Stromberg-Carlson PA amp

[billcreller]

 The guy who had me fix up his old Sunn amp, brought over an old Stromberg-Carlson PA amp, model AU33.  I'm just now looking for a schematic.

Not sure yet what he wants me to do with it.  It has a pair of 6L6G tubes, a 5U4, and I see a 6N7 in there along with others.

It's a heavy amp, and maybe has some good iron for a build, if that's what he has in mind.

The mike & phono inputs are the old Amphenol screw-on connectors, and the output s are octal sockets.
  And of course the power cord is a two prong un-grounded type.

Anyone ever heard of this amp ?

[jjasilli]

Yes there's info all over the web.  Google it for lot's of info & also "audiophool".


I'm a fan of Stromberg-Carlson amps, and rebuilt 5 of them for home theater & hi-fi.  There should be posts on this Forum.

[6G6]

The one I had, had a schematic glued to the inside of the plate that covered the bottom of the chassis.
If you haven't opened that, look.

[jjasilli]

Good point.  I like to collect the old manuals, e.g.: http://www.ebay.com/itm/1951-STROMBERG-CARLSON-AU-33-RADIO-SERVICE-MANUAL-photofact-AU33-SCHEMATIC-/351008504090  They're full of useful info.


It seems this amp dates to about 1951.  Full frequency-range OT's (20 - 20,000Hz) became available around 1962.  The older PA amps may have a frequency range of about 100 - 10,000Hz.  The manual should state; or it could be measured - even just by ear with a signal generator.  Whatever, there are bass amp conversions of this amp mentioned on the web.


The numerous inputs ea have a gain stage.  For guitar, this argues for such designs as parallel or cascading gain stages; a cascode stage, etc. 

For more background on converting PA tube amps into guitar amps, checkout:  http://www.geofex.com/Article_Folders/old2new.htm

[billcreller]

OK ! thanks for the replies and info ! 
The owner sounds like he wants to use it as a PA (for some reason )  I figure I'll have to replace the screw on connectors with phone jacks, and maybe do something different with the octal speaker sockets etc....

[HotBluePlates]

The guy who had me fix up his old Sunn amp, brought over an old Stromberg-Carlson PA amp, model AU33.  I'm just now looking for a schematic.

http://theused.com/wp-content/uploads/resource/stromberg_au-33_ps-33.pdf

[jjasilli]

Nice post, Hotblue!


At 25W and 75 - 10,000Hz it's a bit hard to see this as a bass amp.  Though it might get down to 40Hz with lots of THD.  And it might outdo a 12 - 15W Traynor Bassmate.  OTOH, lot's of interesting possibilities for a guitar amp!

[DummyLoad]

this amp predates what we would now consider minimal HI-FI, and being a PA, so probably is 100Hz-10KHz as already stated.


looks like it'd make a nice fender tweed twin...



--pete

[DummyLoad]

spec sheet HBP posted says freq resp is 75Hz-10KHz and IMO wouldn't  sound good as bass guitar amp: too much bottom shaved off. 


fender tweed twin or... 


--pete

[HotBluePlates]

How did conversion to a bass amp creep in?

[sluckey]

I figure I'll have to replace the screw on connectors with phone jacks, and maybe do something different with the octal speaker sockets etc....

There are adapters available in case you want to keep it looking good. Plenty of them on eBay. Here's one...

http://www.ebay.com/itm/Switchcraft-332a-adapts-microphone-5-8-screw-on-connector-to-1-4-guitar-cable-/121159548342?pt=US_Radio_Comm_Microphones&hash=item1c35abfdb6

[printer2]

Might be fun.


A a different version just for the tubes. I think the original also used the 6N7 somewhere.

[jjasilli]

How did conversion to a bass amp creep in?

That would be me.  billcreller did not state the nature of a possible conversion.  There are a number of posts on the web of converting this amp to a bass amp, so I mentioned it. 

[PRR]

> wants to use it as a PA (for some reason)

It is ** 25 Watts ** !! Seriously, PA?

Know that I did a lot of 2X30W PA in my youth, but that was a long time ago and sonically very far away from today's scene.

