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Post subject: Cleanest Bassman circuit?
Posted: Sun Nov 18, 2012 7:07 am
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Hello, I have a 1967 early silverface AB165 Bassman that has been re capped but not modded (I don't think anyway) that doesn't break up till 7 and sounds amazing. I've been reading about the different Bassman circuits 6G6, AA864, AA165, and AB165, and it seems that most people have different opinions on the tone of each. I see that many say their Bassman amps break up early around 3 but mine stays clean till 7+ and is great for pedals.

I'm looking to either buy or build a backup to my Bassman that has lots of headroom. Any Bassman experts out there know which circuit has the cleanest tone with the most headroom?

Here's the inside of my 67
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Post subject: Re: Cleanest Bassman circuit?
Posted: Sun Nov 18, 2012 5:16 pm
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Like those old Sprague Black Beauties. I've never heard of a properly setup Bassman BF or SF amp breaking up at "3." You'd need to go back to the early tweed Bassman circuits which ran 6L6G and GA's to get OD at "5." At least, if the bias and plate voltages are correct.

These Bassman usually have heavier duty output trannies than their other dual 6L6GC cousins. This make for a cleaner, tighter tone to louder volume. Different OPT's are one main reason for the different tones between these amps.

To get OD below "5" on a consistent basis, prolly need an OD pedal.


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Post subject: Re: Cleanest Bassman circuit?
Posted: Mon Nov 19, 2012 4:01 am
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Lots of work have done on your amp . Many wire are not original and ( sorry ) it is a real mess . Is a piece of black tape I see close the input jack ?

If it play great , live like that.


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Post subject: Re: Cleanest Bassman circuit?
Posted: Mon Nov 19, 2012 10:47 am
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stratele52 wrote:
Lots of work have done on your amp . Many wire are not original and ( sorry ) it is a real mess . Is a piece of black tape I see close the input jack ?

If it play great , live like that.



I'd agree. Never owned one myself but those pics do not look right from a wiring perspective.

Could be wrong.

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Post subject: Re: Cleanest Bassman circuit?
Posted: Mon Nov 19, 2012 1:29 pm
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Yeah the Black Beauties are really cool!!! One thing that stands out to me, is the change to a AB763-type bias control. That's not a bad mod. Infact, I did that to my Bassman. Art

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Post subject: Re: Cleanest Bassman circuit?
Posted: Mon Nov 19, 2012 2:04 pm
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Yeah by looking at it I didn't think it was 100% stock, some caps and resistors look old some look new. I bought it about 10 years ago for $300 and it had just been serviced before I got it. I've used it live the whole time I've had it and have only had to re tube it, sounds killer and has no issues. Not sure what mods have been made to it but the bass side does not have much low end, I've always used the instrument side so I'm not sure how the bass side breaks up at loud volumes.

No electrical tape just shrink tubing, all the work is clean.

Chris

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Post subject: Re: Cleanest Bassman circuit?
Posted: Mon Nov 19, 2012 5:26 pm
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Chris, I like the ceramic power tube sockets!!! The only thing, I see, that might cause the bass channel to have less bass response is the brown tone cap all the way to the right. Although, I can't completely trace all the wiring( it's just the optical situation). Otherwise looks OK. If it works for you, I wouldn't change a thing!!! BTW My Bassman, was an AA165 that I rewired to solid-core cloth covered wire, changed the bass channel to an AA864 and the normal to a generic AB763 channel. Also use .022uf tone caps(3) in the bass channel's tone stack, to better suit guitar. Keep Pickin', Art

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Post subject: Re: Cleanest Bassman circuit?
Posted: Tue Nov 20, 2012 2:49 am
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Stratohawk, prolly best thing to do, is have the voltage points checked out versus the schematic/layout. And the idle bias of the 6L6GCs. That may help you find what section(s) is being overdriven.

I think this amp's circuit board looks barer than most Bassman we are familiar with --- because it's a plain non-verb/non-brato Bassman. those EFX circuits really populate a board.


Good luck. Amp is worth a complete look-over.


