V1 hum in a Pro Junior

[Rhodesplyr]

I've got a Pro Junior that's extremely sensitive to different 12AX7s in the V1 position. Some hum quite a bit, while others are virtually silent. It's the individual tube -- doesn't track with brand.

Here's the problem: The owner is a great player and likes the sound of a GE 12AX7 he's had in it, but the GE tube has gone noisy and microphonic. I have 2-3 GE 12AX7s he could use, but they all introduce too much hum, unlike the previous GE 12AX7, which didn't.

Going on the theory that this is likely heater hum from slight differences in the heaters, will a fix like elevating the heaters to a positive voltage make a Pro Junior more accepting of various 12AX7s? Or maybe a hum balance pot? Or both?

The heater wires in this one are routed away from the other signal wires, and the amp has previously been recapped with F&Ts.

[mozz]

Make sure the heater resistors going to ground are good. I think they are 47 ohms each.

[Rhodesplyr]

Make sure the heater resistors going to ground are good. I think they are 47 ohms each.

Already checked.

[Chuck H]

Before targeting the filament circuit I think it would be a good idea to definitively determine that it is or is not responsible. A lot of tubes hum. Higher gain setting will show this in the first preamp tube with great annoyance. You could temporarily hook the first tubes filament to a 6V battery and that would tell you for certain.

FWIW I find that the Sovtek 12ax7 wa, wb tubes have similar tone and gain characteristics to the old GE tubes. Unfortunately about half of what you buy will also be too noisy for V1 IMHE But it's an option. Buy three and it's likely one will work. Just like the GE they're low in microphonics also.

[Rhodesplyr]

I experimented with it, and tried elevating the heaters to around +33V. This made the Pro Junior much less sensitive to V1 choices in terms of hum, so basically any brand/type is usable.

I added a terminal strip for the divider, and it's done.

[mozz]

Some of these newer Fenders will hum badly if you have the back off. Also, if it has ribbon cables moving them helps, or hurts. Don't move them too much as they like to break just staring at them. I once had to wrap them in copper foil and ground it.

[Rhodesplyr]

People don't seem to want to believe me, but this particular amp was really very sensitive to different 12AX7s, back on or back off. I promise. Scout's honor. With some, it would hardly hum at all, but you had to go through a bunch of them to find one that was acceptable.

Elevating AC heaters to a DC bias is a very old, time-tested technique. The Hammond Organ Co. used it in the late 1930s in their preamps. RCA used it in their mic preamps. Pultec used it in the EQP-1A. I'm aware that it's often not well-understood, but it's not snake-oil. RCA knew what they were doing.

In this amp, the balancing resistors were both 46.5Ω, and I was unable to find any fault that would explain the susceptibility to hum. Google shows that others have had the same problems with V1 and hum in Pro Juniors. No one on here has suggested anything else that worked as a solution. It's possible that the amp needs a hum balance pot on the heaters.

I won't argue with anyone about these amps not being terribly well-designed or built, but people seem to like them, so you have to come up with solutions.

[mozz]

I have one of these, if you look at the schematic you will see no cathode bypass cap on V1, either section. Add a cap on the first section and your hum will go down a lot. You can drop the added gain if it's too much. No need to elevate the heaters, many of us do know that works but i have never used it even on scratch builds. Try small caps or maybe just rewire to fender stock 22uf, try both. Also, one of these fender models has the push pull output tubes filaments out of phase, you need to cut traces to fix that one.

[drewl]

Another common problem is the back panel top middle screw will touch the metal ground/shield plate and cause hum.

Use a shorter screw or washer.
I don't think the back panel screws have washers (cost cutting?) or they were just missing from the ones I've had in for repair.

[Rhodesplyr]

I have one of these, if you look at the schematic you will see no cathode bypass cap on V1, either section. Add a cap on the first section and your hum will go down a lot. You can drop the added gain if it's too much.

This one is a Pro Junior III with a different circuit. It already has the first stage bypassed with a 47uF cap. Much of the objectionable hum was coming from the un-bypassed second stage. You could play around with adding bypasses there, I suppose, but then you'd have to make sure you didn't introduce changes the owner didn't like in terms of how the amp behaves. If he didn't like them, I'd have to open the amp back up and start over.

Elevating the heaters has the benefit of curing the problem without having any effect whatsoever on the audio signal path or gain structure. He likes the amp, just not the noise. He used the amp at a gig last night and emailed me to tell me how pleased he was. I consider that a successful fix.