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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Jan 20, 2013 1:55 pm
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Blues, If you even think about repopulating an entire new tagboard--- borrow a camera with a macro lens. And take photos of the circuit during EACH stage of disassembly. I've seen so many attempts at complete rewiring go haywire when the jumper lines are boobed-up to FUBAR level (yellow ones pressed onto the chassis --- between tubes or pots and the board).

When you pull the top tagboard up, you'll see lines between it and the bottom tagboard. Just be very carefully to rewire correctly. There is no layout of this portion of the board. And the doghouse to the main tagboard wiring --- that I know of. So, I take pictures up the wazoo.

You outta work on a true P2P circuit. One with NO tagboard or turret boards. I have NO IDEA how amps techs did this and not go crazy, in those days.

Just a small portion of my Hickok 539 tube tester's guts:

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Jan 20, 2013 4:23 pm
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BMW2002Ti wrote:
Go with green push-back 18 gauge to the 6L6GC. Then, 20 gauge from the 6L6GC to the 12AT7 & 12AX7s. I used a drill and insert the two lines into the chuck. Then, SLOWY turn the wires to get that nice even looking twist.

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Steve...That is a thing of beauty! You are a real artist!


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Jan 20, 2013 5:31 pm
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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: Sun Jan 20, 2013 6:54 pm
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Late to the party! Chris I'd suggest an R&R for both the eyelet and insulating boards. And yes, all possible component population, done ahead of time. Also R&R any pots, jacks, and switches on the front while the boards are out. The filter cap and power resistors are straight forward. But you may want to rewire this section, while the space is open. Don't have a camera, so I draw layouts and compare the final layout to another old Fender. Got to dig that Beaver!!! I spent a long time rebuilding my Bassman. And there are things I would have done differently(ie prodedures and boards). Weber has good solid wire, as does Alesssandro($). Have at it, keep it clean, have patience, and fun!!! Art

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Jan 20, 2013 9:17 pm
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Hey Art
Always glad to have more input from you.

I looked it over real well and it looks like the main board is fine , the rectifier circuit board is pretty corroded and will need to be replaced.

Wire is a main concern and I've been searching all of the sites for the best deal on the best stuff.
Mojotone is looking the best so far.

I'll check those others you mentioned also.

So other than hunting down and replacing resistors that are way out of spec or burnt and replacing most of the wiring, I'm going to leave as much in original condition as possible.

Sat back tonight and played my Bandmaster Reverb for an hour or so and remembered why I love vintage Fenders so much and why its worth getting this bassman back up and playing.

I will change everything over to AB165 specs and make sure the bias pot is a bias control and not a tube matching circuit 8)

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Jan 20, 2013 9:31 pm
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blues bondsman wrote:
I will change everything over to AB165 specs and make sure the bias pot is a bias control and not a tube matching circuit 8)


In that case it won't be an AB165.

More like an AA864 to my way of thinking.

JMOOC

Arjay

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Jan 20, 2013 9:44 pm
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so I should pull up an aa864 schematic and rework the circuit to those specs ?

Hey I'm the student here, gladly following the teachers and hoping to learn along the way.


Still wondering about the cement resistors and the wafer caps on the 6L6's
Maybe this project is too much for someone without theory knowledge ??? :shock:

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Jan 20, 2013 11:27 pm
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Arjay, I think the AA165 has a bias pot and not a balance pot. Which makes this different than the AB165.

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

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


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Jan 21, 2013 12:35 am
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Well he didn't say "AA165" now, did he?

Arjay

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Jan 21, 2013 5:58 am
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Arjay,

Yeah. Funny how I was thinking AA165 all along. Really, not too much diference between the AA165 and the AB864. Except initial voltages off that PT secondary. Which I can't quite figure out, as both SEEM to have the same PT stock number.

Biggest change APPEARS to be the plate and idle bias of the 12AT7 phase inverter. Which could affect the swing voltage onto the 6L6GC. Interesting....



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


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Jan 21, 2013 6:40 am
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Gotta start somewhere.
What is the deal with the very thin greenish white wires going from the last 6L6 pins two and seven to the lamp (jewel assemble) ?
I don't see them on any layout

Also how about the two flame proof resistors going to ground from the lamp assemble ?

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Jan 21, 2013 7:12 am
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Blues, I like to think in terms of the direction of current flow. The current is flowing FROM the PT to the lamp to the 6L6GC heater pins.

The two thin white wires are what I described earlier as the heater lines going from the lamp to each 6L6GC heater pins. These are NOT OEM. They are way too small gauge. Remember you asked what gauge to use? I posted prolly-GREEN cloth covered push-back (PBW) 18 gauge wires replacing those thin white wires from the lamp to 6L6GCs.

Then, 20 gauge green PBW from the 6L6GCs to the 12AT7 and 12AX7s (7025). All of these heater lines should be twisted to help cancel out AC hum.

Those metal oxide resistors from the lamp to ground are heater-line-ends to ground resistors. These resistors help center the heater's AC line. Kinda like a pseudo center tap. You need this to keep the two green heater lines running a symmetric half of the AC sine wave in each wire. This makes the twisting of the line and lowering of AC hum more effective.

Check Valve Wizard's ditty on this (Artificial Centre Tap):

http://www.valvewizard.co.uk/heater.html

You can see those 100-ohm resistors at the bottom right of the schematic, On the secondary side of the PT (TR1) where it says "TO ALL HEATERS AND PILOT LUGHT."

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 7:23 am
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I get what your saying and yes I recall our discussion about how thin they were.

But they don't exist anywhere on any schematic or layout

I can't fin anywhere where the PT wires directly to the 6L6 pins am I missing something here ?

Confused :mrgreen:

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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 7:39 am
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Chris, MOST tube amp manufacturers cheat and do not put the heater lines to tubes, in their schematics. Check out this schemo:

http://www.webphix.com/schematic%20heav ... t_12rb.pdf

See the heater line to each tube (V1 through V6)? Notice that the Ampeg DOES NOT use 100-ohm resistors to ground. Why? Because the PT has a center tap (CT) on those lines. You do not need to make an artificial CT.

Why do some use this CT and others (like most Fender amps) not? Because the heater wire is the most expensive winding in most PTs. This is due to the current that needs to be carried (usually 2-3 amps RMS). And only several hundred mA in the B+ line.

If you rip a PT apart, you clearly see the much thicker gauged heater line.

Now, with a "real" CT --- you'd need approximately twice as much wire to get a 6.3VAC swing as in a PT without a CT. You can get away without using an actual CT on the heater line because it is only going to carry AC current. Therefore, balance of the two ends (or symmetric swing) is not as critical, as in a tap that is going to be rectified into DC.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Jan 21, 2013 7:54 am
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Chris,

Here's a layout with the heater lines. Notice how messy the drawing becomes? Prolly the one reason most amp schematics eliminate these lines.

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


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