Fender Twin Reverb hum at mid volume

[mhamilton]

Working on a 1967 Twin Reverb for a friend, he complained of too much hum. I found that someone messed around with this amp before--replaced a lot of resistors and some caps ('80s vintage filter caps). Lots of non-factory stuff that is going back to original. 220mfd and 80mfd filters going back to 70mfd and 20, etc.

It has this very annoying hum only at mid volume levels, much worse on the reverb channel. Silent at low volume, and only white noise at top volume.

I eliminated a lot of it by putting back the original wiring from the input jacks to the preamps, and dressing the leads as per Fender. Most of the time I have the reverb tank disconnected on the bench, but doesn't change the hum to install in the case and connect the tank. All reverb and vibrato controls are set to zero.

I found by accident that if I put in a shorted RCA in either the reverb or the vibrato jacks that it becomes completely silent. Is the footswitch required for normal operation? Still would have a hum issue with reverb even if that is the case. Or is this more likely an issue with the switch jacks themselves?

Also, this amp had 100 ohm resistors from the heater winding to ground--but if the txf has a center tap should I remove those?

Thanks for any help,
-Michael

[Firestorm]

Working on a 1967 Twin Reverb for a friend, he complained of too much hum. I found that someone messed around with this amp before--replaced a lot of resistors and some caps ('80s vintage filter caps). Lots of non-factory stuff that is going back to original. 220mfd and 80mfd filters going back to 70mfd and 20, etc.

It has this very annoying hum only at mid volume levels, much worse on the reverb channel. Silent at low volume, and only white noise at top volume.

I eliminated a lot of it by putting back the original wiring from the input jacks to the preamps, and dressing the leads as per Fender. Most of the time I have the reverb tank disconnected on the bench, but doesn't change the hum to install in the case and connect the tank. All reverb and vibrato controls are set to zero.

I found by accident that if I put in a shorted RCA in either the reverb or the vibrato jacks that it becomes completely silent. Is the footswitch required for normal operation? Still would have a hum issue with reverb even if that is the case. Or is this more likely an issue with the switch jacks themselves?

Also, this amp had 100 ohm resistors from the heater winding to ground--but if the txf has a center tap should I remove those?

Thanks for any help,
-Michael

You're right to put the preamp filters back to 20uF, but don't bee too quick to go back to the 70uFs in the main filter. Parralleled, that gives you only 35uF (less than in a Deluxe Reverb) which is not enough to surpress 120Hz hum at volume. 220s may be a bit too "tight." You might try 100uF (giving you 50uF).

Check all of your grounds; poor grounds can induce hum in sensitive circuits. You may have a hummy preamp tube, so swap known good ones in to see.

Footswitch is required for the tremelo to work; reverb works with out. When you put a shorted RCA plug into the trem, you are turning it on. Odd that it would affect the hum. Reverb is much more sensitive since the grid of the recovery tube is connected to that jack. I can see how shorting that would quiet the hum. Check the jack grounds, too.

WRT the heater circuit, use either the CT or the 100Rs, not both. The resitors often give better hum cancellation than the CT.

[mhamilton]

You're right to put the preamp filters back to 20uF, but don't bee too quick to go back to the 70uFs in the main filter. Parralleled, that gives you only 35uF (less than in a Deluxe Reverb) which is not enough to surpress 120Hz hum at volume. 220s may be a bit too "tight." You might try 100uF (giving you 50uF).

Check all of your grounds; poor grounds can induce hum in sensitive circuits. You may have a hummy preamp tube, so swap known good ones in to see.

Footswitch is required for the tremelo to work; reverb works with out. When you put a shorted RCA plug into the trem, you are turning it on. Odd that it would affect the hum. Reverb is much more sensitive since the grid of the recovery tube is connected to that jack. I can see how shorting that would quiet the hum. Check the jack grounds, too.

WRT the heater circuit, use either the CT or the 100Rs, not both. The resitors often give better hum cancellation than the CT.

Thank you!

I will go with the 100uf filters in series, that seems like a reasonable increase in capacitance to me. I also won't go too crazy with the hum chasing until I put the correct new filters in place, as the '80s replacements test as slightly leaky. They do reform, but not giving me confidence that they are reliable.

I will check on that jack ground. I didn't realize the tremolo worked when the switch was grounded, so that won't be affecting anything when it's disconnected. I'll definitely check on that jack ground and the other grounds.

How does the copper ground strip ground to the chassis? Is that soldered through? Or is it have a mechanical bond?

I will also check on the heater CT. If I lift the heater CT and the hum cancellation improves, should I just tie off that CT and leave it disconnected? Or stick with original and remove the resistors?

Another question: the cathode bias for the 7025 2nd half is shown as 820 ohm--I found a 1.8k ohm in place. It looked like a factory resistor, any idea if that value was changed? Measured out at like 1.2k ohm, so still well outside of spec for either value. I put in an 820 to test and did get the listed bias voltage of +2vdc.

[Milkmansound]

The 820 is for a shared ground for the two input stages. See if one goes to another ground tab - they might have raised the value because only one tube is grounded there now

Also, I like 2 80uf in series for a twin. It's stiff but not too stiff.

If there is noise you may want to check the 3m3 reverb mixer/divider. Usually that carbon comp resistor is noisy as hell! It can create a nightmare scenario - I use a carbon metal film in that spot and that spot only.

[mhamilton]

Well, I think I got this one wrapped up. Turned out the hum was mostly from a lead dress issue on the reverb channel, on the input from the volume wiper to the preamp. Changing the lead dressing while the amp was running to minimize hum worked, but still wasn't perfect. Found I had more hum when back in the case. Finally realized the metal screen shield on the top of the case was not grounded without the chassis tight, so I added an additional ground wire from the screen to chassis and now it is silent.

Changed the p/s caps to 100uf/series and 22uf. All the film caps got replaced, along with the bad preamp cathode resistor. The 3.3M resistor was already changed to a newer type by a previous tech.

Found that the heater winding had no center tap to ground. So left those 100 ohm resistor in place.

Adjusted the bias to -52v and all seems to be working well! Very nice sounding amp, I think my friend will be very pleased to have this working like new again

Thanks again for the help!
-Michael