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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 02, 2013 8:04 pm
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Chris,

1.) The 82k-ohm/100k-ohm PI issue I think I may have discussed this before. The AA864 used an asymmetric V3 (7025) output into the V4 (12AT7, PI) section. Then balances the PI output stage by running appropriate voltages onto the two anode plates of the 12AT7.

This is true in a LOT of original early BF circuits. The idea was to get a bigger swing in the PI section to drive the 6L6GCs. Without running the 12AT7 too hard. At least that is what I have read.

Look at the components around the 12AT7 PI section in the AC864 versus the AC568. Notice how much simpler the symmetric AC864 12AT7 section looks? I think that this is one of the big reasons the earlier BF circuits sound so much different compared to the later SF circuits.. In the same model Fender amp.

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

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

You would need to change all components of the AC568 7025/PI sections (V3 and V4) to the AA864 to get the benefit of that 82k-ohm/100k-ohm plate loading resistor tandem. This means changing the local FB on the preceding 7025. Plus, the input resistor and cap onto the PI section's signal grid (pin 2). And the grid-to-ground setup on pin 7.

The motorboating or hissing noise that you are getting is prolly not due to the lack of an 82k-ohm/100k-ohm plate load arrangement onto the 12AT7.

Prolly this motorboating is a lead dressing, worn socket, poor solder, grounding issue(s) around those 12AT7 and 12AX7 (V3 and V4). Maybe, even around V1 and V2 --- the input tubes.

2.) Try putting the 47k-ohm back in with the 100 pico-farad and 0.1 mfd/600VDC cap back into the GNFB circuit and see if this solves some of the motorboating If the amp did NOT motorboat with NO 47k-ohm GFNB resistor (no GNFB) --- let sleeping dogs lie. The less global feedback, the better the amp.

3.) I think that the popping issues of the STANDBY SWITCH are a separate problem. Exacerbated by all this turning on& off testing. It really sounds like that switch is starting to fail on you. You can TRY opening it up and cleaning the contacts with fine crocus cloth. Just be sure to clean off all metal shavings, before reassembly.

Good luck!


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 02, 2013 8:23 pm
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Well.......
As soon as I removed the 47K the blast went away again so........
I don't think its the switch.

I'll install a new socket in the 12AT7 and see where it is tomorrow.

It really helped in V-3 for sure.

I may have to rebuild the NFB circuit, but I hope not !
Other than a bit of noise it sounds meaty as all get out about half way up the dial !!!

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 02, 2013 8:24 pm
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Chris, motorboating= oscillation. You're talking about two 47K resistors that apex on the board, and then run to pin#1 & pin#6 of the PI, right? And there is still no negative feedback circuit? Am I correct? Art

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sat Feb 02, 2013 8:54 pm
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Art,

No. I think Chris is talking about the 47k-ohm resistor in the original GFNB loop of the AC568 cricuit. The network with the 100 pico-farad bypass and 0.1 mfd/600 VDC Brown Drop inline cap.

The 47k-ohm plate loading resistors to the 12AT7 have been changed to 100k-ohm resistors (to lower plate voltage back to AA864 standards). It's not a complete retrofit. As the frort end (signal grid portion) of the PI has NOT been reverted back to AA864.

I've seen many SF Fenders (though not a Bassman) with AC568 reverted back to earlier BF circuits --- without completely overhauling the V3 and V4 sections. The tone is not completely pure BF. But, more of a intermediate tone between the BF and SF.

If Chris can run that amp with NO GNFB, the better. Bassman run very nice OPTs. So with good 6L6GC and original iron you get a much slower, easier OD tone. Almost a GZ34/dual 6L6GC tone.

The last amp I setup like this was a Bandmaster Reverb head unit. Went back to more of a Super Reverb AB763 circuit from the original AA1069 (TFL-5005).

The dropping of the plate voltages onto the 12AT7 made the amp more flexible throughout the volume range. And a bit Bluesier. Althoug the PI sections between the BMR AA1069 and the SR AB763 is not as much different --- as Chris' Bassman AC568 and the BF AA864.


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: Sat Feb 02, 2013 9:38 pm
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OK man!!! That was what I was getting at. A partial revision won't necesssarily work well. The amp should be reverted entirely to the 864 design and then things like lead dress and down-stream grounding issues, can be more easily addressed. Beemer, to be honest I didn't want to put too much on Chris's plate. But I think if you walk him through the process, it'll be fine. I'm gonna bail and let you guys work through this. "Too many cooks spoil the stew" !!! Art

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 03, 2013 7:54 am
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Don't know if you guys noticed but I've been slowly rewiring with 22 gauge cloth from AES.

The issues at hand are pulsing 6L6's and especially V-5 and the constant swish noise which is really almost liveable.
The amp overdrives at a much earlier level than say my Bandmaster Reverb
Most of the rattle at higher volumes was taken care of with the new socket on V-3.
A new socket is going into V-4 later today, I'm hoping it solves some or all of the issues.

Now the question is: rebuild the NFB or go on with the rest of the mods ????
I'm game if you guys are and if you feed it to me in little bites please, if your weary of all of this, its OK no harm no foul 8) or if you think Its too far over my head.
I can rebuild the NFB in a couple of minutes so no biggie there.

Thanks Guys !