Yes, I do know some very small low-key lounges where a singer/pianist duo could work with 25W. But who is gonna carry this beast home? Not the pianist: he might hurt a finger. Not the singer, she's only 90 pounds.

Yes, it could be a "special effect". Neil Young had a tune where part of his lyric was sung through a bullhorn. As his hands were full, he had a nasty-tone PA horn on a swing-arm. The film Jersey Boys just came out; I could see a tribute act doing the pre-fame Four Seasons through one 25W PA (to contrast with the 1960s and 1970s sound with the bigger PAs).

Unless a better idea comes along, I think it should find a better home. (I do not know who.)

[billcreller]

Thanks for the link to the manual HBP ! 

  And I'm familiar with those adapters Sluckey.  I have a National steel guitar that I bought new in 1950, and Valco loved those pain in the ass screw-on connectors.   They would save some work in this case...

[PRR]

> those pain in the ass screw-on connectors.

They are sex-less. If you need to extend a cable, you back-off one ring and then the other connector will go on. No more finding which end of a cable. No bag of M-M and F-F adaptors for odd situations.

And they were very cheap.

[billcreller]

Kinda sounds like I don't need to fool with this amp.
 My plate is too full to build something from the parts.
I'll ask what he wants to do with it.....

Thanks for all the replies...

[stratavox]

Since we're in a S-C thread...AU57 pa, has two rectifiers, a 5U4, and a 6X4. I have no experience with these circuits- can someone explain the reason/ use of two rec tubes? Thanks...

[Willabe]

can someone explain the reason/ use of two rec tubes?

2 rectifier tubes provide more B+ current with less voltage sag. Will give a tighter bottom end.

But with 2 different rectifier tubes it might be that amp has 2 different B+ rails.   


               Brad   

[DummyLoad]

Since we're in a S-C thread...AU57 pa, has two rectifiers, a 5U4, and a 6X4. I have no experience with these circuits- can someone explain the reason/ use of two rec tubes? Thanks...

usually done if there is enough of a variance in the plate and screen supply voltage requirements. in this case, the 5u4 provides power for the power amp tube plates (425V), and the 6x4 for the power amp screens and preamp (340V, etc.).


--pete

[PRR]

The lower voltage is Choke-Input.

This is for G2. G2 in many 6L6 plans wants to be lower than plate (old specs: 250V G2 360V Plate), but G2 current will vary a lot on 6L6. The choke-input gives about 63% the voltage (skewed here by rectifier drops) with much better regulation than a resistor.

[stratavox]

DummyLoad and PRR, thanks- makes that clear now. The 6x4 is on a separate secondary, and does in fact pass through a choke...there are several 12A...'s on that part of the circuit, and the 6L6's are with the 5U4...this had  been parked in a church for decades, all the tubes (but the 5U4) are S-C branded...

[HotBluePlates]

The 6x4 is on a separate secondary, and does in fact pass through a choke...

Look at the second schematic in this post.

The 425v 5U4 output and the 340v 6X4 output both share the same 330-0-330v secondary winding. The voltage of the 6X4 output is lower because there is no filter cap before the choke (making it a choke-input filter instead of a cap-input filter).

A 6X4 could be used because the current draw for the output tube screens and preamp is well below that drawn for output tube plates. The 6X4 is also a physically much smaller tube than a 5U4.

[stratavox]

HBP, right as always, I missed that. Thanks again, continuing to learn.

[billcreller]

Lots of interesting info here.  I haven't touched this one yet.  I think I'll make a small turret board for replacing  the filter can caps, and leave them mounted. I should replace the cathode bias caps too ?  They are all the wax dipped type.  Looks like a radio from the 30s !  Maybe replace ALL the caps ?







Bill

[HotBluePlates]

I should replace the cathode bias caps too ?  They are all the wax dipped type.

Most of the cathode bypass caps are electrolytic and probably should be replaced. There is one that is a small value, non-electrolytic, and may be okay. But see below.

They are all the wax dipped type.  Looks like a radio from the 30s !

If a radio repairman looked at the amp, they would assume all wax-dipped paper caps are leaky and replace them. Because they usually are bad. You can try replacing just the electrolytics first and seeing how the amp performs, or you can swap all caps. Your call.

