Bell amps ground hum

[Randall]

With the variac dialed back to put 6.3vac on the filaments, the B+ is 331v. All tubes are American made, I swapped them from amp to amp with no difference. I will try clipping in a test cap next.

[Leo_Gnardo]

With the variac dialed back to put 6.3vac on the filaments, the B+ is 331v. All tubes are American made, I swapped them from amp to amp with no difference. I will try clipping in a test cap next.

B+ sounds comfortable for 6V6's. You could always select higher value dropping resistors in the hi voltage chain to bring the voltage to spec'd values in the preamp, no harm in that.

[Randall]

Clipping a test cap across each cap makes no difference.

[Leo_Gnardo]

Drat, I'm running out of tricks. Is there any reason Bells would need to be turned up to 7 - 8 out of 10 where this motorboat effect starts up? If you're well into clipping before that point, you could pad the volume controls to stay out of the trouble zone.

[Jazz P Bass]

Have you replaced all of the power supply caps?

The ones further down the preamp chain are considered decoupling caps.

If not that, I vote a bad interstage coupling cap.

[Randall]

Interestingly, the Bells as they are now are fed directly from the mag cartridge, so there is no clipping going on on the inputs. I believe there should be an RIAA pre-amp between the turntable and the 12AX7s, but I'm not 100% on this. If so, I think this should be enough extra juice to where the volumes could be run at lower levels. At least I hope so. I don't want to spend the rest of my days on these, helping out a buddy. Still would like to know why it is oscillating, tho. I too, am running out of rope here.

[Leo_Gnardo]

Interestingly, the Bells as they are now are fed directly from the mag cartridge, so there is no clipping going on on the inputs. I believe there should be an RIAA pre-amp between the turntable and the 12AX7s, but I'm not 100% on this. If so, I think this should be enough extra juice to where the volumes could be run at lower levels. At least I hope so. I don't want to spend the rest of my days on these, helping out a buddy. Still would like to know why it is oscillating, tho. I too, am running out of rope here.

I can see what looks like a passive RIAA sort-of EQ following the 1st stage 12AX7 triode. You've replaced the cathode bypass caps in the preamp too? 2nd & 3rd stage triodes share a cathode RC pair. If that cap is defective you'd have unexpected feedback thru the common cathode resistor. If that turns out OK, I can only recommend 2 things in general, less low frequency response: cut the values of cathode bypass caps, reduce value of interstage caps. And/or limit the volume control with a pad resistor to stay out of the trouble zone.

If there was no RIAA or similar eq built in, your magnetic cartridge would sound super tinny, all screeching highs & no bass. Have you played a record thru this rig, did it sound that bad? If it sounded reasonably good, you may well not need an external RIAA pre. And besides it looks like there's plenty of gain in the Bell to me.

Hold on cap'n, I sees sumpin' weird: take a look at the 3rd stage triode's grid, I don't see any grid leak resistor! Neither do I see one on the 4th triode, marked 2nd AF AMP - PHASE INV. Hm, if that's really the case, you could expect instability. As for the 3rd stage, if you put a shorting plug into the crystal cart or radio inputs, the resistors leading to those jacks would serve as grid leaks. As for the 4th triode, there just isn't one, how they get away with that? Slap a 1M resistor across that grid (marked NO 5 OR = orange?) to ground, see wha happens. Also wouldn't hurt to put a shorting plug into unused crystal & radio inputs, then triode 3 will have a resistive path from ground for its grid.

[Randall]

It does not sound super tinny with a nice turntable. And it does the oscillation even with the 12AX7 missing. So it's not that stage. What would make the plate voltages swing that hard?

[Leo_Gnardo]

It does not sound super tinny with a nice turntable. And it does the oscillation even with the 12AX7 missing. So it's not that stage. What would make the plate voltages swing that hard?

It has more to do with the cartridge than the table, but your report is useful: if it sounds good, it IS good. And likely does not need an additional RIAA pre. It's easy enough to find a schemo for a passive RIAA and compare that to the filter that follows your 12AX7 triode, I think you'll find them similar.

Check paragraph 4: in the next stages there's a couple of grids with no leak resistor. You could verify that with an ohmmeter quick enough, but what I see in the schematic is alarming. Also, the shared cathode resistor/cap in the 2nd & 3rd stages, reminds me a bit of similar shared Rk/Ck in Fenders. If cascaded stages have this setup, and the cap goes bad or gets disconnected, you get some strange behavior - could be the same here. Please check that bypass cap & its associated 2200 ohm R, you can clip a cap on there as you did for the power supply ones. And let's find out whether there's some hidden grid leak R's that I'm not seeing on the schematic. One quick check, you could remove the 2nd tube 6SL7 and see if the wobble stops.

What a long fezzle. I just had a BA115 Ampeg in here, same thing, the repair party never ends. "How much can go wrong?" Oh crap, there's another one demanding my attention.

[Randall]

Ding Ding Ding, we have a winner! Leo, I grounded the radio input and it stopped oscillating. Man, what a slog that was! Now I have to figure out why now this amp is way louder than the other amp, and why some of the hum returned. Everything I measured was within bounds, and almost every component has been replaced.

[Leo_Gnardo]

Ding Ding Ding, we have a winner! Leo, I grounded the radio input and it stopped oscillating. Man, what a slog that was! Now I have to figure out why now this amp is way louder than the other amp, and why some of the hum returned. Everything I measured was within bounds, and almost every component has been replaced.

Yay! Did you ground the radio input on the other amp too? Now it's down to "what's wrong with this picture?" One must be different from t'other in some way. That dangling grid on the next stage still gives me the heebie jeebies.

You would think - a pro manufacturer like Bell would have seen to this, even if they put in a very high value resistor for the "gone to hell" situation of having no resistive ground path to control the grids. I guess they assumed the radio input would be plugged into the output of a - radio - or other device that had a resistive path between ground and signal line. Like maybe a volume pot at the output. Crystal cartridge input? Couldn't expect much of a resistive path there, crystals generally have super high resistances more or less infinite.

I see our fellow MEFster eneumann has as his byline Hofstadter's law. It definitely applies in this case: It always takes longer than you expect, even when you take into account Hofstadter's Law.

[Chuck H]

Ding Ding Ding, we have a winner! Leo, I grounded the radio input and it stopped oscillating. Man, what a slog that was! Now I have to figure out why now this amp is way louder than the other amp, and why some of the hum returned. Everything I measured was within bounds, and almost every component has been replaced.

The subject amp may be louder than the other AND have more hum simply because of higher gain. There may have been some leakage of signal at any shared components (especially cathode circuits). Did you replace ALL the electrolytic in the amp, or just the power suppply caps? Sort of skimmed, but if it's fixed bias you'll definitely want to address the bias filters and for my money, in any amp that age, any other electrolytic for sure.

You may still have a hum issue that wasn't apparent before you "fixed" the signal bleed issue on the test amp. That still may be improved by replacing the cathode bypass caps.

[Jazz P Bass]

My vote is on 'coupling' caps.

[Randall]

Pretty much all components have been replaced on both amps, and they were close in output with a sine wave on a scope before I started unsoldering and measuring. It seems to me the instability was around V2. I checked everything that was suggested and all seems well. Lifting the .05 C8 made the wobble stop, and I read no leakage there. The cathode bypass R and C look good on V2 and V3. I did swap some tubes between amps, maybe there is a gain issue with one of them?

[g1]

Try swapping the whole lot of tubes from one amp to another. Then you will know if it's a tube or not.
Also, if you have an idea of what size input signal the amps expect, try to guess whether one amp is weak, or is one amp too 'hot' ?