Transformer hum?

[R.G.]

When you've exhausted grounding as a source for the moment, start looking at capacitive pickup or magnetic pickup.

Try the same turn-off-the-power trick. Does the remaining hum vanish or get smaller? If yes, the AC power in the box is doing it somehow.

Get a 6V lantern battery and power the heaters from it for a short test, opening up the heater winding out of the transformer (be careful and don't short the heater winding doing this). Does the hum go down a lot? If yes, you have heater issues. If no, the heaters are not contributing.

Short the grid of the input tube to ground right at the tube socket. Hum vanish? If yes, it's coming in on the input wires. No, it's not from the input tube section.

Divide and conquer.

[txstrat]

I unsoldered the wires of the heater winding and clipped a lead in between transformer and the unsoldered wires.
Turned the amp on and opened the heater winding by unclipping the leads. The deep frequency hum was still there.
I assume it's not from the heaters then? Haven't tried the battery test yet.
When I short the grids of the preamp and PI tubes with a clip lead the following happens:

Input grid on preamp tube: slight buzz, deep hum increases with opening the vol pot, so does the buzz when full open.
2nd grid on preamp tube: same buzz, deep hum, not increasing with opening vol pot.
1st grid on PI tube (pin 1): NO MORE buzz, slight hum (like before) no reaction on vol pot.
2nd grid on PI tube (pin 4): less buzz than without clip lead, hum increases with vol pot.

Interchanged the triodes (first grid is now pin 4) - NO MORE BUZZ.
Still hum, which increases with the vol pot.

I'd say it's the same pitch like the buzz, just deeper. Like E-String 3rd fret on a bass guitar instead of a guitar.

edit:
This is actually better than I was hoping for. There is absolutely no buzz when the vol pot is full down (even with the ear on the speaker) and starts a little at around 5 por 6 but absolutely nothing I would worry about.
Still the deep hum could be less.

[tubeswell]

Glad you (almost?) got it sorted Matt. So how about a soundbyte or two?

[txstrat]

Thanks TW
I'm about to haul it to the rehearsal tonight anyway, to see how bad the hum is compared to a bass signal with a decent 2x10 cabinet.
I'll see what I can do regarding the soundbytes.

[txstrat]

Just let me think aloud:
In post #79 I mentioned when I pull the first tube, the hum disappears, the buzz stays.
I sorted out the buzz but the hum is still bugging me. The hum increases with the volume pot. That means the source of the hum must be before the pot.
If I ground the 1st grid the hum stays the same (see post #92) so it's not coming from the input of the tube, right?
I mean there's not much left between the grid and the volume pot. I thought about a defective coupling cap. Here's a link to the schematic again: http://www.webphix.com/schematic%20h..._portaflex.pdf
Any ideas?

[tubeswell]

You tried a DC heater for that tube?

[txstrat]

Yeah, I tried a DC heater on both 6SL7 a couple weeks ago to no avail. You think it'll be worth to try again now, since the buzz disappeared?

[txstrat]

Tested the amp in the rehearsal room yesterday and my band mates say it's quiet enough. OK it's less humming than our solid state PA system but still there is an easy audible hum.
I 'm at a loss...

[kg]

maybe it just doesn't know the words?

[txstrat]

maybe it just doesn't know the words?

Maybe it helps when I hum along.

[txstrat]

I think I'm done with it. I don't feel like getting into it again at the moment.
Changed the wires from the first stage coupling caps to shielded wires which took the hum slightly down again, although it might just be my imagination.
The sound clip I recorded with my laptop computers inbuilt microphone was a Fender Precision bass (all pots open) and a 2x12 Celestion guitar cabinet. Don't expect something HiFi.
The hiss is from the open microphone (amp was 3 feet away). The amp does not hiss at all.
Heres the amp settings:
1st run - vol:2-3 treble:10 bass:8 ultra high switch: on
2nd run - vol:2-3 treble:10 bass:8 ultra high switch: off
3rd run - vol:2-3 treble:10 bass:10 ultra high switch: off
4th run - vol:3-4 trebel:10 bass:10 ultra low switch: on ultra high switch: off
Oh and excuse my crude playing.

[Tom Phillips]

Well. There is no hum apparent on the sound clip.
Sounds like you have a good amp there.

[tubeswell]

Sounds very nice Matt. What speakers have you got in that cab?

[txstrat]

Well. There is no hum apparent on the sound clip.
Sounds like you have a good amp there.

Yeah, it's really low level now. I can live with it (and have to anyway). When I find the time I'll try to record the hum.
Thank you Tom, for helping me through this.

Matt

[txstrat]

Sounds very nice Matt. What speakers have you got in that cab?

The speakers are two 12'' Celestions which have no name tag or anything as far as I remember (just Celestion). They're from the early 80s when I bought that cab. They are 25w 15 ohms each (yes, fifteen ohms)
Thank you too TW for contributing to my success.

Also R.G. thank you so much for your teaching which led me to understand more than just this amp.

All others be thanked too.