Ab568 with cathode resistors

[68strat]

Has anyone restored reapaired or played a Fender ab568 2 6L6 amp?

This ab568 super reverb has the 2 150 ohm 7 watt resistors ground to cathode.
This is a cathode and fixed bias setup.

Paul C designed a amp for heritage with cathode and fixed bias.
He says you get chime at lower volume, then fixed bias sound turned up.

What is your experience with ab568 using this dual bias setup.
Does it sound good? verses the usual cathode to ground fixed bias.

I am truly asking for a friend, lives in another state. Has SR ab568 on bench.

Thanks so much! ( love the ampG. ) - 68strat

[sluckey]

I had a Bandmaster Reverb AA768 back in the mid '70s. It has the exact same circuit as your AB568 except for speakers, OT and PT. It had a very clean Fendery sound, but I was no longer with a band while I had it, so I never had the volume higher than about 4. I thought it was a good amp but it didn't have the crunch you could get from a Band Master AB763.

[Stevem]

I which I still had mine!

I found that the slight cathode biasing gave the amp a warmer sound which was fine for me since I only used the amp for clean tones and chord work.

The cathode biasing does seem to knock 4 watts off of the total wattage of the amp.

I did also rework the general layout of the amp so I could clip out those two shimmer killing 2000 pf caps that are in the cathode circuit also.

[68strat]

Shuckey and Stevem, I appreciate the great responses.

Very well explained, helpful, and I'll pass this information
along to the tech. The SR is in pristine condition looks wise.
With y'alls help, keep the circuit as stock as possible too.

I will Report back on this amplifier outcome. Thanks again.

[pdf64]

…
I did also rework the general layout of the amp so I could clip out those two shimmer killing 2000 pf caps that are in the cathode circuit also.

There’s a widely held misapprehension that it’s fine to do that, that negative consequences are unlikely.
However, unless the amp can be checked afterwards, under load (including inductive load) to verify that it’s unconditionally stable, it may well be affected for the worse, eg




https://www.thegearpage.net/board/index ... t-32062697

[68strat]

Thanks pdf64 for the interesting information.
Very helpful ! Appreciate you chiming in.

Really does make a person wonder why CBS
implemented the additional cathode resistors
and capacitors to the output section in that run/era of amplifiers.

Who really knows for sure except Leo and his team on the exit perhaps.
I always suspected not being able to source quality 6L6 in mass quantity.
But never the transformers.

Will update on amp. It may take a little time, but will Report back. Thanks!

[68strat]

Update on this ab568 Super Reverb Amp Thread.

An intermittent connection. Cracked solder on the
brass ground plate

Tech said amp sounds good.
No modifications will be done.

Thank you guys for your help He said!
He appreciated ya'll helping and not to mod info.
68strat says Thank you too !