[billcreller]

Today the guy gave me the amp.
He said he didn't really have a use for it ( and I don't either )  Some good parts there I suppose.  I'll keep it around until I get a brain fart to build something.  I'm down to 5 amps and a Valco clone in the works, so I don't need any more.....

[stratavox]

Sorry to pull an old thread up, but I'm working on the SC AU57 now...I replaced the filter caps and dropping resistors, changed the outputs to 1/4" jacks at 4 and 8 ohms, and swapped one input jack for a 1/4". Issue- first powering up had output, and within a few moments v5 and v6 went dark. I have voltages, albeit low (still on the current limiter) where I should have them- any suggestions as to where to begin? I have swapped them (v5 and 6) for known good tubes, and swapped recto tubes. Thanks all...

http://el34world.com/Forum/index.php?topic=13654.msg127840#msg127840

[stratavox]

A little more info...voltages on the tubes (v5 and v6) in question...off the limiter...test speaker ver'd good, all tubes good.

Getting 420 volts at the recto...

6l6's are are at 419 plates, 343 grids...meter probe causes a "pop" at the plates...


12au7 pin 1  56v, pin 3  2.3v, pin 6 56v, pin 8  2.3v,  heaters at 2.25v

12ax7 pin 1  176v, pin 3  1.7v, pin 6  238v, pin 8  1.8v,  heaters at 2.25

Any thoughts? Many thanks...

[sluckey]

Measure voltage for all six of the B+ nodes (called source on the schematic). What are they?

I suspect you are measuring filament voltage incorrectly. You cannot measure filament voltage in reference to chassis ground in this amp because the filaments are not referenced to ground. Instead, they are elevated to the voltage on the cathodes of the output tubes and that voltage will be about 25VDC in a healthy amp.

You must read the AC filament voltage by connecting one probe to a filament pin and connecting the other probe to the other filament pin. That would be pins 2 and 7 for the 6L6s and pins 4/5 and 9 for the little tubes. Are V5 and V6 still dark?

[stratavox]

Well, thanks first. Appreciate your help Sluckey...as for filaments...I get a 25vdc reading at 2 and 7 of the 6l6's, AND a 6.3 v AC reading. Is this a product of the "elevation" process you mentioned? And I'm sorry I used the term "dark", I meant it seems they're not functioning. They are in fact lit...as to power rail voltages- the schem in the attached file shows voltages, but my unit has a different schem glued to the inside of the bottom, showing no voltages. Having said that- at the first two 30 mfd caps, I read 422 and then 344v. The next two 30's are 242 and 182, and the two 15's read at 60 and 140. Hoping some of this makes sense...and again, thank you. I continue to benefit from the generosity and knowledge of those here...

[stratavox]

I question as well the output connection...there is/was a screw terminal strip, which I removed and replaced with a pair of 1/4 " jacks. I ran the 4 ohm and 8 ohm o/t leads to them, and the "common" wire to each, but they are on a piece of circuit board material, not grounded to the chassis. Any thoughts?

[sluckey]

The schematic shows the common grounded and since there is a NFB loop, you must ground the common to chassis.

[stratavox]

Sluckey, grounding the outputs made no difference. The schem I have is a bit different than the one in the above post, mine doesn't show voltages, for example. At present, if I probe the plates, I get a very faint "pop", and if I touch the "tip" terminal of the input jack, I still get a very faint, but audible, hum. It almost seems like the volume is just turned nearly off...I listed the voltages I read after replacing the filter caps, and again, all the tubes are good (subbed/swapped). Stumped now- anyone? Thanks...

[jjasilli]

Some thoughts:

I think that probing the power tube plates should produce a crisp, not a faint, pop. This leads me to wonder if the OT secondaries are connected to the wrong taps.  This could cause output power to seem weak.


The filament supply is confusing.  Unless you're certain all is well, then maybe disconnect the existing filament supply.  Temporarily run the filaments off a "standard" outside supply.


Use a listening amp to trace signal to where it fails.  Or, bypass the preamp in sections and inject signal at progressive points in the amp to isolate a point of failure.