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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sun Apr 29, 2012 7:46 pm
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What I mean is that you'd be converting the entire power section to AB763 Twin Reverb(Showman) specs. So you'd have to compare what you have in the amp already, with the Twin's schemo. This would include the voltage that feeds the phase inverter plates. That's where the resistor between the filter caps would need to be examined. If you don't touch the pre amp section, you may have to make adaptations, to keep those voltages inline with the original(769) design. BTW I like the test point set-up! Art

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Mon Apr 30, 2012 5:02 am
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That's the odd part about it Art is that the AB763 (blackface) layout has the bias adjust output tagged as -52V. I have -54V so it would seem like the bias adjust is ready to go but the resistor value for the piggyback resistor on the pot is so different. maybe when all is said and done it'll fall in place but knowing how things like this can go I'll be looking at a lot of voltages all throughout this amp. According to Tino Zottolas book all the changes happen in the PI and bias adjust. It looks like I'm on the hook for changing 8 resistors which is a minor thing really.

The test points are nice to free up my hands and safely probe the bias. I'm glad I could make the test points from salvaged era correct parts. To me, they look like they were intended to be in there. I don't really need the extra points... yet, to poke around and take other measurements but I may find a couple that I need in the future. I have a matched set of tubes plugged in and I'm glad to see that the bias current and plate voltages for both pairs are nearly identical.

Before I bought the MoJo tone blackface cap kit I scoured the AA769 and the AB763 schemos and noticed the only differences were in the PI so I figured I didn't need to go the whole route right away if I didn't want to but this bias issue has me wondering if I should have. I don't know what these resistor changes will do but I'm jumping in with both feet now. That reminds me. I need to make a real cap discharging tool.

Arc.

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Mon Apr 30, 2012 3:07 pm
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Right, that resistor on the low side of the bias pot is just a safety feature, in case the pot fails. A 27K would do the job. Once the new resistors are in place on the board, the voltage at the PI's 220K junction would be my only concern. Keep at it ! Art

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Mon Apr 30, 2012 6:44 pm
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So digging around some more I found out that switching the standby switch on AFTER the amp is switched off discharges the caps in this amp. I usually don't jump right in after turning it off anyway and I checked with a meter to see the voltage dropping down to under a volt. I finished it off with a short anyway to be sure and no spark happened.

Arc

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Mon Apr 30, 2012 10:12 pm
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Yep ! Bleed resistors doing their job. But keep that meter checking periodically. The filter caps can recharge. I usually leave a 470K 1watt(short) lead in the amp during work. I have a couple of Tino Zottola's books, that I use for reference. Very good info! Art

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Mon Apr 30, 2012 11:39 pm
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Filter caps definitely can recharge, after the amp is turned off. Leave the STANDBY switch turned on, while bleeder resistor is connected-- when amp is on bench. This insures ALL caps will stay discharged.

Just remember to turn all switches off & REMOVE bleeder before lighting up the unit. You think this is funny, but I don't know HOW many techs relay the "hey, my amp is showing a short" story after recapping it. It's why a use a BIG WW resistor as bleeder (wired to alligator clips)-- big thing is always staring me in the face.

Keep safe!


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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Mon Apr 30, 2012 11:54 pm
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BMW2002Ti wrote:
Just remember to turn all switches off & REMOVE bleeder before lighting up the unit. You think this is funny, but I don't know HOW many techs relay the "hey, my amp is showing a short" story after recapping it.


LMFAO!

I once replaced a couple of burnt-out dial lights in my old Pioneer SX750 stereo receiver then couldn't understand why, during the post-procedure function check, the amp refused to power up.

After thirty minutes or so I finally discovered the problem......

I'd plugged the unit's AC line cord into its own back-panel convenience receptacle.

DOH!!!

:lol: :lol: :lol:

Arjay

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Tue May 01, 2012 1:04 am
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Arc, I think you were thinking about putting together a drain line. Like Beemer said, a wire wound resistor is cool. Which ever you choose, it is best to leave it connected while you're in the amp! Always take the safe route!!!! It isn't all that important about circuit results(they can be changed). But your personal safety is the paramount consideration!!! You seem to know your way around the amp, so have fun! Art

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Tue May 01, 2012 4:50 am
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OK... you guys have me convinced. When I took apart that bench power supply it had a couple of 1.25k WW resistors in it. I'll turn it into a BIG goofy looking discharger that I simply cannot forget to remove before powering up the amp. You guys know better than me about these animals and I respect your advice. The parts for the BF conversion have been ordered as well as new external hardware and tolex. I'm just not sure about the grill cloth yet. That will be the last piece of this project.

Arc

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Tue May 01, 2012 6:30 am
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I always drain the caps with the standby on and a little wire with a resistor and alligator clips one each end attached to pin 1 of a preamp and the chassis...I always remember,or have so far,to unhook it,but there's no telling,I'll probably forget next time.
I have forgotten to put the power tubes back in when I've removed them for some reason when putting everything back in the cabinet. :lol:


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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Tue May 01, 2012 6:59 am
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Unfortunatley I think we are all prone to a brain fart now and again and that's OK as long as we don't zap ourselves or anyone else in the process. It's been many moons since the last time I've been zapped by line voltage, 400hz or 7.5kV. I've been fortunate enough when zapped hard to just need a walk around the building a few times to get my composure back. I don't need any more wake up calls at this time in my life so I will be making the new shorting tool tonight.

Ken... AKA Arc.

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Tue May 01, 2012 8:15 am
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Arc-n-spark wrote:
I'm just not sure about the grill cloth yet. That will be the last piece of this project.


How big a piece do you need?

Arjay

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Tue May 01, 2012 9:08 am
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Not much and since this will be blackfaced and thus not restored to original specs I guess I could put on anything I like however I think you told me that the original grill cloth is not available am I right?

Ken

It looks like this only not so much turquoise.

Image

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Tue May 01, 2012 7:12 pm
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I took these things I had laying around...

Image

and turned them into this.

Image

I think I already feel safer

Ken

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Wed May 02, 2012 2:18 am
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Way to go, man!!!! Ken, be sure to have fun with the project!!! Art

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