1967 Fender Bassman

[Greg_L]

This came to me last weekend. My buddy and drummer in one of my bands inherited this amp from his late father. It had been sitting in hiding for who knows how long and it is in need of a freshening up. It is 100% factory original. This things a fucking time capsule. But unfortunately pretty much every single capacitor and resistor in this thing has drifted far out of spec or flat out failed. These old parts in old amps are often okay if you keep the amp working over the years. Once they start sitting idle collecting dust and moisture, things go south.

Anyway I've decided I'm gonna just get it working with quality parts but not necessarily "restore" it. I'll keep what originality I can, I'll use period-correct stuff where I can, but working is better than not working. This is the sort of weird AB165 Bassman circuit but I'm gonna tweak a few things to make it more modern-life ready - like giving it a truly adjustable bias and PPIMV.

First some inspection and date codes....


Internal chassis stamp - 36th week of 1967


Original filter caps under the doghouse...all that brown crud and warts and stuff are the guts of those caps removing themselves from the party. It'd be really cool if they worked but they gotta go for sure. Those big brown dropping resistors actually measure okay as they sit there with no current passing through them, but I wouldn't trust them in a working situation. They're going away too.


Power transformer - 28th week of 1967


Choke
27th week of 67


Output transformer
30th week of 67


So I got a parts order showing up later this week and we'll start digging in.

[Lt. Bob]

following

[JD01]

Cool. Will be intrigued to see what this sounds like when you have it cranked.

[Greg_L]

Nice. Will you / can you do any refinishing of the old corroded metal bits or just leave them as they are?

No I'm leaving it. I'll clean the dust and crud off but I'm not trying to revive any metal.

[rayc]

I've mentioned it before - I had my hands on an OLD Bassman in the late 80s. Borrowed it & the owner forgot about it. I could've been mean & kept it but I reminded them & returned it after about six months.
I didn't know about the magic etc so it was just a cool, old tube jobbie to me at the time. That and my experience with a Vardis amp in the mid 70s are as exotic as I go.

[Greg_L]

Digging in a little closer tonight...seeing a lot of this kind of stuff....

To look at the rectifier diodes, they just look old and crusty...


But in reality they fall apart with just the slightest touch.


Same with resistors. This what-should-be 470k resistor wasn't reading anything. Upon further inspection it disintegrated under the tips of my multimeter probes.



So as I expected, I can expect pretty much everything to be bad and so far that seems to be the way it's going. I've found exactly three resistors that are still in spec. The big blue molded caps are still in spec, and thank goodness for that because they are cherished in Fender amp lore. But we'll see once voltage hits them.

[Tadpui]

Wow, what a mess! I always wince when I see vids where the tech starts clipping out old components and replacing them. But holy hell, this is the most righteous example of replacing everything that I've ever seen. I don't think there's any shame (or alternative) if they're literally disintegrating. May the force be with you!

[Greg_L]

Wow, what a mess! I always wince when I see vids where the tech starts clipping out old components and replacing them. But holy hell, this is the most righteous example of replacing everything that I've ever seen. I don't think there's any shame (or alternative) if they're literally disintegrating. May the force be with you!

Ha well I think the general consensus is an amp that works is worth more/better than an amp that doesn't. If you want a museum piece then by all means just leave it be. If you want a working amp, which is the only reason to have one of these things, then sometimes things have to be changed. Most of this stuff that I'm changing is stuff that is expected to be changed. Almost everyone knows that electrolytic caps don't last 60 years. The filter caps and cathode bypass caps are way way out of range (I'll show that later). The crumbling resistors and diodes are negligible as they can be replaced with new but similar components and no one would even know unless they look closely.

The good news is that just via measurements the transformers seem perfectly fine. As long as they're okay the rest is just small parts and it will live again.

[Greg_L]

Update...still tinkering with this thing, but I have made progress.

The filter cap board completely redone...


Rectifier and bias board completely redone...


Everything on the board that needed to be changed got changed but it still looks mostly stock and unfucked with. Cleaned a reflowed grounds on the brass plate.



Removed weak and loose ground tabs and made secure chassis grounds directly to chassis where necessary.


Rewired the power cord to modern safety standards and added a PPIMV in the extra speaker jack.


And...



It is alive. The voltages all look great. No shorts, no smoke, everything seems well as far as the high voltage stuff goes.


But there is a problem in this area....


There is a "short" of some type happening in this region of the board that is freaking out the output tubes. It's some type of oscillation or crosstalk or signal shorting where it shouldn't. Everything is nice and quiet but if I tap on this area of the amp the output tubes go into freakout. If I pull the phase inverter tube it does not happen, so it's something before the phase inverter and it seems to be related to the second stage of the bass channel. I suspect there is an ancient blob of solder or flux that has broken loose and is intermittently making contact where it shouldn't. I'll need to remove the top board from the backer board and see what's going down in there.

[rayc]

Pity about the freak out but you knew something was going to be wrong and you've limited the location by diagnostics and logic so you'll do what has to be done.
Looks excellent.

[JD01]

Shame about the annoying snagging, but that looks like a work of art.