# Stromberg-Carlson AU 42 overhaul



## Greg Ellis (Oct 1, 2007)

Here's an early 50's PA amplifier I picked up for cheap, built in Rochester NY around 1953 as best I can tell.

GCForum - Google Drive

It's a "lunchbox" sized amp in a style that was very popular way back when, with the ins and outs and controls mounted right on the chassis, and a metal cage over the tubes and transformers so you don't burn your fingers.

Inputs for mic and phono/aux appear on one side. The back of the unit has some crazy old multi-prong speaker jacks that fell out of fashion a very long time ago.

This particular amp was a part of a series that worked like this:

AU-29 = 1 x 6L6 for 6W, 5Y3
AU-42 = 2 x 6L6 for 15W, 5Y3 << my amp
AU-34 = 2 x 6L6 for 25W, 5U4
AU-36 = 4 x 6L6 for 50W, 2 x 5U4

15W on a pair of 6L6 output tubes seems pretty light, but I'm sure it will be plenty. I'm guessing they were saving money on the rectifier and the transformers to offer up an amp at a certain price point.

Being close to 70 years old, this amp has been through a lot. 

The output transformer is not original. The spec calls for a Stromberg-Carlson 46317, with 4.5 K primary impedance (290 ohm DC resistance) and secondary taps for 4, 8, 15 and 500 Ohm output

The replacement is a Hammond 1615, with a 5K primary and output taps for 4, 8 and 16 Ohm loads. It's rated for 15W. I can't tell how long ago this replacement happened - i don't see any date codes or anything - but I'm going to guess that the work was done in Canada, since the transformer is mounted with Robertson screws, lol.

The power transformer is not marked anywhere I can see. I'm guessing based on appearance that it is the original part, a Stromberg-Carlson 46316 with a primary spec'ed for 117VAC at 0.85A, 680V CT .120 ADC, 5VAC @ 2A, 6.3VAC @ 2.8A

Rectifier was originally a 5Y3 of some sort, but it was replaced at some point with a solid state rectifier in an octal tube package, an SI S5251. This is obviously conjecture on my part, but it seems possible that this SS rectifier, combined with the general rise in AC line voltage since 1953 may have had something to do with the original output transformer blowing up.

One of the reasons I chose to buy this particular amp, rather than some other old junker, is the wealth of documentation I found on the internet. I've got a full parts list of values and ratings for all the caps and resistors, a schematic, a table of voltages measured at various points, etc. That should make life a lot easier.

General layout of the amp is as follows:

The mic input passes through a 6SF5 (single triode) before it is joined by the phono input at the grid of a 6J5 (also a single triode tube) to be amplified some more. 

Output of the 6J5 is coupled to the grid of one half of the 6SC7 driver/phase inverter (twin triode tube), along with the tone pot circuit and what looks like negative feedback from the 15 Ohm secondary winding of the output transformer.

Output of that triode is coupled to the first power tube, and a (presumably small) portion of the signal is passed through a 330K resistor back to the grid of the other triode in the 6SC7 to create the opposite phase for the other 6L6.

The power tubes are cathode biased with a 200 Ohm resistor to ground, bypassed with a 50 MFD cap.

Some of the expected voltage readings are a bit curious (from the spec, I haven't measured anything):

6SF5 (mic pre) 135VDC on the plate, 1.6VDC on the cathode
6J5 (audio amp) 50VDC on the plate, 1.8VDC on the cathode << only 50V on the plate?
6SC7 (driver/PI) 85VDC on both plates, 1.3VDC on the common cathode
6L6 (output tubes) 330VDC on the plate, 300VDC on Grid 2 (pin4), 22VDC on the cathode
5Y3GT (rectifier) 320VAC on the plates (pins 4 and 6), 340VDC on the filament (pins 2 and 8)

The can caps look like replacements to me, but I'm not sure what vintage they are.

They are Spragues marked as follows:

D6488 96
40MFD 450VDC
30MFD 350VDC
15MFD 300VDC << not used, and does not appear on the original parts list
50MFD 50VDC

D6475 116
10 MFD 350VDC
10 MFD 300 VDC
10 MFD 250 VDC
20 MFD 50VDC

One of the caps on the second can is not connected, but it looks like it used to be (wire is clipped). I'll have to trace some wires to figure out what happened there.

Is there any easy way to determine the age of the Sprague cans?

I'm waiting for some power tubes to arrive before I can do much of anything.

I might untangle some of the rats nest of wires that were added to mirror the OT taps to a set of screw terminals. It looks like someone removed the second speaker jack and replaced it with a custom power input jack too - that will all need to be undone. 

My current plan is to remove all of that mess, cover the openings with a metal plate and mount a couple of output jacks for 8 ohm and 16 ohm speakers, plus a new 3 wire power cable.

I can see quite a lot of caps that look ancient and dangerous - I'll need to start a shopping list for those.

Any comments or advice much appreciated. I'm expecting this will be a long slow project with a lot of learning for me.


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## Greg Ellis (Oct 1, 2007)

I've been chasing some circuits through the amp while I wait for supplies to arrive, and I came across something that seems a little odd to me.

