>On Thu, 11 Sep 97 03:29:34 -0800, Rich Measures <measures@vc.net>
>wrote:
>
>>>On Wed, 10 Sep 1997 19:06:30 -0500, w5ec@digitalexp.com (Hawkins,
>>>Bill) wrote:
>>>
>>>...snip... ... Had I kept it, I would have cut
>>>a hole in the side of the cabinet and mounted a fan just for the tank
>>>coils.
>>>
>>Sounds like a good idea, Bill. Heath did it right when they blew
>>cooling air over the tank in the SB-220 - despite the subsequent
>>blatherings of naysayers.
>>
>>>I now have a homebrew 8877 and an Alpha 91B with the optional fan,
>>>either of which will run brick-on-the-key at 1500 watts out,
>>>guaranteed.
>>>
>>Does the 91 have an inductance in series with the grid termination R? If
>>so, how much L? What is the value of the GTR?
>>thanks
>>Rich---
>>
>_______________________________________________________________
>No coil in series with the grid terminating resistor.
Interesting. Very interesting. - - I'm wondering why the guy in
Wisconsin told me that there's an inductor in series with the GTR when
there obviously isn't? ... ... ... Some people tell the truth and other
people do not.
- So far, I have seen such circuit only once - - from the U.S.
distributor of Svetlana tubes - which raised my eyebrows when I saw it.
.
>From the grid(s), there is a 1 mHy coil to the bias supply, a pair of .02 mF
>caps in parallel to the GTR, and from them, the GTR directly to
>ground. The GTR is 50 ohms, 90 watts. The cathodes each have a 12
>ohm 5 watt resistor directly to ground, with nothing else.
The 12 ohm resistors seem like a good idea, Bill. With -60v of grid
bias and a 50 ohm GTR, more than 36W rms of drive power is going to
cause grid current - which increases distortion. However, with 12 ohms
of cathode R, at 2.5a of peak cathode current., 30v of peak feedback V is
added - which means that 90v of drive [81W rms into 50 ohms] is not
going to cause grid current.
It might be interesting to know how much total L is in the leads between
the grid terminal, the 0.02uF caps, and the GTR. It might be interesting
to couple a dipmeter to the 0.02uF caps and look for the rotten VHF
resonances that NEVER appear on the diagram. .
- If Alpha had fed the bias through the 50 ohm GTR, they could have
saved the price of the 1mH RFC and, with triple-bypassing, provided a
lower-Z VHF path from the grid to RF ground.
thanks, Bill
Rich---
R. L. Measures, 805-386-3734, AG6K
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