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[AMPS] The things you see Homebrew Amplifiers

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Subject: [AMPS] The things you see Homebrew Amplifiers
From: wlfuqu00@pop.uky.edu (Bill Fuqua)
Date: Tue, 05 Oct 1999 22:20:09 -0400
  
  

At 11:15 PM 10/5/99 +0100, you wrote:
>At 09:07 AM 10/5/99 -0400, William L. Fuqua wrote:
>>
>>   Just bought a HB amplifier. A pair of 4-400 in GG.
>>It looks very nice but a few things I see need to be changed.
>>This one is just fine except for the following:
>>
>
>Hi Bill, the following observations are based on using a similar GG 4-400
>amp for about 30 years:
>
>>1. Clip on plate caps of the type usually used with 807's.
>>   You would think a pair of 4-400s would
>>   have plate caps with fins. 
>
>No problem, works fine with adequate side cooling.
 
 This has a muffin fan. But I guess he did not run it
at higer powers. Glowing plates. The main problem being the
heat conduction thru the plate lead to the seal. I'm concerned about
cracking the seal. However, I'd like more input about this. 
I wonder if any readers have cracked anode seals on 4-400's or
4-1000's and what they believe led to the failure.
  I have heard of holes sucked in the side of some glass tubes.
This had to be  done by radiant heat like a hot spot on the
anode. 
  


>>
>>2.Tubes are fan cooled on the side and I see nothing providing air 
>>  flow thru the bases.
>
>This works fine as long as plenty of air is moving around the bases. If you
>melt the solder out of the tube pins, however, it's not so good.
>
>>
>>3. BC-375 tuning unit band switch. This was used in a 100W transmitter.
>
>Typical milatary ratings. Mine has been running for more then the 30 years
>I've had the amp. 1200 to 1500 out depending on band, HV, drive, etc. This
>switch makes a nearly indesdructable antenna selector switch by the way.
>
Contact area on the tubes is quite small for
a KW+ amplifier with high Q tank. This has a PI and
not a lower Q pi-L. It actually has more current flowing thru
the switch contacts than a modern amplifier.
>>
>>4. No internal shielding. Just a slide on cabinet cover held with
>>   6 screws.
>
>If it keeps you from getting electrocuted, it's probably good enough. I
>would change that myself, but only because I don't like the looks of that
>sort of thing.
Also the internal shielding is to prevent cabinet radiation.
Keep the harmonics inside to prevent interference to other 
radio services. Such as cordless phones, cell phones, etc..
This cover offers NO shielding only has screws in the base and no other
electrical contact.
You did not comment on the panel mounted variable. The problem
with this is that the RF current flowing thru the PI input capacitor
flows thru the front panel making for an excellent antenna for
all the harmonics that the capacitor should be filtering out.

  73
Bill wa4lav


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