Gibson GA-15 RVT Noisy Reverb Recovery

[g1]

Yes, but when you run out of things to try, even triple checking is worth it to eliminate things. I was pretty much convinced the tank was ruled out when you grounded the grid of the recovery tube and the hum remained. But ground loops and hum can be very difficult.

I'm not sure if we've talked about the footswitch and whether it's always connected? When you did that ground the grid check, was the footswitch disconnected?

[bobloblaws]

Yes, but when you run out of things to try, even triple checking is worth it to eliminate things. I was pretty much convinced the tank was ruled out when you grounded the grid of the recovery tube and the hum remained. But ground loops and hum can be very difficult.

I'm not sure if we've talked about the footswitch and whether it's always connected? When you did that ground the grid check, was the footswitch disconnected?

I can't remember the first time, probably, but I just tried it again, in both cases, with footswitch connected or disconnected, grounding the grid has no effect on the hum that is present when the reverb signal is not grounded at C15/R15. It stands to reason I would think as there is no practical difference between the footswitch cable being unplugged and it being plugged but not grounding C15/R15.

[Helmholtz]

surely we've ruled that out as a possibility by unplugging the input cable and observing no change in the hum in question? I asked that same question in #24 and I know you have been skeptical yourself that it has anything to do with the tank.

Agree. As you already tried shorting the grid of the recovery tube, we also know that the hum doesn't enter this tube via the grid.

[bobloblaws]

Agree. As you already tried shorting the grid of the recovery tube, we also know that the hum doesn't enter this tube via the grid.

Let me throw another angle at you. Other than the hum, the reverb seems to work reasonably Ok. BUT, according to amprepairparts.com the replacement tank for this model amp is a 4FB2A1C, input impedance 1475 ohms, output impedance 2250 ohms. However, on the tank in this particular amp I measure both input and output at about 175 ohms. Could this be a factor?

[The Dude]

You're measuring resistance- not impedance, and your measurement seems reasonable. I don't think that has much to do with it. FWIW, here's a chart that shows DCR for different impedance tanks. Values will be approximate depending on transducer wind, so don't sweat a slight differnence.

https://amprepairparts.com/reverbdcchart.htm

[bobloblaws]

You're measuring resistance- not impedance, and your measurement seems reasonable. I don't think that has much to do with it. FWIW, here's a chart that shows DCR for different impedance tanks. Values will be approximate depending on transducer wind, so don't sweat a slight differnence.

https://amprepairparts.com/reverbdcchart.htm

I realize some tanks have impedances in the range of a couple of hundred ohms. My point is the impedance of this tank is way off the specs of the one that is supposedly a replacement for this amp. I think you'd call it an order of magnitude? Granted, it could still be the case that it has nothing to do with the problem I'm experiencing.

[Helmholtz]

I realize some tanks have impedances in the range of a couple of hundred ohms. My point is the impedance of this tank is way off the specs of the one that is supposedly a replacement for this amp. I think you'd call it an order of magnitude? Granted, it could still be the case that it has nothing to do with the problem I'm experiencing.

How do you know the impedance of your tank?

What the Dude meant is, you can't measure impedance with a DMM/Ohmmeter.
What you measured is the DCR which is always much lower than the impedance. Look at the chart.

I think you have the correct tank.

[g1]

Yes, you need to look at the small print on that chart, it mentions DC resistances. Your measured DC resistance numbers are good for the impedances given.

[bobloblaws]

Yes, you need to look at the small print on that chart, it mentions DC resistances.

Oops, I missed that. I thought it was similar to measuring impedance of a speaker with a DMM ohmmeter. The DC resistance typically shows as less than the rated impedance but is pretty close.

[Helmholtz]

I thought it was similar to measuring impedance of a speaker with a DMM ohmmeter. The DC resistance typically shows as less than the rated impedance but is pretty close.

Speaker impedance is only slightly inductive at 1kHz, while reverb transducer impedance is almost completely reactive/inductive. Reverb transducers are essentially inductors.

A wrong impedance tank would cause weak reverb signal. Certainly would not cause increased hum/noise.

[The Dude]

^^^^^^ Yes that. I'll add: The ratio of impedance to resistance is not linear. More winds will generally have higher impedance because of coil interaction, but not effect DCR much. There isn't much difference in resistance of 1' of wire compared to 50' of wire.

[bobloblaws]

Ok, so we've ruled out the tank for sure I think and there is no difference if footswitch is connected or not, bottom line is C15/R15 is not grounded there is significant noise added to the main signal. What else could it be? I've replaced C15, R15, R27, and R25. I don't think it's a bad 6EU7 tube as swapping V1 and V2 does not change the scenario, but I'm almost ready to buy a couple of replacements just so I can rule the possibility that both of them are flakey on the B side. Any opinions on that idea? They $30 USD each, that's quite dear for a preamp tube.

[bobloblaws]

Hey guys, I needed a break from this amp but I'm ready to give it another go. I ended up buying one new 6EU7 to try in V1 just to be able to rule that out conclusively. As you probably expected, no change. A pronounced hum when un-grounding the C15/R15 junction.

So I'm humbly appealing again for any suggestions.

One thing I've wondered , is it possible that it's inherent in the design? The hum is unacceptably loud so it's seems hard to believe this and other amps of this model always have that hum when the reverb return is not grounded. On the other hand, there are not a whole lot of variables and we've investigated everything I can imagine already. If you look at a typical Fender amp from this era the design is somewhat different with the reverb level pot being on the return side as opposed to the send side. Even still, other than that there isn't that much of a difference and I wouldn't expect an unusable hum even if the reverb pot was dimed on a Fender. I recently sold my Twin Reverb so I can't play around with that circuit and compare.