Ampro PA-3 head

[CrocMule]

I spotted this thing in my friend's shed 25+ years ago.  He was more of a collector back then, so he told me its story (it was part of the sound system for a gigantic "home model" 16mm carbon-arc projector that he saved from going to the dump), and then the conversation moved on to other things -- clearly it was his and he wanted to keep it. 

But since getting remarried he's been under pressure to thin stuff out.  I asked about it again yesterday while there, and he gave it to me.  That and a 12" Operadio alnico speaker in a junky cab.  Woohoo!  Happy day!

Luckily the schematic pasted inside the Ampro is still in pretty good shape.  It was drawn in 1943.  I found 2 other identical PA-3s that were listed on Reverb as 1948 models.  I'm seeing a lot of 6s as the 4th digit in what look like date stamps inside.  1946? 

This thing has 3 rectifier tubes and 7 transformers and coils.  And a veritable forest of cap cans.  Complex circuit.

I haven't had much time to explore it yet beyond taking photos.  It needs a cleaning before anything else.

This will keep me entertained for a while!

[CrocMule]

Gut shot

[CrocMule]

Schematic

[WimWalther]

Wow, crazy. This thing must have been big $$ back then..

The 5Z3 is the main B+ rectifier. The 5V4s are used to derive a C- supply for fixed output tube bias, very unusual. The 6H6 is used as a rectifier to drive an output level meter. There's also an AC line meter.. but it doesn't read the AC line directly, it uses the 6.3V AC heater winding.

Coupling transformer does the PI.

What's the "exciter" circuit for? Is this part of the projector setup? ETA: OK, it's an oscillator formed by section 2 of the 6N7 & L5. It's used by the projector.

There's also a saturable reactor in there, also part of the osc circuit? ETA: Nope, it's in the C- supply.. wth?

Yeah, this thing is quite a box of parts.. they did stuff differently back then.

[CrocMule]

they did stuff differently back then.

Sure looks that way to me.  Design maximalism.  Different manufacturing philosophy?  Still trying to prove to the world that they can make the best?  Check out the low/normal/high voltage switch on the mains input -- that's thorough.  Even the thumb screws that hold the cover on are heavy and beautifully made.  It'll be interesting to dig into.  This is going to be a job to savor slowly.

Thanks,
Russ

Do you know what the paper burrito is that's below and to the right of the big adjustable wire wound resistor?  It has two wires coming from each end.

[WimWalther]

Do you know what the paper burrito is that's below and to the right of the big adjustable wire wound resistor?  It has two wires coming from each end.

The schematic shows one 3-section lytic cap, C21. My first guess is that's what you're looking at: 4 leads = 3 caps with 1 common negative.

Trace its connections and compare to the schematic to verify. Also, if none of the can caps have 3 sections, that would be another clue.

[WimWalther]

Can you tell me which part this is? I'm thinking it's one of L4, L5 or L6..

Also, can you identify L3, the driver xfmr? I'm assuming that L7, Choke, is topside with the PT & OT?

[shooter]

that's a barn find!
take it slow n easy, you get her up n working, you'll have a pretty loud "neighbors can hear me" amp.

[WimWalther]

that's a barn find!
take it slow n easy, you get her up n working, you'll have a pretty loud "neighbors can hear me" amp.

Should we hold a contest to see who can guess what it weighs? ; -)

[CrocMule]

Can you tell me which part this is? I'm thinking it's one of L4, L5 or L6..

Also, can you identify L3, the driver xfmr? I'm assuming that L7, Choke, is topside with the PT & OT?

There are 3 hefty transformers topside. I haven't started tracing the circuit to identify parts yet, but I'd guess the choke would be smaller than the driver transformer and OT.  There are 3 chokes/coils that I can identify inside the chassis -- one looks toroidal (to the right of the burrito).  The largest one looks like the right size for a choke.  But I'm just guessing from my armchair at this point -- I'm not even out in my shop.

Unfortunately, I'm not going to be able to do anything with it today.  Working on taxes is way more fun.

I haven't weighed it yet, but I can say it's definitely not light!

Thanks,
Russ

[Platefire]

I guess the Beast will clock in around  49 pounds

[CrocMule]

Okay, I've got 2 bathroom scales lined up for the official weigh in.  I'll average the two.

Platefire's is the first vote: 49 lbs.

Anybody else?

I've looked a little more closely. 

The black wad (which I take to be a toroid) to the right of the burrito seems to be L4, the tone inductor coil.

The burrito is the oscillator coil L5 (6 leads).

The saturation reactor, L6, is another tiny coil with only 2 leads inside the chassis.

The choke, L7, is in fact the largest coil inside the chassis.

The PT, driver xfmr, and OT are all topside.

[CrocMule]

I'm really impressed with the attention to detail in this amp.  Check out the tube sockets, labeled on the sockets themselves, and on tiny riveted plaques next to the ceramic sockets for the 6L6Gs.

The three 6J7 sockets are all mounted on rubber isolation/damper washers (still intact), and they are all mounted in a plate that is also isolated from the rest of the chassis.

take it slow n easy.

Slow and easy is definitely the plan.  I was thinking of pulling all the tubes and putting it on the variac, through a light bulb limiter, to test voltages from the PT as a first step.  Does anybody see a reason not to?  It's a complex circuit -- I'm not sure if everything getting power will be fit to get powered.  The HV CT will still be feeding the oscillator coil.  Not sure if that's a problem if the HT leads dead end at an empty socket.

Looks like I'll need to jump some contacts in the speaker jack to make it work.

And I need a new fuse -- it came with a 4A installed.

[CrocMule]

Woops.  Photo!

[PRR]

42 pounds.

[CrocMule]

Alright, two votes! 

Does this mean I need to come up with a prize?

[PRR]

Wow, crazy. .. The 5Z3 is the main B+ rectifier. The 5V4s are used to derive a C- supply for fixed output tube bias, very unusual.

I admit I am lost, but I wonder if it is a Full Wave Bridge rectifier. It seems to have screens (and small tubes) at half-stack from plate supply. The diode filaments are wired the way they have to be in a FWB.

The transformer and especially the 6F6 driving it say this is the AB2 condition 47 Watts at 360V. Or maybe more: while RCA had to walk-back the 400V rating on the first 6L6 they got it sorted quickly and may have quietly told brave designers to push 500V at plates.