Ab763 question

[spufuz]

Hello!

I have a question about the Fender AB763-circuit.

Assume that i would build a Super Reverb based on the AB763 and want to exclude the Vibrato-channel, the vibrato-circuit and the reverb-circuit.
Meaning that i only want to build the Normal-channel stage and the power-stage.

Now take a look at the attached .PDF!
ab763mod.pdf

Would it be possible to simply exclude the circuitry that is not highlighted?
Or would it require major modifications in the circuit?

//David

[überfuzz]

Just to make sure. You intentionally left out the transformer and the speakers. But don't forget the electrolyte cap in the bias circuit.

[spufuz]

Oops! Forgot that one.
Seems like it would require som minor mods anyway.
A tip from another forum was to build a BF Bassman instead.
Maybe that´s an better option.

[überfuzz]

That's cool!

I'm in the middle of building a 5F6-a:ish amplifier. Can't wait to fire it, to hear it come alive.

Förresten är det spufuz från elektronikforumet..? Oj ja, kul att se dig här! (Some Swedish to welcome a new user.)

[Wilder Amplification]

You could also eliminate the 220K resistor at the output of the normal channel just before the phase inverter input. Those are "mixer resistors" that keep the channel outputs somewhat isolated to prevent loading but aren't needed if the 2nd channel isn't there.

[spufuz]

That's cool!

I'm in the middle of building a 5F6-a:ish amplifier. Can't wait to fire it, to hear it come alive.

Förresten är det spufuz från elektronikforumet..? Oj ja, kul att se dig här! (Some Swedish to welcome a new user.)

Ok! Thats a Tweed-model right?
I bet you look forward to fire it up, and it would be nice to see a biuld-thread later on at "elektronikforumet"!

(Jepp det är jag, tänkte jag skulle dra igång ett stärkarprojekt på riktigt nu. Men är ju inte riktigt säker på VAD än bara.)

[spufuz]

You could also eliminate the 220K resistor at the output of the normal channel just before the phase inverter input. Those are "mixer resistors" that keep the channel outputs somewhat isolated to prevent loading but aren't needed if the 2nd channel isn't there.

Ok thanks!

[d95err]

Why not start with the Deluxe (non-reverb) AB763 circuit:
http://www.schematicheaven.com/fende...b763_schem.pdf

It should be easy to remove the vibrato channel from there.

[cbarrow7625]

You could also eliminate the 220K resistor at the output of the normal channel just before the phase inverter input. Those are "mixer resistors" that keep the channel outputs somewhat isolated to prevent loading but aren't needed if the 2nd channel isn't there.

You absolutely can do that but it will modify the gain structure of the amp a little bit. To keep the gain structure exactly the same as the original you really need to consider the voltage divider that the signal "sees" at the junction of the 220k resistors. Think of how the 220k resistor, 0.1uF cap & 100k plate resistor form the other channel form an AC voltage divider with the series 220k resistor from the normal channel. Also consider the effects of the 50k vibrato pot connected between the 220k resistor & the 0.1uF cap. You basically have the 0.1uF cap in series with the 100k resistor (the 0.1 cap only affects frequencies below 20Hz in this evaluation so it can pretty much be ignored) in parallel with the 50k vibrato pot = approximately 33k. That 33k is in series with the 220k mix resistor = about 250k. So you have a voltage divider that has a 220k series resistance with a 250k shunt resistance. The voltage at the junction of the 220k resistors is almost cut in half.

Again, no problem leaving the 220k resistor out but if you find the volume doesn't match the original or it breaks up too soon you could always go back and add the voltage divider or (god forbid) a 500K master volume pot to dial it in just right. I found I needed to add it on a low wattage amp I built with this preamp channel to reduce the drive back to where I expected it to be.

[JoeM]

When I think of the AB763 circuit, it's the Vibrato channel, not the Normal channel that comes to mind. IMO, the extra gain stage (from the reverb recovery, etc) with the 3.3M/10pf is part of the Fender sound. My (non reverb) Princeton is built like this, but sans reverb or trem. It is a subtle difference, and requires an extra tube however.

[spufuz]

Ok, so i think ive decided to not modify a Super reverb circuit but build a 5F6-A bassman instead.
I´ve only played a silverface bassman, but i REALLY liked it.
And it seems like the 5F6-A tweed circuit will fill my needs. - A clean nice fender tone that breakes up really nice when you crank it.
Any thoughts on that??

[JoeM]

Ok, so i think ive decided to not modify a Super reverb circuit but build a 5F6-A bassman instead.
I´ve only played a silverface bassman, but i REALLY liked it.
And it seems like the 5F6-A tweed circuit will fill my needs. - A clean nice fender tone that breakes up really nice when you crank it.
Any thoughts on that??

I think you'd really like the 5F6A.

[überfuzz]

What caps and resistors are you going to use? The 5F6-a probably had polyester in them and carbon resistors. Nowadays polypropylene caps are quite popular and often works a replacement caps.

I think I mentioned that 'm building a amp with the 5F6-a preamp circuit. I use polyprop and polyester caps carbon resistors, except for one resistor that is sitting right on top of a tube socket. (This is in the preamp.)

[txstrat]

A clean nice fender tone that breakes up really nice when you crank it.
Any thoughts on that??

You might wanna have a look at this: YouTube - '59 Bassman
I've build a 35w amp with a bassman channel already and am currently building another (20w) one. Great sound.

[spufuz]

What caps and resistors are you going to use? The 5F6-a probably had polyester in them and carbon resistors. Nowadays polypropylene caps are quite popular and often works a replacement caps.

I think I mentioned that 'm building a amp with the 5F6-a preamp circuit. I use polyprop and polyester caps carbon resistors, except for one resistor that is sitting right on top of a tube socket. (This is in the preamp.)

Gaahh, made a quite long reply but must have pressed the wrong button!
Lets try again.

I wrote that we all know the debade about difference brands and component materials, but i really havn´t tested the difference between carbon and film resistors myself.
I think ill go with carbs because ive read and heard that they sound better. Maybe a bit noisier but what the h*ll, they look so good i cant resist them!

You must mean the 100k resistor on the 12AX7?
Why not a carb. on that one?

Btw. are you building yours in a tweed combo-cab or as an head.
I a bit ashamed to tell you but i plan to build mine in a 2HE 19". hehe...