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Post subject: Re: NAD - 1969 Super Reverb - Finished (Almost)
Posted: Wed Aug 07, 2013 5:45 pm
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Retroverbial wrote:

Are there any EIA codes stamped on those CTS speakers, "Arnold"?

Arjay


All I have is:
CTS custom design
10E8540
8 Ohm
137 7323
No marking on cone, just the magnet.

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Post subject: Re: NAD - 1969 Super Reverb - Finished (Almost)
Posted: Wed Aug 07, 2013 5:53 pm
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Definitely not made for Fender. All CTS ten's purchased by FMI for use in the Super Reverb carried the Fender P/N of 064121.

Arjay

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Post subject: Re: NAD - 1969 Super Reverb - Finished (Almost)
Posted: Sun Aug 11, 2013 2:28 pm
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Got hold of Jerry's amp. Broke it open. Original Mallory electrolytic caps under the doghouse (note date).

Has anyone ever worked on a Fender amp with thin, colored, lacquered wire? All appears OEM.
.
.
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Post subject: Re: NAD - 1969 Super Reverb - Finished (Almost)
Posted: Sun Aug 11, 2013 2:46 pm
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Damn, those 'lytics and 'zistors under the doghouse look to be in pretty good shape!

That neoprene wiring does look a skosh "underscale" but it appears to be OEM.

IOW, if it ain't broke don't "fix" it.

Everything else seems to be unmolested.

Rawk on!

8)

Arjay

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Post subject: Re: NAD - 1969 Super Reverb - Finished (Almost)
Posted: Sun Aug 11, 2013 3:13 pm
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Arjay,

No these are actually lacquered cloth covered wire. Single stranded. Quite fancy. Never seen this in a Fender amp.

The caps have leaks and pimples on the side. So they'll need replacing. More photos to follow. As soon as the iron heats up.

:mrgreen:


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Post subject: Re: NAD - 1969 Super Reverb - Finished (Almost)
Posted: Sun Aug 11, 2013 3:19 pm
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BMW2002Ti wrote:
The caps have leaks and pimples on the side. So they'll need replacing.


Absolutely! I merely meant my remarks as a commentary on the extremely nice condition of the chassis overall. It doesn't look like it was ever abused or played in desperation. Jerry will have a real nice primo example of a Super Reverb when you've completed the overhaul.

Arjay

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Post subject: Re: NAD - 1969 Super Reverb - Finished (Almost)
Posted: Sun Aug 11, 2013 4:55 pm
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Wow! I didn't think Steve was going to get into my amp that quick. I just delivered it to him this morning.

I know he will do a great job !!

Jerry


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Post subject: Re: NAD - 1969 Super Reverb - Finished (Almost)
Posted: Sun Aug 11, 2013 6:20 pm
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Nice and clean! Makes me want another SR :mrgreen:


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Post subject: Re: NAD - 1969 Super Reverb - Finished (Almost)
Posted: Sun Aug 11, 2013 7:02 pm
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Very clean inside...gonna be a really sweet SFSR When you get it back.

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Post subject: Re: NAD - 1969 Super Reverb - Finished (Almost)
Posted: Mon Aug 12, 2013 1:10 am
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OK... update on Jerry's SFSR. Re-did lytic caps and coupling caps (replacing those dull brown caps).

Amp was converted to AB763 properly. Both the bias and the phase inverter setup. Now, to ramp-up the pupp and see how it biases. Then, test-drive.


F&T caps under the doghouse.

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Left Side:

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Middle:

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Right:

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Post subject: Re: NAD - 1969 Super Reverb - Finished (Almost)
Posted: Mon Aug 12, 2013 1:33 am
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You gonna replace those 1KΩ - 5W concrete resistors on the V7/V8 tube sockets with something more appropriate?

Caps look good -- top drawer all the way!

Arjay

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Post subject: Re: NAD - 1969 Super Reverb - Finished (Almost)
Posted: Mon Aug 12, 2013 3:56 am
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Arjay, thanks for the compliment.

Those sandbox resistor are actually on the screen (1K versus OEM 470-ohm). I figure wire-wound is prolly ok, though 5 watt is a bit of overkill. The grid stoppers are the usual 1.5k-ohm, 1/2 watt carbon comp variety. They are under the sandbox ones. And not on the socket (across pins 1 & 5).

Ramp-up went fine. 472VDC on the 6L6GC plates. And 415VDC on the input 12AX7 anodes. Test drive the thing tomorrow. Or I should say later, today.

:D


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Post subject: Re: NAD - 1969 Super Reverb - Finished (Almost)
Posted: Mon Aug 12, 2013 8:32 am
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BMW2002Ti wrote:
Those sandbox resistor are actually on the screen (1K versus OEM 470-ohm). I figure wire-wound is prolly ok, though 5 watt is a bit of overkill.


Leo specified a 1-watt rating here to provide the amp with a "fail safe" in the event of a runaway tube. These resistors are common to nearly all of his 6L6-powered designs.

Arjay

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Post subject: Re: NAD - 1969 Super Reverb - Finished (Almost)
Posted: Mon Aug 12, 2013 3:05 pm
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Arjay,

Yeah. The idea of changing the resistance value (going up), the wattage rating, and going to wire-wound resistors is more of an Ampeg SVT or Marshall Lead "fix." I think the previous tech was thinking in that vein.

As you know, the screen resistor's function is to drop the voltage onto that grid, such that it should nearly always be below the anode voltage. For 450 volts onto the anode, the 470-ohm resistor should drop the voltage about 5 volts. Thus, run about 445VDC –at idle—onto the screen grid. This grid help control electron flow through the tube. Preventing run-away.

The 5VDC drop works fine in good (and the golden word is GOOD) 6VGGTs and 6L6GCs up to 80 watt output. But, in less hardy made EL34s amps and those that run a bazillion volts onto 6550s or KT-88s—thermal runaway still happens with the 5VDC drop. I think ppl found this out with early Marshalls that simply copied the Bassman 5F6 circuit (which OEM is 100-ohms or only 0.2 VDC drop) – but used the EL34 output tubes. The later production “1961” and “1962” JTM-45 had 1k-ohm, 2 watt screen resistors (and these amps first used the great GEC/MOV KT-66. A very tough & expensive tube). Most techs up this to 5-watt rating for thermal stability.

The Ampeg SVT needs at least a 10VDC drop in screen over plate voltages. It’s the nature of parallel output sections (three pairs of 6550s). Where one unstable pair can take out the whole output section (including the output tranny). This amp really requires all wire-wound 5-watt resistors in the output. Both the screen and grid resistors. It runs that hot. Even with the fact the amp uses the toughest tubes made. The TungSol 6550 & the GE 6550A.

Ok. History lesson over. :lol:


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Post subject: Re: NAD - 1969 Super Reverb - Finished (Almost)
Posted: Mon Aug 12, 2013 3:56 pm
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+1

Yeah, that final-drive stage in the SVT is brutal on output tubes -- a real "muthah" for sure.

Only the hardiest of 6550's need apply......

:mrgreen:

Arjay

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