Tube Amp - Capacitor Cross Reference?

[jpatti75]

Doubtful this fits the category of "best of old tube technology", but I recently picked up a 1960 SK70 3-channel Motorola Tube Console Stereo. While it powers and the tubes at least fire, it needs a thorough going through and I imagine at least some of the caps need replacing. Finding replacement tubes isn't a challenge if that becomes necessary, but determining modern caps is a head scratcher.

According to the Photofact manual, there is an electrolytic can capacitor in the amp, and an axial electrolytic capacitor in the pre-amp. I haven't yet determined which of the other 27 caps are paper-type that would be ripe for replacement. I built a worksheet (zipped Excel attached) and was hoping I might be able to find a website that offers some modern cross-reference numbers, but so far nothing I have found has pointed me to modern part numbers. I am also having trouble determining the capacitance ratings based on how they were noted in the Photofact manual. Not sure if I am dealing with micro or pico farads (or something else), and I imagine I am going to have to build a replacement for the can electrolytic cap in the amp.

Before I go too far down this path, I'm trying to determine if finding replacements is going to be a royal pain, and if this job is even worth contemplating. I have also considered gutting the unit, and filling the cabinet (which I really like) with more modern components. I have attached a picture of my capacitor worksheet, in case anyone has guidance on how I can determine the modern cap replacements or which ones are actually worth swapping out. But I am also open to recommendations to avoid the job, if it is a waste of money.

Thanks for any recommendations!
JP

 

[gadget73]

paper caps? replace all of them, problem solved. Leave the ceramics alone, they're in the tuner and messing with them will affect alignment.

Those old part numbers won't likely cross to anything now. You'd have to look up value and voltage at whatever vendor you like. Voltage can be equal to or higher. I honestly don't stock anything but 630v just because they will work in anything I mess with. Guessing most of yours aren't going to be over 400v but a 630v will still work fine. Modern parts tend to be fairly small so fitment won't be an issue.

Value should be within 20%, but most of the time there will be a modern value within 10%. Most of those look like they would be available as an exact value which makes it easier. I use a lot of Illinois MPW caps, but honestly any film cap will serve just fine. If you want somewhere with a less daunting selection than Mouser or Digikey, try here

https://justradios.com/
https://www.oldradioparts.net/
https://www.radiodaze.com/film/


no affiliation with any, have used each at different points. They sell basic boring yellow caps that will get the job done without much fuss. I have a lot of things with those caps.


mmf = micro-microfarad, or picofarad now
mf = microfarad, uF (the long tail Greek u)
Normally there will be a notation on the schematic saying what things are. Something along the lines of "values in microfarad unless otherwise specified". Also typically has a remark about resistors, 1/2 watt unless otherwise specified.

https://antiqueradio.org/recap.htm

might also be worth a read

 

[jpatti75]

Thanks very much for the response. While I was looking at the capacitance table in the manual, I hadn't looked closely at the schematics, which actually do a better job listing the values, which clears up lots of my concern. Thanks for the article link -- it's very informative.

My biggest concern on the recap is going to be replacing the can electrolytic capacitor in the amp. From the Antiqueradio.org article, it looks like I can recycle the can base so it plugs back into the amp like the original, but that part of the job is a bit intimidating. I restore old outboards as another hobby, and I only ever have to replace two simple caps that screw in place in the magneto. That's more my speed!

Also, I'm glad to hear your recommendation to avoid the caps in the tuner. There appears to be a tube out on the tuner, but I'm hoping that's its biggest issue along with the dial bulbs. I don't have a diagram for that chassis, and I haven't been able to locate one.

 

[cademan]

If the amp uses mostly those disc (ceramic) capacitors, you can usually leave those alone however you can replace them if you wish. The can capacitor could or probably should be replaced for reliability but as long as there is no hum, it is fine for now and for testing purposes.

 

[jpatti75]

I’m going to open up the amp and pre-amp in the next day or so and see what type of caps are in there. I believe it has some ceramic and mica caps, but I am going to focus on the electrolytics and paper caps. The unit does power on and there is a hum, nothing awful, but some hum. Separately, the tuner does have some issues — it appears one tube is out, and when you switch the unit to OFF, it doesn’t actually shut anything off, I have to unplug it. Can’t get it to tune in a station, but it is in my basement workshop and doesn’t have a good antenna hooked up. Might just not be getting a strong enough stereo signal ... I may need to set it to mono, which is how I think the previous owner had it set when he demo’d it.

Thanks for all the help!

 

[gadget73]

There are a few ways to do the multi cap. I usually mount a terminal strip underneath and just put them there. The can is disconnected and abandoned. If there isn't room you can cut the can open to re-stuff, or if it mounts to a wafer, these are a trick solution

https://adapt-a-cap.com/

I just bought some of these to try out in a TV chassis that just has no room to do anything and it worked out fairly well. They don't fit well if you don't have a wafer mount though. The ring of connections is larger than the square or triangle cutout that some chassis use.

Hayseed Hamfest also can custom make cans in any value you need for a drop-in. Not the cheapest, but the prices aren't unreasonable either.

https://hayseedhamfest.com/

looks like they're backed up at the moment so not a good option if you're in a hurry.

 

[Dave451]

Gadget, that adapt-a-cap looks pretty nifty--I think I'll get a few to have around.

+1 on Hayseed Hamfest--they are my go-to for full sets of replacement cans, but they do one-off and custom sets as well. If I only need one, a lot of times the CE can multi-section caps will work if you can get close in value for the sections and the physical dimensions permit (diameter and height being the most critical). Antique Electronic Supply carries the CE cans (no affiliation).

