6V6 Hum Issues

[cbj591]

Good evening all,

I just finished rebuilding my first 6V6 pp amp and I am having some hum issues.

The first build I had a hum/buzz that would be present when the amp started up but when a source was plugged in it would go away. Also when a source was plugged in there was a very slight hum when you put your ear to the speaker. I blame the lower hum on using a non-conductive material for a chassis and will address that after the main problem is found.

The second build I made a turret board with 1/2 spacers to keep it away from the heater wiring. I am still getting the same hum/buzz when a source is not connected, but it goes away when I connect an iPod. Also if I short the input terminal, the buzz goes away.

The input jacks are isolated, the earth ground is tied to the 3 transformer chassis for safety (not connected to the PS Ground).

Does this sound like a ground loop problem? Or a PSU problem? Both channels do the same thing. I am using a GZ34 rectifier tube.


Thanks
Brian

[rmyauck]

Hi,

Maybe post your schematic.

[SemperFi]

That's a very nice build!

Schematic would be helpful, but I'll give it a shot...

Buzz.
Since the buzz goes away when the input is shorted or driven by a source, it is not a gnd loop problem.
So since the buzz happens when the input is floating, it is picking up stray HF. This can be adressed with (either/and/or) shielded wire to the input pin, grid stoppers at input (oh dread...), or something else...hehe, much help eh?


Low hum.
This is likely heater hum, but your wire dress is good to go, so my guess is the heater is grounded? If you bias the heater to a voltage >10volts higher than the cathodes, the hum should go away.
If the hum is not heater related, it may be layout, such as power tranny and OPT orientations, and many other things regarding layout.

My pennies...

Good luck

[SemperFi]

A quick peek at the pictuers again and I see the rectifiers which are probably for DC heater supply. So the hum is likely not heater related.

Buzz. Those diodes are relatively close to the input, perhaps HF radiation coming from those?

[chrish]

Or was the diode bridge and resistor to power a front panel LED rather than DC heaters?

[cbj591]

Good morning - Yeah, I guess a schematic would help...

The mass diode bride bridge is to power the LED. I got lazy and didn't want to make a trip to Radio Shack so I used these 6A diodes that I had.

The input jack wire is a 2 conductor. 1 inner wire with a copper 'ground/shield'. They are both connected at each end.

I looked at a couple of things. There is a voltage difference of .2-.4 +/- volts between the input leads (+ and -) without a source connected. When a source is connected, the voltage is the same. That is why I thought maybe a ground loop.

Also since shorting out the + and - makes the hum go away, I used a resistor (250R) to touch across the leads, and the hum went away as well. I have the 470K installed per the schematic, but adding the 250R across the leads stops the hum. Could the 470K resistors be an issue?

Thanks for the help
Brian

[SemperFi]

Well, I see no cathode resistor to gnd in the PI. Is this a typo? (should be in parallel with the 16V/100uF cap).

The input resistor 470kohms is high and therefore you risk picking up stray noise when input is floating. All modern sources will easily drive 10kohms, but to go somewhere in between I'd use 100kohms for that resistor.

(Actually, if it works fine when connected to a source I wouldn't worry about the buzz, but it aint mine so...)

[SemperFi]

About that slight hum.

The schematic doesn't show if the heater wires are gnd'ed or floating...???

Gnd'ing heater power, or better the center tap of the heater power, is a left-over from filament tube days. Per RCA manual, and many other litteratures, a indirectly heated cathode will not pick up hum if the heater potentail is above the cathode potential. 6.3VAC has a peak voltage of about 9volts, so biasing the heaters to at least 10volts above the cathode potential, will hinder any hum from entering the circuit. This done properly will be just as silent as DC heating, believe it or not (most don't believe it, I have tested and can confirm).
Anywhere between 20-80volts is fine for heater bias.

[chrish]

Originally Posted by SemperFi

Well, I see no cathode resistor to gnd in the PI. Is this a typo? (should be in parallel with the 16V/100uF cap).

The input resistor 470kohms is high and therefore you risk picking up stray noise when input is floating. All modern sources will easily drive 10kohms, but to go somewhere in between I'd use 100kohms for that resistor.

(Actually, if it works fine when connected to a source I wouldn't worry about the buzz, but it aint mine so...)

From what I can see, there is no cathode resistor on the tag board for the input/phase splitter, and there is none on the schematic. With my limited tube knowledge, just having a capacitor here (the 16V/100uF) you will have an AC ground, but no DC ground, so I don't know how (or if) it works as shown.

[SemperFi]

The 10kohm feedback resistor gives a slight DC path...but how does this sound? The PI is way underbiased, it really shouldn't sound great, but who knows?

I'd stuff a 1kohm in place of the 16V cap you have now. It will bias the PI more correctly, and w/o the cap you get better phase splitting.