Deluxe 6G3

[tubeswell]

On the hum, remember that you can run the 100R pair from your heater winding to the 6V6s cathode (to elevate the heaters w.r.t. ground).

[Fretts]

You might try a JJ 12AX7 S with the spiral filament. It helped me with my Gibson Disoverer rebuild that hummed until I subbed that tube.

[Fletcher Munson]

Yes, I use two 100 ohms 2w resistors as a virtual tap. I'm gonna add a wire to the star ground for the OTs ground and see if that'll change anything.
I use the same grounding scheme in all my builds. One bus wire from the far right input jack over all pots to the star ground. From the first preamp stage to the second (to the third if existent) to the PI to the star ground. Power tubes cathodes over 1 R (for bias reasons) to star ground. Bias supply to star ground. OT (black wire) to star ground. And the ground from the power socket to star ground. Heaters virtual tap of two 100 R resistors to star ground. In this case I use a transformer with a CT that goes to star ground as well (BTW the star is bolted to one of the PT bolts).
If the wire from the OT to ground won't change anything I might suspect the transformer. This is my first build with a hammond PT and the first with a CT on the secondary. AND tube rectifier.
Where should I send the corel file?

You can send the Corel file here:

dnahojom <at> yahoo <dot> com

Many thanks for that. Your gounding scheme sounds solid and your build looks really clean. Perhaps it is the transformer or some other component. Is my understanding correct that you're using a true single-point star ground? I've had good luck using a multi-point scheme advocated by Doug Hoffman.

http://www.el34world.com/charts/grounds.htm

I also tend to follow the original Fender (multi-point) approach when building a clone. Not real sophisticated, but they work... the result of lots and lots of trial and error.

[txstrat]

Thanks for the link Fletcher. I think I know now where to look first. I grounded the preamp filter cap with the others - I'm gonna change that ground to the bus wire on the pots. Same with the board grounds. I'll take it from the star ground to the bus wire as well. That should "decouple" the preamp ground from the power sections ground.

[txstrat]

Put the last two filter caps and the board ground to the bus wire on the pots and the amp got way more silent. While playing around a little (it's new, anyway) I realised, that the hum depends on the intensity pot.
With the tremolo ON the hum goes up and down with the oscillating (depending on the speed pots level).
With the tremolo OFF the hum was at a steady level when the intensity pot was turned up more than half.
With the intensity pot turned down it's the most quiet amp I've ever heard.
The bias feed has it's own secondary tap (50v) and it's own wire to the star ground.
Any ideas?

[tubeswell]

Put the last two filter caps and the board ground to the bus wire on the pots and the amp got way more silent. While playing around a little (it's new, anyway) I realised, that the hum depends on the intensity pot.
With the tremolo ON the hum goes up and down with the oscillating (depending on the speed pots level).
With the tremolo OFF the hum was at a steady level when the intensity pot was turned up more than half.
With the intensity pot turned down it's the most quiet amp I've ever heard.
The bias feed has it's own secondary tap (50v) and it's own wire to the star ground.
Any ideas?

Hmmm... the intensity pot changes the bias of the output tubes, albeit on a recurring cycle. On the part of the cycle when you have more wave amplitude in the signal, you might be picking up latent hum that is always there, but just doesn't show up until the wave amplitude is increased, and because you hear it contraposed to the part of the cycle where there is no hum, the impact is more noticeable.

So the 'trem depth' hum could be a symptom, rather than the cause of, the hum. Have you tried elevating the heaters?

On the other hand the hum could also be emanating from your guitar or from your input cable. (Try a different guitar or cable?)

(And this one is a bit of a long shot but) it just also occurred to me that the hum that you can hear in the trem might be partly a function of the bias diode switching on and off as part of the rectifying cycle, in which case if you used two diodes in series, and maybe put 100nF caps in parallel with each one, would that help to cancel any such switching noise? (If anyone has any thoughts on this idea, that would be interesting.)

