Gibson GA19RVT Tweed v. Crest ... compare/contrast

honkyjonk

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I have the same thing. Weber 12a125, basically got it on your recommendation. Another thanks, because it sounds really great.

Just to clear things up concerning the rectified voltage, what I meant was I have 300VDC AFTER the 470ohm 1 watt resistor (at pin 4 on the 6V6's) . On the schematic at this point it indicates 290VDC. Additionally, instead of the 470ohm resistor, I have that string of resistors that totals 2.9K in resistance, and that drops the voltage down to 300VDC at this point.

Anyway, my concern in this regard isn't sound, but rather the potential that the extra voltage could wear out the 6V6's too early? (As you can tell, I know just enough about tube amps to be dangerous)

I'd love to try a les paul through this thing because it sounds like it could easily do an awesome late 60's/early 70's Neil Young interpretation.
 

capnjuan

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Okay; my mistake; yes, 290VDC after the dropping resistor on the schematic but you're getting 300VDC after the resistor. So long as the voltage immediately out of the rectifier is a higher than 300VDC, it's not that big a concern; you just need some voltage spread between the plates and the screens.

Back in the day, 110VAC was the standard household voltage ... it's probably stamped somewhere on your chassis. If you set your meter to AC voltage and put the red lead on the fuse post and the other on the chassis, you are probably getting something between 117VAC and 120VAC ... the modern standard. Since there is more AC voltage on the primary, there will be more secondary AC to be rectified and the rectifier will produce correspondingly higher DC. It won't be exactly proportional because there's voltage drop across the rectifier. When I'm not using it on the bench, I have a variac between the house supply and my old Gibsons so they don't see more than 110V.

Voltage on 6V6s; once upon a time, there was this amp dude from Southern California ... Leo something-or-other. A bright guy and he read between the lines in the RCA and other tube manuals. RCA and other mfrs used to recommend modest voltages for 6V6s. Gibson, because they weren't trying to set the world on fire with their amp designs, were happy enough to just follow along ... what the heck, who needs warranty problems with blown tubes (and they didn't want their mid-fi/mid-price amps out-performing their more expensive stuff).

This Leo guy, he tested the factory 6V6s to see how much voltage they'd take before they self-destructed and he found out that the tube mfrs were sand-bagging ... purposely understating many tube voltage-handling standards. Considering that not all 6V6s went into guitar amps, it was the sensible thing to do. Anyway, two of Fender's most famous designs run 6V6s at much higher voltages than Gibson amps ... higher than most everybody else too: twin 6V6 Fender Deluxe running at 375V and the AA764 single-ended 6V6 Champ running at 360V. Later Champs run at better than 400V. Some of the current JJ 6V6s, the ones designed to mimic 6L6 tone, can run at nearly 500V ... :shock: Moral: you can't hurt your amp or tubes if the voltage is a little higher than design.

Everybody feels differently; some like Juicy Fruit ... others like Spearmint but if you want a speaker that is most similar to the best commercially-available alnico speakers 60 years ago, you have it ... the Weber 12A125; Weber's dupe of a Jensen P12Q.

Sounds like you've got yourself a killer amp! 8) 8)
 

capnjuan

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Another Gibson GA19RVT on eBay. One of the three Gibson tweed-era amps with on-board reverb; the others the GA30RV and GA77RV. The amp uses the now-expensive 7199, a two-tubes-in-a-bottle tube, for reverb drive and recovery and three 6EU7s that also appear in nearly all the other Gibson tweed-era amps. Two 6V6s and a 5Y3 rectifier.

Front01.jpg



Oh the goodness of Gibson tweed (cotton-backed clear vinyl) ... the handle insert is clear lucite or equivalent with a patch of tolex with 'Gibson' painted on both sides. The handle (mfr long gone) was also used on the sister Epiphone amps.

handle01.jpg



Oh the badness of Gibson tweed ... the cotton backing held mold spores that when coming in contact with the water-based tolex glue, blossomed into the orange-brown stains commonly seen on these amps. Regardless of which of the less-expensive models; GA9/14/18/19/20/30 .. any with mold-free tolex typically sell for over $1,000.

Back01.jpg



Pics of my (former) GA19RVT

The GA18 and GA19 both used 'racks' instead of the phenolic eyelet boards found on the sister tweed GA20Ts and GA30RVs. Good; most of the components are easy to get at ... Bad; some of the components are hard to get at like the silver mica coupling caps (ugh) that are folded under. These racks also use #22 jumpers underneath the components so if there's a bad connection, it's the pits trouble-shooting. This amp was from my 'Orange Drop' period; I later switched to Mallory 150s and haven't looked back.

I also switched out the Jensen P10R for a Weber 12A125 which meant not just a new baffleboard, but cutting the lower baffle batten to allow the lip of the speaker to just about touch the bottom of the cabinet. Had the center of the 12" speaker stayed the same as the 10", the speaker bell would have interfered with the tubes. The tube covers are polished aluminum; over time they tarnish but can be made to sparkle with Flitz or Brasso. Every tube, resistor, and capacitor in that amp was new; it will last another lifetime.

GA19chassis-1.jpg



The tweeds were the last Gibson amps to have mahogany footswitches. With a little steel wool and lacquer or varnish, they can brought back to a furniture-quality finish:

ga18d.jpg
 

capnjuan

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Another tweed GA19RVT on eBay. This one, in better physical condition and with a better-written auction; this auction has 4 times as many bids and the price pushed up to twice the top-bid price for previously-linked amp ... and it has more time to go.
 
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