[Collins] 30L-1 Repair

[Carl]

>
> 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.

** Its no different than running many other vintage design glass tubes 
especially if highly unmatched or different manfacturers.


>> 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.


** It doesnt even work well there for the same reasons I mentioned above.


>>
>> 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.


** Its a fallacy perpetuated by the later ARRL and wannabee engineers that 
dont understand amp design.
Even a pair of GG 3-500Z's is only marginally stable on 10M as any simple 
tuning test for minimum Ip with Ig peaked will show. An amp with 3-4 of them 
is asking for trouble.
The length of the grid wire has little to do with attaining adequate 
neutralization and it is a lot simpler in GG than with PP Class C where 
those same 30's designed tubes were widely used into the 70's

>>
>>> 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.


There were plenty of better tubes than the 6BA6 by the time the 75A4 came 
along. The use of a single triode was a poor choice for a mixer, a dual 
triode such as the 12AT7 would offer more conversion gain and a high degree 
of overload handling followed by improved gain distribution..


>
> 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

OTOH Collins didnt have a decent AGC for any mode on any model until the 
75S3C. I modified both of my contest grade R4C's using that circuit plus 
added a Medium position.

The 75A family was even poor on AM for ham use, other brands were much 
better; heck my 1941 SX-28 or NC-200 is much better. The HRO-60 is a dream 
to use as a battle conditions AM radio. The A4  AGC was a joke on CW/SSB and 
was one of the first things I changed with mine in 65.

Carl
KM1H