Hum after bias adjustment

[rblp]

I've been messing around with a Dean Markley CD-60. It had been modded and repaired and left pretty messy, but working. I reversed all the mods and replaced most of the large caps and with the original 6L6s I have a very quiet, but dull sounding amp. I tried some JJ 6L6s that I ordered from Eurotubes.com for my Peavey Triumph. I needed to adjust the bias pretty far (I was increasing the resistance of the 5k bias pot) to get them to 32ma. I noticed as I increased the bias pot a hum started and then became pretty loud. so I put the other tubes in and rebiased and the hum went away. I didn't like sound of the JJs anyway so moved on to some Groove Tubes that came in my Egnater. They are supposed to be 5881s, but they are nothing like my Tung-Sol New Production 5881s (which I hear aren't really like the old 5881s, but I like the sound of them). I had to increase the bias resistance again, but not as far as the JJs and noticed a little hum. But they really sounded good. So being that I can't leave anything alone, I decided to modify it to be able to run EL34s. They work great. I changed the 5k bias pot to a 50k pot and adjusted the bias way up and the hum is back.

I started checking voltage on the tube sockets and when I touch pin5 of one of the tubes the hum completely went away. When I touch the other tubes pin5 it gets louder. So I wiggled the tubes with no noise or scratches. I had just cleaned the tube sockets earlier this week. So now I'm thinking that this could be normal and I just don't know why it does that.

So my 2 questions are (if you made it this far). Why would the hum change when I put a multimeter set for DC Volts on Pin 5? And is there anything I can change in the circuit to help the hum? I was thinking of a hum balancer like my Peavey has. I can get a schematic if someone needs it, but would be happy with general information about this.

Thanks
Bryan

[tubeswell]

1st guess - Did you try another EL34 in that socket in which the noise stopped when you put the meter on the signal grid? (or at least swapping the tubes around to see if the same thing happens on the other socket?)

[Enzo]

My thoughts:

You have some hum coming through the amp, it is therefore on your output tube grids. When you touch the grids - pins 5 - with something (your meter probe I assume) that ALSO injects hum into the system. The two sides of the push pull output are 180 degrees out of phase - opposite polarity. On one side your extra added hum is opposite polarrity from the existing hum, about the right amount to cancel out the existing hum, so the overall total hum is gone. But touch the other side and your injected hum is the same polarity as the existing hum, so they add up, and that means the total hum in the system will be larger.

So we are still left with your existing hum.

First, do ANY of the controls affect the hum in ANY way? If so, the source of the hum is before the control that affects it, and the hum is coming in after controls that don;t.


Pull your power tubes out. Measure pin 5 to ground for DC - your bias voltage. Does it adjust up and down on both sides? Now set your meter to AC volts and measure again. What is there? SHould be none. If there is AC voltage ther - and it doesn't need to be very much - check at the bias supply filter cap. If the bias supply is not clean, neither will be your sound.

Power tubes back in. Still hums, Pull the phase inverter tube. Does the hum remain or stop?

[rblp]

Thanks for the replies.

I pulled the phase inverter with no change. No change when moving any preamp knobs.

Swapped tubes. At first the left tube socket made the hum better and right made it worse. Now after the swap the right socket made it better and the left made it worse. So the tube swap changed it. The only adjustment that makes the hum louder and softer is the bias adjustment.

Pulled power tubes and showed 46.0 DCV and 46.1 DCV on pin 5 of each tube. Both tubes showed .000 ACV.

Tried another "matched pair" of el34s. Hum is louder. Touching pin5 makes the hum better on one side, but not gone like the other tubes.

It seems like tubes that need the bias adjusted higher makes more hum. Before I did the mods to use el34s, the JJ 6L6s needed a big adjustment on the bias pot and had a loud hum. The original tubes didn't hum much when I backed off the bias for them.

So both the 5k pot and 50k pot (after the el34 mod) make a hum when it has less resistance. More resistance equals less hum.

Any ideas?

[Enzo]

So how much rippple is on the plates and screens of the power tubes? That is measure the AC on these DC supplies. The plates could have a little, but the screens should be real clean. If you pull the PI tube and the hum remains then obviously it is in the power stage or power supply.

I don;t know this amp, two power tubes? Are you checking the current individually in each tube? Are the screen resistors the same resistance on each?

Just for science, is there any hum when all the power tubes are removed?

And does this amp have the pair of resistors to ground from the heater windings? Are they intact?

[rblp]

The plates have 3.75vac the screens have 0vac.

It's a 2 power tube amp. I have a bias probe that checks the current through pin3.

No hum with tubes out.

82Ohm resistors on pin7 and pin2 to ground. They each read 40.5 which make sense if the filament puts them in parallel.

The plate current on the tubes I had been trying was within 2ma of eachother. As I started swapping them out I found a pair that was within 1ma and the hum got much quieter. Still not as quiet as the original 6L6s or my other amps, but playable. Is 2ma so far off that the amp should hum real bad? Is it possible to use the bias pot for the larger adjustments and have pots coming off of the resistors on pin5 to fine tune the current for each tube.

[rblp]

I started testing and replacing components and found a .1uf cap that showed .o1uf on the schematic. I replaced it and the one next to it. they were connected to pin2 and 7 of the phase inverter. This took the hum down to the normal fish tank type sound. Still audible, but within what I would expect from this amp.

Thanks again for all your help!