Antique guitar amp restoration

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Antique guitar amp restoration

Postby erichayes » Sat Jul 22, 2006 12:14 am

Hi All,

I was commissioned last week (indirectly) by Humboldt County musical legend Brooks Otis to resurrect a 1930s era guitar amplifier he has, and I thought it might be fun to take y'all on the ride with me. I'll post pictures a little down the line, but the essential spec's are:

1st audio, 57; 2nd audio, 56; phase inverter, center tapped AF choke (more on that later); output, push-pull 45s; rectifier, 80.

I figure this thing is probably good for around 12~15 watts at obscene amounts of distortion. The field coil magnet 8" speaker appears to be in good condition, but the amp, itself, has been attacked by more than one turkeytech over the years, so my job's cut out for me.

This should be a real interesting journey . . .
Eric in the Jefferson State
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Postby dhuebert » Mon Jul 24, 2006 7:05 am

Keep it comin'

Don
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Postby EWBrown » Mon Jul 24, 2006 8:13 am

That name came from one of the vintage Cheech and Chong LPs...

Sounds pretty much like a "ground-up" restoration, should be interesting.
First thing I'do is replace that "field coil" speaker with a newer (Eminence?) 8 incher and a choke to fill in for the field coil. The Eminence speakers are quite efficient, and rugged. Maybe replace the 45s with 2A3s, they give 2X the power and are much more available.
Of course, if keeping it as "pure" and close to original, then these modernizations don't aply here. Yellow_Light_Colorz_PDT_02

One can wring about 1.75 watts (2W with a stiff tailwind) out of an SET 45, 3.5W with a 2A3 / 6A3 / 6B4G and 8-10W out of a 300B, in SET mode, and about 10 "clean" Watts out of a PP pair of 2A3s. For guitamp use, where (controlled) distortion is a good thing, factor in another 40-50% output power before the "smoke fairy" and Red Plate disorder arises... (I'm not counting those "creative accounting" inflated power ratings that guitamp manufacturers of olden tymes liked to claim).

I'm thinking of ginning up a PP (cheap) 300B guitamp, just the perfect use for those Valve Fart 300Bs and EH/Sovteks, which tend to fall a bit short for "audiophile" purposes (or porpoises, for that matter). I SWAG thatI could wrestle about 35-40 watts out of a PP pair before that dreaded red-plate syndrome and "the smoke fairy" attacks. Yellow_Light_Colorz_PDT_06

/ed B in NH
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Postby erichayes » Tue Jul 25, 2006 12:52 am

Hi All,

This is one of those projects that is neither no win nor no lose, as the amp, itself, bears no resemblance to its original incarnation. The chassis, speaker and cabinet appear to be the only original components. What I plan to do is construct what I think was the original designer's circuit and see what it does.

I thought about 2A3s, Ed, but I don't think the PT could handle the load. Ironically, the chassis is stamped with "45" in front of two sockets, and "2A3" on a third, but one of the sockets in question has "56" stamped on it.

As I said, this is gonna be veeerry interesting.
Eric in the Jefferson State
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Postby EWBrown » Tue Jul 25, 2006 5:20 am

Just some more early morning rambling, er, musings... Yellow_Light_Colorz_PDT_09

Could have been that there was a 2A3 SET version of this amp, they just used a single chassis design for both SET and PP versions.

I dunno if the power trannie would like this, but an 83 MercVap rect in place of the 80 might be interesting, and then there's that cool blue glow... One caveat: 83s should be oriented pins down, and definitely not upside down - if indeed that layout was used in the original config. I know, Hickok tube testers can get away with a horizontal 83, but then the rectifier is running at a really light duty cycle.

/ed B in NH
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