Dave

 

[jpatti75]

Thanks for the information on the cans. I was able to get all of the components out of the cabinet last night so I can work on everything on my bench. This all seems less daunting to have everything out. I’ll see if I can salvage the can, or at least the base and retrofit new caps to it.

I was able to get the unit working better before I pulled it apart. I got FM reception, and in the process found out the right channel is only static, but I did get decent sound from the left and center. I’m going to test the right speaker on a different system to try to isolate whether this is the speaker (and the electrolytic crossover cap), amp, or possibly related to the one tube that is out on the tuner while I try to find specs on the tuner chassis.

Thanks for all the recommendations!

 

[FlaCharlie]

According to the Photofact manual, there is an electrolytic can capacitor in the amp, and an axial electrolytic capacitor in the pre-amp. I haven't yet determined which of the other 27 caps are paper-type that would be ripe for replacement. . . . I imagine I am going to have to build a replacement for the can electrolytic cap in the amp.

Before I go too far down this path, I'm trying to determine if finding replacements is going to be a royal pain

Since there's so much available space underneath it's easy enough to go with individual caps rather than a replacement can. This is a much, much less expensive way to go and you can choose parts that are higher quality than you get with most replacement cans. Hayseed is the only can that uses similar high quality parts.

I don't mess with tuners, so I can't offer any advice there, but on the amp section (and the preamp, if you're using it) if you replace all the electrolytics and the coupling caps you should be fine.

Modern values are a bit different than vintage but it's not an issue. The can has four sections. Replace the three stock 40uf sections (C1 A,B,C) with 47uf 450v parts and use the stock value (100uf) for the cathode bypass cap, which is C1D. The stock can uses a 50v part here, which is fine but it was probably used because they got a good deal on that particular can. The schematic shows only 7v on the cathode, so you could use a lower voltage part if you want - either 16v or 25v. Replace C2, which is located in the preamp section, with a 10uf 450v. Use radial lead caps as you'll pay much more for axials and the selection is not as good.

For electrolytics, I usually go with ones that are rated for 105 degrees and 10,000 hours. On small parts orders you may end up paying more for shipping than for the parts. Digi-Key is a major distributor and has a free shipping option.

http://www.audiokarma.org/forums/in...hipping-on-parts-orders.739513/#post-10018562

 

[gadget73]

with the tuner, pull and re-install the tubes. I've had a lot of issue with crusty connections causing them to not work. Wouldn't explain one dead channel though. That could be a fault in the multiplexer (stereo decoder) if it works normally in mono though. Assuming it is FM stereo at least. That didn't happen until 1962, and many consoles were mono tuner, stereo turntable.

easy check to see if its a problem with the amp or before it, plug in something with a volume control direct to the amp. Ipod, phone, whatever you have is fine. If it plays from both channels, the issue is in the tuner or preamp. If both channels work on the turntable, its something specific to the tuner. If its out on the same channel with the turntable, its in the preamp.

 

[jpatti75]

Thanks for all the responses. The electrolytic cap replacement details are much appreciated, FlaCharlie!

Gadget73 - the right channel is static in either stereo or mono mode — the tuner does say “Stereophonic FM.” Also, I’ll pull all the tubes per your recommendation. I have a can of QD electronic cleaner I plan to use on all of the contacts.

Unfortunately, the turntable was DOA, so I haven’t been able to test the channels that way. I’m hoping the turntable is just gummed up with old grease, but the motor doesn’t even hum. But, that’s for down the road.

 

[jpatti75]

Tube question .... my manual seems to be somewhat of a hodge podge of info. For the amp and pre-amp it lists 12AX7A on most of the diagrams, but on one page it lists the 12AX7. For these types of amps and pre-amps, would either work, if I end up need to replace any?

 

[cademan]

12AX7(A) are the same tube. The "A" designate heater warmup time for circuits like series string chassis'.

 

[jpatti75]

Thanks for clarifying!

 

[jpatti75]

Hope I’m not a pest. Was hoping to just get input on selecting new crossover electrolytic capacitors based on these oldies:

I understand I need 5uf, just want to make sure I look for the correct voltage.

Thanks,
JP

 

[Dave451]

I would replace this with a 5uF film capacitor, not electrolytic. I think film caps sound better than electrolytics in crossover networks. Looks like 25 "WVDC" (but the clip covers up part of the rating). Something of at least 50V rating for the film cap would work. Physical size becomes limiting as the voltage rating of film caps goes up, but they are in generally pretty small compared to the old types. No polarity to worry about with a film cap in a cross over, either.

 

[jpatti75]

Thanks very much, Dave!

So, that begs another (possibly dumb) question. Since I can switch to film capacitors for the crossovers, does the one electrolytic cap in the pre-amp and the can unit with 4 electrolytics in the amp need to be replaced with electrolytic caps, or should/can I use film caps for those, too?

 

[RWood]

Film caps would be too large in the values you will need, both capacitance and voltage handling. We would use film caps if they would fit. E-caps will be fine.

Could you post the amp chassis #? HS- something. It will be helpful to have this, down the road.

 

[gadget73]

in theory any electrolytic can be swapped for film, but size and cost make it not practical most of the time.

Other option for the crossover is a non-polar electrolytic. Thats what the original is. Cheaper, but the film is the better option.

 

[jpatti75]

Thanks for all the explanations on the caps. I’m ready to order them now.

RWood - The amp chassis is HS-960A, pre-amp is HS-959A.