Mismatch and Biasing Questions

[b-strung]

1)Can I run a 66 bassman into a 8 ohm cab? I feel like bassman run 4 ohms. I haven't gotten my hands on the amp yet, but am planning on getting one.

2)Can the 6l6's in the bassman be replaced without biasing? Any tube recommendations?

Thanks.

[phsyconoodler]

1) yes,but it will have reduced output

2) no,it is not suggested to replace tubes without at least checking bias

[Swarty]

Also, a '66 Bassman will have the CBS era bias-balance circuit. A few wires will need to be moved to convert it.

[phsyconoodler]

CBS bias balance in a 66 bassman? Are you sure?I have my doubts.I haven't seen those in 69 bassmans'.??

[Swarty]

Possitive. The Bassman bears the distinction of being the first amp to receive CBS era "enhancements". The goofy bias circuit appeared in early '65... and they never went back.

[Blue Strat]

AB165's definitely have the goofy bias balance circuit. It's unclear which circuit this amp has.

[Swarty]

A '66 would be an AB165, no question.

[phsyconoodler]

What year was the AA864?It didn't have the balance system.

[Blue Strat]

Originally Posted by Swarty

A '66 would be an AB165, no question.

Yes, but often when people quote year of manufacturer, they're wrong

[Old Tele man]

hint: AB = ciircuit design revision and 165 = month/year, or: January(1) 1965(65)

...thus, AA864 is "first" design (no revisions) of August(8) 1964(64) circuit.

[Blue Strat]

Originally Posted by Old Tele man

hint: AB = ciircuit design revision and 165 = month/year, or: January(1) 1965(65)

...thus, AA864 is "first" design (no revisions) of August(8) 1964(64) circuit.

Ok. Now explain how you'd tell that a 165 made in '68 is really a '68?

[John Phillips]

Parts codes .


The AA864 was made only in 1964 and possibly early January 1965, although it couldn't have been for long then because there are two January 1965 circuits - AA165 and AB165 (which indicates they were having trouble with it, FWIW).

The fact that AA165s are not totally unknown I would think makes it likely that it was made for the bulk of the month. There may also be a lag (fairly short, since they clearly were revising things on a weekly basis) between the circuit design date and the changes actually reaching the production line, and to confuse things even further they also used up left-over tube charts from earlier models. Many "AA165s" are probably early AB165s with older left-over charts, since presumably they printed a batch of the charts expecting to use them for a fair while. You may even find AB165s with AA864 charts. Fender really did mean it when they said "specifications are subject to change without notice" .


You can run a BF/SF Bassman into an 8-ohm cab, but you will lose power and bottom end (maybe not a bad thing, depending on the sound you're going for) and there is a small risk to the OT. Don't run one at 16 ohms, there's a large risk to it.

You can probably change the power tubes without biasing it, but it would be wise to at least check as already said. If it's a AA/AB165 you can't actually adjust the bias on one tube though - that particular version of the bias-balance circuit is a real bodge - and you should probably have it modded to a bias adjust circuit anyway.