'68 silverface twin reverb questions

[gravityhigh]

I just bought a "68 silverface twin reverb, it appears to be stock! I think its the blackface circuit with the 150 ohm 10w resistors on cathodes of the power tubes and the added capacitors mounted on the side of the chassis. Does anyone know what that does to the circuit? does it basically just make it a "mixed" bias configuration? how does that affect tube life and headroom? Anyways, I'm a pedal steel player and have played through another early silverface TR (a '69 i think) that looked like the same circuit and loved it, which is why i'm excited about this guy. I think I'm going to keep this one as is and not "blackface" it, but I need to address a couple things:

-there is reverb leaking from the normal channel into the vibrato channel. Its very faint but definitely there. any idea of why? and how to stop it?

-the filter caps all look ok, no bulging and stuff. I hate to change anything on this amp. am i really gambling to run it on these caps, do I risk blowing a transformer or other part of the amp?

-also, any suggestions for a quad of new production power tubes? she still has (possibly original) blackplate RCAs but they are a little rickety. was thinking JJs or SED, can't spring for NOS at the moment

links to any info regarding early silverface twin reverbs would be greatly appreciated. Thanks!

[pdf64]

The mixed bias arrangement should act to extend tube life, compared to fixed bias, but will probably reduce the maximum continuous power output capability, though that is unlikely to be significant.
Electrolytic caps have a design life of a few thousand hours.
Yours will very likely have exceeded that and so will probably be out of spec.
The various failure modes could be causing your other problems (eg crosstalk, weak output), so the 1st step is to replace all electrolytic caps.
One cap failure mode (ie catastrophic insulation failure) could result in damage to the transformer/s.
Pete.

[mac1amps]

The R/C network in the output stage of those '68 Fenders had a tendency to significantly reduce the overall "dynamic" response of those models, in an attempt to increase "headroom". For 98% of the GUITAR players that I've removed the R/C network (i.e. directly grounded all power tube cathodes), they've preferred it without the R/C network.........'just sayin'

[gravityhigh]

so if I bypass the 150 watt cathode resistors and extra capacitors that are additional to the AB763 circuit do I need to change anything else? will that change the bias?

[Tom Phillips]

so if I bypass the 150 watt cathode resistors and extra capacitors that are additional to the AB763 circuit do I need to change anything else? will that change the bias?

Maybe. Given Fender's habits with transitional designs and the fact that the amp is 44 years old I think it's best to make no assumptions and just check everything against the AB763 service information. (It doesn't really have "150 Watt" cathode resistors. Right?)
The bias should be checked and set anyway. Even if you don't make any circuit changes. 1968 was still a good year for wiring dress so you should have a nice amp when you are done.
Tom

[JoeM]

Your amp should also have the bias balance arrangement, instead of a bias adjust. I would recommend changing to bias adjust and get rid of the 150ohm cathode resistors.