[Collins] 30L-1 Repair

[Dr. Gerald N. Johnson]

On 7/6/2014 8:58 AM, Carl wrote:
>>>>>> 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.

However you do admit they can be a bit less than perfectly stabile.
>
> .
> 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.
>
Its important, but its a single band result. Its a failure with multiple 
bands.
>
> 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.

Neutralization is a patch instead of a cure, but it can work though the 
length of the grid wire can make it difficult to get perfect 
neutralization on all bands. Fortunately the stage gain in grounded grid 
is low so perfect neutralization isn't required.
>
>
>> 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.

When I was in Cedar Rapids, the whole HF department worked on military, 
broadcast, and VOA, and ham gear was an occasional sideline. They had 
designs that didn't get to the prototype stage after the S-line because 
Art thought they were not fancy or expensive enough. The Collins 
products that did get to the ham market had qualities the competition 
didn't imitate for decades, like better than 100 Hz frequency 
calibration and stability and better than 30 dB 3rd order intermod 
products on the transmitters. So many a KWM-2 and S-line served on 
military frequencies as did slightly modified KWS-1 and 75A-4. Military 
and government sales did add profits to the ham gear.

What would you have designed differently?

I think I would searched for a remote cutoff pentode for the 75S IF with 
more gain than the 6BA6 so the two stages didn't have to be run up at 
their power dissipation limit to get the desired gain and I would have 
complicated the power supply to have a 250 or 300 volt output for a more 
common 6AQ5 output stage that was cathode biased instead of fixed biased 
from the fragile bias supply to have gotten the desired audio power with 
less heat from the output tube. I would have never used a selenium 
rectifier for the bias supply either.

By the time the 30L-1 got to Collins engineering it pretty well was 
defined from a home project of a couple of the department engineers.
>
> Carl
> KM1H
>
73, Jerry, K0CQ