[Collins] Collins ham gear redesign

[Carl]

>
>
> On 7/8/2014 12:14 PM, Carl wrote:
>>
>> ----- Original Message ----- From: "Dr. Gerald N. Johnson"
>> <geraldj at netins.net>
>> To: "Carl" <km1h at jeremy.mv.com>
>> Cc: "Glen Zook" <k9sth at sbcglobal.net>; <collins at mailman.qth.net>
>> Sent: Monday, July 07, 2014 6:33 PM
>> Subject: Re: [Collins] Collins ham gear redesign
>>
>>
>>>>>
>>>
>>>>> 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..
>>>
>>> Ah, the mixers. The 75A receivers used pentagrid mixers that have a
>>> few known characteristics. Easily overloaded, and very noisy.
>>
>> ** I found that out the day I bought my just purchased A4 into the R&D
>> lab at National. It soon had 7360's in both mixers with a 6GM6 RF amp
>> and some gain redistribution; its a very robust receiver that I still
>> use. With recent reproduction and cascaded 800Hz filters it outperforms
>> any ricebox Ive ever tried for digging under the noise on 160/80 CW
>> There is something to be said for a premium level non synthesized radio
>> with no detectable phase noise and "artifacts" present in modern gear.
>> That was also my reason for the TS-830's as expanded below.


>
> The Collins receiver phase noise is not zero, but compared to anything 
> synthesized its several orders of magnitude better.

** No LO has zero phase noise nor did I claim that.
OTOH when the phase noise is below backround noise it doesnt contribute many 
artifacts from every other signal and QRN on the band.
That same 75A4 was also used for 50-432 MHz converters back in those days 
and the very low total receiver noise was a big plus for weak signal 
digging.
.


That being better
> was compromised for strong signal handling by the mixers which are always 
> the weakest link in the receiver. They create much noise, have low gain, 
> and can't stand strong signals. The 7360s were a very much better 
> solution. The KWS-1 brass board I used to have used a TV detector beam 
> switching tube, 6AR6 IIRC for its balanced modulator that didn't make it 
> to production.


** The 6AR8 which had more problems than a YUGO and was quickly replaced.
The 7360 was specifically developed as a SSB balanced modulator for 
commercial use and command a premium these days. I have a low cost sleeve of 
NIB ones I stashed away several years ago.


The diode ring isn't bad though so long as the
> germanium diode thermal drift doesn't pass too much carrier and you can 
> stand the mixer loss inherent in the diode ring mixer.


** Ive usesd Schottky diodes a few times for some homebrew or replacing 
germanium.


>
> I remember a presentation on the new 7360 at an ARRL convention in St 
> Louis while I was still in high school, but old enough to be driving.
>
> I have found some HP/Avgotech quad diodes in tiny packages rated to a 
> couple GHz and some mounting boards to make their use practical from 
> multiple sources and Knepper has a couple for testing in transmitters 
> needing balanced modulator assemblies. The circuit boards are maybe 5/8" 
> square with holes on the corners for wire leads. Mounting the package 
> chips on the boards is a pain, the parts are something like 1/8" long, 
> hard to hand solder. I did them with soldering paste and a hot plate.


** I still have those "tools of the trade"



>>
>>>
>>> The 6U8 and 6AU8 in the S-line were not great choices either.
>
> DUH! I should have said 6EA8.
>>
>> ** The 6U8 was failure prone, the 6U8A fixed that and the 6EA8 fixed the
>> remaining problems and was used in applications for many years as a
>> workhorse tube. While a few say they werent interchangable I havent run
>> across any examples and the swap was either beneficial or no change. In
>> my CE-100V and 200V, for example, the 6EA8 had better performance to 30
>> MHz and emission stayed high a lot longer.
>> It certainly would not be my choice in a receiver signal path however.
>
> Probably the poor life of the 6U8 was caused by running up the screen 
> voltage and plate current to get adequate gain. Then it ran hot and burned 
> the cathode emitting material away, splashing some on the control grid so 
> making it a good emitter of grid current that opposed any external bias 
> and raised the plate current more.

** Similar to what I remember stated at the time.

 Likely even if the data
> sheet didn't say, the 6U8A probably was capable of more power dissipation, 
> or a lower temperature rise for the same power dissipation, the 6EA8 even 
> more.

