[Collins] 30L-1 Repair

[Carl]

----- Original Message ----- 
From: "Dr. Gerald N. Johnson" <geraldj at netins.net>
To: "Carl" <km1h at jeremy.mv.com>
Cc: <collins at mailman.qth.net>
Sent: Saturday, July 05, 2014 10:36 PM
Subject: Re: [Collins] 30L-1 Repair


>
>
> On 7/5/2014 7:08 PM, Carl wrote:
>>
>> ----- Original Message ----- From: "Dr. Gerald N. Johnson"
>> <geraldj at netins.net>
>> To: "Carl" <km1h at jeremy.mv.com>
>> Cc: <collins at mailman.qth.net>
>> Sent: Saturday, July 05, 2014 3:02 PM
>> Subject: Re: [Collins] 30L-1 Repair
>>
>>
>>>
>>>
>>> On 7/5/2014 7:18 AM, Carl wrote:
>>>>
>>>> ----- Original Message ----- From: "Dr. Gerald N. Johnson"
>>>> <geraldj at netins.net>
>>>> To: <collins at mailman.qth.net>
>>>> Sent: Friday, July 04, 2014 5:10 PM
>>>> Subject: Re: [Collins] 30L-1 Repair
>>>>
>>>>
>>>>> 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.
>
> 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, but series resonating is one way to get 
> better stability. Neutralization is very difficult when the grid leakage 
> from not being bypassed AT the grid is different for each band.
>
> Art probably could have stuck his nose into that but that's too technical 
> for his knowledge base.
>>
>>
>>
>>>> 20 to 30 MHz was high
>>>>> enough in the 30s that the power rating of the 811A was already
>>>>> reduced from its low frequency rating.
>>>>
>>>> ** While the 811A is a 811 with a more robust anode it is still capable
>>>> of good performance to 6M in a well designed circuit. I regularly
>>>> convert various 572B amps to 6M and it is simply a 811A with a thick
>>>> graphite anode.
>>>
>>> Providing the grid is resonated.
>>
>>
>> ** This is GG, no resonating involved. Bypass the grids hard with a .01
>> and neutralize.
>
> That won't fly with 811A, the wire to the grid is too long inside the 
> tube.
>>
>>
>>>>
>>>>
>>>>>
>>>>> I note in the Collins 30L1 manual that they want a 20.5 foot long
>>>>> piece of RG58C/U between the exciter and the linear. Rumor has it that
>>>>> had some effect on linearity, but it also might have a big effect on
>>>>> match, being about 1/2 wavelength at 20 meters in polyethylene coax,
>>>>> not foamed coax. And will improve the SWR seen at the exciter just
>>>>> from the loss in that coax. The round trip loss would be about 0.8 dB
>>>>> reducing the reflected power a bit. And interacting with the input
>>>>> tune circuit to affect the match seen at the exciter end of that coax.
>>>>
>>>> ** The input circuit was poorly designed as was the norm in the 50's 
>>>> and
>>>> the specific length of coax was adopted by many.
>>>
>>> It may not be a benefit with a driver other than a pair of 6146 with
>>> limited load range as is typical of KWM-1, KWM-2, and 32S. But it
>>> might have an effect when the SWR is so poor. I'm not sure the reason
>>> for the cable length was ever made public, its been argued a few times
>>> since then, probably helped intermod performance of the 32S 30L-1
>>> measurement on the band it was tested on and really didn't help any
>>> other bands. But it might be a useful test to install it in this case.
>>
>>
>> ** There are loads of 6146 and sweep tube rigs that still need that
>> cable at least in some 30L1 production runs.
>
> What are the symptoms of its need? Hard to tune the exciter? Excess 
> intermod, (hard to detect on the air with the poor intermod performance of 
> the sweep tube rigs)? Low PA gain?
>>
>>
>>>>> As noted recently on this forum mica capacitors are pretty good but
>>>>> can fail.
>>>>
>>>>
>>>> ** Thats fairly common when the bandswitch is set to a different
>>>> frequency than the exciter. Those micas arent built to be part of a
>>>> 50-100W dummy load.
>>>
>>> Actually they have to carry the RF plate current/cathode plus grid
>>> current. They are handling the RF plate current of a KW worth of
>>> tubes. They are working hard. Silver micas dipped or molded won't
>>> survive there, the current carrying surfaces are too thin. They need
>>> old fashioned foil and mica or large low temperature coefficient
>>> transmitting ceramics.
>>
>> ** The subject was about input network caps going bad as I described.
>> Failure of the grid bypasses is a different cause/subject.
>
> I'm talking cathode circuit capacitors they get the same RF current as the 
> other end of the electron stream at the plate. They don't get the RF 
> current in the plate to grid capacitance.
>
> The grid capacitors get that RF current in the plate to grid capacitance, 
> which is most of the tube output capacitance in grounded grid.
>>
>>>
>>> What can be as hard on the cathode circuit is to be on the right band
>>> where they are resonant, but the plate voltage isn't on for some
>>> reason, then the driver can develop high voltage in the one next to
>>> the PA cathode and damage that capacitor.
>>>>
>>>> Check to see if the whole slug in in the coil, if it got broken
>>>>> off or a wrong slug inserted it won't tune properly either.
>>>>> Unfortunately the parts list doesn't give the measured inductance
>>>>> range of the coil to check for the proper slug.
>>>>>
>>>>> 73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association
>>>>
>>>> ** While the 30L1 isnt the easiest amp to work on it is still rather
>>>> basic. If not already done its a good time to replace the various
>>>> electrolytics and check the other high failure items as mentioned on
>>>> several web pages.
>>>
>>> For sure.
>>>>
>>>> Carl
>>>> KM1H
>>>>
>>>>
> 73, Jerry, Technical Adviser to the Collins Radio Association
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 2014.0.4716 / Virus Database: 3986/7806 - Release Date: 07/06/14
>