Gibson EH-150 Hawaiian Lap Steel/Amp

[groovyfingers]

Just aquired a 1941 set, one Hawaiian Lap Steel and matching amp made by
Gibson. The amp needs a new power cord and new caps, the original filter caps somewhat exploded. By that I mean the end seals have pushed-out, and I'm sure this is an indication that they are dead. I did power it up, the tubes glowed, but no sound. The speaker is the oddest I've seen, it has like a tube socket and matching plug, and I'm clueless about this. Should I leave it alone, have it a museum piece, or go through it so it can be another tool in the sound arsenal? What do you all think?

[Chuck H]

I think your speaker could be a "field coil" type. With the choke coil for the power supply acting as the speaker magnet. If the speaker or the choke are the reason the amp is dead you would need to either find a good working vintage replacement part or replace the speaker with a permanent magnet type and modify the power supply to replace the choke before the amp will work. Since you have the two pieces together as a "set" it's a shame to modify the amp circuit. But I don't think you'll find a suitable replacement speaker. It's a bigger shame if the amp never works again. So if it comes down to the speaker I would do the modification. I would replace the filters first and then do some voltage testing to see what else might be wrong.

Chuck

[Bruce / Mission Amps]

Not only what Chuck says but the speaker "octal plug" also is the shorting strap connection for the heater wires... so if it is not plugged in... the tubes get no filament voltage.
If the speaker is blown, Orange County rebuilds them for about $70.00.
Replace the filter caps with any suitable modern parts and start from there.
If you have screen and preamp voltages, then the electro-magnet winding in the speaker (not the actual voice coil) is probably OK.

[Enzo]

Is this remotely similar to your unit?

http://www.schematicheaven.com/gibsonamps/eh100_1.pdf

They have a couple versions, and they are clearly marked speaker field coil serving as B+ choke.

[groovyfingers]

Yes, although mine is an EH-150, with an 80 rectifier tube, two 6v6's, one 6N7, and two 6C5's. I have a schematic out of an old copy of Aspen Pitman's The Tube Amp Book 4.1 ed., and you are right, they show the same speaker field coil acting as the B+ choke. The schematic in Pitman's book shows an EH-150 w/an extra 6C5, for a total of seven tubes. I'll check schematicheaven for a copy of my setup. A big thank you all for your replies. It's really helpful.

[groovyfingers]

What's so odd is that the tubes look brand new! I'm thinking they are the originals, and what a trip. This amp is a closet-kept one for sure.

[Enzo]

Duh, I looked right at your EH-150 and my eyes saw RH-100. Getting old. The EH150 drawing is newer than yours, but here it is anyway:

http://www.schematicheaven.com/gibsonamps/eh150.pdf

Note the field coil is marked as 750 ohms. I suspect yours is similar, and that is useful data should you need to convert the amp to permanent magnet speaker. it was 1000 ohms in the EH100. Something in that ballpark. Just replacing the fierld coil with a choke would result in elevated B+ voltages downstream.

[groovyfingers]

I'm wrong, I've got 6V6's not 6L6's. So it's like some EH100/EH125 combo-thing. But I did pull out the chassis and have a look-see, and the filter caps are a multi-section tubular electrolytic type (two 10/10 450) which is reflected in the EH150 schematic. Heck, it's probably a conglomerate of all three. I'll probably go with four sprague atoms, 10uf 500V and try to figure out how to mount/install them. That'll kill the museum draw of this amp. but I'm more interested in it as a tool than anything else. Once I do that, I can start checking voltages and such. Is this correct, or can I check stuff now?