ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Newbie: hum problem

3/13/1999 2:53 PM
Giacomo	Newbie: hum problem My Fender SuperTwin is humming loud. Tubes are old. Hum is not affected by volume controls. Hum goes away by turning all clockwise the "output tube matching" pot (I'm not a tech but I suppose that such a setting is not the best). Is the hum caused by lost bias? I will go to change all six 6L6GC output tubes but I am a bit worried about biasing them because it seems to me that I can only "balance" the bias not really set it. Replacing the tubes will resolve the hum problem? Helps and suggestions will be very appreciated. Giacomo

3/13/1999 2:53 PM

Giacomo

Newbie: hum problem

My Fender SuperTwin is humming loud. Tubes are old. Hum is not affected by volume controls.
Hum goes away by turning all clockwise the "output tube matching" pot (I'm not a tech but I suppose that such a setting is not the best). Is the hum caused by lost bias?
I will go to change all six 6L6GC output tubes but I am a bit worried about biasing them because it seems to me that I can only "balance" the bias not really set it. Replacing the tubes will resolve the hum problem?
Helps and suggestions will be very appreciated.

Giacomo

3/14/1999 10:25 PM
Charles	Two things happen on those amps. One is.. the hum balance control, a pot on the back, will burn out if you should somehow get the filament crossed to the hivoltage DC, as when a tube blows. That pot is 100ohms and can easily be replaced with a pair of 100ohm resistors, creating the same artificial center tap for the filament circuit. The other thing, as you noted, is the output balance, which is the bias balance control. If your tubes are waaay out of kilter, the hum will be bad and effected by that control. Although the output balance control can eliminate or reduce hum from mismatched tubes, it's still pretty much essential that the six tubes be matched. You may want to have the control retroverted back to "bias=level" instead of "balance" - and stick with matched tubes. I'm not sure if it's true, but my experience is that amps with UL taps for the screen grids (like yours) tend to hum more if there is a mismatch in the tubes.

3/14/1999 10:25 PM

Charles

Two things happen on those amps.

One is.. the hum balance control, a pot on the back, will burn out if you should somehow get the filament crossed to the hivoltage DC, as when a tube blows. That pot is 100ohms and can easily be replaced with a pair of 100ohm resistors, creating the same artificial center tap for the filament circuit.

The other thing, as you noted, is the output balance, which is the bias balance control. If your tubes are waaay out of kilter, the hum will be bad and effected by that control.

Although the output balance control can eliminate or reduce hum from mismatched tubes, it's still pretty much essential that the six tubes be matched. You may want to have the control retroverted back to "bias=level" instead of "balance" - and stick with matched tubes.

I'm not sure if it's true, but my experience is that amps with UL taps for the screen grids (like yours) tend to hum more if there is a mismatch in the tubes.

3/15/1999 12:08 AM
Giacomo	Thank you Charles, I will replace the pot as soon as possible (I have to understand the wiring) then I'll let you know. Here are more infos about this hum problem: 0- I replaced the filter caps some weeks ago. 1- I made a little mod converting extra-speaker out to direct out. I drilled the chassis (tubes pulled out but much vibration). 2- I turned on the amp, waited the time, standby on... hum, one minute... fuse blown. 3- Inspect the amp and found a broken resistor between pin 1 & 5 of one 6L6's. 4- Replaced the resistor, the hum is still there. 5- Inspected for something wrong in the power supply and found a broken joint at one of the caps which "bridges" the rectifier. Checked the other diode (bias?) by an ohmmeter, it seems ok when I pull it off the circuit. Leaving the diodes in the circuit they runs something in both direction (is it normal?) 6- Hum is still there. Giacomo

3/15/1999 12:08 AM

Giacomo

Thank you Charles, I will replace the pot as soon as possible (I have to understand the wiring) then I'll let you know.

