Help getting a fixed bias circuit working in a factory cathode biased amp

[smackoj]

I am trying to convert what I believe to be an older console phono/radio tube amp to a slave amp for the bass player at church to use. He has a Fender Rumble 75 that has "pre amp out" this old amp is already built as a slave amp like several old Valco types that split the pre from the power and used a lot of wire to connect them. This one seems strange because it came equipped with a 5Y3 recto and 2- 6K6s but after inspection and changing filter caps, when I run it on the current limiter, I am getting consistently 429 vdc from the 5Y3?? (whether someone along the way put 6K6s in it, I don't know and there are no markings on the chassis. The 2 HV wires from the PT show 345-345 vac. I thought because the voltage was so high that I would change it to use 6L6s and maybe a 5u4 and I wanted to get rid of the cathode resistor and polar cap and build an add-on adjustable fixed bias supply. I copied the schemo that tubenit posted (see attachment) and I have rebuilt this Negative voltage device over and over but the best I have been able to get after the diode is -12 to -13 vdc and when I measure at the point where the negative volts are suppose to run to the 2-220k ohm resistors and then onto the grids, I can't get the voltage above -7 vdc. I am not running it with anything but the recto tube in it and now I have taken that out. The good news is I still have 6.3 vac on the filaments and 5 vac on the recto heater and 400+ at the first HV filter cap. When I measure the voltage from the HV secondary to the 4007 diode, I read 190 vac but when I check the dc side of the diode I only have -10.5 dc.


I am stumped and need help. I tried to make a bias supply once before several yrs ago and it did not work at all. I obviously must be doing something wrong?


Thank you amigos

[sluckey]

Your diode is installed backwards! The banded end should point to the PT red wire. ***BUT*** There must be a 220K resistor between the diode banded end and the PT red wire.

Can't tell about the filter caps but be sure the positive leads are connected to ground.

[PRR]

> console phono/radio tube amp to a slave amp for the bass player at church....5Y3 recto and 2- 6K6s

The original application and tube lineup does not suggest much happiness in a crowd, especially for bass. Changing power tubes won't radically improve the output. Changing to fixed bias with no other change (loading) only slightly changes power output. Which is ultimately limited by the output transformer. Especially for bass.

The high idle voltage is because you lost the Radio Tuner and its large power drain.

[smackoj]

Thank you amigos. You guys are the best! I think I will put this one back in the closet. I have a whole bunch of hours invested in this project and very little to show for it other than the learning experience which you gents are always so gracious to steer me through. Very much appreciated.


Jack D

[sluckey]

IMO you have very little to gain by converting to fixed bias. And this small amp will never be suitable for a bass.

[smackoj]

I thank you again SL. I have total confidence in your opinion.


Jack D   

[shooter]

very little to show for it other than the learning experience

I always liked that part best, when the brain filaments started lighting up n neurons started firing