I’m about at the end of my rope trying to fix a diode-distortion problem with a Silvertone Model 4611 (Chassis 101.487, 1937) and hoping that I may get some good advice.
In the process of restoring this nice-looking radio (tube complement:6A8, 6K7, 6B6, 25L6, 6X5, 6U5), I changed the detection wiring scheme to use the 6B6 high-mu duo-diode triode as a simple diode detector, with the diodes wired together. I did this because I couldn’t get it to work using the original schematic which seemed a bit screwy to me (maybe a mistake but I don’t think so). Anyway, it plays ok except for some minor, though irritating, distortion coming out of the loudspeaker. I recapped the lot (even ceramics) and re-resistorized all. Still distorts. I tracked the problem down to the 6B6 tube. Made substitutes, including a 6Q7…same problem. Played my CD player through the grid of the 6B6…no distortion. Also no distortion on the SW bands. From what I read in my Radiotrons (3,4)…problem could be with residual volume effect or “playthrough” via diode-plate capacitance. Seemed plausible since I couldn’t turn volume off completely. Well…when I disconnect Pin#4 diode and use only Pin#5 diode, distortions go down but so does signal. Reversing the arrangement causes more distortion (and lower signal). Other problem I read about could be improper “diode-loading” with high modulation AM stations. I have a 2M volume pot, a 47K diode load resistor and a 0.05 coupling cap+12Meg grid resistor. The volume pot + diode load resistor goes to ground as does the cathode of the 6B6. AVC combo is 1Meg + 0.05 cap. I’ve changed the 2-IF filter caps from 180pf to 68pf/each with slightly better sound with the 68pf combo. I should be using 100pf-caps here but I don’t have any. If I leave the 25L6 unbypassed, radio sounds better but a major loss of throughput. If I disconnect the eye-tube, no difference. Right now, I’m contemplating some delayed-AVC circuits and if that doesn’t work….a 1N34A germanium diode. For the life of me, I cannot figure why this detection circuitry distorts. I’ve got other radios using 75, 6SQ7, 6Q7 duo-diode triodes with this simple scheme and they play OK. What gives?

Hi Rick. You seem to be getting contradictions here. It sounds loke what you've done should work. The fact that it works Ok on short wave indicates it might be a problem in the RF stages, though the signal strength might be different. Try tacking in a 1n34 detector circuit and see if it now works. If it does it indicates the problem really is in the detector stage, if it's still there lokk at the front end. Have you tried a new converter tube? Don.
:
I’m about at the end of my rope trying to fix a diode-distortion problem with a Silvertone Model 4611 (Chassis 101.487, 1937) and hoping that I may get some good advice.
: In the process of restoring this nice-looking radio (tube complement:6A8, 6K7, 6B6, 25L6, 6X5, 6U5), I changed the detection wiring scheme to use the 6B6 high-mu duo-diode triode as a simple diode detector, with the diodes wired together. I did this because I couldn’t get it to work using the original schematic which seemed a bit screwy to me (maybe a mistake but I don’t think so). Anyway, it plays ok except for some minor, though irritating, distortion coming out of the loudspeaker. I recapped the lot (even ceramics) and re-resistorized all. Still distorts. I tracked the problem down to the 6B6 tube. Made substitutes, including a 6Q7…same problem. Played my CD player through the grid of the 6B6…no distortion. Also no distortion on the SW bands. From what I read in my Radiotrons (3,4)…problem could be with residual volume effect or “playthrough” via diode-plate capacitance. Seemed plausible since I couldn’t turn volume off completely. Well…when I disconnect Pin#4 diode and use only Pin#5 diode, distortions go down but so does signal. Reversing the arrangement causes more distortion (and lower signal). Other problem I read about could be improper “diode-loading” with high modulation AM stations. I have a 2M volume pot, a 47K diode load resistor and a 0.05 coupling cap+12Meg grid resistor. The volume pot + diode load resistor goes to ground as does the cathode of the 6B6. AVC combo is 1Meg + 0.05 cap. I’ve changed the 2-IF filter caps from 180pf to 68pf/each with slightly better sound with the 68pf combo. I should be using 100pf-caps here but I don’t have any. If I leave the 25L6 unbypassed, radio sounds better but a major loss of throughput. If I disconnect the eye-tube, no difference. Right now, I’m contemplating some delayed-AVC circuits and if that doesn’t work….a 1N34A germanium diode. For the life of me, I cannot figure why this detection circuitry distorts. I’ve got other radios using 75, 6SQ7, 6Q7 duo-diode triodes with this simple scheme and they play OK. What gives?

Thanks Don:

Actually, I did try a 1N34 with some jumper wires. Sound improved a lot. I'm hesitant to go that route though since that would be a bit like "cheating". It will be my last resort though. Being an engineer, I'm determined to get to the bottom of this little diode "problem". I've also seen same distortion with a 6Q7 substitute tube. Also when I turn volume down really low, distortion is really evident. Not so with my other radios. Drives me crazy.

: Hi Rick. You seem to be getting contradictions here. It sounds loke what you've done should work. The fact that it works Ok on short wave indicates it might be a problem in the RF stages, though the signal strength might be different. Try tacking in a 1n34 detector circuit and see if it now works. If it does it indicates the problem really is in the detector stage, if it's still there lokk at the front end. Have you tried a new converter tube? Don.

