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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Fri Mar 24, 2017 12:48 pm
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Yeah, the ultra-linear's are superb platforms for pedal steel. Plenty loud and clean for days and days and days. The re-issues a little less so but still pretty faithful to Leo's original concept.

Arjay

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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Fri Mar 24, 2017 6:09 pm
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The last TRRI I worked on (around 2005) was for a guy doing lead jazz work. I remember we went with Philips-ECG 7581A output tubes. All RCA 7025 and 12AT7 9-pin minis. And JBL K120 speakers.

He plays a Gibson ES-175 and Super 300. He really likes his TRRI. FWIW...

You have the makings of a nice jazz combo amp.


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Fri Mar 24, 2017 7:04 pm
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There's nothing fundamentally wrong with the older re-issues. My '99 TRRI has performed flawlessly since the day I bought it in January of 2001. All I did to it was replace the 'lytics with Sprague's, plug in a matched quartet of blackplate RCA's, bias it to around 37.5 mA, then install a pair of K120's. I suppose it's logged some 300 gigs or so.

Arjay

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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Fri Mar 24, 2017 8:22 pm
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Thanks guys. My first step will be to get the BF mod complete. For now, while I'm working on it I'll leave the new(ish) Groove Tube PTs and PI that are currently in it. (Those will definitely be replaced down the road! Like most, I no fan of GT) For preamp tubes, I put in a fresh set of JJ brand in the normal values. Again, they'll work for now while I get the circuit tightened up.

Let me ask a question about restoring vintage amps. How does the wire hold up? Should I plan to replace it or just do the normal checks for cracks in the jacket and bad solder joints? Is there any virtue in keeping the old wire just to keep it? Thanks.


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Fri Mar 24, 2017 8:27 pm
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Absent a demonstrable issue, leave the factory wiring in situ.

Arjay

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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Sat Mar 25, 2017 7:35 am
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Yeah, prolly the only thing I would change is the addition of those grid stopper resistors, as per AB763 layout. Be sure amp can be set to a steady idle bias. No crazy hums, thuds, crackles, or bumps-in-the-dark. :D


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Sat Mar 25, 2017 12:17 pm
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Thanks much. Idle bias current holds pretty steady. I set it to 35ma with my EuroTube Bias Probe. I also have another bias tester that I plug into my VOM, allowing me to check two tubes at a time. All four tubes hold steady and are less than a mA apart from each other.

Where do you guys set the idle bias on these AB763 TR? I've got these Fender/GT 6L6GE that are rated for 500v, 30w. IF my math is correct, 50% = 30mA, and 70% =42mA. It sounds pretty darn good at around 35mA. At 30mA it started to lose some fullness.

Thanks.


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Sat Mar 25, 2017 4:32 pm
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SkipBurz wrote:
Where do you guys set the idle bias on these AB763 TR? I've got these Fender/GT 6L6GE that are rated for 500v, 30w. IF my math is correct, 50% = 30mA, and 70% =42mA. It sounds pretty darn good at around 35mA. At 30mA it started to lose some fullness.


Then your sweet spot with those tubes is 35 mA. Run with it.

Arjay

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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Sun Mar 26, 2017 10:18 am
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Guys, I'm going to order some parts for the AB763 and have a few questions:

1. For the 1.5k grid stopper resistors, will any 1/2w carbon composition be okay, as opposed to film or metal?

2. The original 470 ohm resistors seem to be okay - They all test around 480 to 490 or so. Should I leave them alone or change them as well while I'm at it?

3.One of the preamp tube sockets is noisy and not holding the tube very well. Will any 9-pin replacement socket do or are some better than others?

Thanks.


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Sun Mar 26, 2017 12:08 pm
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Leave the 470-ohm screen resistors in place. The 1.5k-ohm grid stoppers should be carbon comp. They have the best low-inductance characteristics, for the price.

You can try tightening the socket tangs with a dental pick. Be sure to have amp off & discharge any residual voltage on the main PSU caps. Belton makes good 9-pin mini sockets for Fender amps.

http://www.ebay.com/itm/BELTON-9-pin-12 ... 1754311498


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Mon Mar 27, 2017 7:53 pm
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Hey guys, What's the normal plate voltage for this Fender Twin? I have a Eurotube Pro One Bias Probe. It reads 412v. It's got 30w Groove Tube 6L6GE power tubes. So, 30/412=.0728. So, the amp sounds good to my ears with both my ES335 and Tele biased at 35mA, which seems a little on the cool side. I know all the business about ignoring the math and letting your ears guide you, I just want to make sure my plate voltage is where it's supposed to be. Thanks all.


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Mon Mar 27, 2017 8:56 pm
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You should have a minimum of 410 VDC at the plates. Pretty low relative to the AB763's 460 volts. That's why these amps only put out about sixty watts (despite the labeling at the output jack). CBS tinkering strangled these things, same with the Dual Showman.

Arjay

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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Tue Mar 28, 2017 5:29 am
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Thanks. I wanted to make sure it within an acceptable range.

Besides the 1.5k grid stoppers, did you see anything else that was non-AB763? I still have to systematically go through the diagram and schematic to check all the values.

Thanks...Skip


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Fri Mar 31, 2017 8:32 pm
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Guys, Concerning the Oxford 12T6s that came out of this amp, as Arjay said I would - I find them dull, mushy and just lacking in clarity. I spent some time tonight reading past posts on this forum about the Oxfords - and with Arjay leading the pack, most agree that after the mid-60s they really went to downhill. So my question is can reconing or any "modernization" save them? I don't want to re-litigate the virtues or lack thereof of these speaker. Suffice it to say, I don't like the way they sound in my Twin, DRRI, or 5E3 Tweed. If there's no way to make them significantly better (meaning, more clear and balanced), I'll just put them up for sale. I know there's a market. Thanks all.


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Post subject: Re: 1968 SF Twin Reverb AB763
Posted: Tue Apr 04, 2017 6:20 am
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From Schematic Heaven site:

"Many amp geeks don't like Oxford speakers found in Fender amps from 1965 through the 1970s. The gap distance was increased in the Oxfords that Fender used later in the decade and this reduced their efficiency (and they were cheaper to make this way)."


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