the unloaded Q and, therefore, the plate tank
> efficiency as high as possible.  At high frequencies, where the tube
> capacity becomes a good portion of the total tank capacity, this precludes
> the use of the normal parasitic suppressor in the plate lead.  As much
> suppression should be done in the control grid circuit as possible with only
> as much suppression as is absolutely necessary in the plate and screen
> circuits.  Suppressors should be made with the lowest value of resistance
> and inductance that will suppress the parasite to avoid excessive loss on
> the operating frequency."

I have seen this scheme and variations of it over the years in several amps.
The 3CX3000A7 has a very high anode C, exacerbated by using the older
style sockets and/or metal chimneys.

The Creative Electronics CE-2500A uses a "U" shaped strap for L/supp with
a 22 ohm 45 watt R/supp across it in the anode lead AND a 4-turn coil wound
around a 47 ohm resistor inserted in series with the RF input lead to the cathode.

The Henry 8-K Ultra uses a unique "in-house" suppressor made of a very large
coil of heavy brass strap that has been coated with chrome plating. This acts as
a suppressor and allows an acceptable  tank Q at 30 mhz. No other suppressors are
used on other tube elements. I buy the coils, sockets, and chimneys from Henry
for use here in all my 3CX3000A7 H.B. amps with repeatable success and stability.

A trick I learned from my Elmer years ago for 4-1000A G.G. amps is to put 10 ohm
resistors in series with 25pf caps from each filament pin to chassis ground, using
shortest possible lead length. This really "tames" the old 4X1's.


(((73)))
Phil, K5PC






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