Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: thelonious on May 31, 2015, 04:26:12 pm
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I found a good condition RCA 416r 16mm projector at an antique store for $16! I got lucky--not only was it in working condition, but the amplifier had three strong RCA 6973 tubes in it, along with a 12AX7 and a 7025. The date codes on the transformers suggest it was made in 1962. I found a PDF of the RCA 400 service manual, but the amp in the 416 is a totally different beast. I was surprised how great it sounds with a guitar plugged straight into the Mic input. When I traced the circuit I found out why: It's already basically a guitar amp. The 7025 makes two gain stages of cathode biased 100K/1.5K/25uF bypass, and the 12AX7 is set up as a floating paraphase PI. Two of the 6973 tubes are used for power tubes, and the third is something else... I think an oscillator for the exciter lamp?
Anyway, the question I have is about the rectifier. I was surprised to find a solid state rectifier in it since it's a 1962 model. Was solid state rectification common that early? As far as I can tell, the power transformer secondary is not center-tapped, and the rectifier arrangement is something I'm not familiar with. I've double-checked the wiring, and I think this is correct. I just have no idea how it works. Anyone willing to explain it and/or correct my schematic? :icon_biggrin:
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Perfectly normal voltage doubler. Google "voltage doubler" for explanation. Very useful, no need for a 5 volt winding, no need for a CT on the power tranny. Yeah, there were rectifier diodes (I assume they are small silicon diodes and not a multi-plate selenium rectifier) in the mid-to-late 50's. They weren't as cheap as they are now but they were likely cheaper than a 5Y3 (even to RCA!) plus a socket. Always a plus, too, to not have yet another heat-generating item in a projector. Those things got HOT.
Lots of BOGEN PA amps used voltage doublers. http://el34world.com/charts/Schematics/files/bogen/bogen_chb100.pdf (http://el34world.com/charts/Schematics/files/bogen/bogen_chb100.pdf) for one example.
What a cool find! You even got a spare 6973 (RCA branded, no doubt) out of it! What's not to like?
Those old projectors were absolutely bulletproof. RCA also made telecine (film to tape) projectors. Google "RCA TP-6" They were 600+ lb monsters with cast iron frames. http://8thman.com/16mm2.htm (http://8thman.com/16mm2.htm)
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Perfectly normal voltage doubler.
Oh, of course! I'm used to working with them in DC-powered circuits like pedals, where there has to be an oscillator of some sort. I never thought about the fact that AC brings its own oscillation to the party. Cool.
Yes, the 1n3194s are Si diodes - see attached pic. (Also check out the point-to-point soldered fuse in the background...! Not meant to be user serviceable, I guess.)
What a cool find! You even got a spare 6973 (RCA branded, no doubt) out of it! What's not to like?
Definitely. It's probably the best find I've made so far in terms of price/value. Until I find one of those TP-6 projectors. Pretty sure my wife won't let me bring that home, though. :icon_biggrin: Thanks for your help!
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That style fuse was popular in consumer electronics at the time. The pigtails were connected to the fuse caps at the factory.
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I suppose it's a good solution when cost and space are important factors.
Here's the preamp, minus the tone stack. The tone control is a dual gang pot. I'm thinking maybe it's some version of a James stack with both controls in one. Unfortunately all of the resistors and caps are built into a... thing I don't know what to call (see pic), so it would be a pain to make the values more suitable for guitar. Doesn't sound too bad as is, though, so I'll probably just leave it alone.
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Those were popular in the very late 50's-60's. You're right, it's a network of R's & C's typical of a pile of tone components. Like an early integrated circuit, except nothing active in it. But it was intended as a wiring aid, not a thing that would keep component values anonymous. If you look it up via part number you'll be able to get the internal schematic. If it's that important, that is. I guess the $64 question is, can the amp section be detached from the projector as an independent unit or are you gonna have to lug a 40 pound gizmo around to jam? :icon_biggrin:
And I am curious: Were you in the market for a projector and if not, why did you assume or figure out that the audio amp in the projector was tube fired?
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I guess the $64 question is, can the amp section be detached from the projector as an independent unit or are you gonna have to lug a 40 pound gizmo around to jam? :icon_biggrin:
Ha! The amp chassis is detachable, but then I'd have to build a different enclosure for it. If I instead remove the motor and other heavy parts, it'll drop the weight considerably. There's actually a speaker built into the lid, but it's pretty anemic (a really inefficient 8" paper cone with a tiny magnet). I'm planning to replace that. I'll post pics when I get it all back together.
And I am curious: Were you in the market for a projector and if not, why did you assume or figure out that the audio amp in the projector was tube fired?
I always keep an eye out for old electronics that look suspiciously tubey. In this case, I looked through its air vents and saw tubes. Then I googled the schematic. I only found a schematic for an older model number, but it was enough to show me that this thing had real transformers and enough stages to do something with. I just got lucky that it was already so similar to a guitar amp though.
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The wife and I went and got new phones yesterday. The guy that waited on us I find out his dad use to run the local theater in town and he has several old Film/sound tube amps from the theater, also an old tube jukebox and lots of tubes. He talked like he might be willing to part with some of it---so I might go check it out. Platefire