[Collins] 30L-1 Repair

[Carl]

>>>>> The cathode circuit and band switch is indeed suspect, but so are the
>>>>> tubes and the grid bypassing. These tubes from the late 1930 have
>>>>> relatively long leads on the grids and the grounded grid circuit
>>>>> depends on getting those grids effectively grounded. A change of grid
>>>>> bypass capacitor type or lead length may contribute to the problem or
>>>>> 811A made primarily for audio applications may contribute.
>>>>
>>>>
>>>> ** The symptom points to an input circuit problem. Any disturbance in
>>>> the actual grid or parasitic circuits wouldnt surface on 20M but rather
>>>> 10M.
>>>>
>>>
>>> Yes it does but the 220 pf on the grid was probably chosen to series
>>> resonate the grid to ground so the grid was at ground better than the
>>> socket pin. Its often necessary to adjust the grid circuit even in a
>>> planar tube at VHF to get adequate isolation from the grid in a
>>> grounded grid stage.
>>
>> ** Collins is floating the grid for DC and some Jr engineer likely came
>> up with that 220pf method which is very frequency dependent. The stage
>> is also poorly neutralized. Did Art stick his nose into this also?? (-;
>
> They switch the bias to the tubes to cut them off for receive and to let 
> them cool and the whole PA to cool.



** Having built, repaired, and/or converted over 500 SB-200/201's, plus many 
30L1's and similar, since the 60's Im sort of familiar with the circuit and 
its shortcomings and cures.


>
> When its three inches from the socket pin to the middle of the grid, 
> grounding the socket pin doesn't ground the grid, especially above a few 
> MHz and for sure not at 30 MHz


** Thats not correct as there are thousands of 811A/572B amps in daily use 
with directly grounded grids that work perfectly well on any band 160 to 6M. 
Some on HF can be a bit touchy on 10M when the parasitic suppressor 
resistors have drifted high and neutralization is most always needed on 6M
The current production AL-811H and AL-572 with 4 each tubes come with a 
neutralization circuit; something QST, Heathkit, and Gonset realized in the 
late 50's - early 60's before the SB-200 went with such a haywire 
design....copied from the 30L1.

.
 but series resonating is one way to get
> better stability.

** That was a half assed Bill Orr/Eimac suggestion that was later recinded 
for the SB-220. Unfortunately several manufacturers copied it for 572B and 
3-500Z amps and those have continued to have stability problems....often due 
to build quality.


 Neutralization is very difficult when the grid leakage
> from not being bypassed AT the grid is different for each band.


** Not true as a simple neutralization will reduce the feedthru isolation 
from about 10-15dB to 25-30dB which is more than sufficient even on 6M for 
complete stability.


> Art probably could have stuck his nose into that but that's too technical 
> for his knowledge base.

** Not having known Art I'll leave that to others but Im certainly not in 
awe of Collins design capability in several products aimed at hams. Too much 
point to point or military requirement mentality.

Carl
KM1H