i have a 1978 Silver Face Twin. Got news for you...those amps are getting heavier as we get older. The JBL speakers will kill you if you drop one on your foot. The coils on those things are impressive.
i have to change preamp tubes and the filter caps (after 40 years they're more than ready to be replaced), but i'm looking forward to cranking it up again as soon as my updates are finished.
Using that amp with my Telecaster will make your ears bleed. When you hit a "Pinch Harmonic" it feels like an ice pick is being driven through your ears, LOL.
The only real problem with the Twins (Pre or post CBS) is that it doesn't distort easily. Those JBL's will take everything you got and beg for more.
Yeah, to get good power amp distortion out of Blackface or Silverface Twins you need to play a very, very large venue. And wear earplugs to boot. So one either has to get a good distortion pedal or get a smaller, lower-wattage Fender like a Deluxe Reverb to get the nice natural distortion. So much easier to haul around too.
RE: ‘78 Twin, Breakup.
If you’re having this work done by someone else anyway, pass him along this list.
Cut that Negative Feedback Wire off the speaker jack inside it.
It takes the signal and re injects it out of phase to tighten and brighten the signal at the phase inverter.
That nonsense belongs in stereo gear and PA/Mixing etc, not in a guitar amp.
Next, IF you 1st discharge the Electrolytic B+ supply filters under the can beside the Output transformer, re regulate the plate voltages on the preamp tubes starting with the Phase inverter backwards to input tube 1 by substituting the dropping resistors between them.
Ph Inv. should be apx 245 VDC with your input tubes at appx 190 VDC.
THEN, lose the JBLs. Emminence, Celestion, whole Bunch of better choices for guitar.
If you still want more breakup, rewire you speakers at 16 ohms instead of 4 (stock) and pull the 2 Outside pwr tubes out of it. (6L6 #1 and 6L6 #4). That’ll cut the pwr in HALF. It’ll break up just fine then.
With those mods alone you shouldn’t need anything more than the vol and tone controls on your guitar.