** Ive no idea what the internal changes were, I was just a user (-;



>> I traced
>>> the noise sources in my new S-3B (it spent a year on its side out of
>>> the case after I moved to Texas) and found that the second mixer was
>>> the primary noise source because the gm of the pentode of a 6AU8 was
>>> low, and as a mixer it was way lower. That limited the dynamic range
>>> of the receiver on both ends. I installed an adapter holding either a
>>> 6688 or 7788 from Amperex with 50,000 umhos in place of the 6AU8 with
>>> 4000 umhos and then was able to drop the signal into the second mixer
>>> seriously.
>>
>> ** Those are serious tubes with a short life and hard to tame as Racal
>> found out. Serious overkill for HF.
>
> Yes even more serious overkill for the second mixer, but so far it was 
> working last time I had that receiver turned on.
>>
>>
>>
>>> With a nuvistor 2m converter in front of it, I took the signal from
>>> the 600 watts of W5WXV at about 20 miles from showing about 20 spurs
>>> in 200 kHz down to three and one of those was where he was supposed to
>>> be and the other two were close to the noise level. I did take the
>>> receiver gain a little further down than was great on HF but I didn't
>>> hurt the system NF with the 2m converter out front.
>>
>> ** I still have Ameco 2 and 6M nuvistor converters somewhere and also a
>> W2AZL 417A 2M converter I built in 66 and preferred over the Ameco. A
>> few years ago it produced a 1.2dB NF at the NEWS Conference which is
>> better than most rice boxes.
>
> There are days with my FT-857D that I wonder about hearing. I tried 
> preamps for the June VHF contest and took them out of the circuit. Didn't 
> seem to hear better after the relay and cable loss.
>>
>>>
>>> A ham at Rippey Iowa who sold prop pitch rotors and remote antenna
>>> switches also sold a drop in replacement for the first mixer using a
>>> double gate MOSFET. He didn't agree that the second mixer was the
>>> limiting factor, but then if he didn't see it raining he would not
>>> believe it if I told it was raining. George McKercher was his name. He
>>> drove a pair of 4-1000A on HF with his S-line. With the plate variac
>>> at half voltage he only peaked at 1 amp and 3 KV on the meters when
>>> chasing DX with a visitor present in the shack.
>>
>> ** These days some use a commercial 1200-1500W amp as a driver stage for
>> a real tube!
>
> There was a ham in Ames that used a KWS-1 when it was new to drive a few 
> 304TL in grounded grid. He was manager at the airport, and one day he 
> didn't show up for work.


A few weeks later they found much money missing
> and he was next heard from in Gatar. I don't think he returned to the 
> states.
>>

** Maybe someone cut his coax or punched him in the face thanks to that 
splatter box. Thats a good reason to head to Qatar.



>>
>>>>
>>>>
>>>> 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 3B and 3C were the same receiver except for the extra crystal
>>> board until very late production. I never noticed a problem with the
>>> AGC in my May 1964 3B. I copied that dual time constant filter to the
>>> Q-5er.
>>
>>
>> ** Back then there was no Internet and all I had was a S3C on loan with
>> its manual.
>> The NCX-3 and NCX-5 also have a superb receiver path and AGC which I
>> didnt appreciate back then as I had no use for transceivers with limited
>> features.
>>
>>
>>>>
>>>> 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.
>>>
>>> There is a problem with the 75A-4 and replacement tubes. I have a
>>> project very slowly in the works to compare RCA 6DC6 to ECG 6DC6. They
>>> give an A-4 wildly different AGC characteristics with the ECG tubes
>>> acting more like 6AH6, sharp cutoff at a lower bias voltage than the
>>> RCA or ordinary 6BA6. Abd that also changes the S-meter calibration
>>> significantly.
>>
>>
>> ** Likely remarked 6AH6's. Relabeling was a big business back then but
>> the 6DC6 never impressed me in any Collins or Hallicrafters. My favorite
>> swap has always been the 6GM6 in the 7 pin family if performance above
>> about 15 MHz was important, it varied by radio. In a NC-300 the 6GM6
>> improved 10M sensitivity over the stock 6BZ6 by about twice comparing
>> several of the hottest NIB of both, but since the AM 10dB SNR was
>> already below .25uV you need a good location and a highly directive yagi
>> in order to benefit. Since I have both it is the usual go to radio on
>> 10AM only, I use my TS-950SD for CW/SSB.
>
> The ECG aren't all that old. Well the two I have for testing are dated 
> 2/86 in the boxes with the federal stock numbers.


** Thats several years after production ceased in the US so washing and 
relabeling was in full swing. Anything marked ECG, IRC, or other "new 
brands" was suspect.


>
> I recall about 1968 getting some 6146 from army stocks where the tubes 
> were sold by the Vacuum Tube Company and the insides looked more like 
> sweep tubes than RCA RF tubes and in a S-Line I was fixing for another 
> government agency, they wouldn't work. RCA 6146s did without circuit 
> changes.

* I have a NOS pair of "Heinz & Kaufman" 6146B's that are Japanese S2001A's. 
In this case those are an excellent tube and likely relabled some years 
after the TS-830 went out of production and were more marketable to the 
general North American market.



> The tube manual page on the 6GM6 looks like one tube could replace two 
> stages of 6BA6 if it didn't oscillate from the circuit layout.

** I do that in the HRO-60 and SP-600's. The 2nd stage in the 60 was factory 
set for just enough gain to exceed stage losses and I retain the 6BA6; the 
extra gain feeding it helps mask the 6BE6 or 6BY6 swap noise. In the HRO-50 
and 50-1 it turns the A coil into a hot performer with decent image 
rejection (single conversion 455kc).
They all did require some basic/simple stabilization.

The earlier Super Pros also benefit from that conversion..



> Definitely a hot tube still with remote cutoff compared to the 6DC6 with 
> sharp cutoff.

** ?? Both are semi remote curves with the 6DC6 being a bit faster/sharper 
resulting in a bit more IMD with a lot of strong signals and a bit more 
pumping.
Ive heard that Art pressured RCA to develop it for the R-390A but there was 
minimal interest by volume users so it is sometimes hard to find. The A4 
makes a nice platform for the swap with minimal changes.


I still don't know the attraction at Collins for the 6DC6.
> Did any other receiver manufacturer ever use them? in radio or TV?

** Hallicrafters in the SX-115 and 117 and maybe a few others. TV's had the 
6BZ6 at the time for IF's which is still a good tube.
The 75S3B/C AGC is a good choice for swaps and sets that need a better AGC 
anyway.

Carl
KM1H