Stromberg Carlson ASR 880 G (Stereo 8) questions

M

mitch01

Member
Hi, I have a few questions about my amp (its the SC that uses 7355's as output tubes, says Stereo 8 on the front panel but ASR 880 G on the bottom plate label).

1. It uses 2 solid state diodes for a voltage doubler circuit. Is this considered crappy design, even with good filtering? Any reason to substitute modern diodes?

2. There is a balance pot for each pair of output tubes. What's the procedure for adjusting it? Look for zero voltage difference between pin 6 (bias) of ea pair 7355s as the pot is adjusted?

3. What does the "phasing" switch do?

4. A few months ago someone was selling an identical amp on Ebay and the auction had the original magazine ad for the amp (looked very interesting). I saved a copy but lost it. Does anybody else have one I can have?

thanks

Mitch
 
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Solid state doubler or (occasionally) full-wave bridge was the usual rectifer in the '60s. Reduces power transformer size and weight by about 30-40% over a tube rectifier for the same rating. And a stereo amp may be over the rating of a 5AR4 anyway...

I'm sure the adjustments are in the Sams folder on my web page. http://www.audiophool.cjb.net/MadeInRoch.html

Phase switch reversed the polarity on one speaker - no need to keep track of which wire is which on the speaker terminals.

Wish I'd seen the ad... I do have a brochure that includes it on my page.
 
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I would definately replace the diodes with modern pieces as they have come a long way since then. I prefer Cree 0 recovery myself, but there are lots of options out there. A voltage doubler with good diodes and caps can be VERY quiet.:yes: Any chance of getting a couple pics? I collect SC gear and have never seen one of these.
 
Thanks very much the card and the folder they are great! I'm surpised that the procedure for setting the balance is so complex. They say to remove a soldered jumper before making the adjustment. I have a scope and audio generator which I haven't fooled with in a while though so this sounds like an educational father and son activity. I had a couple Heathkit mono power amps (ones with the KT-66 tubes) and they had a pot and two jacks for measuring Voltage difference.

About 10 years ago I replaced the coupling caps with Sprague Orange Drop types. I'm thinking about getting rid of the electrolytics. Not sure if I should stuff modern ones in the cans or look for somebody who still makes them (multisecion caps). The one with a paper rather than aluminum body is bulging at the tops so its likely bad..




thanks again

Mitch
 

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mitch01 said:
About 10 years ago I replaced the coupling caps with Sprague Orange Drop types. I'm thinking about getting rid of the electrolytics. Not sure if I should stuff modern ones in the cans or look for somebody who still makes them (multisecion caps). The one with a paper rather than aluminum body is bulging at the tops so its likely bad..
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I would replace the electrolytics. If they are original they are time bombs.

I rebuilt a Bogen CHB100 whose power supply had fried. One of the diodes had vaporized, a filter had exploded shooting it's guts out one end and a resistor was burnt up. Lots of reconstruction. I think the cap had shorted out. Anyway it works like a champ now.

Also there was a multi-section can that I gutted and stuffed with new caps. For the Bogen it was a good idea since the underside of the chassis was crowded.

There are a couple of sources for new multi-section caps.

Try: http://tubesandmore.com/

They also have the multi-section cardboard caps like are found in many old radios.

Your S-C is a very nice amp. I'm envious.
 
Thanks. I see the cans are about $40 each. I looked at DigiKey's website and they have Nichicon 47 uF 450 Volt radial caps @ .83 cents each. So, if its not too hard to disassemble the cans this would be vastly cheaper way to go...assuming they fit. I think the originals were more like 20 uF's, which if I used them instead, would be smaller.

Thanks to everyone who complemented the way the amplifier looks. I really like it too. The top surface is kinda grungy though. I used to have the SC that used 6BQ5's, but I sold it long time ago. They made many similar looking amps.

Mitch
 
mitch01 said:
I looked at DigiKey's website and they have Nichicon 47 uF 450 Volt radial caps @ .83 cents each. So, if its not too hard to disassemble the cans this would be vastly cheaper way to go...assuming they fit. I think the originals were more like 20 uF's, which if I used them instead, would be smaller.
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If the originals are 20uF's replacing them with 47s might be a bit a little much. I'm not familiar with your S-C but bigger isn't always better.

Re-stuffing a can is a pain but you might get a higher quality cap out of it. On the other hand, if there were room under the chassis consider going that route.

Here is a link for re-stuffing the can's. There are others if you look around.
http://www.r-390a.us/filter_capacitors.htm

Perhaps some of the A-K veterans will chime in about sizing the filters.
 
If you mount them under the chassis would you re-use the old mounting points, beneath the caps? That would simplify the replacement but the new caps would be in parallel with the old ones. Or would you remove the the old cans from the ckt and go point to point between whatever nodes the caps used to connect?

mitch
 
Don't reuse the original mounting points if the original caps remain in circuit. If voltage is applied to them, they can and will deteriorate and if they short out, there will be a lot of damage, especially to the power transformer.

Increasing the filter capacitance may put the rectifier over its current rating - they are usually rated for 20 uF and they have to be derated if more capacitance is used.

I have rebuilt two 3-way battery portable radios that showed the same problem - if you leave the original caps in, their leakage can be detrimental to operation and you get serious hum. Remove the original caps and everything works as it should.
 
mitch01 said:
If you mount them under the chassis would you re-use the old mounting points, beneath the caps? That would simplify the replacement but the new caps would be in parallel with the old ones. Or would you remove the the old cans from the ckt and go point to point between whatever nodes the caps used to connect?
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Well you don't want the old caps in the circuit so you can't use those connections.

If going underneath the chassis I would mount a terminal strip for the replacement caps and wire to the lugs on the strip. You could leave the can unconnected or remove it if you don't mind the hole. Either way it's sort of messy.

That's why I ended up re-stuffing the can on the Bogen. It just looked better on top and the wiring was much cleaner underneath. Truthfully I might have been better off to spend the $30 bucks on the new multi-section can but it was kind of a fun project (ok a twisted definition of fun).

Now are you aware that there are LETHAL voltages involved here? Several hundred volts to 450 volts on my amp. So there are safety concerns here. Discharging the caps when you first start working on it, paying attention to polarity, wiring, soldering technique to name a few.

Not trying to dissuade you (and not trying to preach) but urging caution since I'm not sure of your comfort level.
 
With the solid state doubler, one can is insulated - the can is at +220V. This one always seems to fail first since it doesn't have a good thermal path to the chassis to dissipate heat. If you want a different can to restuff, let me know the size, as I have plenty of old ones.
 
Nice find! Jaymanaa, are you the only one who doesn't have one?
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With all the discussion about rebuilding the ASR-880, I think we forgot to mention how this bad-boy sounds. I have my 2nd asr-880. I traded my first one for this museum quality beauty that is better looking than the pictures present. I run Bozak 302's and sometimes use my Magneplaner SMG's. Either way, this unit sounds great. I am sure I could get more out of it if I replaced caps etc., but I hate to alter such a beauty. Here are a few pics:
 

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