Bad Reverb Transformer?

[PfeifferElectronics]

I just finished building a Fender Pro-Reverb based on the AB763 circuit. It works and I can hear some good tone but there is a loud hum when I take it off stand-by. The tremolo intensity knob has a switch and when I turn on the tremolo, the hum gets quieter but is still there. volume knob on trem channel also increases/decreases ampliltude of hum. I removed all the tubes and tested them, also tested the filter caps with ESR meter and everything checks out. I installed the tubes starting with the rectifier and working my way towards the preamps. The amp was quiet until I installed the reverb return tube. So I started checking the reverb transformer with the driving tube removed (12AT7) and found that the resistance to ground from the red and blue winding was changing. It starts at 30M and slowly falls to 100k then moves up again. I'm thinking this winding is shorted. Ideas?

Thanks guys. I love this forum!

[g1]

The reverb transformer goes to the power supply. So you are measuring the supply to ground, probably a cap charging and discharging via your meter.
Is the hum there with the reverb pot turned off? Have you tried swapping the reverb return tube? A tester sure won't show whether a tube has hum or not.

[PfeifferElectronics]

There is hum with reverb all the way down and when I turn the reverb pot up the hum gets louder. I swapped out both reverb tubes and still get the hum. It isn't a minor hum... it is pretty loud like something is grounded that shouldn't be?

Thanks

[PfeifferElectronics]

Still working on this one. I have narrowed it down to the 12AX7 on the reverb recovery circuit. I have a JJ ECC83S in this position. There is a 250mV positive-only ac signal on pin 1. Would that be a positive only duty cycle? Anyway, I have tried different tubes, replaced the socket, replaced the .0033 cap, replaced the 100K resistor coming from D voltage. I don't get the signal on pin 2. Voltages are as follows:

Pin 1 = 238V, Pin 2 = .2mV, Pin 3 = 1.8V, Pin 6 = 237V, Pin 7 = 3mV (goes up to 5mV when I turn up the reverb level pot).

If I remove this tube (V4), the hum goes away, all other tubes are installed. Please help, I've run out of ideas.

Thanks

[Chuck H]

The reverb pan has a straight shot to the recovery stage. If your shield isn't grounded you may get noise and hum. Also, the reverb pan chassis and cable shields should only be grounded at one location. Otherwise you may get a ground loop. Also, your reverb pan orientation can effect hum.

The pot switch for the trem engages a ground that is affecting the problem. I've seen this in other circuits. Where grounding a ground loop can quiet it a little. Reverbs require huge amounts of gain because the pan output is only about 6mV. So avoiding ground loops becomes critical in preventing hum. Check all your grounds for the reverb circuit and be sure there is only one path to ground for the input side of the tank, and only one, but a different path for the return circuit. And be sure the pan chassis and cable shields are only grounded at one or the other location. Not both.

[PfeifferElectronics]

Success! I had two paths to ground from my filter caps. Thanks guys!

[Dandrix]

hi guys

I have the same problem going on with my new fender new build clone. hum with the reverb on zero as I turn it up it get louder. can you break this down for me not as advance as most of you guys on the forum. thanks

12at7:
1:384v 2:0v 3:7.3v 4/5: 3.1vac 6:384v 7:0v 8:7.2v 9:3.3vac
7025:
1:211v 2:0v 3:1.7v 4/5:3.1vac 6:206v 7:hum 0v 8:1.7v 9:3.3vac

when I put my probe down on pin 7 the hum gets louder or if I probe 470k point from pin7 the hum is louder.
dan

Success! I had two paths to ground from my filter caps. Thanks guys!

[PfeifferElectronics]

Sure looks like the same problem I just had. What circuit did you build? Was it the same as mine; a blackface pro-reverb (AB763)?

[Dandrix]

its a fender tweed tremolux 5g9 circuit.

[Chuck H]

Your voltages are fine. You probably have a similar issue. A ground loop. Something is grounded twice. This could even mean that two (or more) parts of the same circuit (in between DC blocking caps) are grounded in different locations. For example... Even though your RCA jacks are probably close together, if they're chassis mounted it's best to isolate the sleeve on one of them. Otherwise the shield and chassis are grounded at one end, and then again at the other. It gets a lot more complicated too if you have isolated jacks on the pan. What kind of pan are you using? MOD, Belton or Accutronics/Belton? The MOD tanks have adjustable jack isolation from what I've read. This could be helpful. Also, don't neglect to try the pan in the opposite orientation it is in now.

[Chuck H]

How did you add reverb to a 5G9 if your not experienced?!? Do you have a schematic of what you built?

[PfeifferElectronics]

A schematic would be helpful as the 5G9 schematic that I found on the internet does not have reverb.

[Dandrix]

accutronics &belton 4ab3c1b, I used a super reverb circuitry for reverb.

Even though your RCA jacks are probably close together

2inches apart, how do I isolate the sleeve?

[PfeifferElectronics]

Have you insured that your large capacitors have only one path to ground?

[Dandrix]

pin7 of reverb of 12at7 to .1uf of (5g9) cap

Y of super reverb circuit to .02uf cap of (5g9) cap

lastly the "guess" Z of super reverb circuit to .1uf again of (5g9) cap. yes, again. wasn't many options left.