Low level hum Hot Rod Deluxe

[Tone Meister]

Early USA model Hot Rod Deluxe

Low level hum on clean channel - increases substantially on the "drive" channel. Sounds like 120 Hz to my ears. We're talking nit-picky levels of hum coming from the speaker, but it's enough that it annoys the owner who is a pro player. I can pull any preamp tube (V1, V2, or V3) and the hum disappears completely. Doesn't matter if the volume is all the way down and does not increase when volume is raised.

Amp has all new tubes - power tubes biased at 60 mV per schematic. I've tried swapping in known good preamp tubes one at a time. A fresh set of power tubes reduces hum slightly. Checked the extension speaker jack.

Measure 2.4 VAC across C31. Measure 41 mVAC at pin 5 of V4 power tube and 32 mVAC at pin 5 of V5 power tube.

Ideas?

[The Dude]

Despite your measurements, which don't look bad, I still suspect one of those crappy Illinois caps they use for filtering. If you have a good cap laying around, try jumpering it across each of those 22uf/500V filters, one at a time, and see if that doesn't make the hum go away. If so, replace that cap.

Edit: FYI, you can just clip in with alligator leads for testing. No need to even solder or disassemble until/unless you find a problem.

[nsubulysses]

Are the wires coming off the PT twisted together neatly or is it a big free for all?

[Tone Meister]

Despite your measurements, which don't look bad, I still suspect one of those crappy Illinois caps they use for filtering. If you have a good cap laying around, try jumpering it across each of those 22uf/500V filters, one at a time, and see if that doesn't make the hum go away. If so, replace that cap.

Edit: FYI, you can just clip in with alligator leads for testing. No need to even solder or disassemble until/unless you find a problem.

Failed to mention that I've already clipped an F&T 100uF across every cap, which made no difference.

Are the wires coming off the PT twisted together neatly or is it a big free for all?

First thing I did was clip all the factory wire ties and dress all the PT leads and routed all the heater wires under the tube PCB far out of the way. Also, shaped the ribbon cables in a 90-degree shape, which is something I always do with those cables.

The only thing that makes a real difference is pulling ANY preamp tube or turning off the amp.

[The Dude]

It sounds like you've covered most ideas. The only other thing I recall running into on one of these once involving an odd hum problem was the connections on the lamp assembly. The connection was loose. If I recall a rivet wasn't tight or something of the sort (it's been a while) causing a poor connection and hum. I found it tweaking around with the lamp assembly. The solution was tightening/crimping the rivet and then adding some solder. It's a long shot, but maybe?

[Tone Meister]

Yeah, that's a good point, and I did check the heater voltage there at the lamp and the voltages are pretty even. I'll definitely check that out that bayonet assembly. I have a new one on hand if that turns out to be the culprit. I'm also going take a closer look at the extension speaker jack.

As a reminder, the volume control does not affect the hum, nor does the reverb control. A fresh set of power tubes did reduce the hum some. Pulling ANY single preamp tube completely kills the hum, but replacing ANY preamp tube and the hum is right back. This isn't a major hum, but it is enough to be troublesome when miked up or when recording. The hum comes from the speaker - it sounds to my ears like physical transformer hum, it does not sound like the hum you'd get from a bad or failing filter cap.

Anyone else have any rabbits I can chase?

[52 Bill]

Yeah, that's a good point, and I did check the heater voltage there at the lamp and the voltages are pretty even. I'll definitely check that out that bayonet assembly. I have a new one on hand if that turns out to be the culprit. I'm also going take a closer look at the extension speaker jack.

How do the filament center tap resistors R80 and R81 look? I've seen them burn up from bad power tubes unbalancing the heater supply.

[Tone Meister]

How do the filament center tap resistors R80 and R81 look? I've seen them burn up from bad power tubes unbalancing the heater supply.

Hey, that's something I would not have checked. However, with the tube PCB laid open and no tubes I get 3.7VAC across both R80 and R81. I reflowed all the solder joints on the tube PCB and can confirm adequate and even heater voltages at both power tubes and all three preamp tubes.

I reinstalled the tubes that were in the amp and the hum is still there, possibly minimized some, but still there.

I made a short audio sample using my phone. It may seem much louder than it actually is here in the room because I had the phone right up to the speaker.

Working with the clean channel only: from 0:00 to 0:10 is the hum with all volumes down, from 0:11 to 0:20 is with the volume at max, from 0:21 to 0:25 the volume is set back to minimum.

[mozz]

I don't know if it was that particular fender model or not, check the filament wiring polarity(ac), i mean pin 9 on 1 preamp tube goes to pin 9 on next preamp tube. The model i worked on i had to cut traces and swap polarity. I also wrapped the ribbon cables in foil tape then grounded that to the chassis preamp ground lug. Also, the back panel has foil, you can add to that or run a ground wire to it. One more thing was no cathode bypass cap on second ?? triode, i added one (small) and that got rid of more hum. Also reduced first cathode bypass cap to 6.8 or 10uf, you don't need 47uf there.

[Tone Meister]

... One more thing was no cathode bypass cap on second ?? triode, i added one (small) and that got rid of more hum. Also reduced first cathode bypass cap to 6.8 or 10uf, you don't need 47uf there.

OK, shotgun replacing all 3 preamp tubes at once cured the hum.

Mozz, before replacing all the preamp tubes just now, at first I did try bypassing R10 (C56 on newer revisions) and the added cap actually increased the hum.

Amp is quiet as a mouse with just the bare minimum of PT hum in a dead quiet room.

[The Dude]

Glad you got it! Just curious: Did you replace with spiral filament tubes?

[Tone Meister]

Glad you got it! Just curious: Did you replace with spiral filament tubes?

Replaced with all TAD 7025WA.

These days it's rare that I use anything other than Ruby or TAD for other folks' amps, since both seem to have the best screening and 6 mo. warranty. Of course, I'll use whatever the owner wants if he presses the issue. But I keep Ruby and TAD on hand.

[Tone Meister]

Can someone explain how the amp can hum with the volume control set to minimum, yet replacing all three preamp tubes cured the hum?

THAT has me quite puzzled.

[Enzo]

Tube after the volume control was one of your multiple sources of hum.