Gibson GA-50 question

[Jason C]

Hey guys, I have just about everthing I need for a Gibson GA-50 amp but I need help getting the right chokes. The schematic I've found just gives a part number and no values so if anyone knows which chokes would work I would appreciate it. Here's the schematic I'm using. Thanks.

http://www.schematicheaven.com/gibsonamps/ga50.pdf

[Jason C]

OK I might have answered my own question. I should have looked here first I guess. I found the schematic for the GA-50T (same amp but with tremolo) and it gives the values which I assume would work for the GA-50 right?

http://www.schematicheaven.com/gibsonamps/ga50t.pdf

[PRNDL]

I fixed a GA-50T last Friday. It's a very cool amp that looked and smelled like an old leather suitcase.

The first choke on a GA-50T is huge. You may want to check on the current rating.

Also, GA-50 uses uses two 6J5 single triodes in the phase inverter. GA-50T uses one 6SN7, which is two 6J5s.

Have you calculated the gain?

6J5 has a mu of only 20, which might not be enough to drive the 6V6's to full power in that circuit. Amps designed in the 50's were designed for low gain to reduce distortion.

Most designs use the 6SL7 dual triode, which has a mu of 70.

[Jason C]

Thanks, the two chokes in that schematic has one at 30H 30ma and the other at 10H 175ma. Is that what you found? Could I use a single 6SN7 as the PI instead of the 6J5's without changing any other values as I have a 6SN7 laying around. I have not calculated the gain and I want this to be jazz amp so I'm not wanting any kind of distortion. Did the one you fixed have have 6V6's or 6L6's?

[PRNDL]

the two chokes in that schematic has one at 30H 30ma and the other at 10H 175ma. Is that what you found?

The 10H175MA choke was big, and may not be necessary for a guitar amp.

Could I use a single 6SN7 as the PI instead of the 6J5's without changing any other values

Yes, they are equivalent.

Did the one you fixed have have 6V6's or 6L6's?

6L6's

I have not calculated the gain and I want this to be jazz amp so I'm not wanting any kind of distortion.

I usually compare other circuits to see if there is enough gain. The circuit is:
gain -- tone stack -- PI -- 6L6

Compare that to a 6G6 Bassman:
gain -- tone stack -- gain -- PI -- 6L6

They have two preamp gain stages using the 12AX7 (mu = 100) instead of the 6SJ5 (mu = 30).

[Jason C]

Would it be better to use a 10H 200ma or 10H 150ma as I can't seem to locate 175ma? Also would a 30H 40ma be all right?

[PRNDL]

Would it be better to use a 10H 200ma or 10H 150ma as I can't seem to locate 175ma? Also would a 30H 40ma be all right?

The answer to both of those questions depends on the current loads, which requires knowledge of tube amp design.

You can cheat by copying from an amp with a similar design, such as a Fender Bassman. The Hammond 194B is rated for 90 mA. The 150 mA rating should be plenty.

It's much harder to do this trick with a second choke, since few amps use them.

These sites may have an answer, or you could post on the Theory and Design section.
http://www.freewebs.com/valvewizard/index.html
http://www.turneraudio.com.au/education+diy.html

[GibsonLover]

GA-50 Chokes

Hi guys - I just rebuilt my 1948 GA-50 and I measured the DC resistance
of the two chokes. The large one was 145 Ohms, the small one was 1.8K.
The GA-50T schematic posts values, but may be suspect. For instance, the
30H / 30mA is rated at 30mA, and inductance increases as the actual current
decreases. Use the posted values as ballpark, then get out your meter.
Cheers

[Ian]

Hi -

Apologies for joining the thread three years late. I'm wondering if anyone has specifics on the GA-50 power transformer and also perhaps the output transformer as well? The DC voltage at the rectifier cathode or the AC voltage across the secondary would be helpful as would suggestions regarding the impedance of the output primary and secondary.

Thanks, Ian

[jmaf]

Just go ahead open your own new thread, that'd give it more visibility IMO.

Edit: Welcome to the forum btw