Converting a Fender AC568 circuit to AB763 - Bias problem...

[CB Amp]

Hello,

I've tried to convert a Twin Reverb AC568 circuit into AB763 (in a 1968/69 TR). I did all the modifications, (new caps, tubes), but at the end, I had a lot of 50Hz hum. I posted a question a few days ago on this forum and tried a lot of things (6.3V heaters, etc...), but always this hum.

After a look on the web I read that it is not possible to modify the bias circuit of an AC568 or AA270 circuit to AB763 specifications. So I measured the bias on the tubes (the current, I had just quickly checked the voltages before ). There is a maximum of 10mA per tube !...

So it seems to be right, with an AB763 circuit, the power transformer of an AC568 Twin Reverb doesn't deliver enough current.
Does anyone knows anything about it ? any experience ?

Thank you !

[jjman]

I'd be surprised if the power tranny of an AC568 can't power an AB763 setup on a Twin. The 6l6's cathodes are now grounded, yes?

What DC voltage are you seeing on the grids?

[CB Amp]

Yes, the cathodes are grounded.
If I set the bias at -52VDC (between the 2x220K resistors), I've got -50.8VDC on the grids... The 1.5K and 220K resistors are ok.

I don't understand, the amp sounds good this way, but with a big hum.
And I also don't understand why a power trans of an AC568 couldn't drive a AB763 bias circuit...

[tubetek]

Howdy CB-
There's no reason that transformer/amp can't be wired to ab763 spec; I've done it myself. The transformer is the same between the 2 models; that leaves only a wiring problem (assuming good, reasonably matched tubes and a good transformer) Can you post a pic of the power supply/bias section?
Tom

[Luthierwnc]

I'm a little confused. The AC568 and AA270 are Bandmasters. The first has a solid state rectifier and is bias balanced. The latter has the more conventional tube recifier and grounded cathodes (although it does have the suppression caps). The AB763 is a Showman -- which is kind of a Twin but no FX.

If you are using a Bandmaster PT to drive a Twin power section, I see more problems than hum. Can you double check your specs and maybe throw a picture up?

Skip

[billyz]

The AC568 twin reverb is notorious for it's sloppy layout and lead dress. I personally have converted this schem to AB763 even though the voltages are a little different. Also, check your choke, balance the filaments and all grounds. It can be done. and you will go from the worst sounding twin to the best . Double check all your work. Some conversions don't cover all the components of this particular conversion, IE the bias resistor from the
the PT 1k on the AC568 470 on the AB763. get a hold of a good layout for the AB763 and follow it. It may help to use the filament balance pot. for some I have found it helps to make sure the filament leads are all uniform in using the same sides.

[Luthierwnc]

Oops! I didn't realize there are a bunch of AA270's

A pic would still help. sh

[CB Amp]

Problem resolved !!...
Tubetek, you were right : not really a wiring problem, but 2 faulty tube sockets... I had tried everything, and when I re-soldered the ground leads, I noticed that some lugs were not firm. They were about to broke.
So now, everything is ok, very quiet twin !

I sometimes look for sophisticated problems, and the solution is always much more simple !! I should know that since long ago...
Sorry for this question, and thanks a lot for your help.