'68 Bassman amp volume

[Justin Thomas]

I'm not sure it's thhe cause of your hum or not, but I question the grounding on your 6L6 cathodes. Do you have an iron big enough to melt those groundnconnections? Usually takes a minimum 50W iron for that. I can see in the pic how the solder is kind of balled up on the chassis. It should be flowed smooth - like your pot grounds.

I"m back at my pretty-much-stock AB165 if you need any measurements.

But given that the hum stops when you pull V2, I think it's somewhere in that stage, or in the mixing right in front of it.

Justin

[Doc Stitches]

I have to find my 50 watt iron. I'll check V2 stage, etc;
Does the grounding switch wiring look OK ?

[Justin Thomas]

Sorry,had a post typed about the ground switch, forum erased it. Anyway, usually adding a 3-wire cord makes the ground switch obsolete... I would just take it out of the circuit - more moving parts to corrode or fail. It's not needed anymore. But leave it in the hole -less temptation for tiny fingers to go "exploring."

Justin

[Chuck H]

I'm not certain of the tab actuation of that switch, but if it's as I think it is the switch isn't doing anything now. Just acting as a sort of terminal strip.

[Doc Stitches]

thanks Justin 'n Chuck .... I'll take it out of the circuit but leave in the hole.

[Chuck H]

If you must. At least it fills the hole right now.?. Of course the connections at the terminals would need to be preserved. I agree that it's aesthetically disagreeable to have a "mystery switch" on an amp. You flip it. And then you flip it again. You ponder.... "What is it doing?" You could always just Dymo label it "MOJO" and leave it there for laughs

[Doc Stitches]

ooooooh ... I'm liking the "MOJO" ... good thinkin'

[Justin Thomas]

I have no technical argument one way or t'other. I just prefer to have as few parts as possible to get the job done. And any switch, to me, falls into "more moving parts to break..." Having worked on cars for a living, I found out that rear-wheel-drive & manual tyrannies & windows are just the way to go...
That said, I like the MOJO idea, too. I'm stealing that.

Justin

[Doc Stitches]

Someone cut the green and yellow ground wire on the PT

Is it necessary ?

[Chuck H]

The green with yellow stripe should be the filament winding CT. The stock circuit does use it. If it's not connected then perhaps someone made a "false CT" with a resistor from either end of the filament winding to ground. Usually a pair of 50R to 100R resistors and usually on either the pilot lamp or the power tube sockets. You'll need to follow the entire filament circuit until you either do, or do not find these resistors. Report back. This could well be the source of your hum.

[Chuck H]

I looked the pics over pretty close and even though I can't see everything perfectly I don't see any false CT resistors on the filament winding. My only guess is that the last guy to work it over had the bias supply wired as a non balance type like the BF model amps and then wired the bias balance pot as a hum balance for the filament winding. That's when the filament wind CT was cut off. Then when you converted the circuits back to stock that could have omitted the false CT for the filament supply. Just wire a 100R 1W resistor from either side of the filament winding to ground. You can probably do it easiest at that terminal strip thing being used as a junction for the tube sockets and the pilot lamp. Just one 100R resistor from each side to ground. If you don't have 100R resistors any value between 50R and 100R is fine, but you must use two of the same value.

[Chuck H]

...

[Doc Stitches]

Got it. That makes a lot of sense, because the 100 ohm resistors were there but I removed them as per layout. I will put them back and report.

[Justin Thomas]

Sometimes in the course of working on an amp, you bump things that shouldn't be bumped. So, while working on the bias board in my own AB165, I nudged one of the pilot light terminals against the chassis and the smoke came out when I turned it on. I torched the heater CT on my tranny, and Tom Phillips said that they may have made the tap wire smaller so that it would fry instead of my whole PT. So mine also has two 100R resistors now, and the charred remains of the former CT are cut off.
I think the AB270 had the 100R resistors stock, maybe the AC568 too... Those iterations came in between AB165 & AA371. I don't think a Fender-installed "Hum Balance" on the heater supply was included until much later, with the UL Bassman 70... since my 74 Bassman 50 w. Master Volume didn't have one...

Sorry, I love Bassman heads...

Justin

[Chuck H]

I never heard a Bassman head I didn't like As much as the SF models can rock, my favorite was an early BF head (AA864?). It had a most perfect and bell like clean tone. Best "Fender" tone I ever heard. Many of the different models are very similar in schematic but somehow they all sound different. I read once that in the early VH years Eddie often used a blonde Bassman head to avoid taking his prized Marshall to shady venues. And you know how he plays... Plug in and crank it up.