1974 Silverface Super Reverb

[MeanMrMustard]

Just acquired this pretty nice piece from a customer who brought in a Blues Deluxe reissue for some work. Said he wanted a Mexican Strat so I bought one and credited him the repair ($120). In it around $700 after ordering a new cab from Mojo and some tubes and other bits. He chopped it down. An abortion really but the chassis is pretty good shape and it was nearly all original. Very clean. Filter caps so original one was bursting at the seams. Strange 330k phase inverter grid leaks. Anyone know why? Can’t see that value on any of the Super schems. Must’ve been modded. Funny the missing bits at the Vibrato channel input. I’m sure they weren’t ALL bad. Checked every single white Mallory electrolytic and they were each around 30uf with minimal leakage so coolness factor will remain lol. I replaced the 715Ps and big metal oxide resistors with the good stuff. 6PS and AB. Pulled the 2kpf caps from the grids of the output tubes. Couldn’t help it. Hope they aren’t necessary but we’ll see when the 100uf 350v F&Ts get here. Speakers are the CTS brown mesh frame AlNiCos. Can’t wait for the cab.

[MeanMrMustard]

More pics..

[MeanMrMustard]

And more....

Also I noticed the PI entrance cap is .01uf and on the proper schematics it’s .001uf.

[Roe]

the pi entrance cap was increased in the 70s. 1000pf works well with two 1m resistors, but removes too much bass with 330k resistors in the pi

[Stevem]

If your going to remove all of those small Pf anti-oscillation disc caps then you should run a separate ground wire for the preamp filter cap and land it back with all of the others near the PT.

If you do not have a big ass iron to do this with or a small torch to first heat the chassis with then just crimp a ring on the wire and bolt it down to one of the PT mounting screws by means of another nut with a washer.

Of course first confirm that the PT screw is making a good solid ground.

[MeanMrMustard]

Thanks Roe. Good to know and I won’t touch it unless there’s a problem. I want it to be mostly stock but only if it sounds good lol. Didn’t get a good idea before because there is a lot updated since just a few days ago.

Stevem, I’ll hear it without and if there’s anything unsettled I’ll pop them back in. I didn’t want to assume it should be permanent since there are significant changes in the PI circuit so maybe they do more good than bad but if all is well cranking it up once again in a combo with my R8 then I’ll probably just leave them out. Speakers have their original cones. Can’t wait.

[pdf64]

Are you able to check the esr of the white ecaps?
If not I suggest to check whether the stage’s gain increases if they are supplemented with a known good cap.
Are you sure this is a 1970 amp? It’s just those 330k LPT grid leak resistors didn’t come in till 1976, together with the distributed load OTs. But the brown HT caps do indicate it’s earlier than then.

[MeanMrMustard]

Are you able to check the esr of the white ecaps?
If not I suggest to check whether the stage’s gain increases if they are supplemented with a known good cap.
Are you sure this is a 1970 amp? It’s just those 330k LPT grid leak resistors didn’t come in till 1976, together with the distributed load OTs. But the brown HT caps do indicate it’s earlier than then.

Yeah I measured them all and got ESR readings for each with my cheap little meter. Highest I saw was 3.8r from a couple that were both high 3s. Others were like 1.7r. The 5uf measured a little above 7uf.

As for the year, honestly I don’t know and should have specified as such. He brought it in saying 1970 and sans master vol I wasn’t too concerned. After seeing the 330ks I was more curious because something in the back of my head remembers reading on these later 70s models that have them. Here’s a pic of the OT and PT codes best I could make out.

[ampdan]

It's a 1974

[MeanMrMustard]

It's a 1974

Thanks for that. Looked up other comparable examples and they look the same. Do these sound good as is generally? Guess I’ll find out in a few days. It’ll be healthy at least.