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.
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AB165 Fender Bassman 50: multiple issues
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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."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
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Originally posted by Chuck H View PostI'd roll with it looking like that just to throw other players off
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"Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
"Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
"All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -
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Originally posted by Chuck H View PostBuying 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%
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Originally posted by mozz View Post
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."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
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Originally posted by Justin Thomas View PostThese are my personal favorite Fenders."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
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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.Last edited by Helmholtz; 07-09-2024, 11:25 PM.- Own Opinions Only -
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Originally posted by Chuck H View Post
The best sounding Bassman I ever played was an AA165 head.
Also, mine came without the 220ks. I added them for grins, couldn't tell a difference, yanked em back put again.
Jusrin"Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
"Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
"All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -
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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.Last edited by Chip tele; 07-09-2024, 05:07 PM.
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Originally posted by Chip tele View PostI'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).
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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.
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Originally posted by Chip tele View PostI'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.
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"Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
"Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
"All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -
- Likes 1
Comment
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Originally posted by Chip tele View PostI'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).
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Originally posted by Greg Robinson View Post
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.
Jusrin
"Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
"Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
"All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -
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