Capacitors in old tube amps?

I hope some of you electronics guru's can help me out. I got a vintage 60's stereo amp from Ebay a month or so ago. It's a little Japanese made Calrad SA-30B with 4 push-pull 6BM8 tubes giving it about 15 wpc. I am having a blast with this project. Waiting for the schematics to come this week, but meanwhile I tested it and one channel is weak and scratchy, the other one is nice and loud, but distorted, so I plan on replacing all the caps. Rat shack doesn't have beans for passive components, but they at least have resistors. There's a bunch of .005 uF paper oil caps in the chassis with TV1200 or 1200TV printed on them, along with the value. Can I use the Sprague Orange drops (polypropylene) as replacement caps. I can get the .0047 or .0056 in either 400V or 600V flavors. The same question applies to the ceramic discs, some of which look mighty ill, with suspicious bulges and corroded leads into the components.

For caps, tubes etc. they will have a reasonable selection

http://www.vacuumtube.com/Products.htm

www.angela.com

he stocks caps and parts for tube gear. He does have a limited selection of vintage caps too!

Another good source of caps is Antique Electronic Supply, www.tubesandmore.com.

Key things to think about tube gear restoration:

1) Power supply caps *need* to be replaced. Electrolyte dries out over time and, essentially, turns them into big resistors whose resistance gets smaller over time. Extreme cases can have the can exploding. Treat these caps as safety items.

2) Interstage coupling caps
- This isn't a safety issue but where you will realize increased quality of sound. Generally, swapping interstage caps will result in more authority, solid bass and crisp treble. Extreme cases of leaking caps will show up as cherry-red plates on the output tubes, with rather disasterous effects, of course.

3) Clean all the tube base connectors carefully and inspect for burns or arcing. I suspect that the Calrad may have used less expensive sockets. You can get replacements at AES, too.

As for values, if a specific value isn't available anymore, choose the next largest size up. For voltage ratings, I always err on the high side there, too. Generally, caps today are 1/3 to 1/2 the size of the originals. This allows you to choose the next higher voltage rating and still fit it into the same space.

I've done a couple of McIntosh MX-110s, an MC-225 and a Citation II so far and they now rival/exceed any solid state amp around.

For most applications Orange Drops will work fine. I'm sure there's plenty of room under the Calrad amp. Also, check all joints for soldering quality. I, for one, like to re-melt all joints with just a bit of added solder to ensure there's no internal oxidation over the years.

Hope that helps. Let us know how it turns out, too.

You can also get help at a couple of other sites:

www.audioasylum.com - check the Vintage and Tubes boards there.

Www.audiokarma.org - check their Tubes boards there, too.

Cheers,

David

Dude... I have the mono version, of almost the same amp... a nice guy from Canada just sent one to me last week! Mine is an Armaco, but I have it on good authority that the same company made both Calrad and Armaco amps. Also 6BM8 outputs, in PP...

One thing- look for out of spec/nearly open cathode resistors in the input tubes (the small triode stages and such). I have seen these go bad in MANY old amps, which can make things drop out. I could see this causing the problems with BOTH channels... sounds like it could be the first input stage's cathode resistor on the weak channel (almost no amplification whatsoever on the first stage), and possibly the cathode resistor on the phase splitter on the other channel (this would make it "lose" the "bottom half" of the waveform on the output... only getting 1/2 the waveform right, hence the reasonably loud output with distortion)...

Mine seems to play OK, but has a bit of hum and a little "midrangy" sound balance. I suspect that the supply caps need to be replaced (too little working capacitance left, which makes it lose the bottom end response, and probably excess DCR in the same caps, which may kill the top end)...

Regards,
Gordon.

Henry and Scott, I appreciate the leads on parts. Boy, once you start looking, there's a lot out there! Yet, you'll find close to nothing too. It took some serious looking to find schematics for this amp (Gordon, if you need a copy, PM me and I'll send you the Sam's Photofacts, don't know if your monoblock has treble and bass controls though, but should be the same. BTW, the same company also sold the exact same stereo amp under the "Olson" name, as I saw one on Ebay recently).

Doodlebug, you're advice was exactly what I was looking for. There's a whole lot of flavors of caps out there and it can be somewhat confusing. I did have a hot plate on one of the 6BM8's that wouldn't go away no matter what tube was rotated there, so I'm awaiting the schematics to replace the caps and input triode resistors for the 12AX7's. The phenolic tube bases don't look burnt and I did clean them. The power supply caps are the three lead kind and I'm researching those as well, for replacement.

Interestingly enough, a couple of years ago, I purchased a portable tube and transistor tester. It's an EMCO 215 (Electronics Manufacturing Corporation) and it's apparently an Lafayette kit. An amazing array of bright red switches, sockets and the big gauge. Taking it apart, I was amazed at the excellent wiring and soldering some anonymous builder took great care to do. The battery holder had come loose and I used 3M super-double sided tape to re-adhere this after changing the ancient Maxell batteries (that thankfully didn't leak). The filament select switch was very hard to turn, but some Radio Shack contact cleaner/lubricant got it working again. I applied the same treatment to all the other pots and switches with good results. I tried to figure it out by examining the wiring and drawing up a schematic of my own. You ain't going to it. It was the big Internet search to find instructions and tube data for this, but I was successful and I got them Saturday.

