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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Feb 04, 2013 8:50 am
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Chris,

Which disc cap overheated? Where is it on your amp?. We need to figure out if this is a feedback issue or something else.

Ok.. We are at a crossroads. Obviously in this amp, you cannot use the AC568 gain stage/PI to drive a AA764 circuit. You can try ONE MORE thing before total reverting-- either back to ALL original AC568 circuit or completely going back to AA764.

Try replacing the 100k-ohm plate resistors on the 12AT7 with the original 47k-ohm ones. See if this helps with the clean tone and volume. YOu may need to go back to the original AC568 GNFB. If this doesn't help:

You are prolly going to need to go back to AA864 COMPLETELY, to get this amp stable and sounding good.

1.) That means replacing all of the parts on the 12AT7 AND the 7025 tube which drives the 12AT7. BTW... nearly ALL European made 12AT7 (ECC81) flash bright on turn-on. Just the way the heaters are placed in the tube. No problem.

2.) Look very carefully at the 7025 and 12AT7 (V3 and V4) of the original AC568 schematic. Highlight the components that differ between teh AC568 and AA864. Not find them on the layout

3.) You'll be ditching the AC568 GNFB circuit (47k-ohm, 100 pico-farad, and 0.1 mfd cap).


Don't worry about posting problems. It's going to take time. Gotta run now. Will check Board later.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Feb 04, 2013 8:58 am
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This one
Image
The big wafer shaped one the .01
I burned it after the picture was taken.

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Feb 04, 2013 10:18 am
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Chris,

That one feeds the V3, 7025 grid (pin 7). This is the tube, whose socket you replaced. Maybe should check the solder points. Also, check to see if the 470k-ohm (grid-to-ground) resistor is ok.


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


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Feb 04, 2013 5:02 pm
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Guys, we have a communication breakdown!!!! On a Bassman of this era, V1 is the bass channel input. V3 is the normal channel input. V2 drives the PI(V4). This set-up is a carry over from the earlier Bassman's, when only the bass channel used the extra preamp tube. It's not intuitive or apparent when looking at the schemo, and the fact that they are not labled in the schemo, doesn't help. OK? Art

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Feb 04, 2013 5:17 pm
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Thanks Art !
So how is that effecting our progress ?
I was pretty sure V-2 was the second gain stage for both the bass channel and the normal, Not so ?

I also A/B'd the bassman to the Bandmaster Reverb I have, what I noticed seems to be a very Mid's heavy sound to the bassman, the bassman goes into OD a bit sooner than the Bandmaster and its not as pleasant sounding.

Hav'n FUN !!!!
By the way guys I scored a pair of RCA 7027A's from that one auction site
and way cheaper than new JJ's (Ampeg related)

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Feb 04, 2013 5:19 pm
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Art,

Ok thanks. Think I should have been more desciptive. Like, the 7025 driving the PI. You know what would have been a WHOLE LOT SMARTER? A picture of the tube chart for this amp. But, that would be way too easy. :D

The V2 (7025 driving the 12AT7, PI) has the 0.01mfd disc cap on its grid (pin 7). This is the cap that overheated, right?

Also, this is the socket you replaced, correct? Prolly a good idea to check all solder points. Looks for shorts and bad grounding points,

How bad was the overheating? You may want to change that ceramic disc cap. You really do not need a capacitor meter. If the disc overheated to the point where it was "sweating" --- I'd replace it.

The amp is ODing sonner prolly because of the PI section current setup and the GNFB loops (or lack of).


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Feb 04, 2013 6:18 pm
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A sylvania 6L6GC just flashed and blew a fuse, volume was at 6 and she was a rock'n , still kinda shrill which is weird for C-rex speakers.

A new pair of used Sylvania 6L6GC's is in there and working

Push it and the tubes pulse big time !

A small thing I may not have mentioned, I re tensioned the 6L6 tube sockets
The tubes are still very loose in the socket (((VERY))) !
could it be as simple as needing new 6L6 sockets ????

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Feb 04, 2013 6:44 pm
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Ok Chris,

Sorry about all the miss- and dis-information. Got my schematics and beer. Ready to go.

Turn off amp. This unit blew a fuse before, correct? I forgot at what point of the modifications.

For now, you have an unstable amp. Prolly the PI section.

Put the 47k-ohm grid-to-ground resistors and the AC568 GNFB network back into the amp. Measure voltages. Check bias.


