Transformer hum?

[txstrat]

Jeez, I'd put some cans around those xformers. That might help with electromagnetically shielding them, but more importantly, it'll help prevent anyone from sticking their fingers on those bare contacts on the PT's secondary!

Yeah I could still try to shield with a metal or something but I somehow believe that wouldn't do it since rotating the xformers didn't help either (I know theres a difference). Maybe there's a hum (other than the AC hum) from the core induced to the wires or speaker jacks without concerning the OT, but I might be wrong. I'll know for sure when I get my recently ordered PT and try the amp with it.
Safety wise the amp will be a head with perforated metal sheets on front and back to prevent touching the xformers and letting air through.

[tubeswell]

Well the fact that you say it is quiet if the PT is further away, but gets hummier when you flip the standby switch on, makes me wonder whether the pre-amp heaters are a source of hum. You could make a FW rectifier off the 6V winding for a DC pre-amp heater supply, (but still take the output tube heaters direct from the 6V winding for AC)

[txstrat]

Wow thats great. Thanks a million TW.

edit:
What if my PT has no center tap. Could I just build a virtual center tap additional to the FW recto? I mean there would be two ground reference points then. Or will I leave them DC heaters just floating (I mean connect to the tube pins with no grounding)?

[txstrat]

km6xz: I owe you a beer.

New PT was delivered yesterday and I tried it just in a short test (hooked it up without bolting it to the chassis just to satisfy my curiosity).
The hum is considerably less. I hadn't the transformer shield grounded (no wire handy) so when I do it might be even less.
I'm also planning to use DC for the preamp heaters. Just have to make space for the parts inside the chassis.
Would it make sense, to use the DC for the power tubes heaters as well?

[km6xz]

Sounds like you are getting closer;>). The higher a stage's gain the more it will benefit, which means the front end most of all. The power tubes will not need it, their gain is low. If the hum 50hz or 100hz. If it is front end entry the volume pot setting will make a big difference. If it is later in the signal chain the volume control will not have much effect.

[txstrat]

I suppose it's around the PI section. When I pull the first 6SL7 the hum is less but still very noticeable albeit no more dependent on the volume setting. When I pull both preamp and PI tube the hum stops completely.
The buzz is also considerably less. Even with the volume pot full open it's barely noticeable.

[tubeswell]

Would it make sense, to use the DC for the power tubes heaters as well?

Its not usually a problem in output stages. Typically any heater hum in the output stage is not the same as it is in the pre-amp stages because it is not a sensitive-a-part of the signal chain in terms of signal-to-noise ratio, and in a PP output stage, any hum riding on the signal would tend to be cancelled out in each side (by each other side) of the OT primary.

[Tom Phillips]

Matt,

Is the "standby mode" hum problem eliminated with the new transformer?

Tom

[guitician]

OK, I recently chased an annoying hum for hours, come to find out it was a noisy tube. Replaced it and the hum was gone. I don't know if your 6L6's could be causing hum, mine was a 12ax7. I've heard steel chassis are better for some types of hum field reduction due to being ferric metal, and yours is a rather small aluminum one.

[txstrat]

Matt,

Is the "standby mode" hum problem eliminated with the new transformer?

Tom

Tom,

haven't completely tested it yet. The short test showed much less buzz and also a heap less hum in both the stand by and play mode even when the PT was sitting on the chassis. For the hum in stand by mode I could very easy live with that. I still believe it can be less, compared to all my other builds. I bought parts in my local electronics shop for the DC conversion of the preamp heaters tubeswell suggested. Can tell you more when I put that in.
Thanks for asking

Matt

[txstrat]

OK, I recently chased an annoying hum for hours, come to find out it was a noisy tube. Replaced it and the hum was gone. I don't know if your 6L6's could be causing hum, mine was a 12ax7. I've heard steel chassis are better for some types of hum field reduction due to being ferric metal, and yours is a rather small aluminum one.

You're right with the small aluminum chassis. The former PT was way more quiet in an amp with an almost double as big 3mm steel chassis. Tubes have all been replaced. With the new PT the low pitch hum is audible but way less than with the other PT (all this in stand by mode).
With the amp in play mode the hum gets louder and increases with the volume pot.

[guitician]

Oh, and another thing, shielded wire can introduce ground loop hum if your not careful about leaving one end un-grounded.

[txstrat]

Have tried the new PT (x-mounted it) AND put in a DC supply for the preamp and PI.
Result is: still hum (somehow I think it's more than before - can't believe this).
Most hum when the vol pot is all the way down and when it's over 6. Between 4 and 5 the hum is least.
What I'm gonna do now is to change the DC heater wires vice versa (don't know if that matters). If that doesn't work I'll take the DC supply out again.
Next and last step I'm gonna change the OT for a different one with end caps. If that all don't work I'm gonna give up and change the preamp tubes for 12AX.. style.

[Tom Phillips]

...Most hum when the vol pot is all the way down and when it's over 6. Between 4 and 5 the hum is least.

This can be caused by hum injection at two different points. At certain settings of the control two hum sources that are out of phase cancel each other thus reducing the hum.

Sorry to hear this is giving you a problem but you are getting a learning experience. For instance, single point star grounding is not a cure all solution.You need to do the wiring, grounding etc troubleshooting. I will look for previously posted hints on this.

Your mains frequency is 50 Hz. Right?
Therefore, it the "operate mode" hum is 50 Hz then work on the heater circuit to correct.
If it is 100Hz then work on the other things I mentioned in post #21. You are in the trap of trying things without specific troubleshooting evidence. The DC heater fix is correct in some situations but it's best to first determine that you have a heater hum issue. I.e. 50 Hz rather than 100 Hz.

[txstrat]

Frankly I don't know if the hum is 50Hz or 100Hz. Maybe I should go an start up that old oscilloscope a friend gave me recently. I just don't hapen to know how to use it. And it quits working after 15 min (that's what he said).
I've attached a layout I've drawn with all (hopefully) the wires and parts as in the actual amp.