From mgarvi–(at)–anix.com Tue Oct 24 11:34:44 CDT 1995
Article: 4634 of alt.guitar.amps
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From: mgarvi–(at)–anix.com (Mark Garvin)
Newsgroups: alt.guitar.amps
Subject: Re: BASSMAN INFO?
Date: 24 Oct 1995 04:39:21 -0400
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In <46hncu$gs--(at)--ewsbf02.news.aol.com> timtub–(at)–ol.com (TimTube) writes:

>Further research has caused me to revise my original post. The AA864 is a
>pre CBS circuit. Perhaps some stuck a later chasis in an old cabinet. Or
>if you are lucky, Someone put a silver face panel on a blackface chassis.
>Once again the transformer dates should shed some light on this. Another
>thing would be the wiring. The wiring on the circuit board in an AA864
>would be all cotton, a 1970 amp would have plastic coated wiring.

Hi Tim, Never can tell with those things. I’ve been pleasantly and
sometimes unpleasantly surprised. Opened a known ’68 silverface
Super and found ’63 wiring, for instance. Cool, but wierd.

I’ve been thinking about ‘non-invasive’ tests for various models…

’64: I’ve already suggested a test for the ’64. The ’64 Bassman
has inverted channels, so bridging them should cause cancelations.
Still waiting to hear how well this test works. (Please post.)
All Bassman circuits after this year have normal phasing, I believe.

’65: Measure resistance from pin 8 (cathode) of the output tube socket
to ground…this will measure 0 ohms in a ’65 and some later models.
The ’65 should measure close to 182k from pin 1 to pin 6 of the phase
splitter (12at7) socket, which is right next to the power tubes.

’68: will measure 150 ohms to ground from the power tube socket pin 8.

’71: will measure 0 ohms on pin 8 (again), but now will measure only
100k or so from pin 1 to pin 6 of the phase splitter tube.

Needless to say, power down the amp before measuring this stuff, PLEASE.
In fact, unplug it and wait a couple minutes. You’ll have to pull a
couple tubes to measure from the tube sockets, but aside from that,
you shouldn’t have to open the chassis at all.

Resistance measurements on phase splitter socket pins may take a second
to ‘settle in’, so hold the probes on there.

If you need to check amp vintage on a regular basis, why not wire up
a ‘dummy’ tube base so you don’t have to poke wires into sockets.

Similar tests will apply to other amps, on a model-by-model basis.

Regards,
Mark Garvin