[Collins] Collins ham gear redesign

[Glen Zook]

The 6EA8 works very well to replace the 6U8 in the Collins S-Line except in the tone oscillator in the 32S- series transmitters and the KWM-2- series.  For some reason, and I don't really know why, when the 6EA8 is used in that circuit there is often a problem with the starting of the tone or a very distorted tone.  Occasionally, the 6EA8 works fine in certain units.  However, more times than not, I have definitely had problems using the 6EA8 in the tone oscillator circuits.  As such, I don't take a chance and always put a 6U8 in that circuit.

Glen, K9STH
website:  http://k9sth.com


On Tuesday, July 8, 2014 1:15 PM, Carl <km1h at jeremy.mv.com> 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 6U8 and 6AU8 in the S-line were not great choices either.

** 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.



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.



> 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.

>
> 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!


>>
>>
>> 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.


>>
> Lots of more recent ham gear has had hardly working AGC. Icom used
> to advertise their amplifier as blowing the receiving operator back
> from the operating table, which it did with their receivers (One IC I used 
> for FD one time appeared to have no AGC at all. Its owner bought a TS-440 
> after that and it was so different the day he got it he stayed on the air 
> for 24 hours.) AGC of the FT-857 is so good that audio is no help at 
> pointing 10 GHz dishes. Aiming is much easier with the AGC turned off.

** For HF I progressed from the 75A4/100V plus 75A4/200V to a pair of C 
Lines, a pair of TS-930's, pair of TS-940's, and currently a TS-950SD and 
one of the highly modified 940's. I might get a TS-950SDX if a good one 
shows up. Ive tested a lot of the so called premium rigs and cant find any 
reason to get one. Having 2 complete stations with amps and a switching 
matrix that permits any rig to use any antenna Im quite satisfied. Maybe if 
SDR improves a lot more I might be tempted but at 73 I might be drooling in 
a rocker before that happens (-;

I use 3 modified TS-830's as the IF platforms for 6M and above. Currenty to 
2304 but gear is available to 10 GHz.
For several years I was supplying 24 GHz mixers and amps to the EME and 
contesters around the world using rejected modules at work that didnt meet 
the Po or gain specs at 23 or 26 GHz and when retuned/peaked for a narrow 
range they exceeded specs at +33dBm.

Carl
KM1H





> On 7/7/2014 2:53 PM, Carl wrote:
>>
>>> One of the first mods I made to my new S=3B was to replace the BFO
>>> adjustment pot that was switched at the CCW end of rotation with a
>>> pot with a push pull switch. So when I set the BFO frequency for my
>>> likes (or for narrow shift RTTY) I could switch to SSB and back
>>> without having to go through the setting process again. I don't
>>> know why it wasn't built that way from the beginning, unless the
>>> engineers never used a modern TV set with a push pull switch. I had
>>> no trouble at all buying the better switch and pot assembly in a TV
>>> parts store.
>>>
>>> The filters of those days had a lousy time response turning
>>> lighting into extended crashes and power line noise into continuous
>>> noise (as seen on a scope that I connected to the jack I added to
>>> the end of the receiver IF strip). A few years later I used a
>>> vintage Q-5er with converter and wired up my receiver/TX patch
>>> panel so I could use either the Q-5er or the S-3B or both with my
>>> transmitters. Often on 75 meters I could copy through thunderstorms
>>> with the Q-5er and couldn't with the S-3B because of that filter
>>> ringing. The Q-5er selectivity came from loosely coupled ferrite
>>> core IF transformers at 85 kHz and had a more Gaussian amplitude
>>> response with a fine time response. Lightning came through as
>>> clicks, not crashes. The modern Collins mechanical filters offered
>>> as options in the Yaesu FT817, 857, and 897 don't ring nearly so
>>> much.
>>
>> Thats why I like to use a NC-300 or HRO-60 under those conditions.
>> One with a 80 KHz IF and the other brute forcing it with a load of
>> 455kc IF circuits. The Collins R-390 is another decent storm radio
>> but not the 390A. The USN RBB and RBC by RCA is perhaps the peak of
>> electrical storm radios but I havent used a R-390 at sea. Yeah, I
>> have those also (-;
>
> 390 and 51J-3 should be better than 390A and 51J-4 because of the lack of 
> vintage mechanical filters. I have a 390A at the barn that the previous 
> MARS member never got working, mice and moisture have probably not 
> improved it in the last 15 or 20 years. I'm no long in MARS but MARS 
> didn't take back any of the equipment though I asked them to, especially 
> the 6' rack TMC independent sideband receiver with 100 tubes. A squabble 
> over a repeated signature for it is why I left MARS though I was Iowa 
> newletter publisher and technical coordinator. Its still with me but 
> filled with mouse nests, I don't think I want to apply power.
>
> I have a 51J-3 with product detector and better AGC added by an ISU 
> professor, W0PFP who is now a SK. I'm not sure I've turned it on since I 
> moved. It was working good except its RF stage will oscillate on the 3 MHz 
> band if peaked. It has a white panel that was tan with pipe smoke when I 
> got it.
>>
>> Another USN radio is the mid 30's RAK and RAL regens, the peak of
>> regen development. It uses an audio peak limiter as a form of AVC
>> that is extremely effective and easy on the ears. I used a RAL often
>> at sea for hamming and have the pair here also. The RAK (it tunes
>> 15-600kc) is better on CW only and feeding a 455kc IF or building a
>> crystal controlled down converter into it would be an interesting
>> experiment.
>
> The RAL and RAK were used on liberty ships through WW2 based on the design 
> philosophy of nothing new, only proven designs and if the operator was 
> careful there was no leaked LO for enemy subs to detect. If the operator 
> let it oscillate it had two Rf stages to cut down that radiation but 
> probably wasn't perfect.
>>
>> Carl KM1H
>>
> I don't know if the modern fancy SDR have come to it yet, but an SDR needs
> two AGC loops. One to protect the D/A converters from overload and another 
> to control the audio level. The front end AGC is seeing wide band signals, 
> how wide depends on the specific design, so if that is the only AGC, a 
> strong signal a distant frequency can be pumping the audio level. There 
> needs to be a post detection AGC, like the limiter of those regens that 
> works on only the audio to the user. I suggested that to one of the SDR 
> gurus in 2006 and he hadn't though about that at all.
>
> That audio level pumping can come from using a narrow audio filter after 
> the RF AGC in most any receiver. Makes the audio or DSP filter relatively 
> useless on a crowded band. Yes, AGC ought to be derived after ALL the 
> selectivity but I sometimes use a passive low pass filter on the speaker 
> wire to cut down on post filtering wide band receiver noise, a problem 
> when the filter is followed by two mixers and 7 stages of wide IF that 
> contribute almost as much noise as the front end passes through the 
> filter. Plus the noise artifacts of the DSP audio filter.
>
> I have a 2m transceiver that has two filters on the IF strip, one at the 
> mixer (its single conversion with a VCXO for the LO), and one right before 
> the product detector. Its called a Hohentweil.
>
> If it had a good tuning range the Hohentweil would be an interesting VHF 
> transceiver and IF for microwave. Not all the Minnesota area tranverters 
> are rock stabile and sometimes there's a want to shift 2 or 2.5 MHz to 
> allow more than one direction of contacts from the central hill top 
> location at the same time. And as I work on higher bands a 432 or higher 
> first IF would be beneficial in image rejection without lossy complex 
> millimeter wave filters.
>
> 73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association
>
>>
>
>
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