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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Sat Jan 17, 2015 12:19 pm
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http://i5.photobucket.com/albums/y166/r ... 592b16.jpg

Could be the bias supply has been modified, from the photo. Still looks like BALANCE pot setup (two lines off the pot).


38-40mA at that plate voltage, would be closer to ideal. 50mA is way too hot. With a BALANCE pot setup, the hum and noise will be magnified --- with the balance out-of-spec.

If I were you, I'd convert the BALANCE bias supply to BIAS pot supply. And buy matched pairs of 6L6GC's.


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Sat Jan 17, 2015 2:31 pm
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Well now that I look at it it's obvious that they have added a fuse to the rear 120 volt socket. They must have done it at the same time the converted the amp to 3 prong. That's what I get for letting my imagination get away from me. ;)

The tubes are all new.
The power tubes are a pair of matched JJ 6L6GCs.
The 12AX7s are Tung-Sol
The 12AT7s are JJ ECC81

I have finally flipped through my copy of "How to Hotrod Your Fender Amp" and it has a really easy to follow section on converting the 1971 balance pot setup to a 1963 BIAS pot setup *but* I ordered new F&T filter and bias caps this morning before I discovered the balance pot setup. I don't mind it being mild for now, the mod is on the list, but not super high on it.

When I get around to making another order I'll add the 25u cap from the '63 setup to it and buy the resistors to do the conversion then. Right now my #1 concern are those filter caps.


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Sun Jan 18, 2015 1:39 pm
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The bias circuit breaks down to a simple voltage divider so my mod was to simply replace the 15k resistor with a 10k resistor and 10k linear pot in series.

This mod will not only adjust the bias on the power tubes but also alter the voltage slightly to the vibrato circuit. I ***THINK*** it is only minor, but I will do some more probing and testing to be sure about it. By the schematics the AB763 design sent -55 volts down that line, compared to -48 in the AA270 and the vibrato circuits seem to be identical.


Steps made simple:
- Remove the 15K resistor that goes from the BIAS BALANCE pot wiper to ground.
- Drill a hole and installed a B10K pot to the chassis nice and close to the BIAS BALANCE pot. I call it the BIAS MASTER pot.
- Tie the wiper lug of the BIAS BALANCE pot to lug 3 of the B10K BIAS MASTER pot.
- Tie one end of a 10K resistor to lugs 1&2 of the B10K BIAS MASTER pot and the other to ground.

I can now adjust this side of the voltage divider's resistance from 10K to 20K which is similar to what the AB763 circuit does.

But I can also still adjust the BIAS BALANCE if I end up with mismatched tubes. Why Fender didn't spend the extra buck or two to do this back in the day is beyond me.

With the old rectifier I could set the bias as high as 75% 438V / 51mV and really low, around 24mV (around 30%)

The final BIAS numbers with the old rectifier are 438V / 44.5mV which on 6L6GC tubes is right in and around 65% and oh boy does it sound even sweeter now that it's dialed in. I had no idea this amp could sound so good though I figured since it was running cool it could be improved.

The caps should be in Tuesday or Wednesday, then it's more clean up, noise reduction and humming and hawing over what to do about the two bad speakers.

So long as I haven't made the vibrato circuit unhappy this seems to be the mod for me. I can always fix the vibrato voltage if it is a problem.


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Mon Jan 19, 2015 12:56 pm
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The vibrato is fine.

I put the new 5AR4 rectifier in and re-biased - 460V / 42mV - though it's a bit tricky, I really had to let the amp warm up a lot before the numbers would settle in. I'm thinking those ancient caps are acting up.

Still waiting for new caps.