Hi Rich

Here is something to check, the coupling cap to the 6B6 grid could be leaky allowing AVC voltage to put the tube into cutoff, causing distortion. For a quick test try a 6R7. This tube has lower gain but will not go into cutoff as easy.

If the coupling cap doesn't take care of the problem let us know.

Norm

: I’m about at the end of my rope trying to fix a diode-distortion problem with a Silvertone Model 4611 (Chassis 101.487, 1937) and hoping that I may get some good advice.
: In the process of restoring this nice-looking radio (tube complement:6A8, 6K7, 6B6, 25L6, 6X5, 6U5), I changed the detection wiring scheme to use the 6B6 high-mu duo-diode triode as a simple diode detector, with the diodes wired together. I did this because I couldn’t get it to work using the original schematic which seemed a bit screwy to me (maybe a mistake but I don’t think so). Anyway, it plays ok except for some minor, though irritating, distortion coming out of the loudspeaker. I recapped the lot (even ceramics) and re-resistorized all. Still distorts. I tracked the problem down to the 6B6 tube. Made substitutes, including a 6Q7…same problem. Played my CD player through the grid of the 6B6…no distortion. Also no distortion on the SW bands. From what I read in my Radiotrons (3,4)…problem could be with residual volume effect or “playthrough” via diode-plate capacitance. Seemed plausible since I couldn’t turn volume off completely. Well…when I disconnect Pin#4 diode and use only Pin#5 diode, distortions go down but so does signal. Reversing the arrangement causes more distortion (and lower signal). Other problem I read about could be improper “diode-loading” with high modulation AM stations. I have a 2M volume pot, a 47K diode load resistor and a 0.05 coupling cap+12Meg grid resistor. The volume pot + diode load resistor goes to ground as does the cathode of the 6B6. AVC combo is 1Meg + 0.05 cap. I’ve changed the 2-IF filter caps from 180pf to 68pf/each with slightly better sound with the 68pf combo. I should be using 100pf-caps here but I don’t have any. If I leave the 25L6 unbypassed, radio sounds better but a major loss of throughput. If I disconnect the eye-tube, no difference. Right now, I’m contemplating some delayed-AVC circuits and if that doesn’t work….a 1N34A germanium diode. For the life of me, I cannot figure why this detection circuitry distorts. I’ve got other radios using 75, 6SQ7, 6Q7 duo-diode triodes with this simple scheme and they play OK. What gives?

Thanks Norm:

I'll try a 6R7 if I can get one. Base diagrams look identical. I'll also try a 5Meg grid resistor on the 6B6. As for a 0.001 coupling cap...first thing I tried. No change. I'll keep plugging away though. Maybe try another one tonight (I'm at work now). What's really peculiar is the difference between which particular diode you use from this tube(6B6). Also...both of my stock of 6B6's seem to test OK on my I-177B.

: Hi Rich

: Here is something to check, the coupling cap to the 6B6 grid could be leaky allowing AVC voltage to put the tube into cutoff, causing distortion. For a quick test try a 6R7. This tube has lower gain but will not go into cutoff as easy.

: If the coupling cap doesn't take care of the problem let us know.

: Norm

Rich

I wonder if there could be leakage in the socket itself? With the wires removed you could try measuring leakage between pins. There should be no difference between diodes.

Norm

: Thanks Norm:

: I'll try a 6R7 if I can get one. Base diagrams look identical. I'll also try a 5Meg grid resistor on the 6B6. As for a 0.001 coupling cap...first thing I tried. No change. I'll keep plugging away though. Maybe try another one tonight (I'm at work now). What's really peculiar is the difference between which particular diode you use from this tube(6B6). Also...both of my stock of 6B6's seem to test OK on my I-177B.

: : Hi Rich

: : Here is something to check, the coupling cap to the 6B6 grid could be leaky allowing AVC voltage to put the tube into cutoff, causing distortion. For a quick test try a 6R7. This tube has lower gain but will not go into cutoff as easy.

: : If the coupling cap doesn't take care of the problem let us know.

: : Norm

I have had similiar problems with some of my radios. Latest example is an RCA 87T1. This radio worked nicely when I got it, but it had been heavily modified. For example the 6H6 detector and 6F5 audio amp had been replaced by a 6SQ7 double diode-triode. So I decided to restore the set to the original condition as much as possible. Most of it went well, but the detector now distorts! The distortion is highly noticeable at all times, but especially during short wave operation with strong fading, as the signals fades out the audio drops out. Really like something was running class-C!
I went forth and back several times, swapping the diodes in the 6SQ7 for the 6H6 with no other mods, and clearly the 6SQ7 works well while the 6H6 does not. Yes, I tried another 6H6 from another manufacturer. Same thing.

I guess I know why this radio was modified! But I still don't know why some diodes distort so much and others don't.

I have not yet given up. Investigation continues.

By the way, as a fun project I'm building a 9-tube radio around glass octals. I had lots of problems with the detector too. One diode simply failed to detect at all! I could have 5 volt of IF across it, and it didn't rectify at all! In another radio this same tube worked well, and in my project a different tube worked.

Empty state small-signal diodes seem to be funny things!

Enjoy,
Manfred.