what's the diference?

[AIRChoco]

I got a Twin Reverb from the 70's. it's the version without master volume. the tube chart sticker says is the amp model AB763 but inside the amp is like the AA270. Would be better to restore the amp like an AB763? And if is better to change the resistors by placing new film carbon with 5% tolerance instead of 10%?

[Teleologist]

That one has the worst changes(AB568) undone, but probably the best thing you can do for it would be the bias pot mod so you can bias the output tubes. BFing the rest is worthwhile, but some of the later SF amps have wire routing problems(lead dress) if some of the 'extra' caps are removed. IMHO SF Twins won't benefit as much from BFing as other Fenders because even the BF Twins are pretty much clean machines. Worth doing if you can DIY or the amp is in for other service.

[AIRChoco]

First of all. Thank you.So.... i should restore it like an AA270? What about replacing all the old 10% tolerance resistors and placing new ones of 5% tolerance of carbon film?
Where i come from is hard to find a good repair service even a fender service... so i'm gonna do all the changes and i'm a newbie in this field. thanks again.

[Teleologist]

If you're going to restore it, you might as well BF it - it's an easy conversion and you'd be replacing many of the parts anyway. Where you may run into problems is with the small parasitic suppression caps attached to the power tube grids and on the reverb return circuit. Try it without them, but if it goes into oscillation you may have to reconnect them. When this happens, a lot of techs will try to use the smallest value caps possible.

Carbon film resistors are pretty far down on the the list of things that affect tone IMO, but if you're doing it, use 'em in the signal path and use metal films everywhere else. Old carbon films tend to be noisey(hiss & snap/crackle/pop rice-krispies sounds) - don't forget to do the ones on the input jacks. Replacing the 'chocolate drop' caps in those amps will make a bigger difference than the resistors, as will some good Sprague Atom filter caps.