Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on October 05, 2017, 11:59:41 am

Title: What identify AB763 in Fender amps ..... the kind of preamp ??
Post by: kagliostro on October 05, 2017, 11:59:41 am

AB763 is used by Fender for twin, il bandmaster, il deluxe reverb..etc.

all have a different power section

is AB763 the identifier of the preamp type ?

Thanks

Franco

Title: Re: What identify AB763 in Fender amps ..... the kind of preamp ??
Post by: sluckey on October 05, 2017, 01:25:52 pm

The whole circuit identifies the AB763. Preamp and power amps are basically the same for all models. The differences between models  are the power output, speaker configuration, PT, OT, and minor changes in the filter caps and B+ rail dropping resistors.

If you compare the schematics for the various models you'll see that they are almost identical. Even the layouts are mostly identical.

Title: Re: What identify AB763 in Fender amps ..... the kind of preamp ??
Post by: kagliostro on October 05, 2017, 02:28:02 pm

Many Thanks Steve

Franco

Title: Re: What identify AB763 in Fender amps ..... the kind of preamp ??
Post by: tubeswell on October 05, 2017, 03:20:21 pm

And if you go to different years, there are different circuit numbers. AB763 circuits came out in 1963, in 1964 you have the AA764, AA864, AA964 and AA1164 for example, in 1965 you have the AB165,  in 1968, you have the AB568, AC568, 1969 you have the AA769, AA1069, in 1970, you have the AA270, AB1270 etc. Each of these circuit types have different things in common, like changes to the tone stack, or the bias supply circuit, or even subtle changes to the trem roach driver circuit. Grab a bunch of Fender Schematics and compare

Title: Re: What identify AB763 in Fender amps ..... the kind of preamp ??
Post by: kagliostro on October 05, 2017, 05:09:26 pm

Many thanks also to you Tubeswell

Franco

Title: Re: What identify AB763 in Fender amps ..... the kind of preamp ??
Post by: PRR on October 05, 2017, 07:05:47 pm

Think "turret board". The same board can be used with 10 or 20 pound transformers, 20 or 22 inch chassis, 2 or 4 power tubes, 47 or 100 Ohm NFB resistor, four 10" or one 15" speaker, etc. Same schematic except notes for number of power tubes, transformer numbers, etc.

Title: Re: What identify AB763 in Fender amps ..... the kind of preamp ??
Post by: kagliostro on October 06, 2017, 12:54:51 am

Many Thanks PRR

Franco

Title: Re: What identify AB763 in Fender amps ..... the kind of preamp ??
Post by: Ed_Chambley on October 06, 2017, 04:24:49 pm

And the often overlooked AA165 Bassman which channels are out of phase and is just asking to move a wire and make one side nasty. Just found one and suffering the delema of to mod or not. It lived its life in a studio. I believe Incan do it and put it back.


I have always liked being able to patch the reverb through the normal channel of the AB763 for eq and volume.

Title: Re: What identify AB763 in Fender amps ..... the kind of preamp ??
Post by: bnwitt on October 07, 2017, 07:42:21 am

Quote from: PRR on October 05, 2017, 07:05:47 pm

Think "turret board". The same board can be used with 10 or 20 pound transformers, 20 or 22 inch chassis, 2 or 4 power tubes, 47 or 100 Ohm NFB resistor, four 10" or one 15" speaker, etc. Same schematic except notes for number of power tubes, transformer numbers, etc.

Indeed.  I was in the middle of my 3rd Hoffman Deluxe reverb build when I received a "need it soon" order for a Super Reverb 4X10 version of the amp.  Since I hadn't yet wired the circuit board in the Deluxe, I just pulled it, swapped out the power rail and NFB resistors, C4 and C10 and boom!  Christmas ham.

So on that note, why do you think Fender went with the 0.022uf caps on the mid pots in the BFSR and BFBM rather than the 0.047uf value?  I know it raises the mid tone frequency point but why was this done on two different speaker configurations?

Title: Re: What identify AB763 in Fender amps ..... the kind of preamp ??
Post by: kagliostro on October 07, 2017, 01:20:43 pm

Thanks also to you friends

all this is interesting

Franco