Fender bassman 100 blowing fuses

[g1]

One more thing you should check is the screen resistors for the power tubes. They are the 470 ohm resistors connected to pin 4 of each power tube. They are often over-stressed when output tubes go bad and will change resistance to a higher value.
As far as the balance control, set the master to 0 and adjust balance for minimum hum.

[Jam]

Thanks on the screen resistors I have 480 to 490 ohms and the balance pot do you mean turn it to zero and and turn it to least hum? Do you have to have anything plugged in to it or any knobs turned up?

[g1]

Those resistors are within 5% so no problem there.
For the balance, the master vol. gets set to 0, then you adjust the balance pot for minimum hum from the speaker.

[Jam]

Thanks again So I have the volume set to zero and I must be dumb because I don't hear any hum and if I turn the balance pot the hum seems to be the same?

[52 Bill]

Thanks again So I have the volume set to zero and I must be dumb because I don't hear any hum and if I turn the balance pot the hum seems to be the same?

I think that the bias voltage is too high (negative) and there is no hum for you to hear. Check the values of the divider resistors in the bias supply to see if you can get it down to a more realistic value.

[Jam]

Thanks Which ones are the divider resistors? Do you mean to check if they are bad? Does it hurt the amp to leave it?

[52 Bill]

Thanks Which ones are the divider resistors? Do you mean to check if they are bad? Does it hurt the amp to leave it?

There is a 3K3 resistor that goes from the bias supply filter cap to the hum balance pot tap and there is a 15K resistor that goes from the pot wiper to ground. These two resistors plus the pot itself form the divider string. With the amp turned off check the resistace of each item.

If the bias voltage is set too high, the power tubes will not be biased correctly, leading to loss of power, high crossover distortion, bad sound, etc.

[Jam]

Thanks the one I get 3.198 and the other one I get 15.25

[52 Bill]

Thanks the one I get 3.198 and the other one I get 15.25

Those values are fine.

You need some method of reading the idle current of the output tubes. Do you have a bias probe type meter?

[Jam]

Is that the only way? I was looking to buy something any thin you recommend?

[Chuck H]

Get some of these:

RS01A1R000FE70 Vishay / Dale | Mouser

Install them between the cathode and ground of all your amps and just leave them there. A mV reading across this resistor with your DMM instantly translates to current in mA.

[g1]

There is another method (actually a few but this is one of the less dangerous ).
First you measure the resistance on either side of the OT primary (power off and filter caps drained).
Then you measure the DC voltage across each side of the OT primary with the amp idling.
Volts divided by ohms will tell you the current through each side of the OT primary (I=E/R).
For each side there are 2 ouput tubes, so divide your current by 2 and you will have the average current per tube.

[Enzo]

BONUS: that method also does not include screen current, an "advantage" over measuring in the cathode leg.

[Chuck H]

Somehow that's escaped a lot of people for a long time! Well... Maybe not, but "I" haven't seen it proposed. I actually considered it once upon a time but abandoned the idea because small recharge/self charge voltage on the caps always messed with my meter readings. If you've done this successfully without unsoldering the OT I'd be interested.

[Enzo]

Sometimes it fools the meter, but usually only on one side of the primary for some reason, like I can read on the brown wire but not the blue. When there is one bias control for the whole thing, one side is usually sufficient. Sometimes when the amp is fooling the meter, reversing the meter leads makes the difference. Or one could go hard core and measuer volts to ground at the CT and at the end and subtract.