what would happen ga 20

[john_t]

I picked up a Gibson ga 20. I believe it is a 1950. I have yet to plug it in as the fuse and holder are missing. When I pulled the chassis some one direct wired to the tranny. No fuse at all. Which would lead me to believe the amp was blowing fuses. My ? is the schematic calls for 2 6sj7 and one 6sl7. yet there is a 6sc7 instead of the 6sl7 . What would this do. I found some info that says the 6sc7 is very simalar to the l7 but pin outs need to be changed. Iam wondering if someone just threw in the wrong tube with out  the proper mods.

thanks in advance john t

[RicharD]

First and foremost:

Never ever plug an unknown amp straight into the wall w/o using a lamp limiter first.

I'd say this goes double or triple for this amp since it's: unknown, had the fuse hack out, and there might be some circuit mods.

Now look at the socket that is supposed to have a 6SL7 but has a 6SC7.  They're both dual triodes.  Section 1 (pins 4, 5, & 6) are wired the same.  Section 2 (pins 1, 2, 3) are quite different.  Both use pin 2 as the plate, but the 6SC7 uses pin 3 as the grid, shares pin 6 as the cathode with section 1, and pin 1 is grounded to the shell.  The 6SL7 uses pin 1 as the grid and pin 3 as the cathode.  Other than the common cathode thang and pins 1 & 3 being different, the 2 tubes are quite similar.  So... I'm guessing the tube in question is a LTPI.  Look at pins 1 & 3.  If pin 1 is open or grounded, it's been modified for a 6SC7.  Look at your schematic and compare.

[john_t]

Thanks for the help. That seems pretty straight forward. I am waiting on some nos tubes 6sj7gt and 6sltgt. I most likely will disconnect the pt before plugging in. I also have a variac so I could fire it up nice and slow. Plus the caps need to be replaced, they appear to be original. Iam hoping this amp is not a total loser. I have read good things on this amp to its a turd. Too late to worry now.

[RicharD]

The variac thang is cool as long as it has an amp meter.  I used to do it that way until I started hanging out here.  Now my variac is strictly for experimentation.  A lamp limiter cots less than $10.00 to build and is just so much safer & easier to use.  I'm sure mine has paid for itself in blown fuses alone.

I just peeked at the GA20 schemo and the 6SL7 is the PI.  Rewired for a 6SC7 would be fine unless you're going for restoration/original circuitry which is what I'd do. 

[john_t]

Butterylicious, thanks for the info. I also found some additional info that some had ga-20s that came stock with a 6sc7. I guess Il find out when I open it up again. It is a bit of a pain to get to as the top panel in finished over with the leatherette covering.

[RicharD]

>I also found some additional info that some had ga-20s that came stock with a 6sc7.
Hmmm... Was Valco making amps for Gibson? 

At a glance, the curves for these 2 tubes are similar.  Not exactly the same but in a LTPI application, you'd probably see very little difference electrically and sonically you'd probably notice as much difference from simply changing tube brands.  Lock at the pins under a magnifying glass.  It's pretty easy to tell if someone has reworked it or not.  As long as the wiring matches up to the corresponding tube, that kitty should purr either way.

[tubeswell]

Hmmm... Was Valco making amps for Gibson? 

http://www.valcopages.com/History.html

Now in Wiki

http://en.wikipedia.org/wiki/Valco

[PRR]

I had the idea that all older Gibson amps were improvised, that many circuits and tube lineups were sold under the same badge.

Which would explain a 6SC7 ("Hey! we ran out of 6SL7!" "Move two wires and use 6SC7!"), and maybe why you "have read good things on this amp to its a turd".... some factory mods were better than others.

This does not explain suspected fuse-blowing. Preamp tubes have large resistors and CAN'T suck enough power to blow a fuse.

Obviously the caps should be replaced, they are just too old to fool with.

[panhead]

At a glance, the curves for these 2 tubes are similar.  Not exactly the same but in a LTPI application, you'd probably see very little difference electrically and sonically you'd probably notice as much difference from simply changing tube brands.  Lock at the pins under a magnifying glass.  It's pretty easy to tell if someone has reworked it or not.  As long as the wiring matches up to the corresponding tube, that kitty should purr either way.

I'm looking at building a GA 20 type amp, and the circuit I found has a 6SL7 as a paraphase inverter - not LTPI. I don't know the difference in gain between the 6SL7 and 6SC7, but for a paraphase inverter, the resistor values in the voltage divider in the circuit that gives the "antiphase" (for lack of a better word) triode its signal are chosen based on tube gain. If the tubes are close its probably not a problem, but if not you may end up with an unbalanced condition (again, not necessarilya problem - you may like the sound of it).