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Fender hum balance...help me

8/16/1999 11:37 PM
jason	Fender hum balance...help me I know what I need is in the archives(how many times have I thought that one lately) but since they aren't currently available I'll ask again. What is up with a Fender amp when the hum balance control has to be turned all the way to one end to get lowest hum. I would think it should be set somewhere in the middle (kind of like the two 100 ohm resistors found on a lot of Fenders. Also could someone tell me if I'm correct in my thinking that the output tubes matching control on a Fender is not really a bias adjustment. Thanks, jason

8/16/1999 11:37 PM

jason

Fender hum balance...help me

I know what I need is in the archives(how many times have I thought that one lately) but since they aren't currently available I'll ask again. What is up with a Fender amp when the hum balance control has to be turned all the way to one end to get lowest hum. I would think it should be set somewhere in the middle (kind of like the two 100 ohm resistors found on a lot of Fenders.

Also could someone tell me if I'm correct in my thinking that the output tubes matching control on a Fender is not really a bias adjustment.

Thanks,

jason

8/17/1999 12:17 AM
Reid Kneeland	Fender's hum balance is a bias balance, not a filament balance. If it has to be way off center for lowest noise, the most likely cause is mismatched power tubes (assuming that the caps and resistors are all OK). Try swapping the tubes and see if you have to turn it the other way. Reid

8/17/1999 5:11 AM
Carlo	FWIW, I seem to remember working on a couple of the Master Volume (pull distortion) Twins that had the output tubes matching control as well as the filament hum balance control.

8/17/1999 8:13 PM
Reid Kneeland	You're right. Now that I've re-read his posting, it's obvious that he's got two different controls. I was speaking from my limited experience (I've never been inside a Fender newer than a silverface) and I shouldn't have. Reid

8/18/1999 9:26 PM
MIKE STEPHENS	Look,read and think. The output tubes matching pot is the control that effects the bias voltage on one tube in a pair. The hum balance control is as named. It is a 100 ohm 1/2 watt pot with the center lug grounded, in that respect if the control is centered it should act as if it were the pair of fixed resistors found in post Blackface amps, prior to that, the pwr. xfmrs. had center tapped filament supplies. To solve this problem replace the pot, as it probably was damaged by a plate to htr. short. Hope that answers your question.

8/18/1999 9:26 PM

MIKE STEPHENS

Look,read and think. The output tubes matching

pot is the control that effects the bias

voltage on one tube in a pair.

The hum

balance control is as named. It is a 100 ohm

1/2 watt pot with the center lug grounded,

in that respect if the control is centered

it should act as if it were the pair of

fixed resistors found in post Blackface amps,

prior to that, the pwr. xfmrs. had center

tapped filament supplies.

To solve this

problem replace the pot, as it probably was

damaged by a plate to htr. short.

Hope

that answers your question.

8/17/1999 5:28 AM
Bruce	The hum balance is not bias Jason, it is form relative zero voltage filament reference/balance. You are right in assuming it is like a pair of 100 ohm resistors (depending on what Fender version of this device)with the wiper in the middle grounded. Imagine what happens to that pot when a power tube goes shorted from plate to filament! The pot is the shortest DC pathe to ground for the entire B+ supply when that happens. Try running +425vdc through a 1/2 watt 50-100ohm pot or resistor to see what happens... no don't do that. POW!there goes the resistor or pot. Probably time to replace it oldman. Now the bias balance control (also helps in the hum department) doesn't really set the over all bias but does give you a chance to balance what bias voltage is availble to the power tubes. Pull the set resistor off and use a resistor who's value is about half or slightly less of the orignal, but also install a small pot equal to the value of the removed resistor (I use a simple 20K to 25K trim pot myself) wired as a variable resistor in series with it so you can adjust that total resistance more or less from what it was stock. Now you have an adjustable bias and balance control . What you are trying to do is shunt some of the bias voltage to ground before it gets to the tubes so you can increase the idle current. If the pot is large enough, you can hold the resistance up higher and shunt less of the bias voltage to ground thus allowing more negative voltage to the power tubes to keep the idle current lower. Sorta lame but works pretty good most of the time. Bruce

8/17/1999 5:28 AM

Bruce

The hum balance is not bias Jason, it is form relative zero voltage filament reference/balance.

You are right in assuming it is like a pair of 100 ohm resistors (depending on what Fender version of this device)with the wiper in the middle grounded.

Imagine what happens to that pot when a power tube goes shorted from plate to filament!

The pot is the shortest DC pathe to ground for the entire B+ supply when that happens.

Try running +425vdc through a 1/2 watt 50-100ohm pot or resistor to see what happens... no don't do that.

POW!there goes the resistor or pot.

Probably time to replace it oldman.

Now the bias balance control (also helps in the hum department) doesn't really set the over all bias but does give you a chance to balance what bias voltage is availble to the power tubes.

Pull the set resistor off and use a resistor who's value is about half or slightly less of the orignal, but also install a small pot equal to the value of the removed resistor (I use a simple 20K to 25K trim pot myself) wired as a variable resistor in series with it so you can adjust that total resistance more or less from what it was stock.

Now you have an adjustable bias and balance control .

What you are trying to do is shunt some of the bias voltage to ground before it gets to the tubes so you can increase the idle current.

If the pot is large enough, you can hold the resistance up higher and shunt less of the bias voltage to ground thus allowing more negative voltage to the power tubes to keep the idle current lower.

Sorta lame but works pretty good most of the time.

Bruce

8/18/1999 1:45 AM
M/J	Bruce, I just finished a 1978 Bassman 10 that had the hum balance and bias control in the back. Evidently there was a short at one time cause the hum pot looked like it had been cooked a little. I installed 100 ohm resistors to the filament leads and the amp sounds great. It did hum till I did that. Also I changed the bias circuit to an adjustable circuit instead of a balanced one. Those two things made that amp sound really good. M/J

8/18/1999 1:45 AM

M/J

Bruce,

I just finished a 1978 Bassman 10 that had the hum balance and bias control in the back. Evidently there was a short at one time cause the hum pot looked like it had been cooked a little. I installed 100 ohm resistors to the filament leads and the amp sounds great. It did hum till I did that. Also I changed the bias circuit to an adjustable circuit instead of a balanced one. Those two things made that amp sound really good.

M/J

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