If I'm not mistaken, the ground lugs from the input jacks, and the grounded end of the input volume pots, are all strapped together with wires and then connected to one of the filament pins on the first preamp tube.

It seems strange to me that you would use a 6.3 VAC power source as a ground point for a preamp. 

Any insights here? Have you seen anything like this before?


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## Lincoln (Jun 2, 2008)

Just that on older amps there was only one 6.3V heater wire, and the other side of the heater circuit on each tube was grounded. Is that maybe what you are seeing?


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## Greg Ellis (Oct 1, 2007)

Lincoln said:


> Just that on older amps there was only one 6.3V heater wire, and the other side of the heater circuit on each tube was grounded. Is that maybe what you are seeing?


Could be, good point.

The schematic shows a secondary on the power transformer for 6.3V that has a center tap connected to ground through a 0.1 uF cap.

I'll chase the heater wires back to the PT and see what is actually happening. Tomorrow. I'm too deep into the wine to make sense of it tonight, lol.

I'm learning as I go here. I've been looking at schematics for a while now - gosh they are so nice and orderly. The actual layout of the amp is another beast entirely.


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## jb welder (Sep 14, 2010)

Lincoln said:


> Just that on older amps there was only one 6.3V heater wire, and the other side of the heater circuit on each tube was grounded. Is that maybe what you are seeing?


I think Lincoln is right, they probably had 2 ways of ground referencing the heater circuit and you probably have one or the other. I don't think you would find both in the amp. So I doubt that your amp has the heater center-tap or the cap connecting it to ground.
I suppose they could use both, but not sure what the point would be.


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## Greg Ellis (Oct 1, 2007)

Alright I'm back at this finally.

Starting over from the power transformer...

This is a very old PT, there's no bottom bell, no markings i can find, and most of the wires are not color coded like you might expect on a more recent unit.

At one end of the transformer, I see five 5 wires coming out. Two are red and are connected to pins 4 and 6 of the rectifier socket. That's got to be the high voltage AC secondary. 

There's another wire in the same spot that is kinda sorta red but not the same red, which is connected to ground at the first cap can. I'm going to assume that is the center tap on the same HV AC secondary.

At that same end of the PT, I see one black wire going to the power switch and then onward through the fuse and out the back of the amp - that's got to be one side of the AC primary. The other black wire is soldered to pin 3 of the rectifier (as a tie point) where a short wire takes it out the back of the amp, so that's got to be the other side of the AC primary.

All good so far.

At the other end of the transformer, there are five more wires. Two are connected to rectifier pins 2 and 8, so that's got to be the 5V secondary. 

Two more wires land on a convenient terminal strip nearby, connected to the heater circuit. And there's one more wire that probably USED to land on the same terminal strip but it's been moved.

And this is where my descent into madness begins, lol....

At some point in its very long history, someone removed one of the 8-pin speaker jacks and replaced it with a 7 pin circular jack that is connected to some really bizarre things.

I'm a bit at a loss to explain how or why anyone would want these sorts of connections on a jack on the back of an amp. Maybe to connect to some other sort of equipment like a turntable or a mic preamp?

pin 1 - AC power line input - ties to the AC primary through pin 3 of the rectifier, also see pin 7
pin 2 - heaters - connected to one side of the 6.3V heater winding at the terminal strip, and onward to the tube heaters, eventually grounded at the input jacks
pin 3 - grounded at a nearby cap can
pin 4 - B+ (whut?!?!) - one wire ties to pin 2 of the rectifier, and another wire connects to the 1st filter cap, first power resistor, center tap of the output transformer primary - this is B+
pin 5 - heaters - one side of the 6.3V heater winding on the PT is connected directly to this pin
pin 6 - heater circuit - the non-grounded side of the tube heater circuit lands here, instead of on the terminal strip
pin 7 - AC power line input - ties to the fuse and then the switch and into the other side of the AC primary

There's a scribbled note on the schematic - "5 and 6 jumpered in male 7 pin plug" which makes some sense. A jumper between pins 5 and 6 would complete the heater circuit between the tubes and the 6.3V winding on the PT.

So... I've seen this sort of idea before, usually implemented as a "jumper in the speaker jack" which prevents the amp from operating unless there is a speaker plugged in. I cannot fathom why anyone would implement this in a POWER jack. If you've unjacked the power, you don't need any protection mechanism. And a jack present in this plug does not guarantee that there is a load on the OT, so again, it's a bit bewildering.

Even more puzzling to me is why you would want to send some of these connections outside the chassis on the same jack you're using to feed AC power into the amp. B+ ? Seriously?

The only thing I can think of is maybe this weird multi-conductor jack was used to connect some other devices. Any thoughts?

Mostly I just wanted to document all this madness before I rip it all out.


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## jb welder (Sep 14, 2010)

Greg Ellis said:


> And there's one more wire that probably USED to land on the same terminal strip but it's been moved.


Where has it been moved to?


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## Greg Ellis (Oct 1, 2007)

jb welder said:


> Where has it been moved to?


