Building a Guitar Amp Out of an Old PA Head - It Seemed Like a Good Idea...

[Rob Mercure]

Hey Bob,

Glad you didn't have one of the truly funky trannies. Like I mentioned, NFB was the "cure all" and coupled with the "strategic materials shortage" of the late 1940s-early 50s manufacturers were doing everything they could to innovatively cut costs and the tranny was a favorite target (and the OT has always been the favorite target of the HiFi set - just look at the number so fiendishly complicated OTL designs that continue to flourish). The interesting thing is that if I had more energy/interest and better tranny engineering background I suspect that it would be possible to use these windings to tweak circuit parameters with some cool results - but with my limited skills/interest I've only found out that these trannies don't lend themselves to the lazy "drop in" type of circuit design that I've become accustomed to (<grin> -lazy me). Fer instance, I wonder if one could take a relatively "high" impedance NFB winding and and use it to drive a "slave" output stage (create a virtual center tap, keep DC out of the winding, etc.)?

Anyhoo, the maroon and silver amp makes a nice conversion - just a tad of impedance mismatch (the Baldwin's 6L6GCs fed a 16 ohm load - the Bogen "A"s feed 8) but for guitar amps my philosophy is "see what it sounds like" before worrying about it.

Rob

[bob p]

OTL? They only seem to make sense for driving electrostatic speakers, as they allow you to eliminate the need for two expensive transformers that perform what are essentially zero sum functions. Beyond that, I've never understood the virtues of the OTL design.

[Rob Mercure]

Bob,

For some reason one of the "bugaboos" of the HiFi world post WWII was "phase distortion" - that any reactive component that altered the phase of the signal was to be hunted down by torchlight by hoards of peasants and burned <grin>. So as many caps as possible were eliminated from the signal path and all transformers were bad mojo - coupled with the fact that good trannies were expensive, the topic of my original response to this post. So OTL was a "holy grail" and there were numerout attempts to create a commercially successful design. Unfortunately to match speaker impedance these almost always involved scads of output tubes in parallel and/or very high impedance speakers - but 450 ohm speakers were available for the task!

Again, the worries of the hifi crowd - we've got it easy in the music production world.

Rob

[bob p]

Wow. I Google'd for info on some arcane topic, and I ended up finding my own amp thread! What's kind of funny is that this isn't the first time that something like this has happened to me.

Finding this thread reminds me that this project has been on the back burner for way too long. I stuffed this amp into a box and shelved it in a back room so long ago that I had forgotten that I still had it. I guess that I need to update my "to-do" list.

What's really scary is that my old posts suggested that I was really up to speed on the technical aspects of the project once upon a time. Today it seems like I've forgotten an awful lot of information. I hate it when that happens.

[dai h.]

sort of the mental version of walking around in a circle

(I know that feelin' )

[Baxtercat]

Every time I take a break from this stuff I have to start all over again regarding which wire goes where...I forget half of it.

Last year I finally got restarted on my junk PA head project and turned it into this fake Marshall combo. My wife found the old '60s PA years ago in the basement where she worked.

It's a Bell-35 with two original RCA 6L6s. The mike input was hot enough for some pretty good guitar noise, so I left it mostly stock inside and just made a combo box for one 10-inch and covered it w/ tolex.





[the sound, live at the pub, running stereo with a second amp]
YouTube- Victory Lane on Cannery Row - Stone's Jam

[J M Fahey]

Hey !!! That's nice !!!!
You sure can turn old worn stuff into new fresh good looking one.
Impressive work.

[Baxtercat]

Thanks mucho...
Hey I just this week finished turning another pile of yonke into a Jetson's amp:

It started as a bare organ chassis and two transformers. I built a box and covered it w/ goofy cloth.
I gigged with it a few times Friday and Sunday...it only plays luau music [kidding!]

[bob p]

Wow, this thread has been brought back from the dead -- twice!

As an update to the original post: when the seller saw the way that the amp arrived after shipping, he refunded my money and told me to dispose of the leftovers. They got 'wrecked.

[bob p]

Thanks mucho...
Hey I just this week finished turning another pile of yonke into a Jetson's amp:

Oh, MAN! That is so cool! You should definitely start a thread to showcase that build.

I've been thinking about doing the same sort of thing, with the trapezoid geometry and with angled legs, but with stubby legs like a tabletop radio. I was thinking about making mine with a pale blue/white speckled formica with gold trim, in authentic pastel period colors. The angled legs are a must. I'd also put a set of rabbit ears on the top, just as decoration.

I want to do this so badly that I even asked a couple of neighbors to keep their eye out for suitable parts donors when they're driving about town. When one neighbor heard what I was looking for, he gave me a set of threaded angled leg sockets.

I love your interior decor. Very period appropriate. Especially the tree lamp and the floral drapery.

[Baxtercat]

Funny happenstance, that background. I was just setting up the amp shot when I realized how the lamps might work....and the Arthur Lyman LP was coincidentally already out...hah!

Blue/white speckled formica...how perfect!

I've got the word out around town hoping to find rabbit ears too.

[tubeswell]

Thanks mucho...
Hey I just this week finished turning another pile of yonke into a Jetson's amp:

It started as a bare organ chassis and two transformers. I built a box and covered it w/ goofy cloth.
I gigged with it a few times Friday and Sunday...it only plays luau music [kidding!]

very cool Bob. All it needs now is a rocket pak

[bob p]

I'd like to find something like this:

[Baxtercat]

Amazing, that looks like the Formica on this homemade old table we found at a garage sale in town [with tortoise shell binding].
Boomerangs!

[bob p]

can i strip the formica off of your table to make a faceplate? please?