Strange noise Fender Bassman AB165

[dude]

Hello all,

Looking for a little help, a good friend is bringing over his BF Bassman with this complaint:

I'll be playing along on it, and it sounds amazing in open position and bar chords in 3rd, 5th and 7th position. But then I go to play a single note above the 10th fret on one of the higher strings, and it just has the most knarly sounding noise underneath the otherwise bell ping type Fender sound - it's really frustrating, just came out of nowhere I noticed it one day, and now I can't get it out of my head whenever I play it!

Any hints on were to start?

He says it's wolfy sounding too, I was thinking of reducing the cathode caps to around 5uf...?

al

[plexi50]

Check the bias on the amp. That can cause bad notes. If all power supply caps are good then you can rule them out. A bad PS cap can cause certain notes to buzz and sound terrible. Changing the cathode caps may thin the amp out more than you like but will tame the woof of the bass. The PI coupling caps are already .022 and that helps to make for a nicer and tighter bass

And as usual check the preamp and power tubes. I am finding more shorted power tubes these days. New dont mean much anymore

[dude]

Today I'll get the Bassman, I'll check the bias, filter caps, voltages and all tubes.

Hopefully I'll get the amp singing again, thanks for the starting point and I'll post the results.

al

[dude]

Thank you Plexi50, the problem was indeed in the bias supply.

The amp is an AB165 with the Hum Balancing pod, someone had changed it over to a standard bias pod but didn't go all the way. There were two 220K's off the plates and 220K's of the bias,  I just followed the "AA"165 which has a bias pod and not hum balance. Bingo, ghost notes gone. Biased up a set of good but old GE 6L6's and clean, clear tones, great punch my friend is in heaven. Such a good feeling to help a someone.

This forum is great, maybe some day I'll be able to contribute more.

al

[plexi50]

Good to hear

Make sure you have (2) 47 or 100 Ohm resistors, which ever you have on hand and solder one to each of the the pilot light heater 6.3 volt leads.

Dont use the power transformer ground lug

Drill a hole right near the pilot light. Put a ground tab and screw it down on the chassis and solder the ends of both resistors to it

Dont try to use a regular solder gun to solder them to the chassis. It wont have eneough heat and will just be absorbed by the metal chassis. You would need a 100-150 watt solder gun like Fender used to get the solder to flow nice and melt into the steel chassis
 

[dude]

Plexi50, the two 100 ohm resistors you suggest.....what's the reasoning behind them?

I didn't look when I had the chassis but the schematic shows a Ct for the heaters?

al