Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Steve_P on April 09, 2012, 06:01:45 pm
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Picked up a tweed/brown cloth covered Guild 99J from cl, covered with shelfpaper. :dontknow: Interesting amp, has the six volt centertap connected to the cathode resistor, injecting dc into the heaters. Later Guilds had dc injection built into the B+, but this is the first amp I've seen in person tapping from the cathode resistor. What really puzzles me is the regular ol' .5mfd 600 volt cap used as the first gain stage filter cap. :huh:
How much filtering can you expect out of that?
At least it has an accordion jack. :m2
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I'm reminded of Rambler. And also a famous quote "640K is enough for anybody." :wink:
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I lucked out with the shelf paper! The tweed is still in good shape, due to being covered and the brown "book hide" covering also has benefited from it's encapsulation. That being said, this amp saw use! there are cigarette burns on the top and discolorations to the tweed that testify to it's use. This was a gigging amp! :wink:
As Shipped:
(http://img221.imageshack.us/img221/3767/001ugur.jpg)
(http://img703.imageshack.us/img703/4700/002jtn.jpg)
(http://img845.imageshack.us/img845/9141/003xfk.jpg)
(http://img11.imageshack.us/img11/1579/004vll.jpg)
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Stripped off the shelf paper and....
She has been used and abused... :twisted:
(http://img813.imageshack.us/img813/9251/img0766q.jpg)
Must have been owned by a smoker. :lol:
(http://img521.imageshack.us/img521/1241/img0767t.jpg)
How the heck do you snap off ALL the speaker studs?
(http://img829.imageshack.us/img829/6747/img0768cw.jpg)
Untouched tweed with the handle studs.
(http://img822.imageshack.us/img822/5940/img0769rh.jpg)
Grille cloth. You know, I thought this type of cloth would be almost impossible to replicate, but the gold squiggles are just a sparkle paint. The cloth itself looks like Fender oxblood grillecloth.
(http://img407.imageshack.us/img407/3505/img0770l.jpg)
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Back to work, :D .
Sanded some of the chassis with 1500 grit sandpaper. Got a little carried away and sanded too much on the back edge, exposing the copper chassis. Yep, copper. If there was any lingering doubt that Ampeg provided parts to Guild, that has been laid to rest for me!
I also have to endorse Evap-O-Rust. It has no smell, it's nonflammable and cleans up with water. It takes the rust off, albeit slowly and doesn't seem to bother the paint. I'll complete the rust removal tomorrow and spray some lacquer to preserve it.
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Replacing the studs on the baffleboard. I carefully removed the grillecloth, which is oldstyle Fender oxblood. The gold swirllies are definitely some type of fabric paint, not stitching. The baffleboard shows no other staple marks, so I can say definitively, "That's the way they made them".
8)
(http://img407.imageshack.us/img407/3505/img0770l.jpg)
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When I pulled out the old can cap, I discovered two wires to a 100mfd/50volt section of the cap. I wasn't surprised to see that one lead went to the 210 ohm cathode resistor, but the other was the center tap for the 6 volt winding. Guild was using the free dc voltage on the cathode bias resistor to feed the tube heaters. I've seen that on an ancient Magnatone schematic, but no where in person. Not surprising that Guild built dc voltage into most of it's mid sixties and later amps as a noise reduction technique.
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Cleaning off the chassis. The rust was bad, but in a lot of 'pinhole" spots. The chrome makes it difficult to take a proper picture.
(http://img138.imageshack.us/img138/6760/img0774hx.jpg)
(http://img716.imageshack.us/img716/1333/img0772tx.jpg)
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Three prong plug. Note the two wires going into the new can cap. The striped wire is the six volt center tap and the other wire is soldered to the cathode of the 6V6. This injects the 20-25 volts dc from the cathode resistor into the heaters, reducing noise. There is also quite a bit of shielded wire used, further reducing extraneous noise.
(http://img23.imageshack.us/img23/9156/img0776n.jpg)
All electrolytic caps replaced, getting ready to spray a fine coat of lacquer to keep the panel from rusting further.
(http://img838.imageshack.us/img838/6845/img0777rn.jpg)
Whoever owned this amp smoked a lot. I have had to clean the nicotine off the turret board with a q-tip.
(http://img526.imageshack.us/img526/933/img0778uh.jpg)
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(http://img834.imageshack.us/img834/2778/img0779fp.jpg)
(http://img152.imageshack.us/img152/2561/img0780lo.jpg)
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Right on! Great stuff, cleaning up nice. Thanks for documenting/posting this!
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> The gold swirllies are definitely some type of fabric paint
There was a paint, you mixed it wrong, put it in a spray gun, it shot "string". Nozzle venturi-cone and worker arm-motion gave a random pattern. It was a short-lived fad, though you can still find imitations in specialty wallpaper and fabrics. Try keywords "atomic boomerang".
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Sanded some of the chassis with 1500 grit sandpaper. Got a little carried away and sanded too much on the back edge, exposing the copper chassis. Yep, copper.
You might have a typical steel chassis.
When chrome plating, the plater typically puts a copper plate layer, then a nickel plate layer, then the chrome plating. Consider the other layers like "primer" to provide a surface for the chrome to adhere to.
I'm guessing you sanded through the chrome and nickel layers, but hadn't gotten all the way through the copper to the steel substrate. I've done the exact same thing myself, but when scrubbing too hard with Nevr-Dull, rather than actual sandpaper.
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> The gold swirllies are definitely some type of fabric paint
There was a paint, you mixed it wrong, put it in a spray gun, it shot "string". Nozzle venturi-cone and worker arm-motion gave a random pattern. It was a short-lived fad, though you can still find imitations in specialty wallpaper and fabrics. Try keywords "atomic boomerang".
Atomic Boomerang came up negatory. I was lucky enough to have an autobody shop on my route today and asked one of the oldschool guys about it.
VEILING -- Also called cob webbing; webs or strings formed from paint drying while spraying from an airbrush or spray gun with an open nozzle with high air pressure. Unreduced paint, very low pressure, in a cup gun is how we did it.
Cobweb Paint
http://www.alsacorp.com/products/cobweb/cobweb.htm (http://www.alsacorp.com/products/cobweb/cobweb.htm)
Thanks for pointing me in this direction! I never thought that hotrods were where Guild, Ampeg and Sano got this idea from!