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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 17, 2013 11:43 am
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Do you suppose anyone well benefit from all of this ???
I sure hope so !

I pulled the V-5 6L6GC socket out of the chassis, solder sucker to the rescue !
cleaned a significant amount of solder from each tab/pin and no I didn't do all of that, however I did add to it without carefully examining the socket for overflow first (bad on me).

So at least were back to 90% stable, I have 34ma for about 20 minutes and running, still getting the faint BZZZP where the bias jumps to 35.5 ish then settles back.

Obviously my drips of solder had shorted something and now were good on that end of it, no more red plate !!!! Ya !!!! :D

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 17, 2013 2:06 pm
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Chris,

Ok, for the oddball oscillation, try the 47k-ohm resistor + 0.1 mfd/600mfd cap, first--- in the GNFB loop. This will save heating and re-heating that 100 pico-farad disc cap. Which seems like more of an after-thought, than much of an effect on the feddback.

That pico-farad cap is most likely there to help reduce ultra-high frequency instabilities. Ones that you prolly aren't hearing.

If the 47k-ohm resistor + 0,1mfd cap helps stabilize the amp. Add the 100 pico farad bypass cap.

If adding the GNFB network makes things worse ---- try one more thing. REVERSE the PRIMARY leads off the output tranny to the 6L6GCs. The Blue and Brown wires to pins #3.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 17, 2013 2:21 pm
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Now to remember exactly how it all hooked up

Image

???
Does this look right, a few things were moved in this area so I'm a bit worried about component placement

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 17, 2013 2:32 pm
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Chris,

Where that 0.1mfd/600VDC cap should connect is to one of TWO 1-Meg ohm resistors (BROWN-BLACK-GREEN-SILVER). I see one in the photo. Prolly another one UNDER the Mallory cap? Next to the 470-ohm resistor (YELLOW-PURPLE-BROWN-SILVER)?

If this is so, then, the 0.1 mfd cap of the GNFB loop should be soldered as in your drawing. 47k-ohm position in drawing is correct. And LONG red line goes to speaker jack.

Not sure what "Hide-N-Eyelet" means?

REMEMBER! If the GNFB makes the amp worse (howling feedback or even more OD) --- REVERSE the primary leads (blue & brown wires) off the PT to each 6L6GC anode plate.

Good luck1


Last edited by BMW2002Ti on Sun Feb 17, 2013 5:45 pm, edited 1 time in total.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 17, 2013 2:40 pm
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it means there is an eyelet under that other .1 mallory in the picture
it had a jumper wire to the next eyelet that I bypassed due to the physical difference in the caps.
I can bypass it again with the other mallory .1

yes the other unseen resistor is the 1Meg in that trio of resistors

I just couldn't remember placement
Thanks

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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: Sun Feb 17, 2013 2:43 pm
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Chris,

BTW... I know this is really anal --- but when you put resistors and caps onto the board ---try to keep the color bands of the resistors and the lettering on the caps in the same direction. Like those 220k-ohm and 100k-ohm resistors feeding the grids of the 6L6GC and plates of the 12AT7.

Polarity of these things won;t effect the performance. It's just one of those things us amps techs look at when doing a critique on previous work.


Last edited by BMW2002Ti on Sun Feb 17, 2013 5:49 pm, edited 1 time in total.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 17, 2013 2:45 pm
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blues bondsman wrote:
it means there is an eyelet under that other .1 mallory in the picture
it had a jumper wire to the next eyelet that I bypassed due to the physical difference in the caps.
I can bypass it again with the other mallory .1

yes the other unseen resistor is the 1Meg in that trio of resistors

I just couldn't remember placement
Thanks



Chris, Ok. just don't let the lead of the 0.1mfd cap touch this eyelet. Either cover the wire with insulation --- or raise it above the eyelet. I have no idea what else maybe contacting that eyelet.

Good luck!


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 17, 2013 2:59 pm
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I noticed my sloppiness with the direction of the bands and I can see why, it lets you see the circuit at a glance without having to do a second look.

I'm learning !

OK here it is in all its glory the NFB

Image

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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: Sun Feb 17, 2013 3:22 pm
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OK !!!
Well the tubes lit up white it shrieked and tried to blow up
something is not right here !!!

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2017 50's Baja Telecaster BSB

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: Sun Feb 17, 2013 3:58 pm
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What is funny as all get out is that Art mentioned the reversed wires from the very beginning and I thought of it as what we might have to do as a last resort, then I read on some other resources that without NFB it won't matter which leads go on the power tubes, however with Negative Feedback it will howl like a wild critter if their wrong.

Guess what blue belonged on V-6 and not V-5, go figure
Dang blue is just a touch short and for now is going over the middle of V-6 to make the connection (easy fix later)

Now the dang thing sounds like a silverface fender, clean and LOUD !!!
As Charlie Brown says " THATS IT " !!!!!!!

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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: Sun Feb 17, 2013 5:38 pm
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Chris,

Cool. You can put the 100 pico-farrad cap parallel to the 47k-ohm resistor. Think about Mallory-150 0.047mfd/630VDC caps to replace those old "brown drops." If you wish to smooth the tone, a bit.

With the polarity reversed in the output stage -- you get POSITIVE feedback Not NEGATIVE (as in global NEGATIVE feedback or GNFB). And that crazy howling.

Idle bias check, please!


:D


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 17, 2013 6:14 pm
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with no further adjustments 30.5 to 31ma
Making sure its stable for a few minutes before adjusting

Yes I'll put 150's in there next week and lengthen the blue OPT wire to V-6 so its not running through the heater wires.

nearly Zero noise now, quieter than the Bandmaster :mrgreen: :mrgreen: :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: Sun Feb 17, 2013 6:27 pm
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Chris,

Very good. Try going toward 38mA (with 470VDC on the plates) = 18 watts per 6L6GC. The tone should become less "SF" and more "BF."

Check red plating, etc.

Good luck!


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 17, 2013 6:43 pm
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Congrats Beemer !!!!!
Talk about your home study in fender amps.
Your an excellent instructor, especially when you consider the student ! HA !!!!

I will take all of the excellent information you gave and continue to study hard untill the theory is all there and I can trouble shoot on my own.
I have learned Volumes during this episode and sure hope it encourages others to learn also.
I just received my copy of yet another G Weber book (Desk Top Reference to Hip Amps) or something close to that, I'm digging in to it and have found some parallel
advise and some new ideas too.

Thanks also to all who jumped in ! Art for his help and his transformer he gave to the project !!!!!

Whats next ?????
A real AB165 1967 Bassman (blowing fuses)
A 1965 Blackface champ some snook drilled out the face plate and installed a toggle power switch :roll:
All it needs for safety is a three prong power cord, it sounds AMAZING !!!!!
Looking for some tips from ARJAY for tolex on both Bassman heads.

I'll continue to report here occasionally, but I hope to figure 95% of it on my own.
Many of the trouble shooting techniques learned in this thread and many e-mails will continue to serve as I buy up every silverface & Blackface I can scrounge up (cheap on C/L)
Thanks
and a couple of parting shots of future projects
God Bless !!!!

Image
Image

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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: Sun Feb 17, 2013 7:29 pm
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Chris,

No problem. Glad your amp seems to be wroking ok.

Hey! Here's a couple of links about global and local feedback. And how to alter parameters.

http://www.aikenamps.com/FeedbackAmp.htm

http://www.aikenamps.com/GlobalNegativeFeedback.htm


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