It was moved to pin 5 of the weird and alarming 7 pin jack, waiting to be jumpered to the rest of the heater circuit on pin 6.

There's an unused wire from the PT sitting on that same terminal strip, which I suspect is the center tap of the 6.3V winding. It's not connected to anything at all.


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## Greg Ellis (Oct 1, 2007)

I think my next step is to remove all of the insanity associated with that 7 pin jack.

I'll move the heater winding off that weird jack and back to the terminal strip where it belongs.

There is a third connection from the PT on that terminal strip, which i bet is the center tap of the 6.3 V winding, but it doesn't seem to be used in the current circuit. It is not connected to anything at all.

One side of the heater winding touches each tube, starting in the power section and backwards through the signal chain to the first mic preamp tube, where it grounds onto the chassis at the input jack. It's pin 7 of the first preamp tube - a 6SF5 - that connects to the ground plane of the mic input and the phone input.

That same ground point of the input sockets is wired to the ends of two volume pots and a tone pot as well.

The other side of the heater winding touches each tube back to the first mic preamp and just stops there.

It seems odd to me to ground a 6.3V AC power line at the same spot where the most delicate signal is entering the amp. Maybe it works? We can try it, right?


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## jb welder (Sep 14, 2010)

Ok, that makes sense. They can't use the heater CT because they have grounded one side of the heater winding. If you ever have to chase down any unwanted hum, you could try lifting the heater to ground connection (you mentioned in post #2) and grounding the heater CT. It has to be one or the other grounding method. If it's not a problem (hum), you don't need to worry about the heater wiring. Some Champs and other vintage Fenders use the one side heater grounded method, like @Lincoln mentioned above.
Am I correct that as long as you jumper pin 5 & 6 of that accessory jack, this unit will function? If so, you don't need to worry about it unless you think it's a safety hazard. Just move the wire from pin5 to pin6.


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## Greg Ellis (Oct 1, 2007)

Oh that entire modded jack is a safety hazard, almost by design. I suspect LSD may have been involved.

And I can use that hole for something really useful like a speaker jack or an impedance selector, so it's going to disappear.

I found a strain relief today that fits the power input cutout on the chassis, so my next step is finding a spot on the chassis for the earth ground.

Is there a spec or a code about that?


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## jb welder (Sep 14, 2010)

You mean the ground wire of the 3 prong AC cord?
A couple things. It is supposed to have it's own chassis connection. That means you don't use a transformer bolt etc. 
It's own dedicated screw/nut with appropriate locking washer. This is often overlooked when updating old stuff to be 3 conductor AC, lots of people use existing bolts for the ground, code doesn't really apply to updates. But since you asked, that is the way it is supposed to be done.
Also, the earth ground wire (green of the AC cord in North America) should be the last thing to disconnect if the AC cord is ripped out of the amp. So when your 3 conductor AC cord is all wired up, the green should be longer than the white and black.


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## @mfahsel (Jan 24, 2021)

Hello there Greg. I was researching my newest project and found your post on the GuitarsCanada website. Thought I'd ask if you were happy with the finished project and if you might have any suggestions for me before I move forward?

Thanks,
Martin


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## Latole (Aug 11, 2020)

Some pictures;


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## Greg Ellis (Oct 1, 2007)

oh wow. 

That's the same amp

Mine is just a decorative conversation piece at the moment.

It's been a fascinating learning experience for me so far, but I haven't got to a point of functionality yet.

Tell us about yours !


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## @mfahsel (Jan 24, 2021)

Greg, I've not yet begun the conversion/restoration process. Here are a few photos of the upcoming project. The case has a fair amount of corrosion, though I think it will clean up well and look fine. I rather like the look and don't intend to repaint. The interior looks basically untouched and the tubes tested to varying degrees of functional.


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## DaddyDog (Apr 21, 2017)

I acquired an AU-34 from another forum. I’ll have an amp tech work on it. It’s in great cosmetic condition.


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## Paul Running (Apr 12, 2020)

Converting to guitar amp?


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## DaddyDog (Apr 21, 2017)

Paul Running said:


> Converting to guitar amp?


Asking me? Yes.


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## @mfahsel (Jan 24, 2021)

That's a beautiful amp. Let us know how the conversion turns out!!


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## Gonflable (4 mo ago)

Hey Folks

I am restoring one of these. I am replacing all the old bypass and coupling caps. 

I noticed at V1 that has the 50uF cathode bypass cap C3 in parallel with R5, one end is to ground. However there is no connection to ground on mine. 

Can anyone tell me how and where this is physically connected to ground? There are probably other components on that first strip also missing the ground connection. A picture if possible, showing that ground connection would be great.

Thank you, CG


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## Gonflable (4 mo ago)

Gonflable said:


> Hey Folks
> 
> I am restoring one of these. I am replacing all the old bypass and coupling caps.
> 
> ...


I figured out it was using pin 1 of V1 as a ground point. Pin 1 was being connected to ground via the input jack ground lug which in turn was connected by being bolted to the chassis. I moved the grounding point to the base of the tube socket.


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## Paul Running (Apr 12, 2020)

Here's another manual:


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