If this doesn't work, I think we are definitely at a crossroad. Time to sell your soul to the Devil. :mrgreen:

Choices:

1.) Go back to the AC568 circuit completely. With the new output tranny (OPT).

2.) Go to the AA864 circuit completely. This requires major surgery to that 12AT7 PI section. In addition, mods to the V2, 7025 section.

NB: That AA864 is MUCH different than the AC568. Another difference in the NORMAL CHANNEL V3, 7025 (12AX7) has a differential feed to V2, 7025 (second gain of the BASS CHANNEL).

The AC568 NORMAL CHANNEL feeds signal to the grid (pin 7) of V2. Plus, signal to the anode of V2 (bypassing V2). Whereas, the AA768 only feeds signal to the anode. Bypassing V2.

What does this all mean? These are two quite different beasts here.

Good luck! Keep us posted. Hope you bought a pack of fuses


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Feb 04, 2013 8:47 pm
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I can't find a layout for either that is clear enough for me to tell the values of the items needing changed.

Very little sleep last night and so it's off to bed for me tonight, the eyes won't stay open.

Sure wish someone had diagrams you could read with the changes needed, i mean come on how many of these have been changed ????
G. Weber says "in my other book" well POOH !!!!

I have a standing order for Mojotone waiting, as soon as I know what I'll need :roll:

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Feb 04, 2013 9:27 pm
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Very cool, guys!!! We're almost on the same page. I believe Chris replaced V1 and V3 sockets(correct me if I'm wrong, Chris). Seems the power tube sockets are candidates for replacement. OK,Chris I'm going to give you a resource that should give you a clue to the different Bassman circuits and changes that are available. It will be in text, don't know how to do the link.....http://tone-lizard.com/Bassman_Modifications.htm

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Feb 04, 2013 9:42 pm
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Hey art ,thanks !
I replaced v-2 and then v-3 sockets

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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: Tue Feb 05, 2013 12:41 am
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Art & Chris,

Very nice site. I forgot about the Tone Lizard mods.

He has summed up the entire 12AT7 PI and 7025 second gain stage issues well. I never realized in this amp there was such a BIG difference between the BF and SF circuits. This is one amp that Fender-CBS really modified to the max.

I cannot find any site on the Net that has the official AC568 circuit layout. I guess what you'll need to do is compare your current layout with the AA864 (which is on the Net). Then, highlight all the differences. Compare the different components to the schematic of both circuits. That way you'll have a record of what you did. Need to do. And how to revert back to un-modified amp. Though, you prolly won't do this, once you can get that amp sounding good and running stable.

Sorry Chris, should have really looked at the two circuits more carefully.

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


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Feb 05, 2013 1:09 am
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Our Canadian brother, Stratele has referred to this site in the past. So we have another guy who knows the gig. Steve, I couldn't find the 568 layout either. But I have the feeling, that the 568 scenario is headed for the ditch. OT, surprised the heck out of me that the site showed up in red. Didn't touch another computer button, after that! Art

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Feb 05, 2013 1:18 am
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Art,

I have no idea what CBS was thinking. The AC568 in the Bassman reeks of hi-fi. Lots of local feedback. Complex GNFB. Differential inputs. Etc.

One Q... did you mod an AC568 COMPLETELY back to AA864? Does Chris have the proper number & alignment of eyelets to add and modified that AC568 board to AA864?

Or do you think he should just start with an NOS or aftermarket AA864 board, to avoid wiring hassles?



Thanks!


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Tue Feb 05, 2013 2:17 am
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Steve, my amp was a 165. It came from a collector, I was working for. Someone tried to turn it into a 50watt Marshall. The 165 board is not exactly the same as the 864, can't say the 568 is perfect(just don't know). But the retro is possible. This issue is why I suggested a new board for Chris's project. Besides, I never liked those brown caps and the ceramics were all subjected to the bean-counters, in that era. I don't mean to say the components won't work, but the only components I liked were the resistors. To be honest, I spent two years dinkin' around with my guy. A bunch of piece-meal efforts in all the sections. Finally decicided to strip the board and power filter board(wiring and all) and rebuilt the amp into a pseudo 864 bass channel and a 763 pro normal channel(less vib). Did my usual tone cap, plate load resistor, and mid freq adjustments to the pre amp. The power section is basically the 763 design. So I changed the NFB to taste and added the Ken Fischer cap trick to get full presence. Other than that crap, it's stock!!! Sorry folks, too much information. Got to get back at the tele, dates comin' up next week. "People Get Ready" is on my agenda. Peace, Art

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