Fender Bassman 135; what to do with that voltage divider?

[Bernardduur]

Hey all!

I just wanted your opinion. I notice that in most Bassmans (100, 135's) I get in the voltage divider (2k7 in line, 30k to ground) on the high voltage is shot. Mostly the 2k7.......... why was this design added? And how to repair? I would understand that adding a new 2k7 would induce the same problem later on.........

[Enzo]

That all depends. What was it that failed downstream to burn up that 2.7k in the first place? Replacing the bad part, and repairing what caused it, will leave the amp in stock condition. That new resistor will only fail if something draws too much current through it. Suspects might be 20uf caps going bad.

That is a 10 watt resistor being asked to dissipate only 3 watts.

[Bernardduur]

Ah yes, I thought so too.......

I just wondered; I see more of these amps with this part failing...... dunno why...... bad part? too much current? Too much used?

Dunno........

[Enzo]

If you start by replacing just the bad resistor, then how does the amp perform? Or when that part fails do you find other things have failed as well? I am no longer looking at the drawing, but if I recall, the resistor was being asked to drop 500v to 410v - 90 volts. 90v across 2700 ohms is 33ma. if the voltage across the resistor is larger than 90v, then so is the current larger than 33ma. Easy way to monitor current then is voltage across the resistor and Ohm's Law.

[Bernardduur]

Works like a charm!

[pdf64]

Consider the peak voltage rating of the 2k7 resistor - there will be a limiting element voltage which needs to be at least 500V. Many wirewound ceramic types have a lower rating than this, which may act to reduce the expected life.
Whilst it operates at 90V, at switch on there may be >520V that will reduce as the downstream caps charge up. Pete.

[Bernardduur]

tnx!