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Must see!! Stromberg/Carlson

1/13/2001 12:56 AM
Frank D
Must see!! Stromberg/Carlson
Hey guys,  
I just got back from my friends antique shop and after telling him I was trying to build a tube amp from scratch, he said: "I have some old tubes here, let me find them." He started diggind around in an old storage shed and came out with an antique phonograph amplifier. He let me have the entire thing! I need to know what I can build with it.  
gain control  
big,red mallory  
2x 6f6gt-looks like 6l6  
2x 6sn7-looks like 6v6gt  
1 5y3 five pins?  
fidelity switch (looks like it bypasses a choke on inside)  
treble/on-in same switch.  
Power/Output trannies look excellent!  
She's in mint shape-just dusty.  
Giant red mallory cap that says-vac 450 450 450 350 on it  
All cloth wired, caps on term strips, large 5/6 terminal cap on back wall  
The caps look like old cigars and cigarrettes!  
One resistor going accross v4 (or v3) says IRC on it with an ohm symbol over the R-it looks like a domino!  
There are also 2 chokes on the inside of it. None of the caps are bubbly or distorted looking and there is a mint schematic on the bottom lid of the chassis(it's yellow, though).  
C'mon guys! I'm excited-what can I build with her? It seems as though I could just wire in a 1/4" i/p in place of the phono i/p. What do you think?? I haven't "fired" her up yet and discharging the caps seems like a daunting task because they're all over the place.  
How can I tell if the trannies are any good? How many watts is she(though a rear plate says 85)? I need to know because I want to build her into a cool blues amp. I'm going to have to rip out all of the guts and start over-sockets, pots, jack, etc...there's not much room in her-10"x6"x2.5". Any ideas? If the trannies test well, is it a matter of shoehorning a circuit in here and then trying to make it compatible with the power section? I don't know where to start.  
1/13/2001 1:13 AM
Ted Pimm

Looks like a nice amp for about 8-10 watts in push-pull. Stromberg-Carlson generally were very good quality products. I have one with a big Hammond 25-cycle power transformer (1951). 6F6G's were nice push-pull tubes which can handle around 350V without too much problem. They like higher load impedence than 6L6 types though. You could probably mod to a 6V6 output and the output transformer would be OK. A "deluxe amp" might be a reasonable circuit to mimic, although, stay with the 6SN7 twin-triodes (think octal 12AX7). Set-up/biasing of these triodes is probably close to what you want already. The 5Y3 winding is probably only rated at 1.5-2.0 amps so dont sub a 5U4 type which draws 3.0A  
Please do yourself a favour and replace all waxy or old coupling caps. All electrolytics too despite the fact that they seem fine. That amp probably hasnt been used much in a lot of years and the caps ARE likely to fail with consistent use and can take out your PT in the process.
1/13/2001 1:50 AM
Frank D
Thanks for the reply. I just tried to fire her up and its a no-go. I checked the power cord and it's ok. This is my 1st project so I don't realy know where to go from here. I'm ordering new tubes for her right away-just to be abe to rule that one out. Guess I can't do too much until they arrive. A plate on the back of the chassis says "85 watts...50-60 cycles". So does this indicate the output of this amplifier? THis thing has quite a large transformer on it. Am I pretty much dead in the water until I get new tubes?  
Thanks plenty,  
Frank D
1/14/2001 8:54 AM
85W will be the draw from the mains.  
1/13/2001 2:47 AM
JaySG Re: Must see!! Stromberg/Carlson
Sounds like a PA-37. I have one. Mine has been a variety of different circuits since I started doing evil things to it. Most recently, I replaced the OT with a Tweed Deluxe OT. The stock OT sounds better the way I'm running it. The phase inverter is an older design than most. I think it has a gain of 8. I've kept that part stock. I've replaced the common cathode resistor (250 or 350?) with a 500 ohm. It was dissipating too much plate power (18W per 6v6!!!!). I replaced the filter can with a new Aero from Antique Electronics -- I think a 475V 20-20-20-20.  
The stock circuit is ultimately, a phonograph amplifier. If you want a guitar amplifier, most of it will have to go.  
I used the Fidelity hole for a presence control. I used the input gain adjust pot hole for the input jack hole. I used a 6sl7(I think) as the first tube. Whatever I used is octal, has a higher mu, and can approximately use 12ax7 values. I bought a NOS 5Y3 and gained 25V in the B+ over the original. B+ is 465V with diodes in place of the 5Y3. Pretty high for 6V6s. The plates are run after the choke, not before, so there's a bit of a drop there. The schematic is on the inside of the bottom plate and is a bit of a pain to follow. I was able to get 18W from it in one of its earlier incarnations. It's at 12W in the Tweed Deluxe reality space.  
Good luck. Never part with the thing. Mine's given me a lot of enjoyable hours.
1/13/2001 6:55 AM
Frank D

This one's actually an ar-37. I have not a clue how old it is- though you wouldn't believe some of the crazy phonograph websites they have! They really have something for everyone, here on the net. It's a wonderful thing.  
Anyway, I'm getting philosophical...where do I start my diagnosis from? All Iknow is the crummy powercord works. There's hardly any room to work with, but it seems like such a worthy challenge. What tube variety did yours come in? And how do I test the tranny on it? I swear, those old cloth wires almost all look brown! Did you have to replace any sockets? I feel a few loose solder lugs...I know I'm getting a little too emotional, bu she smells like some good ol' WWII electronics...reminds me of my grandfather. Keep them coming guys.  
1/13/2001 7:26 AM
Frank D
Cap can
The gig cap can next to the 5y3 is a red, white, and blue Mallory. On the side it says: 40,20,20,20 mfd. This is simply a case of it? So I replace it with a 100mfd one, right?
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