Fender bassman 100 blowing fuses

[Jam]

So much info and many options and Im not a pro at this so I don't know where to start my next step ? So are bias meters a waste of money? Thanks

[g1]

They are not a waste of money, you can even get inexpensive kits and build your own: http://music-electronics-forum.com/t32089/
The limitation is that they usually can only handle a few types of power tubes, and you need different versions to be able to handle a wide range of tube types.

As for your next step, you need to find out your idle current in that amp. You can either install resistors as suggested in post #41, or get a bias probe, or use the method from post #42. If you are not comfortable measuring high voltage, don't use the last method.

[Jam]

Ok I got a bias meter and I dont understand it. The first tube I had 05.2 ma ,05.6 ma ,23.8ma, 23.1ma and the plate was 461. Thanks for any help

[52 Bill]

Ok I got a bias meter and I dont understand it. The first tube I had 05.2 ma ,05.6 ma ,23.8ma, 23.1ma and the plate was 461. Thanks for any help

If i understand you correctly, the first power tube has a bias reading of 5.2mA, the second tube reads 5.6mA, the third tube reads 23.8mA and the fourth tube reads 23.1mA. Is this what you are saying?

If it is, two of the power tubes are barely turning on at all (#1 and #2) and the other two (#3 and #4) are on but could be biased a little higher. Getting the power tubes biased higher will also bring down the plate voltage some.

Because the readings are similar with each of the two pairs of tubes, I don't think that the problem is tube related, but to be certain swap the positions of the two inside tubes (#2 and #3) and retest the bias settings. If the readings stay about the same as the first test, then you will need to examine the bias circuit.

[g1]

Adjust the balance (output tubes matching) pot till all 4 readings are close, post results.
Here is the balance pot circuit drawing, once you get the balance set for equal bias at all tubes, some resistors can be changed to set the overall bias.

[Jam]

Thanks guys The tubes are brand new and I adjusted the balance and all 4 are very close the same about around 9.8mv Is that super low? Thanks

[g1]

Yes, that is quite low, around 5Watts per tube.
I think you will need to raise the value of the 3.3K resistor that feeds the balance pot, or add a pot in series with it to make it adjustable bias.

[The Dude]

Also, are we now straight into the wall or isolation transformer- no light bulb limiter? Seems obvious, but just checking. Low line voltage will also cause this symptom.

[Jam]

Thanks I don't have it hooked up to the limiter . What size of resistor should I go up too?

[Chuck H]

Try a 10k pot. That'll give an adjustable bias. Which is a good idea now since it seems like all the different 6L6 tubes bias up very differently from each other. When Fender designed that circuit there were closer tolerances for tubes and most often you could just plug in a set and it would be close enough to your last one that it wouldn't matter much. Not so now.

[Jam]

Thanks where do I get the 10k pot? Does it get wired were the 3.3k resistor was?

[Chuck H]

10k pots are easy to come by. You should be able to get one at Fry's or Radio Shack. See if they have a multi turn, board mount type pot rather than a panel mount pot. Better adjustment accuracy and less likely to end up out of adjustment. Replace the 3.3k resistor with the pot. Wire the pot as a variable resistor by connecting one leg and the center at one end of the circuit and the other leg at the other end of the circuit.

[Jam]

Here is what I got a 10k so do I take the resistor out and the center leg and one of the other legs go where the resistor was? what do you do with the third leg? after installed how do I set it? Thanks

[bsco]

I will never buy ruby tubes ever again.

IMHO JJ tubes are far more consistent.

here we go again!!! god has spoken....

[Jazz P Bass]

Don't poke the bear!