Last night, I had a chance to actually use the tube tester and using the tube data I set all the dials and switches in their correct positions and set the shunt voltage and first checked for shorts. Three of the four 6BM8's are bad as well as one of the 12AX7's. The rectifier is ok, despite it's very dark glass all the way to the bottom. All in all, a fun and fascinating process, the little needle either reading "good" or more often "bad" or "?". The instructions recommend tapping the gauge when in doubt! New ones aren't going in until I replace/repair many of the passive components in the amp. Neat little hobby, that I thought would be cheap, but $100 or so later, I've at least got the basics (dual wattage soldering iron, soldering supplies, magnifying lamp, etc, etc). Half the fun is in the treasure hunt for tubes, data and components, plus the helpful advice!

Here's a link to a source for custom-built electrolytic caps that I have personally used a couple of times.

Zack Engineering is the name but here's the web site: http://www.vibroworld.com/. Ask for Zack.

What you'll need to provide are the values and voltage ratings you're looking for as well as the type of can you want to replace. Its a good idea to also provide the height of the can from the deck of the amp to the top point. Otherwise, you may get a can that's too tall to reinstall the cover. Ran into this with my 2 MX-110s. He got it all to work. Normal price for a 4-section can was about $25 ea. Not bad when you're facing the prospect of having to rejigger the underside to try to get discrete parts in there - not an option with the MX-110s.

I have seen some guys hollow out the old cans then stuff new caps in them but I broke off the tabs trying to get them out so that wasn't really an option either.

Hope that helps.

Cheers,

David

Regis, if you have trouble finding any, I think I have some usable 6BM8 tubes. The tubes I have, tested OK on a Sencore Mighty Mite tube tester and worked OK when I tried them in a new Eastern Electric MiniMax tube amp. PM me, I'll let 'em go pretty cheap for LHF members...

Regards,
Gordon.

Im thinking of buying one of these amps. what is a good price? this is a very clean unit.

(eight years later...)
I recently (also) acquired a very clean old Japanese Calrad SA-30B integrated stereo tube amplifier.

There's a good bit of information on the web posted by "Triodeguy" (Richard Sears) about the SA-30, but I don't know how it differs from the "B" version. I asked him but he doesn't know.

Mine needs work. One channel is distorting pretty badly and maybe, after it's fixed, I might also implement some improvements suggested by Triodeguy if they apply.

But before I do, I need to know what I'm doing and for this I need the schematic or at least a good understanding of how the 30 differs from the 30B.

Gordon, If you have a schematic you'd be willing to share, (assuming you're still perusing this forum) perhaps you could email me and let me know how I can have a look at it - or perhaps someone could direct me to where I might find one.

Thanks
Marc Stager
) ) ) ) ) )
Stager Sound Systems,
New York Cityhttp://stagersound.com/silver

email: [email protected]

Quote:

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Originally Posted by Regis

I hope some of you electronics guru's can help me out. I got a vintage 60's stereo amp from Ebay a month or so ago. It's a little Japanese made Calrad SA-30B with 4 push-pull 6BM8 tubes giving it about 15 wpc. I am having a blast with this project. Waiting for the schematics to come this week, but meanwhile I tested it and one channel is weak and scratchy, the other one is nice and loud, but distorted, so I plan on replacing all the caps. Rat shack doesn't have beans for passive components, but they at least have resistors. There's a bunch of .005 uF paper oil caps in the chassis with TV1200 or 1200TV printed on them, along with the value. Can I use the Sprague Orange drops (polypropylene) as replacement caps. I can get the .0047 or .0056 in either 400V or 600V flavors. The same question applies to the ceramic discs, some of which look mighty ill, with suspicious bulges and corroded leads into the components.

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You might want to send a PM or contact Gordon directly ...
I hadn't seen him around here for a while and when I checked his stats, seems his last visit was before Christmas!


Join Date 05-19-2003
Last Activity 11-21-2011 07:27 AM

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Quote:

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Originally Posted by Stager

(eight years later...)
I recently (also) acquired a very clean old Japanese Calrad SA-30B integrated stereo tube amplifier.

There's a good bit of information on the web posted by "Triodeguy" (Richard Sears) about the SA-30, but I don't know how it differs from the "B" version. I asked him but he doesn't know.

Mine needs work. One channel is distorting pretty badly and maybe, after it's fixed, I might also implement some improvements suggested by Triodeguy if they apply.

But before I do, I need to know what I'm doing and for this I need the schematic or at least a good understanding of how the 30 differs from the 30B.

Gordon, If you have a schematic you'd be willing to share, (assuming you're still perusing this forum) perhaps you could email me and let me know how I can have a look at it - or perhaps someone could direct me to where I might find one.

Thanks
Marc Stager
) ) ) ) ) )
Stager Sound Systems,
New York Cityhttp://stagersound.com/silver

email: [email protected]

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