Fender Bassman 70 Hum Control

[buddyrama]

Hi, this is my first post after observing from the side lines for a while. Lots of great info available here. I just bought a 1978 bassman 70 head and 2x15 cab. The speakers have a blue label that says fender special design on large square magnets, could these ba alnico? Also the hum balance pot is missing. Do I need this if I add a bias pot? Any info greatly appreciated. Thanks.

[Structo]

I would install the hum balance pot.
It helps dial out the hum from the heater winding.

If you don't use it you will have to create an artificial center tap with two 100 ohm resistors from the winding to ground.

[Firestorm]

If it's missing the hum balance pot, someone may have already installed the artificial center tap. The other pot labelled "output tubes matching" is the one you might want to convert to a true bias pot. A few later Fender models (but not the Bassman 70 as I recall) had dual adjustments: one to set the bias, one to adjust the bias balance. The square speakers are probably CTS ceramics.

[buddyrama]

Thanks for the info. This is my first project, learning to bias and so on. I found a bias mod on bassman70.com. I will use that. Thanks again.

[buddyrama]

I found something else. The power tubes will not seat correctly although the amp sounds great and it is quiet. I looked and over each power tube socket there is a small transistor? soldered too close over the hole for the center post of the tube. Can I jumper a little wire to these or should I replace. I thought I might add noise, they look to be original parts. The former owner loaded it with new Sprague and Ruby caps which sound very peppy. Everything else looks original including two big hackin transformers. Thanks again.

[Randy Magee]

I found something else. The power tubes will not seat correctly although the amp sounds great and it is quiet. I looked and over each power tube socket there is a small transistor? soldered too close over the hole for the center post of the tube. Can I jumper a little wire to these or should I replace. I thought I might add noise, they look to be original parts. The former owner loaded it with new Sprague and Ruby caps which sound very peppy. Everything else looks original including two big hackin transformers. Thanks again.

Maybe you mean resistors? I'd just replace them with 470 Ohm 3 watt metal oxide and 1.5K 1/2 watt carbon film or metal film... position the 470 Ohm one off the side of the tube socket between pins between pins 4 and 6... the 1.5Ks go between pins 1 and 5... raise them just a bit over the socket...

[Structo]

I don't see how a screen resistor could keep the power tube from plugging all the way in.

Can you post a picture of it?

[Randy Magee]

Something else to check... I bought an old Fender Bandmaster once that had a broken off tube guide pin in the socket that prevented the tube from going all the way in until I forced it a bit and broke the screen resistor...

[selloutrr]

check the key and make sure nothing is stuck in the socket preventing it's insertion. Also make sure you are lining the tube key and socket key up correctly. It should be a firm snug but not forceful fit. If it have the bear clamps you may need to gently spread them with your fingers as you install the tube.

[buddyrama]

Thanks for all the info!!!

[David Root]

I believe the square magnet speakers are ceramic (ferrite) not alnico.