Help Identify Fender SF Bassman Circuit

[marcumh]

I hope this finds you all well.

I have a SF Bassman ('68 via Xformer codes). It appears to have elements of AB165, AA270 and a Pro Reverb AB668 <shrug>.

I mention the AB668 because of the 25/25 cap and the 150ohm resistors on the 6L6 cathodes (not shown on the bassman scems).

I'm not trying to reinvent the wheel, so I will likely go back with stock "pull off" values, but I was curious if anyone has some insight. Is there a schem missing from history perhaps?

The bias circuit does not seem to conform to either schematic. As you can see from the photo I did replace the 1k 1/2w (tested way high out of spec) and went with the 470ohm 1w from the AB165 schem (but wonder if I should go back to the 1K or just stick with it if my -bias V is in the ball park.



Thoughts?

[g1]

What is different from AB668 ?

[marcumh]

My AB668 schem is for a Pro Reverb. As the bias circuit goes it seems the bias cap value is 50/70 compared to 50/50 and has a 2.7k? (my copy is blurry there) resistor instead of 470 (or 1k). From the 10K resistors on I would say it is fairly identical as far as I can tell.

[52 Bill]

My AB668 schem is for a Pro Reverb.

Look up a Bassman AC568 schematic. I think I have one somewhere, but you may find it faster than I can.

[marcumh]

That's the winner.

Not sure how I missed it in my searches last night and had to put at least a little more than a "normal google" effort to find this one.

Now,...I wonder if I should change that bias cap to be a 50/70 (even though there was a stock 50/50 in it?


Thank you so much for your guidance.

[Tom Phillips]

...I wonder if I should change that bias cap to be a 50/70 (even though there was a stock 50/50 in it? ...

I would install a cap with a higher voltage rating if it were mine. 50uF/100V or even 100uF/100V.
Also attached is a copy of the schematic for other interested readers. If the copy you found is better than this one please post it.
Cheers,
Tom
Fender AC568 Bassman.pdf

[marcumh]

Thanks Tom,

I'll change that cap out.


Here's the schem I found.Fender_BASSMAN_AC568.pdf

[Tom Phillips]

...I'll change that cap out...

Although there there hasn't seemed to be an issue with Fender under-rating the voltage rating of the bias caps I have always been perplexed that they blatantly list a typical circuit voltage reading on the schematic that is higher than the specified cap rating. 55V on a 50V cap in this case. There is probably someone out there who thinks that there is mojo associated with a negative safety factor.