70’s Fender Pro Reverb - AC noise on output, stumped!

[bw1985]

Hey all,

I’m currently really stumped on an issue with a 70’s Fender Pro Reverb Ultralinear amp where I’m measuring 4Vac across one power cap and another 2Vac across the other.

The amp has been recapped including the bias circuit and caps across the solid state rectifier. Also replaced the diodes just to be sure.

Are there any other sources I should investigate that could be the culprit for this AC voltage? It’s producing a light hum and I’d like to kill it if possible.

Thanks!

[potatofarmer]

That amount of ripple doesn't seem unreasonable - what's your B+ voltage?

Any ripple that makes it to the screens will be amplified. These amps require the bias balance control to null out the ripple noise. You turn the master to 0 then adjust the bias balance "power tubes matching" control for minimum noise, then turn the master to 10 and adjust the humdinger "hum balance control" for minimum noise again.

The other approach to keeping ripple off the screens is what Sunn did - filter the plate and screen supplies through a massive choke.

[Mick Bailey]

+1.

Expect to see around 4v AC on the first PSU node. This is largely cancelled out in the output transformer if the output tubes are balanced/matched. Removing the PI tube will tell you if the noise is coming from the power amp section - the amp should be quiet.

Are your tubes good?

[Enzo]

Filter caps is only ONE source of hum among MANY. DO not assume a humming amp just needs filter caps.

[bw1985]

That amount of ripple doesn't seem unreasonable - what's your B+ voltage?

Any ripple that makes it to the screens will be amplified. These amps require the bias balance control to null out the ripple noise. You turn the master to 0 then adjust the bias balance "power tubes matching" control for minimum noise, then turn the master to 10 and adjust the humdinger "hum balance control" for minimum noise again.

The other approach to keeping ripple off the screens is what Sunn did - filter the plate and screen supplies through a massive choke.

Having the master turned all the way down and adjusting the output tube balance pot still produces the hum on the speakers. I've also tried to pull the phase inverter tube, and the hum is still there. So that would mean it's somewhere in my power supply correct?

What else could be the culprit for this? Should I replace the resistors across the caps? I feel like I'm running out of components to replace here!

[Enzo]

How much ripple is on your bias supply at the grids?

[g1]

Also, you did the output matching adjustment, but you did not say if you did the hum balance. Does the hum balance make a difference? They often get cooked which may add hum.

[Mick Bailey]

Also, a badly mismatched or failing tube may still hum. My usual suspect is tubes first rather than power supply - verify your output tubes are OK.

[Enzo]

Or one side is not conducting, make sure there is B+ on screens of ALL power tubes.

[bw1985]

Also, a badly mismatched or failing tube may still hum. My usual suspect is tubes first rather than power supply - verify your output tubes are OK.

Yes, I've tested the tubes and also swapped for a different pair. The hum is exactly the same.

Adjusting the matching pot, it only makes the hum worse as I turn it up. The humdinger can be adjusted to successfully get rid of the preamp buzz when I adjust it. And pulling the PI tube should confirm it is not in the preamp, correct?

Here are my voltage readings on each tube:

1- -56.7v (touching probe increase hum)
2- 9.6vac
3- 468v
4- 466v
5- -56.8v
6- 469v
7- 5.9vac
8- ground

1- -56.8v (touching probe decreases hum)
2- 9.66vac
3- 464v
4- 465v
5- -56.7v (touching probe decreases hum)
6- 464v
7- 5.9vac
8- ground

I can also see the 4Vac on pins 4&6 of power tubes.

Definitely something weird about the oddly high heater voltage...I wonder where that would be coming from.
Although measuring across 4&5 + 9 on all tubes gives me the 6vac in the range im expecting.

Here are some pics of whats going on:

[The Dude]

Something seems strange. I see a "4, 7, something" (last band is hidden by the wire) resistor from filament to the pilot lamp. I don't see a filament CT wire, so I'm thinking there should be two 100 ohm resistors for an artificial CT. It also looks like one wire of the filament winding is grounded. Can you get us a better picture of just that terminal strip near the lamp or draw up a quick schematic of that area? Something definitely looks funky there. Also post the AC voltage measured directly at and across the filament winding.

[bw1985]

Something seems strange. I see a "4, 7, something" (last band is hidden by the wire) resistor from filament to the pilot lamp. I don't see a filament CT wire, so I'm thinking there should be two 100 ohm resistors for an artificial CT. It also looks like one wire of the filament winding is grounded. Can you get us a better picture of just that terminal strip near the lamp or draw up a quick schematic of that area? Something definitely looks funky there. Also post the AC voltage measured directly at and across the filament winding.

Yes I thought the resistor was strange as well, should I remove it? I'm not with the amp at the moment but can check tomorrow, however I don't believe that the heater is actually tied to ground at that terminal strip.

You are correct, no CT on the filament windings, I did attempt an artificial CT earlier with the 100ohm resistors with no luck. Hum was still present.

I do recall measuring across the heaters at that strip and the voltage was around 6.15v...which seems healthy...totally confused...

[The Dude]

Oh wait, I just noticed this is an ultralinear version. It should then have a hum balance control, which is basically the same as the 100 ohm resistors only adjustable. Make sure that pot is good as g1 suggested. Also, make sure the 4.7 ohm leading to the hum balance is not open. That version also has a 4.7 leading to the lamp, so my apologies for the wild goose chase. To avoid further confusion, here's a schematic for the amp in this thread (I believe).

[g1]

It also looks like one wire of the filament winding is grounded. Can you get us a better picture of just that terminal strip near the lamp or draw up a quick schematic of that area? Something definitely looks funky there.

This is my take as well. It looks like the green wire is going to a terminal that may be grounded (incorrect) and also to something on the master pot/switch which is probably also supposed to be grounded.
The resistor for the lamp looks correct for schematic, check that the hum pot is wired as shown in schematic.

[The Dude]

The other thing I don't see, which is more of a safety issue, is a grounded power cord, although maybe it's just not visible in the photos.