Twin Reverb Silverface: Hum in Reverb Loop

[jpglyde]

I have a Fender Twin Reverb on the bench with a hum in the reverb circuit--only audible when reverb knob is turned up. I'm not sure exactly which model the amp is. It is old and seems to match the AA270 schematic.

I've replaced the 12AT7, the power supply caps and checked the pan and cables. The hum seems to be present in the reverb send. I can hear it if I plug the reverb-send signal into an external amplifier. If I ground the signal pin of the out-to-pan plug, it goes away. Otherwise the reverb works fine.

Two things I've noticed: The B power supply (which feeds the 12AT7 through the reverb transformer) has an ac ripple of about 3 volts. It is still present even with the output tubes removed. Is that too much ripple?

Also, the dc resistance of both sides of the reverb transformer seems low. Aren't they specified at about 23k primary and 8 ohms secondary? I'm reading about 1.5k primary and 2 ohms secondary.

What do you think? Bad reverb transformer?

AA270 Schematic: Davidson Amplifier Repair, Nashville, TN : Schematics: Fender: Twin Reverb Amp AA270 Schematic

[Jazz P Bass]

I have seen this on a few Reissue Twins.
But the reverb has to be above '6'.
Never found the cause.
That 3 volt ripple can be ruled out by simply clipping another cap in parallel.

[52 Bill]

That 3 volt ripple can be ruled out by simply clipping another cap in parallel.

He sez he replaced the power supply caps. Could be a bad new cap, but probably not the cause of the problem.

If the hum is in the to the tank drive signal, I doubt that the tank will reproduce it as a hum in the from the tank return signal.

What side of the tank is under the power transformer, input or output? The drive transformer resistance readings sound about right to me. Remember that dc resistance is not the same as the ac impedance.

[Jazz P Bass]

I did read that about changing the caps.
It struck me as odd, 3 Volt ripple.

[nickb]

How about a try lifting (one end) of the 500pf coupling cap to the reverb driver grid to see if it's present on the input? If it goes quiet you've ruled out entire driver stage and pan.

[jpglyde]

I did try a different cap to no avail. I even tried stepping it up from 20uF to 30uF. It reduced the ripple (and the hum) slightly but not significantly.

I didn't take note of the orientation of the pan although I made sure to put it back in the same way it came out. Also, moving the pan or the amp has absolutely no effect on the hum.

[jpglyde]

Tried that. The hum didn't change.

[nickb]

Tried that. The hum didn't change.

I that case I believe it's the ripple on the power rail you are hearing. If you add a filter 470 ohm 5 watt in series and 47uF 450V to 0v in the supply to the reverb driver it will reduce the ripple current in the transformer to 2.5% of the original value.

[diydidi]

Hi. Did you try swopping the two RCA plugs on the tank? Only one needs to be grounded to the tank body.

[Bruce / Mission Amps]

Hi. Did you try swopping the two RCA plugs on the tank? Only one needs to be grounded to the tank body.

Swapping the shielded cable leads will just kill the reverb but the poster might be on to something... it could be a replacement tank with one of the input RCA jacks improperly grounded for this circuit.... seen it myself on old Fender reverb amps in the shop for tune ups... etc.

[jpglyde]

I tried an experiment and moved the transformer power supply connection to the "C" power supply instead of "B". A little less voltage but nearly no ripple and NO HUM. Also, a little less reverb but still way more than most guitar players require.

[Ted]

I've known a dead electrolytic on the reverb recovery tube cathode to cause hum.

[pdf64]

And (with an AB763 type V4 shared cathode) oscillation howl as the reverb level is turned up (due to positive feedback around the cathodes shared by cascading gain stages).
Pete