Fender SF Super Reverb Bias problem

reticon · Post by **reticon** » Mon Nov 07, 2005 7:51 pm

Hello i am new here and this is my first post. I found a 1972 Fender SF Super Reverb at a local pawn shop. It hummed and crackled but it was only $120 dollars so i bought it and retubed it with all JJ tubes and set the bias and now here is my question.

On socket B i get 36ma when is what i want with 460 plate volts. On socket A i have only 27ma, yet the plate volts is also 460!

I thought it was poorly matched tubes so i put them in my 57 twin and they bias perfect at 37 for socket B and 36 for socket A

I am clueless but as far as the amp goes, it is AWSOME and very quiet. If i wasn't a bias checkin freak i'd never thing it had any issues. All original inside except for one filter capacitor which was replaced in 1975 with a GE brand of the correct value. Any ideas???

reticon · Post by **reticon** » Mon Nov 07, 2005 11:46 pm

Ok i spoke too soon... finally found an old 2 button foot switch and after plugging in into my super reverb, the tremolo does not work. I can hear the "clicking" get faster and slower as i turn the speed up and down, but its not doing tremolo to my notes. LOL!

Flames1950 · Post by **Flames1950** » Tue Nov 08, 2005 12:32 am

CBS Fenders usually use a bias balancing control -- one tube can be adjusted to match the other tube, which is "fixed" at whatever it happens to draw. If the 36mA tube is the fixed tube you should be able to adjust the other one up to get close. If the 27mA tube is the fixed tube you'd need to modify the bias circuit to bring its current in line, then adjust the other to match as close as you can.

reticon · Post by **reticon** » Mon Nov 14, 2005 1:29 pm

Heya! Thanks so much man you were right!! Its a balancing pot, neither one is actually fixed i found, so when i balanced both out it came out to 31ma for each tube. The bad news is thats down in crossover distortion and with 465 plate volts i wanted it to be at least 36ma.. So here is my question...,

I had an idea... even though the tube chart says this amp should have a 5U4G rectifier, every super i've seen until this one has has a GZ34/5AR4 in it so i removed the 5U4G and put in a JJ GZ34 and wow!! My plate voltage went up to 485 per tube, which makes my current 39ma on each tube and the amp sounds INCREDIBLE!

What did i do technically speaking??? More importly, can i always swap i GZ34 in place of a 5U4G? Assuming i have an amp where i can adjust the bias if it gets too hot of course

Metropoulos Forum

Fender SF Super Reverb Bias problem

Fender SF Super Reverb Bias problem

update

Thanks!!!!!!!!!!!!!!!! Oh and one more question!