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Post subject: '69 Dual Showman Reverb questions
Posted: Sat Mar 31, 2012 8:15 am
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Hello all,
I have a few opening questions about my new acquisition but first a short story. I overheard a guy at work say that he never throws anything away. Knowing that this guy has been playing since '74 and gigging every weekend for the past 30 or so years I asked him if he had any old tube amps laying around that he might want to get rid of. He told me he has a Fender Dual Showman Reverb that was repaired 24 years ago and has been laying around ever since. It moved from garage to shed to the shed and he said he needed to clean it out before giving it to me. I guess some small mammals made a home in it along with numerous insectoids and arachnids. It could also pass for a elementary school project as a mold experiment. Needless to say I have my hands full. I gave it a good cleaning without going overboard and when hooked up to the speakers in my Peavy Renown 400 I was amazed at how quiet this amp sounded. It all works OK which is a nice place to start. Now for the questions. I want to make this amp safe and revitalized. I will change out all the tubes to start with since all the 6L6 tubes are different. What electrical parts should also be replaced? How should a grounded cord be installed? I realize this next question may be more of a mater of opinion but should I keep the amp as much "as is" or should I try to restore this with new tolex, hardware, knobs, etc? If I decide to restore it would enhancing it with a master volume control or other mods be cool or would it detract from it's value as a vintage amp? I don't plan on selling it but plans can change.

Any and all replies are much appreciated. Thanks for reading

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sat Mar 31, 2012 9:27 am
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Judging from the cosmetics, the amp is a '69. The ground issue is easy enough to address -- merely swap the OEM 2-conductor cable for a modern 3-conductor type. You'll find them ready-made and available at Lowe's, Home Depot, and other hardware stores. Tie the green wire to one of the mounting bolts used to anchor the power tranny to the chassis. There will be a Blue Molded .047uf cap attached at one end to the "ground switch", with the other lead soldered to the chassis. Disable and/or remove it. You may wire around that switch now or bypass it completely since it will no longer be necessary.

At a minimum, I would strip out all of the electrolytic caps under the "dog house" and replace them with Sprague Atom's or F&T's. Inspect the resistors there as well -- replace as appropriate with metal-film types. The bias supply circuit should be treated to a similar upgrade (note the reversed polarity of the cap). Barring any other issues, you're ready to re-tube and re-bias. A matched quartet of power tubes is recommended. This amp uses a bias "balance" control in lieu of the traditional adjustable bias pot, ie: you're balancing the paired tubes of the output stage. You'll need a quality DVM for this.

The pre-amp tubes will determine the amp's sonic character so choose those that please your ear. High-quality 12AX7A's should be used where originally a 7025 was specified (those are no longer manufactured). Be sure your phase inverter (V6, a 12AT7) has balanced transconductance to maintain symmetry within the output stage. The schematic for this amp will be identical to the same-year Twin Reverb so if you cannot locate one for a DSR, that will suffice. Likely, the circuit for this amp is the AA1069 revision but check the date codes on the trannies or stamped on the chassis to confirm.

HTH

Arjay

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sat Mar 31, 2012 10:06 am
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Thanks for the quick reply Arjay.

The amp is definitely a '69 with a stamp of what looks like F071669 on the cabinet interior and I got the serial number series from a site that stated my number falls in the range of "A16500 to A22400" - 1969. The schematic you referenced is for a different amp as my amp has 4 6L6 tubes in it. I don't know if surgery will begin this weekend or not. I was thinking of another option to restoration and that is to fit this into a custom made 2x12 solid wood cabinet.

AnS

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sat Mar 31, 2012 10:22 am
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Arc,

Take a look at my posting "bandmaster questions". I've got the wiring diagram you need posted there for converting from a two prong to a three prong power cord.
I just converted my bandmaster without any problem.

Abe.

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sat Mar 31, 2012 11:17 am
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All Twin Reverbs have four power tubes -- just like your DSR.

