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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Jan 21, 2013 8:02 am
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so I did find a picture of the AC568 and it does indeed have the wires
The correct green pbw in the correct gauge.

So I'm guessing this will not hinder in any way my conversion to AB165 ????

First step is completed
I removed the Monster mega power cord.
Don't you find it funny that they used an industrial power cord and skimped on the heater lines ?

Image

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Jan 21, 2013 8:31 am
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Chris,

I didn't mean to imply that reverting this amp to AB864 (or AA165) would be impossible. Actually, it is quite possible.

1.) Correct those heater lines.

2.) Re-do the bias supply to AA165 (or AB864) standards with pot being a bias pot.

3.) Eliminate the cathode-tied sandbox resistors from the 6L6GC.

4,) Notice the setup around that phase inverter? Quite a bit of difference between the AC568 and the AB864 (or AA165). Plate voltages and global negative feedback onto this stage. Prolly one of the reasons Fender went to snubber caps and cathode resistors on the 6L6GC of the AB568 --- for some stability. As the AB568 runs the PI much hotter than the AB864. This section will need to be set back to AB864 correctly.

That section will require care. B+ voltages and cathode voltages will need to be changed back via plate resistors, cathode resistors, and coupling cap changes. To get you to BF tone.

This is a REALLY good project. You will learn a lot about the inner workings of an amp. Things that can be apply to nearly ever amp.

And once all is done, I'm sure that amp is going to sound much more flexible.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Jan 21, 2013 8:41 am
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And just to make sure you understand where I'm at with this.

For the moment I need pictures and advice like put tab A into slot B :lol:

I'm ordering proper wire today next step heater lines !

Then maybe a little more hand holding for the PI ? PLEASE !!!! :mrgreen:

This is where I would understand if you guys said go get educated and I would love to but......... at my age and with my 24-7 job it just isn't gonna happen

Thanks !

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1968 Bandmaster with 2X12 cab C-rex speakers
VHT Special 6 Ultra combo

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Jan 21, 2013 8:55 am
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Chris, I'm glad you brought up this project. I've always wondered what that AC568 hybrid circuit was all about. THE PHASE INVERTER. Just one look at that stage and you would not believe that the AC568 and the AA165 or AB864 carries the same name "Bassman."

And all the "fixes" needed to compensate for the extra horsepower derived from that stage. Even a pico-farad cap in the global feedback. Almost like one of those Internet "hot-rod" mods gone wild. :lol:

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

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


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Jan 21, 2013 9:36 am
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blues bondsman wrote:
What about filter caps ?
Last I heard there were problems with sprague atoms
Still an issue ???

That is one this I can start with for sure.

I'm itching to dig in, I think I'll go back to replacing needed components and get the wiring as correct as possible.

I'm pulling out all my notes from the 65 and 66 bassmans I did filter caps on, this is by far the biggest project I've tried so far.

Thanks for being patient with me !

Beaver Caps ?????? Must have been Canadian eh ?


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Jan 21, 2013 9:42 am
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Dang. Only took 5 million attempts to post this. :lol: :lol: :lol:



T2... thanks, for comment on the wiring!

Chris, did you find any hair on that Beaver cap? :?:

Go with F&T caps. I like the 100mfd/450VDC in the first stage (two caps in series). Yields 50mfd total capacitance at 900VDC. This is the most critical stage of any amp. So, higher voltage rated cap usually = longer life and happier amp.

22mfd/500VDC are fine for the other stages:

I like 100mfd/100VDC in the bias stage. The extra capacitance will help filter out any diode hash.

If you decide to change the diodes, go with UF4007. Ultra-fast are cheap and less noisy.

I use Sprague Atoms for bypass. 25mfd/50VDC ones.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Jan 21, 2013 9:55 am
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BMW2002Ti wrote:
Go with F&T caps. I like the 100mfd/450VDC in the first stage (two caps in series). Yields 50mfd total capacitance at 900VDC. This is the most critical stage of any amp. So, higher voltage rated cap usually = longer life and happier amp.


+1

Beefier caps for the first-stage reservoir will tighten up the LF response of the amp and sonically enhance the mid-range focus.

Arjay

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Jan 21, 2013 3:59 pm
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I have the wire, caps, resistors and a replacement OPT either here or on the way.

Arjay thanks for offering to hunt an original OPT, I just went with this one

http://www.classictone.net/40-18011.html

It meets specs and mounting is exactly like the original so........

Been reading up on these monsters all day.

Biggest leason learned is (((DONT TRUST FENDER TUBE CHARTS))) !!!!
The kid that pulled this off on me, well, I should have known better, sometimes ya just gotta slow down and check !
The fact that Fender fooled me with a tube chart, That kinda stinks !

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Keepn' the Blues Alive

2004 50th Ann. Limited edition AMSE Stratocaster
2017 50's Baja Telecaster BSB

1968 Bandmaster with 2X12 cab C-rex speakers
VHT Special 6 Ultra combo

Visual Sound Route 66
Cmatmods Signa Drive


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Jan 21, 2013 7:02 pm
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I keep all my vital amp data loaded on a palm pilot for when I "travel" in search of bargains.

Arjay

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Jan 22, 2013 8:51 am
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Hey Arjay,

Did we ever figure out if my find from 2011 was a AB165 or AC568? Still sounds great but those caps are getting mighty old. Some were replaced and some are over 40 now.

http://forums.fender.com/viewtopic.php?f=12&t=55487&start=0 And I still haven't changed out those two IC caps.

Art


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Jan 22, 2013 12:56 pm
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Hi Art, seems to me, to be the AA371 Bassman circuit. This is based on the bias circuit. The IC 22uf caps, seem to be a substitution for the old 50uf cap. JMO Art

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Jan 22, 2013 3:01 pm
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You may be right, Art.

It doesn't quite match the AC568 paradigm.

Arjay

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Jan 22, 2013 3:10 pm
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Art, did the 371 circuit have cathode tied resistors and snubber caps off those 6L6GC?

Thanks!


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Jan 22, 2013 5:21 pm
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Now I need to check all those date codes again. Thought we had determined this to be a '69.

Art


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Jan 22, 2013 7:24 pm
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GOOD NEWS !!!!
My 12-12 1966 2x12 cabinet is done !!!!

All that's left is to find some speakers, I put my Swamp Thang and Tonker in it for now and to remove the add on casters someone previous to me installed (makes her wobbly )

Here she is !!!!
Image
Image
Image

Thanks again to Arjay for the excellent tips on stretching the new grille cloth.
The aged blackface stuff from Mojotone is very cool !

And yes it sounds amazing too !!!!!!

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Keepn' the Blues Alive

2004 50th Ann. Limited edition AMSE Stratocaster
2017 50's Baja Telecaster BSB

1968 Bandmaster with 2X12 cab C-rex speakers
VHT Special 6 Ultra combo

Visual Sound Route 66
Cmatmods Signa Drive


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Profile
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