TheRadioBoard

A receiver project I began a few weeks ago, the Super Gainer from the Frank C. Jones Radio Handbook of 1936, is giving me fits. In fact, I asked a tube substitution question about it on this forum not long ago. The subsequent thread wandered off into a discussion of suppressor injection, and so I thought it might be better to start over again.

The mixer isn’t working. First, here’s the schematic:



It all works fine with the exception of the mixer section – with L1 and the tuning cap disconnected, a modulated 455 kHz signal injected at the grid of the 6C6 comes booming through (no surprise there). However, when I reconnect L1 and the mixer tuning cap, I can barely pick up a strong signal from my sig gen, set to the middle of the intended tuning range, say about 6.5 mHz. I can also tune this extremely faint signal across the intended band as I vary the sig gen output frequency.

I have checked and rechecked a thousand and one times. There are no wiring mistakes. I have tried three different 6C6’s (the one I’m using is NOS). There are no exposed leads that could cause an accidental short. Through-chassis connections are protected with a rubber grommet. I even went so far as to disassemble the entire mixer circuit, built it all over again with new components and wires and substituted a new (NOS) 5-pin base for the plug-in coil. The plug-in coils are well-built (if I may say so myself ).

Here are some diagnostics: The LO works just fine and with the present LC combination the LO tunes from from 5.2 to 7.1 mHz. The i.f. transformer was lifted from a Halli S-20R parts chassis where it once served as the mixer-to-first i.f. transformer. My A-B-C supply tops out at 113 vdc, and so with everything plugged in and hooked up, the 6C6 pins read as follows: 111vdc on the plate, varies from 0 to 74 vdc on the screen depending on the setting of the screen pot, 112 vdc on the suppressor grid (it’s connected directly to the plate of the 76—that’s why), and 1.8 vdc on the cathode. All this looks about right to me.

Here’s something else: My freq counter shows the mixer stage oscillating from 5.6 to 7.8 mHz if the screen pot is advanced enough. I guess this isn’t too surprising, since it is a regenerative mixer.

Anyone got suggestions? Anything I might be overlooking?

Many thanks!
Tom

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Tom

The only thing that comes to mind is a possible tracking problem. If you have the signal generator set to 6.5 MHz, then the LO needs to be running at 6.955 MHz in order to get a 455 kHz IF, and from what you've said, 6.955 MHz is near the top of your LO range.

Hi,
I received a project radio a while back with a similar mixer configuration. It used a 12SK7. I never could get it to work I always felt that having full B+ on the suppressor grid wasn't the right thing to do but that's not based on any great amount of knowledge.

Good luck!

try putting a 10 pf cap in the tank circuit of the 6c6 (in series in the grid gap lead) and see if it makes any difference good or bad.
Just fought through something sort of similar with a set here. It might not help but it would only take 30 seconds to try.
Don VE3LYX

It's certainly not a tracking problem--I can pick up this very faint signal from my sig gen all the way from about 5.6 to 7.5 mHz. The 6.5 mHz was just a representative example. Actually, with the sg gen set at 6.5, the LO would need to be set at 6.045, since it responds with the LO set at the low side. I'll try the 10 pf cap in series with the 6C6 grid see what happens.

I might also try different methods of LO injection, although I was hoping to replicate the receiver exactly as published. It's kind frustrating after all this effort!

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Tom

it is possible the tube is overloading. the 10 pf will rell but another quick and reliable checkis touch the ant with a steel screw driver. it should make a very distinct audible click. (I know it is not connected except for the gimmick cap but just try it. ) If there is no click the problem is most likey too much gimick cap coupling. That will move the tube beyond good sensitivity.) I now use this all the time to test and adjust. If the coupling is too tight that paert of the circuit wont work right and if you do hear any signal it will be very quiet even though you may have your sig gen smokin. I spent about three weeks working through an old circuit trying eveything and thinking like you "it should work ,it used to" trying all different tubes. Oh i know it is too simple and I know I may be wrong but what if I am right? I am right there with you. i have felt the frustration and annoyance of building something as close to the original as possible only to have it not work. Probably absolutley nothing wrong with what you did. It is the stuff like this, the little stuff no onetels you that vcan drive you nuts. Your success is my only wish.
Don VE3LYX

Hmmm,

It sounds like a tracking problem to me as well. But since you state that it is not a tracking problem I'll will take your word for it.

The only other thing that I can think of that will give you a conversion LOSS instead of a conversion GAIN, would be that you somehow have the wrong LO injection level(power).

In that case you might try verifying/changing the "grid leak" resistor of the LO or some other way to vary the oscillator output level.

