How to change bias on vintage 1950 deluxe amp and 1964 princeton amp?

[Guitarman555]

Any body knows, how to change bias on a tube of vintage 1950 deluxe amp (only curious) and 1964 princeton amp(problems with vibro effect)? Thanks!

[oid]

You have to take some measurements and calculate what the cathode resistor should be to get the desired bias point, then you replace it with the new value. You probably should not do this unless you have fixed the other problems you were having with the amps first and do the needed maintenance.

More importantly what makes you think the bias needs to be adjusted?

I assume the amps are in the same state as they were a month or so ago?

[Guitarman555]

You have to take some measurements and calculate what the cathode resistor should be to get the desired bias point, then you replace it with the new value. You probably should not do this unless you have fixed the other problems you were having with the amps first and do the needed maintenance.

More importantly what makes you think the bias needs to be adjusted?

I assume the amps are in the same state as they were a month or so ago?

I got an black face Princeton, with weak tremolo, this maybe a question of a bias. Or not? But what I really needed to know: once you calculate the bias values, WHERE IN AMP YOU CHANGE IT? tHERE IS NO TOOL TO CHANGE ANY VALUE AROUND THE TUBE ENDING INSIDE THE AMP!! IT IS A MYSTERY TO ME!

[oid]

It is probably not the bias, or at least not just the bias.

In these amps you have to REPLACE the resistor that sets the bias point, as I said. It is not adjustable without some knowledge and skill.

All of this anyone can learn, but we want through this in the other thread.

[Guitarman555]

It is probably not the bias, or at least not just the bias.

In these amps you have to REPLACE the resistor that sets the bias point, as I said. It is not adjustable without some knowledge and skill.

All of this anyone can learn, but we want through this in the other thread.

I see, the bias resistor should be the last solution, thanks, I got it.

[Guitarman555]

But what else it could be apart from bias on a tube? I really don´t know, vibro works but really not enough, it is hardly heard even in maximum value.

[oid]

could be any or all of the dozen or so resistors and capacitors associated with the tremolo oscillator, the power supply, the oscillator tube, the power tubes, the power tube grid resistors, the power tube bias, the bias supply, the coupling caps on the phase inverter or just a bad solder joint.

Generally on an amp of this age it is a combination of most of the above, if not all the above and fixing just one cause is temporary and the problem or a new one crops up in the near future and occasionally a fix of one causes much worse and much more expensive damage, such as fiddling with the bias. Fixing all of them is not a must, but you can not just assume, you still need the check and make sure they are in a reasonable range.

You have 3 choices, starting reading up and learn enough to fix the amp yourself.
Take it to a tech.
Find yourself with 2 dead and very expensive to fix amps due to neglect.