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Old 22nd March 2013, 02:41 PM   #1
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Default Selecting 5881's question

Hi everyone-

I am new to tubes.

I have a 1960's vintage Schulmerich audio amp (marked Model 6-190E, evidently used with Carillons) that needs four 5881 output tubes, two for each (independent) channel. I have had no luck finding a schematic, it has not been powered on yet, and I have little knowledge of the actual circuit requirements.

At this point I just would like to attempt to repair it enough to work, then later perhaps improve/tweak. This is a learning project. I have replaced all electrolytic caps and done a few other repairs. I need to buy four 5881's for this. Could you recommend which ones and which supplier?

I have been recommended to Bob Dobush, in Ohio, of findatube.com. He has four used US green print black base Sylvania's for sale for $15 each plus postage. His opinion is that they will sound better than the Sovtek 5881's (through yenaudiostore on eBay for about that same price). He is evidently reputable, and checks his tubes, but I am unfamiliar with what that might actually mean in his case, or what to ask. Maybe you could suggest questions for me to ask him?

I don't want substitutes at this time, i.e. 6L6 variants etc, since the amp is designed for 5881's. And please, if you recommend other suppliers, don't use acronyms- I won't know who you are referring to! $15 is probably as high as I would want to go for these, at this time.

The other tubes are: 6SJ7 and 6SN7. Neither the power supply transformer nor the output transformer are marked with any info. The outputs are 8-pin octal sockets, and I suspect has taps for 70V as well as 16ohm and 6ohm (not sure yet).

Different question, if I may; why would an amp like this require two separate power line inputs, one for each channel?
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Old 22nd March 2013, 03:13 PM   #2
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Personally I would use a set of cheap soviet 6P3S-E for my initial testing.

The two power lines are possibly for each channel (easy to verify) or possibly one for the amp and a feed-through for the preamp.

Post some photos of the underside.
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Old 22nd March 2013, 03:18 PM   #3
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Yes, one power line to each channel. But why would it need that? Photo is before I replaced caps, etc.
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Old 22nd March 2013, 03:30 PM   #4
TheGimp is offline TheGimp  United States
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Totally conjecture, but I suspect to save power when using one channel as a mono amp.
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Old 22nd March 2013, 03:52 PM   #5
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Hi,
If you haven't done so, change out the black capacitors with the red writing on them; especially the ones that connect to pin 5 of the 5881 sockets. They become leaky and are the main culprit for destroying output tubes.
The way the amp is laid out, it looks like a dual mono amp on one chassis. Very neat.
Hope this helps...
Daniel
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Old 22nd March 2013, 04:07 PM   #6
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Hi Dan- OK. I assume those with the red lettering are mylar, and might not need replacing... but I do expect to check all of them at some point. I certainly didn't know about those particular ones contributing to the demise of the output tubes, so that's a definite. Thanks!

TheGimp- yea, OK, that's possible. With these power amps (to me) it seems silly to quibble over the cost of power, but then again, if these were used for Carillons it may be that one channel was left on 24/7? I just wanted to make sure there wasn't some electrical reason. Since I plan to modify these for home use, I might as well remove that option and use something more convenient and common.
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Old 22nd March 2013, 05:00 PM   #7
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Originally Posted by keithostertag View Post
Hi Dan- OK. I assume those with the red lettering are mylar,

They are paper caps and usually go bad because of age. As a rule of thumb, they get replaced because more often than not they are leaky.
They orange/yellow ones are probably bad also. Electrolytics...
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Old 22nd March 2013, 05:06 PM   #8
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A Sovtek 5881 has slightly larger Anode and can dissipate an extra 5Watts as a result. Apart from that they are electrically similar to a 6L6GC.

Last edited by JonSnell Electronic; 22nd March 2013 at 05:06 PM. Reason: Addition
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Old 22nd March 2013, 06:03 PM   #9
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Looking at the photo of the bottom it looks like the bias voltage is taken to the output sockets (blue wires from output tube sockets connected to the bias pots).

That should make it easy to adjust the bias for whichever tubes you install but it is done per pair, not per individual tube.

If there is enough latitude in the adjustment you may be able to bias it for the lower dissipation 6L6s (23W).
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Old 22nd March 2013, 06:33 PM   #10
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Quote:
Originally Posted by TheGimp View Post
Totally conjecture, but I suspect to save power when using one channel as a mono amp.
My guess is they wanted 100% redundancy - if anything fails, there's another full unit ready to go... the chimes must go on or nobody will know it's lunch time!
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