With a build date of week 16 in 1969, the circuit is the AA768 revision.

Your chassis is a drop-in fit to a Twin Reverb cab -- no muss, no fuss. All you need is a screwdriver. It will however require a longer set of reverb-tank cables. You can even substitute a TR faceplate if you wish and convert it to blackface cosmetics. Fortuitous since that 1969-era grill cloth is not available from anyone.

Arjay

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sat Mar 31, 2012 3:30 pm
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I took some more pictures. Are these the caps that need replacing? Do they need dischaarging before replacing?

Arc

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sat Mar 31, 2012 4:39 pm
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That is them.

Yes Discharge them ! now ! or sooner.

Again look through some old posts here to learn all you can Before you get yourself killed !

You should easily find "how to build a safe Cap discharge tool", "how to change the filter Caps" etc.........

Looks like a fun project, careful its very addictive.

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sat Mar 31, 2012 6:41 pm
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blues bondsman wrote:
That is them.

Yes Discharge them ! now ! or sooner.

Again look through some old posts here to learn all you can Before you get yourself killed !

You should easily find "how to build a safe Cap discharge tool", "how to change the filter Caps" etc.........

Looks like a fun project, careful its very addictive.


+1!

9th week of '69......they're *only* forty-three years old!

:lol:

Arjay

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sun Apr 01, 2012 5:05 am
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After some surfing through the net I found that the schematic is AA769. I was thrown off by a mod that was done to the balance pot. I'll have to look it over a bit to understand why it was done. Time to order some tubes and caps.

Arc

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Sun Apr 01, 2012 8:23 pm
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Can't see a mod in the pics, Arc. Just what looks like a wire splice. Guessing the wire got burned when the bias cap was replaced. The 68K resistor is clearly visible. So it seems stock to me. Art

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Thu Apr 05, 2012 6:49 am
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I just spoke to the guy who gave me this head. I asked he and his buddy had bought it with the cabinet and he told me that his bass player cut it in half so they could get it in cars. :shock: They still use it to this day.

Arc

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Thu Apr 12, 2012 6:56 pm
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I got parts in the mail today. I have replaced the 5 big caps under the hood. I just replaced the power and standby switches and a cruddy reverb pot. The cord is getting replaced later and the cap removed. New orange and mica caps are going in tomorrow. I have a new faceplate for it too. The grounded rear outlet should get here tomorrow or monday. Then I should have a safe revitalized amp to break in. Speaking of breaking it in is there a run in procedure for the new caps?

Arc

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rob


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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Thu Apr 12, 2012 7:06 pm
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Were this amp mine, I'd also replace the 50uf/70VDC bias/balance cap as well.

Arjay

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Fri Apr 13, 2012 1:42 am
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Arc, seems like you're on the way. Bedding in the power caps may or may not matter to you. If they are fresh caps, they were quick charged by the maker, and are good to go. The idea of slow charge, with a variac( which is a random method IMO) or a current limited draw(the most accurate forming method IMO), take many hours and the end results may only provide you with a little extra quiet operation and horsepower. So use your own judgement ! JMHO Art

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Post subject: Re: '69 Dual Showman Reverb questions
Posted: Fri Apr 13, 2012 7:15 am
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Ahh... I love the smell of rosin in the morning. So far it's going well. I have the power cord in and the cap removed. I have replaced the 470 ohm and 1.5k ohm resistors on the output tube sockets. All of the 470 ohm resistors read high... 515, 525, 530 and 550. the 1.5ks read a little high but one read 2.4k. I redressed some of the wires slightly. I am about to continue with the electrolytic cap replacement. I have a 50uf 50v cap in the kit I bought. Arjay: would that be the cap for the bias that you spoke of? I have a 100uf 100v sprague atom in there now but I have no idea how old it is. I'd say it's at least 24 years old and not powered up much in that time. The schematic calls for a 50 50 in that spot.

Arc

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