I'm new- I need help with a GIBSON amp I'm building

[xodus]

Hi everyone.
This is my first post.

I amp not an amp builder, but I recently got a CHASIS to a 1939 GIBSON EH-185

They are great amps.

I am currently trying to build the CAB. that holds the chasis.

I am told that these amps used a 12in FIELD COIL speaker.
Can someone tell me would it be a problem if I used a different type of speaker. Say something like a ALNICO 12in speaker

I know the originals had old JENSENs in them

Please if someone could give me advice on the speaker I should look out for, and what would give it a great tone/and also be reliable to last.

(the amp I'm trying to build is this one
http://uv201.com/Audio_Pages/gibson_eh185.htm
I have the EXACT chasis)

[zhyla]

I hadn't heard of field coil speakers before so glad you brought this up. A quick google found this thread:

http://www.audiokarma.org/forums/arc...p/t-90189.html

2nd post seems informative. My guess is you'd want to:

1. Put a resistor across the field coil's jack to mimic the load of the field coil. How you'd figure out what resistance I'm not too sure... probably just play with it till the B+ taps are at what you think is the correct voltage(s).

2. Reverse engineer the output transformer's impedance ratio to select the right speaker impedance. From there selecting an actual speaker is the usual voodoo and budget considerations.

Though who knows, you might be able to find an original speaker for it.

[stingray_65]

or get a 12" field coil speaker

they are still around, even if you have to have it reconed or rebuilt it might be worth it to preserve the amp

If you're looking for an inexpensive amp, you may be able to sell that amp to a collector and purchase one.

I haven't did any research on that particular amp, but it seems a shame to hack on possibly irreplaceable vintage piece

just my .02

it's your amp

but if you would like some help researching it and seeing what it would take to restore it to its former self we can help with that too.

got some pics of it?

Ray

***edit***

I did a quick search of completed listings on EBay and found several jensen 12" field coil speakers that were sold for less than $50

[xodus]

Yea, I plan on sticking with the original stuff.

I just looked on ebay and there are a BUNCH of 12in Field Coil speakers.

http://search.ebay.com/search/search...ker&category0=

Does anyone see a paticular speaker that looks good?

[PRNDL]

You may want to rethink your build.

A field coil speaker has the output transformer with the speaker, usually mounted on the side, or in the bell.

Modern amps (a strange term since we're talking post 1960 or so) have the output transformer mounted on the chassis.

Here's a field coil speaker. You can see the little transformer mounted in front of the bell.


What's the difference? Nothing (and everything).
The only difference is where you mount the output transformer.
If you want to use a modern speaker, check to make certain there's room to mount the output transformer on the chassis.

If you want to buy a used field coil speaker, make certain that the transformer impedance matches the output tubes.

The other difference, which is huge, is most field coil speakers are OLD, and usually alnico. Also, they may or may not be suitable for guitar. Indeed, most would be voiced for PA or music.

[xodus]

you definatly bring up some great points.

again, I'm a total NOVICE with this stuff.
I've been told that these old GIBSONs all used FIELDCOIL speakers.

Heres some pics of the chasis. HOPEFULLY this will help you further in determining what type of speaker I should get

[xodus]

so do these pics help anyone suggest a decent speaker for me?

[TD_Madden]

drop a note to Rob at www.buffaloamps.com

He's strictly a vintage Gibson-circuit guy.....in fact, he's building me a BA-70.

[xodus]

thanks,
I sent Rob a message

[tubeswell]

so do these pics help anyone suggest a decent speaker for me?

Not any useful suggestion on the speaker above & beyond what the others have said about mounting the OT somewhere on the chassis - but they do suggest that some of your caps need replacing. Some of the caps (are they Astrons?) look sort of bulgy and swollen