What are these two caps in this Super?

[KeithC]

After much looking at SF layouts I guess this is a AB568

The two ceramics discs that I have circles appear to be going to no where. Any idea what they are?

The bigger square caps that have I have marked in blue would be removed if doing a BF mod right?

There is another cap that isn't shown but it ties the two tube sockets together and I can only see it in the AB568 layout.

Thanks

http://i43.photobucket.com/albums/e365/blueprintk/copy3copy.jpg

[Old Tele man]

...those are the "high-frequency" to ground decoupling capacitors associated with the CBS-era "mods."

...those two 150 ohm resistors (marked with blue) look like cathode resistors (is one end connected to chassis/ground).

...the "non-polarized" cap should connect between the "tops" (ie: tube cathode ends, pin 8 on tubes/sockets) of each cathode resistor.

[KeithC]

Yes, sorry! The resistors are connected to chassis.

And the discs that are disconnected at one end were to "un-CBS era mod" it then I guess?

When I look at the AB763 layout none of that is present. Not the discs and not the cathode resistors with the non polarized cap either.

Or am I just missing it somehow.

I REALLY appreciate your patience!

PS: here is a not so good pic from another angle showing the 7watt resistors and the non polarized cap.

http://i43.photobucket.com/albums/e365/blueprintk/SuperChassis001copy.jpg

[Old Tele man]

...no, you're correct, that *is* the infamous CBS-mod circuit, but unless you plan on ALSO changing the bias circuitry (which *IS* different than AB763 chassis), then that's about as far as you can go (disconnecting the hi-freq caps).

...the "normal" AB763 bias circuit supplies SAME bias voltage to each tube; but the AB568 circuit supplies a "fixed" bias voltage (derived from two resistors) to ONE output tube, while letting you adjust the bias to the OTHER output tube with the bias pot.

[KeithC]

Got it!

I am going to do the bias change over from bias balance to bias adjust.
I think I have it pretty well at hand.

After I do that then I can remove the extra stuff then?

I know I have to add two 220k resistors on the bias supply and remove the others that show up on the AB568 circuit.

Thanks again.

[KeithC]

Just double checking. I have done the bias adjust mod succesfully.

Can I now remove those extra resistors and the non polarized cap between them that are the CBS changes?

Looking at the AB763 they are not there and the bias change I did should make that part of the circuit safe to remove that extra stuff?

That would be a "Blackface" mod I guess if I do that?

Thanks!


Edit: I would still need pin 8 on each tube grounded to chassis.

[Swarty]

Yes, just make sure to ground the cathodes. Also, check the value of the plate and cathode resistors on the PI.

[KeithC]

Those should be 100k and 82k?

If so, those have been changed to those values.

Thanks

[KeithC]

How about the rectifier difference?

The AB763 uses a GZ34 but mine calls for a 5U4GB.

After the BF changes can I use the GZ34 or is there a PT conflict?

If so what would be the up-side to switching to the GZ34?

Thanks

[Old Tele man]

...use the rectifier the label calls for; the original AB763 chassis with GZ34 had 360VAC PT, but later models with 5U4GB rectifier had 365VAC PT...so, unless you plan on changing the PT also, you might as well just leave the 5U4GB in there, as using the GZ34 with the higher PT-voltage can possibly push the voltage ABOVE the "working-voltage" limits of the installed power supply capacitors...which can cause their failure.

[KeithC]

Thanks. I will just leave the recitifier as is.

Really the amp sounds great as is.

Only thing is with the bass control up past say 5 I get some really strange woofy overtones/ghost things going on. Not just overly bassy sounding.

I am hoping maybe changing out the almost 40 year old filter caps might help that.

But, with the bass down in the 3 range where I like it anyway the amp really sounds great.

I'm a'learning as I go! With safety the first order of business :)