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Post subject: Re: Cleanest Bassman circuit?
Posted: Wed Nov 21, 2012 4:38 pm
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Chris, for the cleanest Bassman, I'd suggest the 165 with NOS 7581 power tubes. The 165 has a couple of negative feedback circuits designed to keep it real clean. JMO... Art

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Post subject: Re: Cleanest Bassman circuit?
Posted: Wed Nov 21, 2012 5:27 pm
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I agree with Art... 7581A are killer tubes for tight bass with tons of headroom. Need slightly hotter biasing (about15-20% above NOS 6L6GCs) to hit the sweet spot, in most amps. My favorite are GE pink based. Even good used ones put some current new tubes to shame.

BTW.... that is prolly an AC568 circuit (or some clone). Those "sand-box" resistors going from the 6L6GC cathode-to-ground seem to point to this. And none of the resistors look OEM. Almost like a DIY kit on Fender breadboard, with perhaps OEM PT, but perhaps NOT output tranny. The hold-down screws don't look stock. You'll need to really go over the schematic and your amp's layout to figure out just what you have there.

Good luck with the new addition!

http://www.webphix.com/schematic%20heav ... _ac568.pdf


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Post subject: Re: Cleanest Bassman circuit?
Posted: Wed Nov 21, 2012 11:38 pm
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Yep!! Looks like the top fiber board has been lifted and some wiring has been rerouted and replaced. Obviously, the balance pot was removed and a true bias pot installed near the OT. Actually, doesn't look too bad. It's just the intentions of the rebuilder, that I would want to know about. I'm inclined to think the choke was changed(from the machine screws being used), but that's OK if it's up to spec. As for the PT, think I'd get that silicone out of the wire exit hole(let the heat vent), unless there was some insulation fraying going on. Otherwise, these guys were built like tanks and can really deliver the goods!! Like I posted earlier, Chris, if this guy is working well for you, I wouldn't mess with anything other than tubes( and like Beemer suggested, the bias is tantamount to overall performance). Yeah Beemer, you're right about those cath resistors. Don't see the cap. But yeah, I'd let those guys go and take the cathodes straight to ground. Don't see the necessity, especially with the 763 bias pot. And this set-up is not inherently unstable. JMO, Art

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Post subject: Re: Cleanest Bassman circuit?
Posted: Fri Nov 23, 2012 6:36 pm
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Chris,

For comparison, here is a "mostly" stock 69 Bassman.

Image

Art


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Post subject: Re: Cleanest Bassman circuit?
Posted: Sat Nov 24, 2012 7:15 pm
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My Bassman has a smaller OT (same as Super Reverb) and I am told that contributes to it's low headroom. By the time the volume is at 5 you're getting quite a lot of distortion. By that point my distortion pedal is simply a "saturation" pedal.

Maybe maybe changing your OT can lead to more headroom but I've read that is a complicated and expensive mod and I'll leave it to the experts to answer whether it is actually a good idea or not.


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Post subject: Re: Cleanest Bassman circuit?
Posted: Sat Nov 24, 2012 7:55 pm
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Daniel, was the original OPT changed out, in your Bassman? As far as I know, most BF and SF Bassman used the 125A13A OPT. I believe heavy duty with 4-ohm outout. Not the 2-ohm output of the SR.

To "try" another quality OPT as an experiment is kinda $$$$. Think he should rule-out all other potential issues, first.

Just my opinion.


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Post subject: Re: Cleanest Bassman circuit?
Posted: Sun Nov 25, 2012 6:01 pm
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BMW2002Ti wrote:
Daniel, was the original OPT changed out, in your Bassman?


Yes in fact, by a previous owner. You can see four holes in the chassis where the original would have been mounted, and the replacement, which uses two screws, is much smaller. However I gave the wrong model; apparently it is the same as that used in a Pro, Bandmaster or Vibrolux amp. It is definitely 4 ohms.

Supposedly the Bassman is one of the most frequently modified Fender amps. As well they were built with several different circuit variations. This may explain why there are some Bassmen with little headroom and some that stay clean all the way up to 10.


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