My First From Scratch Repair - 67 Bassman + Mods

[tybone]

Hi everyone. I thought I would share a recent win I had with an "as is" bassman I purchased. When I received it, the amp powered up but had a very major 60Hz hum that wouldn't go away unless you had the amp cranked AND were playing. It also seemed a little quiet.

So I started with cleaning it up and cleaning the pots. Then I checked the grounds...all seemed well. Checked the gefilte caps and found that the 16uf450v that should have been an 8uf cap had a broken leg....replaced it...no change to the wicked hum but I felt better. While checking the voltages on the two 70 uf caps it was clear that one had a much lover voltage across it than the other..mmmm. I put a 100uf cap in parallel with each cap in turn, rechecked the voltages and there was no change in voltage or Hummm. Darn! Ok, so Ill move down the chain a bit. Pulled the power tubes and started checking the voltages on each pin. WHOA something is not right here cuz they are drastically different. Pin 4 looked majorly low…. 0v. Ok so let’s check the voltage drop on the 470ohm resistors. Ok, clearly one of them was wide open. Replaced it and "OOOOooooohhhhhh Yeeeeaaaaaaaa" to quote Fender. I had clear clean tone from my Fender. Cost about 5 bucks.

THAT was a lot of fun. I like immediate feedback you know especially if it is positive.

So why stop there. There are a boat load of NTD (not too destructive) mods posted on the net that can be done to this amp so I;

1) Changed the bias circuit to a bias circuit. At lest I can set the bias for BOTH tubes now.

2) Got rid of the plate resistor bypass caps. - This noticeably increased the lower treble frequencies and upper mids.

3) Got rid of the 220k local feed back resistors in the plate and grid of the power tubes. - This kinda changed the feel of the amp and brightened things up a little bit.

I am not sure I like the amp better with its new brighter nature but I guess that why they put treble controls on the flippin thing.

Next change will be to remove the 470k feedback resistor on the last gain stage. But, for me, that would be like taking out one of the many little Fender bits that made into Dumbly DNA. WTF, I'll give it a try and let you know if I kept the change.

Thanks to the people that wrote all the articles and post and stuff. It was VERY helpful.

Cheers
Larry

[tlpruitt]

It has been my experience that removing that 470k feedback resistor in that gain stage adds too much gain and noise.

If you haven't done it already you should consider blackfacing the phase inverter. If you do don't forget to switch the wires on the secondary of the output transformer to avoid the "SQUEAL!".

-Tim

[tybone]

Thanks for the input. And yes.....removing the 470 added WAY to much gain. Clean tones with any sweetness to them at all were out of the question.

Once I get the parts and redraw the layout I will definitely blackface the PI. Will that have any effect on gain?

[tlpruitt]

Thanks for the input. And yes.....removing the 470 added WAY to much gain. Clean tones with any sweetness to them at all were out of the question.

Once I get the parts and redraw the layout I will definitely blackface the PI. Will that have any effect on gain?

Check the web for some info on blackfacing the PI of the SF Bassman. I have seen a few web sites and at least 3 books that illustrate how to do it. It can be a little confusing because you are not only replacing parts but also moving some connections around to new locations.

-Tim

[tybone]

So I put the 470k ohm resistor. Thank goodness too cuz now the amp is not fully saturating with the volume on 2. While I was in there I changed the .22uf coupling caps to .1uf sozo's. The amp has much less bottom end in a good way. It sounds more like a guitar amp now. Kinda like a Marshall really.