NAD - 73 Quad Reverb hatchet job

[tubeswell]

So I just picked this do-up chassis in a head cab up off a mate who has too much vintage gear to look after it (in case I run out of things to do with the rest of my life)


Today's job - looking at it scratching my head (thinking 'Why do people do this?)


AFAICT, its mean't to be a 100W MV export model. I think this is the correct schematic (At least the xformer codes match up)? The EIA dates point to around/after 3rd week Oct 1973.


I've never seen that little xformer next to the PT before ( Wondering if that's that part of the thermal protector circuit?, and whether there is any point bothering with keeping that in, seeing as how its practically going to amount to a virtual rebuild)?


I have a DSR cab load with a pair off orange frame bels that I can run it with (if I keep as a head), seeing as how its practically the same as a DSR from the same era.

[PRR]

> that little xformer next to the PT

Choke?

[tubeswell]

Hmmm, yes I did wonder - There is a proper choke underneath (but whether it works is another matter)


(Edit: I thinks its a separate bias supply transformer someone has added. There are two sets of leads coming of it. a 2 x yellow twisted pair and a blue and brown twisted pair. The yellow wires are going to the bias supply circuit. The blue wire is grounded, and the black wire is attached on one of the main PT leads. Maybe they've used it to buck the bias supply up?)

[tubeswell]

Some of the other carnage inside

[tubeswell]

The filter caps have never been replaced - which could explain some of the other 'repairs' (like the bias supply transformer). How bad will this get? time will tell.

[tubeswell]

Progress report.



At some stage in the past the lamp holder's insulating cork had cooked, leaving charred contacts. The lamp had been disconnected from the heater winding. Also one of the 6L6 1R cathode current measuring resistors was fried, so it seems that there was a power tube failure. I did't trust the old lamp holder nor the 6L6 sockets, so I replaced them with new parts, and generally cleaned up around the chassis with isopropyl. (I even reused the former 'as-found' wiring  :) )  I also re-did the crappy SF soldered-PT-bolt chassis mains ground with a proper crimped ring terminal lock-nutted to a dedicated chassis bolt. I also replaced the filter caps with new FTs, and replaced all the other e-caps inside the chassis, but the rest of it is stock for the time being.


I'm also leaning towards a replacement PT. Something bad did happen (in the past) with the old lamp circuit and something fried one of the 6L6s, and for some reason they whacked in a bucking tranny for the bias supply, which all adds up to power supply issues. So methinks the 73 vintage PT may have to go    Time will tell about the PT when I fire it up with a new quad of 6L6s.


On the positive front, the vintage OT passes the requisite Pr:Sec VAC ratio (and is evenly matched on both sides of the Primary), so being able to keep that will be good for the sonic mojo.


I tolexed a DIY head cab for it yesterday.

[bnwitt]

Maybe a good candidate for some Dumble-esc modding

[tubeswell]

Grille cloth and badge arrived in the post this morning. Took me all of 1/2 hour to finish the grille.


Now I just need to find a few extra witchhats and the cosmetic stuff is done.


Still got inside stuff to do

[bnwitt]

TW,
That is really starting to look nice.   Love the white feet.  Makes the amp look like it's wearing spats.

[tubeswell]

Did some more work on the guts today - replaced old roach and re-did bias setup. Now got ballpark voltages. I'll stick with the old main board for now and see if its going to tow the line. The Eureka Moment will happen soon LoL

[bnwitt]

What type of dummy load is that?

[tubeswell]

What type of dummy load is that?

A DIY one using 8 x 100W bolt-on resistors and 2 heatsinks. Can get 8R @ ~400W or 4R @ ~800W. All parts stolen from eBay

[tubeswell]

Finally chucked some tubes in. There is still some hum to chase down (from previous owner) . Found some replacement witch-hats

[mresistor]

Looks great Tubeswell. Are there jbls in that speaker cab?

[tubeswell]

Yep. Orange frame D130F bels circa 1970. (The Serial numbers are only 4 numbers apart!) I think that came as a factory option with the Dual Showman Reverb cabs.

[mresistor]

Awesome!   You're gonna hear every nook and cranny in the sound from that amp..

[TIMBO]

Nice work mate.

Another oldie back in business.