Good Luck
kb0lxy

I had to give the benefit of the doubt to the original designer of the circuit, but I agree with Exray, that having a high positive voltage on the suppressor is operating the tube way out of its normal range. We discussed that in the other thread. I wonder if there may have been a manufacturing change in 6C6 tubes sometime after that circuit was invented that would have rendered this arrangement unworkable.

I'd be inclined to add a capacitor between the plate of the oscillator tube and the suppressor (100 pF or less), and then connect a resistor (10 - 50 k) from the suppressor to ground, which would bias the suppressor back to where it should be.

At one point I did insert a small cap between the 76 plate and the suppressor grid of the 6C6 (maybe 50 pf) but that didn't help. However, I didn't add a resistor from suppressor to ground, so this is the first thing I'll try today. I'll also play with LO grid leak resistor values and see what difference that might make. Thanks guys!

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Tom

Update: I added a 15 pf cap between the LO plate and the 6C6 suppressor grid, and grounded the suppressor via a 33K resistor. The results were a significant improvement, i.e., the signal from the sig gen was no longer extremely faint. Now it's just at a fairly low volume. However, connected to antenna, it is completely deaf.

I am frustrated enough with it that I'm going to move on to other things for the time being. Looks like this Super Gainer will end up a project that will look pretty on the shelf, but that's it. Thanks again for all your suggestions.

Tom

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Tom

I dont suppose you could find the heart to try my suggestion. You do have after all a regenerative detector and all the quircks they come with.
Don VE3LYX

The "super-gainer" is NOT a "regen set"! It is a "superhet" !

I don't know all of the details of the set, so it MAY be that the detector is regenerative( if L5 were placed inside the IF can for instance).

I am not sure that is the case, and even if it were, it is still a superhet.

Also the problems as described, sounds like a tracking problem!

By making the RF amp/mixer regenerative(to get that "super gain" ), causes the front end selectivity/tuning, variable and sharper than the LO selectivity/tuning.

This further complicates Tracking of the RF tuning with the LO tuning.

Whether or not the LO is on the low side or the High side of the RF, the Difference frequency must be "Exactly (or very very close to ) 455kHz".

If it is off by only a little( a few kHz), then the IF can will filter it out causing exactly the described problem. Everything working separately but very little to no IF, getting thru the "can"! This would be especially true if indeed the IF/Detector section is made regenerative, because the detector selectivity would be VERY sharp. The schematic appears to show a "selectivity control" potentiometer there.

A selective level meter or a spectrum analyzer will tell just what IF frequency IS being produced, and if the IF frequency stays the same when the set is tuned thru the band.

Without that equipment the only thing I can think of to try is to use separate tuning caps on the RF and LO and see if an I.F. can be produced that will go thru the can.

73
kb0lxy

Here's the one I had.



I didn't design it or make the drawing so don't blame me! It came with the radio. And it was an accurate schematic reflecting how the radio was wired that some third party had drawn up. I have no idea if the radio ever worked.

I tried everything in the book to make it work. And it "sorta did". The best attempt was taking the B+ away from the suppressor.

Back Story - About the same time I was working on a Hallicrafters SX-140. It uses a regenerative IF for "Q-multiplier and BFO" and the "regeneration" control is a pot to the suppressor that provides a couple of volts of variation. 6BA6. It dawned on me that if a couple of volts difference on the 6BA6 could transform it from a well-behaved Q-Mult to a well-behaved BFO why the heck would a design use full B+ on the suppressor.

Long story short...since the old dawg had the 'infrastructure' for an AA5/6 set I just rebuilt it to a different (conventional) topology and it works great.

I did a bit of research on this thing a couple of days ago. I wasn't able to locate the write up on the original, but found the circuit and write up for the "Improved Super-Gainer." Both the RF and Detector stages are regenerative. The pot that is in parallel with L5 is the regen control for the detector. I originally thought that it was some sort of oddball volume control. The write up for the Improved Super-Gainer says that the coil is not magnetically coupled to the IF transformer. So, maybe the detector feedback is due to Miller capacitance in the detector triode, and the L5/10k pot combination simply shunts some of that feedback signal to ground. The coil winding instructions in the later circuit call for 35 turns of #26 DCC jumble wound on a 1/2" porcelain insulator. With no specified winding length, the resulting inductance could vary significantly.

Now that brings up another thought. Since the RF stage is also regenerative, I have to ask whether the leads may need to be reversed on the tickler winding L3.

In any event, the circuit for the improved Super-Gainer, dumped the 6C6/76 combo for a 6J8G converter.

I am also a bit puzzled about B+ DC on suppressor grid in the original design. After reading Joe Souza posts at radiomuseum, it looks like normal pentodes actually need negative DC on the suppressor grid for meaningful suppressor control operation:
http://www.radiomuseum.org/forum/dual_c ... todes.html

My suggestion would be to use a dedicated dual control pentode like 6AS6 with zero or slightly negative DC level on the suppressor.