[ARC5] Solid State Conversion of a BC-779

[Tom Lee]

Occasionally you may encounter the term "pushing", usually in the 
context of supply-voltage sensitivity ("my VCO shows a lot of supply 
voltage pushing"). I don't know when that term gained currency; I don't 
recall hearing it during my undergrad years, but maybe I was just not 
paying attention.

-- Tom

-- 
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 11/4/2020 19:23, Richard Knoppow wrote:
> There are a couple of types of pulling. One happens mostly in 
> converter stages, that is  a stage that is both local oscillator and 
> mixer in one tube. The presence of the RF can cause the oscillator to 
> change frequency. Another kind of pulling is caused by the variation 
> of voltage on the oscillator. That is the kind I am referring to. The 
> B+ changes when the plate current in the IF and RF stages varied due 
> to the bias changing. So, the local oscillator can change frequency 
> when the RF gain changes, weather controlled by AVC or manually. In my 
> receiver my modified LO used an electron coupled circuit, in which the 
> plate and screen voltages counter-act each other. If the correct ratio 
> is chosen the oscillator will not react to varying voltage. In my 
> receiver I also used a VR tube. When I rebuild this set again I will 
> use the VR tube but with the original triode LO since I will probably 
> use the original mixer which needs the large LO voltage. This is close 
> to the original circuit. The plate resistor on the LO is normally 12K 
> but if a VR tube is used it should be reduced to about 8K. That keeps 
> the voltage on the plate steady at +150 volts, which is the correct 
> value. That removes the pulling although it does nothing about the 
> temperature drift. In the last of the series, the SP-400, about a 5pf 
> N1500 ceramic cap was placed across the main tuning capacitor just 
> under the cover of the tuning compartment. These are hard to find, 
> normally an N750 is the most negative of the TC caps. Even an N750 
> will reduce the warm up drift. There are probably other causes for 
> temperature drift in the Super-Pro.
>    Also, the circuit in the SP-210  which goes to 40 Mhz is different 
> than the ones with 20Mhz higher limit. In the 20Mhz version the RF 
> stages are series fed, in the 40Mhz version (and I think the 400) the 
> stages are shunt fed to get the DC out of the cores and increase the 
> Q. Any single conversion receiver with a 455 Khz IF is struggling for 
> decent image rejection much above about 10 Mhz. RCA obtained decent 
> image rejection in the AR-88 by using very low loss formers for the 
> coils. Still it can't hold a candle to a double conversion receiver.
>     Hammarlund resorted to their very complex guillotine band switch 
> to reduce loses, however, I think RCA did about as well in the AR-88 
> with a conventional bandswitch but using very low loss wafers and an 
> attempt to reduce lead length. The best of all is the HRO which does 
> not have a bandswitch. Nonetheless the HRO specs for images is no 
> better than the RCA receiver.
>     Of course Collins blew everyone out of the water with the 75A-1, 
> with its double conversion system.
>
> On 11/4/2020 5:28 PM, Hubert Miller wrote:
>> I thought 'pulling' had as much to do with difference between signal 
>> and HF osc. as AVC voltage. I think
>> 'pulling' occurs even without a signal, doesn't it, as you for 
>> example, change the RF antenna trim control ?
>>
>> Interesting article. I have been saving such articles and schematics 
>> of receivers for years, mostly still
>> puzzled about AGC circuits in the conversions. My preference would be 
>> to use tube-base plug-ins.
>>
>> Ben, did you publish your procedure with the Trio receiver?
>> I would love to see that too.
>>
>> I bought a couple 'transistor conversion manuals' from a fellow who 
>> has some videos on Youtube. I do
>> have some differences though. I see no need to patch in a new power 
>> supply  when there's already one
>> right there. And I see no need to use a whole lot of 2.5 mH RF 
>> chokes. Nevertheless, I consider my money
>> well spent when I get some information and perhaps gather some new 
>> ideas.
>> -Hue Miller
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