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Mon Jan 19, 2015 6:03 pm
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The TRRI has both HUM and BIAS pots. The hum pot is on the heater line.

http://ampwares.com/schematics/65_twin_ ... manual.pdf


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Tue Jan 20, 2015 3:02 pm
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BMW2002Ti wrote:
The TRRI has both HUM and BIAS pots. The hum pot is on the heater line.

http://ampwares.com/schematics/65_twin_ ... manual.pdf


Sweet. I'm glad someone at Fender knows how to wisely spend a buck.
----------------
The caps didn't show up today, they've taken a small detour and will be in tomorrow.

I'm done scrubbing the loose rust off the head with wire brush and finished it up with a scrub pad and mild cleaner (actually a gun cleaner.) The rust stains will stay, this thing IS road worn, not just something new that pretends to be, and these badges of honor are part of it's character.

I finally used contact cleaner on all the pots and the RCA jacks. Why I waited until now to do this is beyond me. Oh wait, I couldn't find my can of contact cleaner, that's why. :D

I also cleaned up the power and rectifier tube sockets. They were finicky and I thought I might have to replace them but cleaning them worked. If this was to be a gigging amp I WOULD replace them and put Marshall style tube retainers in as well.

I went to town on the reverb tank which was functioning but still had flaky corrosion on the outside and a well rotted, moldy original cardboard bottom cover. I fabricated a new bottom cover, added in some make-shift vibration isolation, bagged it and installed it back in the cabinet. I've only tested at low volumes but the reverb seems fine. Will have to see if there is mechanical feed-back at higher volumes. If there is I have lots of ideas on how to better isolate it from the cabinet.

I'm getting slightly different voltage readings from yesterday. I *think* it's because the filter caps are old. I'm hoping it's not a sign of a flaky transformer. I will have to keep an eye on that after the electrolytics are done.

I found a deal on two Eminence Copperhead 10" speakers and they're in the mail. I'm going to do an X pattern with the original AlNiCo and 1028K AlNiCo speakers and see how it sounds. Some guys online say it's a good combination and I can't pass up the opportunity to buy speakers.

When I do eventually get the originals reconed, the Copperheads can go into their own 2x10 cabinet, or on eBay, or maybe I'll build a combo amp...


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Wed Jan 21, 2015 5:03 pm
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New filter caps are in and they made a world of difference. The bias cap has over 50v going into it and I bought 50v replacement so that's another oops on my part.

The 2 copperheads are in the cabinet with the 2 AlNiCo speakers and the sound is quite bluesy.

I still wasn't happy with the rectifier socket so I spent more time refurbishing it.


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Wed Jan 21, 2015 5:29 pm
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Sounds good. Yeah, use a 100mfd/100VDC cap in the bias supply. F&T or Sprague Atom.


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Fri Jan 23, 2015 1:40 pm
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Back in one piece for the first time in well over 10 years. In need of a face lift and a very little more non essential surgery but in my eye and more importantly my ear, a thing of beauty.

Image


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Fri Jan 23, 2015 5:28 pm
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jruddy wrote:
Back in one piece for the first time in well over 10 years. In need of a face lift and a very little more non essential surgery but in my eye and more importantly my ear, a thing of beauty.

Image


Congrats Jruddy! Some new grill cloth, a little elbow grease cleaning and you got yourself one helluva tuneful amp!

T2

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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Fri Jan 23, 2015 7:21 pm
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Think that you are going to love this amp! Well worth any work you put into it.


:D


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Fri Jan 23, 2015 10:57 pm
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A new all-pine cabinet with solid planks, finger-joined corners, and screw-in baffle board would be most appropriate for this project.

Arjay

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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Sun Feb 01, 2015 12:57 pm
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jruddy wrote:
Back in one piece for the first time in well over 10 years. In need of a face lift and a very little more non essential surgery but in my eye and more importantly my ear, a thing of beauty.

Image

HOLY CRAP!! That is one sweet amp!! Man, if I came across one of these and the guy sold it to me for $0-$200 it would be GONE!! Pretty unrealistic budget, but I just bought a Mustang 3, so now I am on a TIGHT recreational budget :lol: . Well, I hope you enjoy the amp!!!!


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