Here are more infos about this hum problem:
0- I replaced the filter caps some weeks ago.
1- I made a little mod converting extra-speaker out to direct out. I drilled the chassis (tubes pulled out but much vibration).
2- I turned on the amp, waited the time, standby on... hum, one minute... fuse blown.
3- Inspect the amp and found a broken resistor between pin 1 & 5 of one 6L6's.
4- Replaced the resistor, the hum is still there.
5- Inspected for something wrong in the power supply and found a broken joint at one of the caps which "bridges" the rectifier. Checked the other diode (bias?) by an ohmmeter, it seems ok when I pull it off the circuit. Leaving the diodes in the circuit they runs something in both direction (is it normal?)
6- Hum is still there.

Giacomo

3/15/1999 1:25 AM

Bruce

Yes, check the hum pot ASAP.

quote:
"2- I turned on the amp, waited the time, standby on... hum, one minute... fuse blown.
3- Inspect the amp and found a broken resistor between pin 1 & 5 of one 6L6's.
4- Replaced the resistor, the hum is still there."

Oh-Oh,.. I don't like that either.
If you loose the bias of one tube for too long and then after correcting it, it hums, there is a good chance you worked that singular tube so hard that it is very soft now and drawing excessive curent at idle...that would unbalance current through the OT quite a bit and you might be hearing that too.
Have you checked the idle current through each power tube to determine what each is drawing?

Bruce

3/15/1999 7:53 AM
Giacomo	Bruce >Have you checked the idle current through >each power tube to determine >what each is drawing? Unfortunately I don't know how to do that. There is som web page or similar where can I learn? Giacomo

3/15/1999 10:31 AM
Giacomo	Charles I replaced the Hum Balance pot with two 100 ohm resistor. No good news for me, hum goes on. I tried the "quick & dirt" OT method, but I think that my analog VOM multitester has too high resistance, in fact I find 20-22ma for each group of three 6L6. Furthermore when I attempt to measure the current across each OT's half the hum decrease very much. I guess that much of the current go through the meter. I should now measure the idle current of each tube. Is it the same of "cathode resistor" method? Giacomo

3/15/1999 10:31 AM

Giacomo

Charles

I replaced the Hum Balance pot with two 100 ohm resistor.
No good news for me, hum goes on.

I tried the "quick & dirt" OT method, but I think that my analog VOM multitester has too high resistance, in fact I find 20-22ma for *each group of three* 6L6. Furthermore when I attempt to measure the current across each OT's half the hum decrease very much. I guess that much of the current go through the meter.

I should now measure the idle current of each tube.
Is it the same of "cathode resistor" method?

Giacomo

3/15/1999 9:09 PM
Bruce	Yes. If you have room, install a 1% to 5% 1 ohm resistor on each cathode in series to chassis ground. If all the resistors measured the same resistance, you will be able to measure the voltage drop across them, respectivly, and determine the aproximate idle current of each tube individualy. If one side of the OT is drawing much more current then the other, then I would first assume one or more power tubes on that side of the OT is going bad. Another thing to consider is the screen resistors. If one screen resistor has failed on one side of the OT, it will effectvly shut that tube down and the opposite OT side's tubes would be drawing more current and unbalancing the OT. That could manifest as a hum too. Does your analog VOM measure milivolts very well? I would be looking for around 28mv to 38mv across each resistor. Bruce

3/15/1999 9:09 PM

Bruce

Yes. If you have room, install a 1% to 5% 1 ohm resistor on each cathode in series to chassis ground.
If all the resistors measured the same resistance, you will be able to measure the voltage drop across them, respectivly, and determine the aproximate idle current of each tube individualy.
If one side of the OT is drawing much more current then the other, then I would first assume one or more power tubes on that side of the OT is going bad.
Another thing to consider is the screen resistors.
If one screen resistor has failed on one side of the OT, it will effectvly shut that tube down and the opposite OT side's tubes would be drawing more current and unbalancing the OT. That could manifest as a hum too.
Does your analog VOM measure milivolts very well?
I would be looking for around 28mv to 38mv across each resistor.

Bruce

Page 1 of 2

Next>

Last Page>>