AB165 Fender Bassman 50: multiple issues

[Chip tele]

Well, I'm afraid I spoke too soon. It sounded good for about 30 minutes, then started developing a static or humming sound. It sounds like the white noise that used to come through the television late at night after the stations signed off. It's not dependent on the volume knob--stays at the same level whether I turn the volume up or down. Does that sound like filter caps? It's been ten years or more since I last changed the filter caps. Otherwise I'll follow Rob Rob's suggestion and pull tubes one at a time (with the amp off) to see if that eliminates it. If it does, he says the issue is between that tube and the one preceding it.

[Chuck H]

Clean all the unsoldered contact points in the amp with an electronics appropriate de ox product and use techniques itterated here on the forum (available with searching).

EDIT: You can't imagine how many amps I've "fixed" by simply cleaning contacts.

[Chip tele]

Thanks, Chuck--I'll keep working on it.

[Justin Thomas]

I'd roll with it looking like that just to throw other players off

Plus nobody will steal it because they don't know what it is. They'll think it's a homebrew.

These are my personal favorite Fenders. I had to sell mine but as soon as I had parts I built a clone. It's still a work in progress but I'll get there. Maybe once I get my 2x15" home I'll be motivated. Mine had a Twin Reverb OT in it & the 220k feedback resistors in the power amp were removed; other than using standard-value filter caps, it was stock. That's a magical preamp & I modded my Bassman 100 with another tube to implement it there. It started crunching around 4 with humbuckers & got "worse" from there; my stoner-doomie-sludge-playing friends loved it. Don't forget to jump the channels!

Jusrin

[mozz]

Buying used tubes on *bay is a careful dance. I've done it to get the RCA 12ax7's I adore but I always look for some indication of "working", age and a decent picture of the tube itself to see that it maybe hasn't spent time in a bad environment (both operating conditions and physical). So far it's been worth it if not always 100%

Just picked up 8 pieces of 12ax7 at a local hamfest. $15. No boxes, probably used but all test good, including a Mullard MC1 long plate.

[Chuck H]

Just picked up 8 pieces of 12ax7 at a local hamfest. $15. No boxes, probably used but all test good, including a Mullard MC1 long plate.

This is a great way to do it. Though I never have. But when I lived in San Jose, Ca. I had an electronics salvage shop up the road in Santa Clara called Halted. One whole end of their warehouse was gerter shelves with boxes and boxes of salvaged tubes. Usually only vaguely identified. There was A LOT of digging through to find 12ax7's and these were often marked with one of the many alternate designators. They had a tube tester on site so you could at least test your finds to see if they were grossly bad. They typically charged a couple of bucks for tubes without a known ID and four or five for tubes that were marked 12ax7, ECC83 or 7025. They didn't often add tubes but there were so many there to dig through that I made perhaps a half dozen trips before the juice wasn't worth the squeez. So for a time I had a pretty good stash of UOS (used old stock) preamp tubes from makers like Bugle Boy, Phillips, Telefunken, RCA, etc. plus some that had equipement manufacturers branding but were otherwise made by known tube makers from the golden era. Even used these tubes were typically superior in both stability and gain to the Chinese and Rusky product being sold new and I usually reserved them for V1 in amps. Alas I've used them all now. This was over twenty years ago.

[Chuck H]

These are my personal favorite Fenders.

The best sounding Bassman I ever played was an AA165 head. It had a benchmark "Fender" clean tone that's never been beat to my ears. It crunched up well too but the clean tone was really that amps strength. So I can see why you pulled the 220k feedback resistors off the power tubes. I think Helmholtz determined that they shouldn't make much of an audible difference. I'd have to hear that circuit both ways and decide for myself.

[Helmholtz]

The interesting part of the preamp is the wiring of the mixer triode.
It employs plate to grid voltage NFB (just like the power tubes).
This makes the grid a virtual ground node, perfect for mixing.
Because of the local NFB the gain of this stage is very low (around or even below 1).
The NFB also causes the stage to have low output impedance, which in turn significantly affects the amount of global NFB of the power amp.

[Justin Thomas]

The best sounding Bassman I ever played was an AA165 head.

But that one's different. unlike the AA763 & AB763, it's (the AB165) a different beastie from its predecessor.

Also, mine came without the 220ks. I added them for grins, couldn't tell a difference, yanked em back put again.

Jusrin

[Chip tele]

I'm glad y'all mentioned pulling those 220k resistors. Wouldn't it give more gain on the power tubes without those two NFB loops? I might try it but I want to get the static out of it in stock form before I make any mods. To try it, all I would need to do is unnsolder one end of each 220k resistor, right? Those were two of the resistors I replaced recently, as the old carbon comp resistors were both a little out of spec (tested at like 270k as I recall).

The local ham club in my area used to have a ham fest annually, but I checked their website; and it looks like they're mostly inactive now.

[Helmholtz]

I'm glad y'all mentioned pulling those 220k resistors. Wouldn't it give more gain on the power tubes without those two NFB loops? I might try it but I want to get the static out of it in stock form before I make any mods. To try it, all I would need to do is unnsolder one end of each 220k resistor, right? Those were two of the resistors I replaced recently, as the old carbon comp resistors were both a little out of spec (tested at like 270k as I recall).
.

Removing the local NFB will increase power tube as well as PI gain.
But the global NFB will equalize most of that.
Generally a higher forward gain means less stability margin.
In other words, power amp gain will slightly increase but there's a risk of HF oscillation.

[Justin Thomas]

I'm glad y'all mentioned pulling those 220k resistors. Wouldn't it give more gain on the power tubes without those two NFB loops? I might try it but I want to get the static out of it in stock form before I make any mods.

Thanks for jogging my memory... When I built my clone, I had a nasty buzz when the amp was on. I disconnected those resistors &bit disappeared. I haven't scoped it yet to figure out where it was coming from, but it won't hurt anything to disconnect them. Maybe the buzz will go away too... My original 1967 Bassman didn't get added noise when I reinstalled them. Don't know why...

Edit: to Helmholz's point, these amps are quite sensitive to wire layout. Move around the wires as little as possible to avoid oscillation.

Jusrin

[Greg Robinson]

I'm glad y'all mentioned pulling those 220k resistors. Wouldn't it give more gain on the power tubes without those two NFB loops? I might try it but I want to get the static out of it in stock form before I make any mods. To try it, all I would need to do is unnsolder one end of each 220k resistor, right? Those were two of the resistors I replaced recently, as the old carbon comp resistors were both a little out of spec (tested at like 270k as I recall).

What did you replace them with? In that position the resistors are subject to extremely high voltage stress, which the original carbon composition resistors are uniquely suited for. Using typical carbon or metal film resistors I would expect internal breakdown due to arcing. I'd wonder if that could be the cause of your new noise issue?

[Chip tele]

I used metal film 1/2 watt resistors in place of the carbon comp 220k resistors. I saved them so will try putting them back in and/or disconnecting them.

[Justin Thomas]

What did you replace them with? In that position the resistors are subject to extremely high voltage stress, which the original carbon composition resistors are uniquely suited for.

Thank you, my man! I'll take notes!

Jusrin