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 03, 2013 9:06 am
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Chris,

Try it without the GFNB network (the 47k-ohm. 100 pico-farad, & 0.1 mfd cap). The reason I'm kinda rock-headed about this, is that the GNFB loop on the AC568 is NOT your typical resistor-style network.

First off, most GFNB feedback to the PI or earlier gain stage run through via a resistor network divider. Like the AA864 circuit. See how it feeds not only the 12AT7 grid, but also the 100-ohm grid-to-ground resistor?

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

It create a voltage differential on that grid. And varies the signal gain, keeping the output tubes as much in linear range as possible, throughout the volume range.

Anyhow, the AC864 GNFB is much more complex. Prolly a product of the amp original design for bass use. And how hard the 12AT7 is being driven & the original hybrid mehtod of biasing the 6L6GCs. It feeds directly to the grid of the 12AT7.

You have removed the cathode-tied biased resistor to the 6L6GCs (150-ohm, 7-watt sandbox). In addition, you've lowered the plate voltage of the 12AT7 to BF levels. And will most likely be playing guitars and not basses through the amp.

Anyhow, if the amp bias is stable. And the tone is good. Try the amp with & without GNFB. It will not harm the amp, if you go without the GNFB. As long as the output stage is stable.

Little ditty on GNFB:

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

Good luck! Keep us posted.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 03, 2013 3:35 pm
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Checked the bias again after installing a new V-4 (12AT7) PI
Its jumping around from 32 to 35 ma

I'm going to rebuild the NFB and see what happens
The real question is, do i have to re-install the sand box resisters and the other wafer caps ????

here we go

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 03, 2013 4:15 pm
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Chris, NO! Do NOT reinstall the sandbox resistor or 2000 pico-farad caps. These components are unique to the AC568 circuit and bias power supply layout.

Drifting around 23-35mA is ok. Prolly more of a function of thermal heater and socket contact. Try reading the amp about 30 minutes into use. And then again, in an hour or two.

You may want to temporarily put that GNFB netowrk into the amp. Be ready to pull it. It may not result it any change or may cause other issues. That's because the 12AT7 PI is idling at a much lower point, than before (closer to BF levels, than the SF levels).

So the amount of influence the GNFB network will have on the 12AT7 changed.


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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 03, 2013 6:05 pm
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rebuilding the NFB had nearly zero effect of the amp and I was using used parts that have been heated a few too many times so........

Cranking the bias pot all the way clockwise only yields 36ma

The amp is very bright, and I'm playing it through a pair of C-rex

One of the large .01 uf wafer style caps got over heated (cooked)

and again the amp is going into Distortion way too early !!!!!

Thanks

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 03, 2013 7:07 pm
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Chris, you're on the right path!!! I'm not weary, just thinkin' one guide is best. Keep at it, man!!! Art

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Sun Feb 03, 2013 9:43 pm
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I put a 5751 in v-3 an RCA 1958 12AX7 in V-2 and a nice GE 12AX7 in v-1

Still have the (testing) tubes in the power section, they were new GT 6L6GE's

I'm getting sweet warm Fender tones at volumes up to about 2.8 then it gets kinda harsh and it gets real fizzy with any pedal attempts past that volume too.

I'm past the idea of re building the NFB for a third time 8)

Still wondering about the tubes topping out at 36ma on the new bias pot ??????

I'm thinking we are very close to a final inspection here !
Some fine tweaks maybe ?
The 12AT7 (I've tried four different ones) flashes fairly bright when the power switch is flipped to on, the standby problem went away after the new socket was installed in the 12AT7

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Feb 04, 2013 12:15 am
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OK dude, I'm gonna jump back in. V2 would be my choice for the 5751. The amp is running both channels through this tube. For the PI, I would choose a GE 12AT7-halo getter tube(balanced if possible). Turn-on flash is different among tubes and is not usually a problem symptom. I think, the power amp section needs to be viewed as a seperate entity, in this case. Beginning at the pre amp's input to the PI, is where the scope of examination and changes must occur, and it won't end until the output transformer's hook-up to the speakers. In my estimation, you have already done 70% of the work. If you can bring the PI and negative feedback circuits up to 864 specs, I think you'll find that the amp is easier to manage. JMHO Art

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Feb 04, 2013 3:42 am
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Thanks Art

I printed another copy of the AA864 layout to see if the changes are evident to me.
I'm afraid half of the problem is that I'm not well versed in some of the terminology and so it is difficult for me to understand the direction of the information shared here at times.

But yes the amp is sounding much better ! and I'm learning a great deal in the process.

One somewhat frustrating thing about being a beggar of information is that many times after searching for the information myself, I find that the answers are given as if to a trained tech and not a student, which is what I would certainly consider myself.
However when asking for a free education one should expect to have to dig in !

Thanks again to all who have spent their own time and energies without much hope of reward.

The thrill of taking one of these amps and making it sound great is what makes it worth the stress to me !!!!

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Keepn' the Blues Alive

2004 50th Ann. Limited edition AMSE Stratocaster
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VHT Special 6 Ultra combo

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Post subject: Re: 68-69 drip edge Bassman /circuit changes
Posted: Mon Feb 04, 2013 3:45 am
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PS
Art didnt we eliminate the NFB circuit ?
and what is left to do with the PI circuit ???

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VHT Special 6 Ultra combo

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