[Fletcher Munson]

At least you know what part of the circuit the hum is coming from. I'd suspect that you're picking up latent hum that is always there, as tubeswell said, and would look at the grounding in that part of the circuit. In the original Fender layout most of these grounds go to the brass plate (preamp ground bus) and are separate from the first filter caps and tranny center taps.

What are you using to switch the tremolo on/off? A pedal or a shorting jack? The Fender tremolo pedal is essentially a "remotely controled ground lift" and they can be noisy in my experience.

The only other thing I can think of is the lead dress going to the "intensity" and "speed" pots. The Fender practice was to take the blue/brown/red wires from the circuit board and twist them into a bundle that was no doubt intended to have a hum supressing effect.

[txstrat]

I use a plug still to short the trem but intend to use a footswitch later on.
It hums even when the switch is in trem OFF mode and the pot is turned up.

TW
Haven't elevated the heaters, yet. Can exclude the guitar and cable as hum source since the hum is there even with no cable plugged in.

Fletcher
I didn't notice the cables were drilled around each other in the original layout. That's what seems to be the closest possibility for me except of a too small filter cap.

Will try in this order and see if it changes something.

Thanks so far.

[txstrat]

Drilled the cables around each other - no change.
Put a 2nd bias capacitor parallel to the first - no change.
Changed the bias circuit ground wire from the main filter caps to the bus wire - no change.

TW
I use the 1 ohm cathode resistors for easy biasing - wouldn't lifting the 100R resistors of the heater wires to cathode level have a negative effect on that?

Somehow I think this could be a normal hum, since the pot has an effect on the bias anyway.

Matt

[tubeswell]

TW
I use the 1 ohm cathode resistors for easy biasing - wouldn't lifting the 100R resistors of the heater wires to cathode level have a negative effect on that?

Matt

Of course! (Slap on the head) - its fixed bias! (Geez wayne! - I forgot that when I suggested using the output cathode to elevate the heaters earlier). So raising the heater a few mV above the ground won't make any difference to the hum!. So the way to elevate the heater circuit would be to create a +ve bias supply of 20V or so (you don't want the heater to cathode voltage in your tubes to be too high, but about 20V above ground is ideal for this). Tap a (say 100k) 5W-10W series resistor off one of the HT windings (pin 4 or 8 of the rectfier) and a diode to +ve rectify that, then a 10uF-100uF 100V cap (and if you want a better filter, then (another) resistor and a second filter cap) and then a voltage divider to get it down to about 20VDC, and run your heater 100Rs to that. (If someone's got a better idea for raising the heaters I'd like to see it.)

[txstrat]

Thanks TW for spending your thoughts on this. I really appereciate it. I just believe it would be too much effort to add still more parts for a minor hum problem like this. It's really not THAT bad (you know, when you know it's there you always gonna hear it ).
You can form an opinion when I found the time to record a clip with this amp and post it here.
Would be interesting, if somebody owns an original brownface amp with this bias vary tremolo who could tell if the hum is sort of normal.

[txstrat]

It's getting on.
The cabinet is made of birch plywood. I just throwed it together to apply the speaker and play the amp.
The black smearings on the baffle board are from painting the inside black. Speaker is a Jensen C12Q.
Haven't ordered the tolex and other parts (handle, feet etc.), yet.

[tubeswell]

Hi txstrat

How does the C12Q sound with it?

[txstrat]

The sound is adorable. Although I expected it to be less grainy. The amp starts to break up at 3 on the dial. With turning it over 3 you just add more gain - no headroom or volume. Since the circuit is exactly like the schematic I suppose it's meant to be this way.
I'm gonna record some soundfiles tomorrow with different pots settings.

[txstrat]

Here are some soundclips I recorded today. Recorded with the notebooks mic. Nothing professional.
All clips recorded with a strat into bright channels input 1

clip1: neck + middle - vol:3 - tone:7-8
clip2: neck - vol:3 - tone:7-8
clip3: neck - vol:4-5 - tone:7
clip4: neck - vol:6-7 - tone:7

I tried to rec the hum when the intensity pot was turned up but couldn't get it audible on file. Seems the hum is not loud enough.

Hope you enjoy.

Matt