Old Newcomb G-12 guitar amp

[pchilson]

Hey guys,

I bought an old Newcomb G12 guitar amp today. I'm guessing I need to recap this baby but have not run across this type of capacitor before. They are a ceramic shell axial style capacitors that are color banded. I do not have and cannot find a schematic for this amp that I can pull the values from. Can someone give me a crash course on reading the color bands so I can compile a list of the cap values I need to get?

What exactly is this type of cap called and do I need to replace them?

Here is a shot of the inside of the chassis. Some of the colors are hard to tell from the pics but I can see them cleary on the unit itself. I just need to know how to decode the bands. Thanks

[ATTACH]4164[/ATTACH]
[ATTACH]4165[/ATTACH]
[ATTACH]4166[/ATTACH]

[oc disorder]

This site should help
http://www.antiqueradio.org/recap.htm
Couldn't see any electrolytics maybe they are on the top side.
Probably wise to replace them.
Think they are waxed paper like the sought after "Bumblebees"
that were in tone circuits in guitars.
Hey you could sell them on feebay for millions then move house..!
Seriously, maybe an ole' timer can correctly identify them.
Tubular polypropylene rated at 630 volts should be fine and they are also axial
type (leads either end) so should be a breeze to fit.
Looks like you have a good conversation piece there.

[pchilson]

Hi,

Thanks.
I will see if I can decipher the color bands from the chart you linked.
Anyone recognize these caps?

The electrolytics are in a can on the top side of the chassis. 4x unit 20@475v-10@475v-10@475v-25@50v.

Anyone here able to supply a resource to find a schematic for this model?
"Newcomb Model G-12 Instrument Amplifier"
"MFG. BY NEWCOMB AUDIO PRODUCTS CO. - HOLLYWOOD CALIF."

Also, the output transformer has been replaced at some point in the amps life. Without a schematic how can I tell what is an appropriate replacement OT for this amp circuit. I would prefer a multi-tap unit so I can have my choice of speaker. Any suggestions?

Label on top of existing OT

[ATTACH]4170[/ATTACH]

[ATTACH]4171[/ATTACH]

The top view of the two RCA 6V6GTs in the amp. What is the squiggly on the one?

[ATTACH]4172[/ATTACH]

General view of the amp

[ATTACH]4173[/ATTACH]

[pchilson]

Forgot to add the tube compliment in the amp is:

1 x 5Y3
1 x 6SF5 (metal)
1 x 6SC7 (metal)
2 x 6V6GT

10 to 12 watt output?
Single ended or push/pull?

[oc disorder]

I'd say the squiggle was done during valve manufacture I sometimes see
that type of thing in valves maye its to show the assembler whuich way upor it could be graffiti (50's).
It has 2 x 6V6 output valves so its push pull.
There is another thread on this forum re converting a Newcomb PA amp
to guitar but its a more modern unit.
It looks good in the photos.
Yes there are many choices for a output transformer to suit 6V6's which have multiple output taps.
This early Fender schematic while not the same is representative of this style of circuit.
Using this as a reference why don't you try to draw out the schematic ?
Once you work out the pin numbers on the valve sockets its plain sailing...
You may have to look up the 6SF5 to determine its pins well the heater pins are easy then there are only 6 left !
Have you tried it ? I would suggest a light bulb in series (search light bulb limiter in the forum) 'cause if it has a major short the light bulb will light up preventing damage to the amp.

[pchilson]

Thanks oc, I'll see if I can get to some of your suggestions.
I took the tubes in and put them on a tester yesterday and found the 6SC7 shorted. The others came away with good marks. I now need to find another 6SC7 and give the amp a go to see where it stands with all good tubes installed. Any good sources for the 6SC7?

Thanks

[pchilson]

I've figured out my cap values and have placed the order for replacements and also a NOS 6SC7GT tube. Should be in late next week and then I'll be able too better evaluate the amp. I'll post back then.

Thanks for the links oc disorder.

[Yakobatzi]

Just found the site.

The schematic is on the inside of the bottom cover. If it is missing here it is.

[Muzicdepot]

I got a G12 for sale mike. Muzicdepot@aol.com

[TimmyP1955]

Great handle on that amp. Does it look to be original?

[Muzicdepot]

Yes the handle looks like an ol oven handle very cool it looks like the amp is all orignal and in great condition