New AB763 Blackface Mods Page (includes DRRI & 68 CDR)

[robrob]

I have a new AB763 mods webpage up and as always suggestions and corrections are welcome.

https://robrobinette.com/AB763_Modifications.htm

[g1]

Under "Firm Up the Deluxe Reverb's Loose Low End", you mention upping the first filter stage value, but do not mention a limit for the 5AR4. It would be good to at least mention the max. spec. of 60uF. Especially the way modern tubes can be.

[robrob]

Good suggestion g1, I'll put that in there. Thanks for looking.

[nickb]

Nice work - keep it up.

My $0.02, the page would be much more understandable if you were to include a marked up schematic for each mod.

[robrob]

Yes, that is true. I'll work on some schematics.

[robrob]

I put up schematics for most of the mods.

[pdf64]

Under "Firm Up the Deluxe Reverb's Loose Low End", you mention upping the first filter stage value, but do not mention a limit for the 5AR4. It would be good to at least mention the max. spec. of 60uF. Especially the way modern tubes can be.

At the bottom of that section, note that two 70uF caps in series create an effective capacitance of 35uF.

I don't think that coupling cap values at the 2nd stage begin to affect the low end until they are reduced to <22nF on the normal channel / <1nF reverb channel.
So I can't see that the normal channel's 47nF cap will give any more perceived bottom end than the 22nF of the Reverb channel.

The situation in which those two cap values becomes significant is with the 'Fritz' channel link mod, in which each channel's 2nd stage is loaded by the output impedance of the other channel's 2nd stage, but via their respective coupling caps.
If their values were the same this wouldn't be significant, but the differing values creates a frequency dependent potential divider, and results in a noticeable bass boost in the normal channel and bass cut in the reverb channel.

See Fred's profile page at FDP http://www.fenderforum.com/guestbook...me=Fritz_D_Cat which shows that and other mods.

My preferred mods for any twin channel BF reverb amp (in addition to the 1st channel link type mod) is to get rid of the 10pF cap, which I feel adds a nasty top end to the tone, and to reduce the power tube grid reference resistors to 100k, which acts in 3 ways to reduce bias shift (reduces the time constant, reduces the LTP gain, reduces the max LTP output signal).

[robrob]

Thanks for the great input pdf.

[Steve A.]

Thanks for posting a better print of the 1968 Custom schematic. Do you have schematics of some of the other new Fender amps like the Supersonic 22?

Thanks!

Steve Ahola

[robrob]

Hey Steve,

Here is the Super-Sonic 22 Layout:



Here is the Preamp Schematic



Here is the Power Amp and Power Supply Schematic:

[Enzo]

Steve
WHile we appreciate robrob putting up more schematics, you ought to check out nickb's web site for schematics, so many of them are already up there.

http://bmamps.com/Schematics/fender/..._schematic.pdf

[robrob]

Enzo, pdf files are more difficult to work with online. It's easy to copy and paste a .jpg file into a forum thread.

[Enzo]

Sorry, I didn't mean it as criticism, I got the impression it was a "find" to come up with that one, and I was pointing out that nick's site has a huge collection that anyone ought to check first if they are searching. Wasn't implying it was necessarily a better option.

Just an aside, my own preference is pdfs, and I can expand them and otherwise manipulate them. I generally download the image file for use rather than viewing the forum display directly. JUst me, not speaking for anyone else's preferences. I copy and past the URL of a pdf.

I often find myself responding to someone's request for a schematic by a link to one in nick's collection.

[big_teee]

You can usually save a gif as a pdf.
I saved the one in post 10.
right click on the image and select, save as, then change the file type gif to a pdf file type, and save.
Then when you open it will be viewed as a pdf.
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