Ampeg B15 (Circa 1961) Bad hum w/chnl 1 hum w/no instrument

[Mars Amp Repair]

Hello Folks,
I have to think numerous folks have run into this issue and I haven't worked on a B15 for quite a few years to recall:
With no instrument plugged into channel 1, you get major hum w/ turn up volume even to 1/4 for chnl 1. Possibly the owners manual back in 1961 may have actually suggested turning that input down when not in use which makes sense. I've seen that kind of instruction from 1950's and such manuals to get around a design flaw or situation they couldn't easily deal with. Sure it would have worked but is still annoying & with a very low gain input, the hum gets more noticeable. BTW, all tubes check fine, no shorts.
FYI, input jack 1 appears to have the shunting lever cut off but this is actually how the jack is designed so that they could use that tie-point on the jack like a terminal strip to connect all the resistors between the jacks & such.
I finally decided this is just the way it is & as it is with older amps sometimes the drifting of components, corrosion, etc makes them do things they didn't do when they were new. I just don't question anymore. So, I just decided to change the hi gain jack to a real shunting jack & wire the inputs like a Fender Twin replete with the 68K & 1M resistors. Worked like a charm.
Schematic included for reference.
Any feedback is always welcomed.
g

[Leo_Gnardo]

You did the right thing. Ampeg and other amp manufacturers that didn't use the ground shunt switch, well you get hum what can you expect? Why they took their good old time fixing that problem, who can tell? Might have lost some sales because of it, way too late to worry about it.

[Enzo]

50-60 years ago hum wasn't a thing like it is now. Today, the slightest noise sends them screaming into the hills. Back then, it was accepted, stuff hummed. Remember it was also the era of two wire mains plugs, ground flip switches and chassis shocks. You set your stage up, then went around flipping ground switches until you were no longer getting shocks, then any further hum reduction was a bonus.

With a two wire plug, you could flip the ground switch or turn the plug over in the socket. Roughly equivalent, but they were not the same. SO you might get better results with the wire and switch one way than with the "equivalent" of reversing both.

I doubt they lost any sales over it, in fact, at the time it wouldn't have been considered a problem.

[The Dude]

We always said back in those days that if you could hear hum, you just weren't playing loud enough.

[g1]

As you inferred in he first post, I think back then, the manufacturer would probably have said "but why would you have ch.1 volume turned up when your accordion is plugged into ch.2?". Lots of other amps of that era had the same issue.
By at '68, or maybe even sooner, they had addressed the problem as seen in the later revision attached.

[Mars Amp Repair]

Yep...switching ground switches & flipping ac plugs the norm back then. This one even with grounded pwr cord the hum was very objectionable, but I like the philosophy of why would you have channel 1 turned up if you accordion was plugged into channel 2
Thanx for the responses...glen