Amps
[Top] [All Lists]

[AMPS] Blown TL922A... What to do?

To: <amps@contesting.com>
Subject: [AMPS] Blown TL922A... What to do?
From: W8JI@contesting.com (Tom Rauch)
Date: Wed, 6 Sep 2000 19:26:22 -0400
> But then why does nearly every current-day HF amplifier manufacturer use
> "1935 technology" with the inclusion of the classic coil turned around a
> carbon composition resistor for the purpose of serving as a parasitic
> suppressor?

1935 technology was to use distributed losses in the system, with 
or without additional lumped components.

There was a valid reason for that in the 30's, tubes had long internal 
connections and shielding was poor. Components were poor 
quality, frequency performance was limited by stray reactances in 
the system. Amplifiers built with 1930's technology and 
components tended to oscillate at or near the operating frequency, 
builders had to reduce system Q at or near the operating frequency.

If you tried to suppress oscillations near the operating frequency, or 
even at twice or three times the operating frequency with 
conventional suppressors used today, the resistors would have 
produced too much heat. A more simple way to get the loss was to 
distribute the loss...and the heat.... in the wiring.

As tubes have higher and higher upper operating frequencies and 
chassis became metal with better wiring techniques, and as 
components became better, the tendency to oscillate was not only 
reduced...the frequency also moved higher.

If you have a peek at amplifiers, you'll see tubes like the 572B or 
811A (old long-in-tooth designs a few steps removed from the 30's) 
have to use large inductors in the suppressors, diverting more 
current into the resistors. They have to use more resistance in the 
resistors, to affect the impedance of the more reactive anode to 
tank path caused by the long skinny leads inside the tube. These 
same tubes often need neutralized in grounded grid amplifiers, to 
prevent oscillations at HF!!! 

You see tubes like the 8877 or 3CX800, or 3CX1200Z7 often don't 
require any suppression at all....or at the best minimum 
suppression.
 
That's because those tubes grid are anti-resonant up around 500 
MHz or higher, where tube gain is suffering. The anode systems 
are usually resonant far below the grid's "problem" frequency, and 
so the stage is stable or easily stabilized.   

> If parasitics are of little concern, who on this list (manufacturers
> included) is perfectly comfortable with running (or manufacturing) their
> multi-band HF amplifier without a parasitic suppressor?  One manufacturer
> we know of uses an 8877 without the use of any suppressor, but other
> amplifiers in their product line do.  Does this infer that some of their
> amp products are prone to potentially damaging parasitics while others are
> not?

Exactly. Let me tell you what I've been doing the past few months. 
I put a 1200Z7 in an RF deck of the AL1200, and with multiple 
fingerstock contacts the tube was totally stable with no 
suppression at all. That allowed me to add inductance between the 
tube and the tank, to reduce Q on ten meters and bring efficiency 
way up. The only reason that works is because the Z7 has virtually 
zero grid impedance to the chassis. A 1200A7 will oscillate like 
crazy with no suppression, or with suppression and that series coil.

I did the same to a 3CX800A7. I added a grid collet instead of a few 
fingers on the grid, and I could remove all suppression on a 
3CX800. That let me use the same technique to put an AL-800 
amp on six meters (bandswitched from 160-6 meters) and get 1200 
watts out on six meters!

I have a 3CPX800A7 with 3000 volts on the anode, no suppression, 
a VHF Q of over 1000 in the tank system, and it is completely 
stable. That PA runs 1100 watts out on 2 meters with 30 watts of 
drive, and is rock stable. The reason? The grids are very well 
grounded both inside and outside the tube. The grid "problem" is far 
above the operating frequency so stability isn't an issue.

Now ask the engineers on this reflector who disagrees with what I 
have just said. Watch how it stacks up.


73, Tom W8JI
w8ji@contesting.com

--
FAQ on WWW:               http://www.contesting.com/FAQ/amps
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com


<Prev in Thread] Current Thread [Next in Thread>