gibson ga15rvt and ga20rvt amps

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both amps need some work.

1. ga15rvt has an electrolytic that is green at the solder joint. do i just resolder or replace the cap? see image below.

2. ga20rvt needs quite a bit. it has incorrect power transformer, T1. incorrect choke, L1. needs output transformer, T2. also needs recapping.

the ga20rvt probably wouldn't get more than $300-400, so is it worth it to buy new parts(i don't trust ebay used parts) from heybour, mercury or hammond IF they are available?

recommendations?
thanks,

An externally hosted image should be here but it was not working when we last tested it.


ga20rvt schematic.

An externally hosted image should be here but it was not working when we last tested it.
 
These amps are a labour of love to restore, so I think it depends on if you want a working servicable amp, or a museum piece. If museum piece then on the GA-15 you may not want to touch anything. \The GA-20 has been modded so need to evaluate if its working, maybe just bring up to scratch on electrolytics, etc. You have not detailed if they are actually working on not, so you would need to get more details, pictures of the amp etc..
Edcor is reputable and reasonable cost for transformers and chokes.
 
These amps are a labour of love to restore, so I think it depends on if you want a working servicable amp, or a museum piece. If museum piece then on the GA-15 you may not want to touch anything. \The GA-20 has been modded so need to evaluate if its working, maybe just bring up to scratch on electrolytics, etc. You have not detailed if they are actually working on not, so you would need to get more details, pictures of the amp etc..
Edcor is reputable and reasonable cost for transformers and chokes.

i'd like to have working serviceable amps. the ga15 works fine so would probably only need that cap and a couple others replaced, if necessary.

the ga20 needs, minimum, the output transformer before i can even plug it in.
thanks,
 
i found that i need 8-10000 ohm input on the output transformer. unfortunately, the edcor appears to be larger than the original and with 4 screw holes so i may have to just attach only two of the four or look to hammond or heybour for the OT. mercury is just too expensive for this project.

costis: yes, i'll definitely replace the electrolytics.
 
Don't know if you have chosen which supplier, but you should be fine with Hammond or Heyboer. Surprised that Edcor did not have a tranny frame that would be compatible though. Better I think not to have to start drilling out the amp chassis.
PS Do you think the market price 300-400 range for that amp in working condition may be a little low? I have no idea what the outside looks like, and I'm not in a position to do anything other than ask.
 
Don't know if you have chosen which supplier, but you should be fine with Hammond or Heyboer. Surprised that Edcor did not have a tranny frame that would be compatible though. Better I think not to have to start drilling out the amp chassis.
PS Do you think the market price 300-400 range for that amp in working condition may be a little low? I have no idea what the outside looks like, and I'm not in a position to do anything other than ask.

i got the hammond 1750e at a decent price thru amplified parts. since there was no output transformer in the amp, i'm having to rely on the schematic. there are 3 wires, blue, brown and red. i can figure out the output to the speaker but the input/primary has me confused. obviously i need to get it correct.

the outside is in great shape except for a couple of small holes in the grill cloth.
thanks,
mikeB.
 
That's great to hear it's in good shape cosmetically.
The OT primary as per the Hammond drawing: Red is the centre tap (generally that is the colour code used for that going to +supply)
The phase of primary with respect to secondary is important.
So secondary the COM is black, and green is ''hot'' for the speaker side. ''Com'' is the side grounded to chassis.
 
Brown on primary is in phase with Black of secondary
Blue on primary is in phase with Green of secondary.

The dot shown on drawing on each side of the windings indicate which colour wires are in phase.
If you reverse that phase on an amp with NFB, you will get squealing (positive FB).
 
shanx: thanks really appreciate it.
in reference to the schematic, then the blue would go to pin 7 of v6 and the brown would go to pin 7 of v7. the red would go to a terminal strip connecting with L1 filter choke.

with the secondary, the green would go to the terminal strip connecting to the positive of the speaker while the black would go to the terminal strip connecting to the negative of the speaker.

thanks in advance.
mikeB.
 
Hi Mike,
Yes that is the way I read it from your schematic posted. That will be ''phase correct''. As the amp in this particular case does not show a negative feedback tapped off of the secondary of the OT, it would not go into oscillation (positive feedback) if the speaker were reversed, but the phase would be reversed.. so always good to check that.
Looking forward to see how this progresses. There are probably a few more checks you need to do prior to doing final connections, and powering up. If the old transformer was removed, was it blown previously? I assume yes, so good to check that components on the final section push pull and the preceding transformer to be sure no faults there.
 
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