Picking up RTA-12Cs

bobdod

I am going to check out some RTA-12C this week. I am new to Polk and vintage speakers, though I've restored a few vintage amps. I am looking for advice on what to look for, issues to look out for, and advice on transporting them. Thanks for any advice you may have.

CH46E

Make sure the drivers move freely. No voice coil rub when you easily push them in them. Like any other speaker, just make sure they sound good and all the drivers work and adjust your offer accordingly.

Fyi, the 6 1/2" woofers are crossed over at different frequencies. So if you put your ear up to one woofer it will not sound the same as the other. This is normal.

Not sure how much you know about the 12C but that big flat woofer is actually a passive radiator.

Once you have them we can give you a lot more info on them. There are some guys here that know them very very well.

I have the 12B. Very happy with them and the 12C is supposed to be even better.


Welcome to Polk Forums!

CH46E

Oh and as far as transporting them. I'd lay them flat on thier back with some cushion and careful driving. If you put them on thier side you run a risk of a magnet shifting that causes voice coil rub.

bobdod

Well, I wound up getting them. Looked pretty much brand new, all drivers worked, and they sounded pretty good. They are 12Bs though, not Cs like I thought, or at least they are labeled as Bs. A few photos are below. I have not set them up yet.

CH46E

You got yourself some nice speakers. Make sure you set them up right. One is for left side and one for right. Designated by "R" and "L" on the labels on the back. You pictured your right speaker.

Did you get the hats with them?

The crossover you pictured, is it disconnected and sitting in a box?

Nightfall

That or a homemade cardboard top hat 😂

bobdod

CH46E wrote: »

Did you get the hats with them?

The crossover you pictured, is it disconnected and sitting in a box?

Yes, hats are here, along with the grills. Both are like new. The crossovers were removed for transport, so yes, sitting in a box untill everything is set up.

I do want to do some upgrades, though I am not sure what is worth doing.

CH46E

You would need four 4.4 uf caps, two 55uf caps, eight 27uf caps to recap the pair.

I think there is one 7.5 ohm resistor in each crossover.

I dont know much about inductors so i do not mess with them. And I think most would say leave them alone.


I'll post a pic of the schematic for you in a sec.

They are certainly worth upgrading.

bobdod

CH46E wrote: »

You would need four 4.4 uf caps, two 55uf caps, eight 27uf caps to recap the pair.

I think there is one 7.5 ohm resistor in each crossover.

I'll post a pic of the schematic for you in a sec.

They are certainly worth upgrading.

Thanks, I am definitely going to try to order caps right away. I've recapped a bunch of amps, but never speakers oddly enough. Any suggestions on caps is appreciated, though I am not looking to buy any mega buck high end stuff.

I've also read about newer tweeters as a possible upgrade. I am not sure which ones are in these. Any idea if it's worth getting new ones over what is in there currently?

CH46E

At the top of the vintage speaker section is some threads with all the schematics.

There has been a lot of threads on the 12b lately. Use the search function and search RTA12B. Should bring up a lot of good info.


This is a link. https://forum.polkaudio.com/discussion/38755/polk-audio-speaker-wiring-schematics-amp-more-all-models-except-sda

And here is a pic for your speaker with the fuse vs a poly switch.

CH46E

You already have the better tweeters. Those are the oem Peerless and they are well regarded. There is no drop in upgrade unless you want to make changes to the crossover.

Sonicaps are well liked here for vintage polk. I used Sonicap 4.4uf for mine. I also really like the Jantzen Superior Z-caps I used in another speaker.

Jazzhead

Good find! Replacing the stock binding posts with Cardas posts was a big improvement for my original 12's. That and the other Polk Club recommended mods. Be sure to glue the driver magnets to prevent magnet shift (a problem with older Polk drivers). If you wish you can dynamat the driver baskets (prevents ringing), and adding a strip of Black Hole 5 or Sonic Barrier behind each driver can really help clean up the midrange (some think the 12's can sound "shouty" without it). Make sure all the polyfill stays up behind the divers and there is none lower down near the passive radiator. Search the forum and there is much good advice and specifics.

Faustin

Great find. As suggested for new caps, Sonicaps are used quite a bit around here, as well as Clarity caps. The cost can start adding up quickly, so another option is Dayton 1% caps. Either way you will be replacing old caps that are way past their prime. Keep in mind that the new poly caps are going to be quite a bit larger than the old ones, and you may need to get creative fitting them on the boards.

bobdod

Preliminary look at caps definitely caused a little sticker shock. I am thinking of going with the following:
-Sonicap for the 4.4uF
-Jantzen crosscap for the 27uF and 55uF
That adds up to what I paid for the speakers haha.

Am I going to be disappointed? Do I pay an extra $100 for better 55uF caps?

I can afford it, but my general rule for my vintage audio hobby is to try to not get into gear for more than its realistic market value.

westmassguy

RTA-12s done with Sonicaps, note the rather cramped space.

CH46E

bobdod wrote: »

Preliminary look at caps definitely caused a little sticker shock. I am thinking of going with the following:
-Sonicap for the 4.4uF
-Jantzen crosscap for the 27uF and 55uF
That adds up to what I paid for the speakers haha.

Am I going to be disappointed? Do I pay an extra $100 for better 55uF caps?

I can afford it, but my general rule for my vintage audio hobby is to try to not get into gear for more than its realistic market value.

Thats almost exactly what I did with the caps. Sonicaps for the 4.4uf, Jantzen cross caps for the 27uf. I went with a 56 uf Mundorf. I dont remember what made me go that route over the cross caps.

The Jantzens fit well.

CH46E

bobdod

That's some very tidy looking work @westmassguy . Those Sonicaps are just too much $$ haha... About $600 in caps by the look of it.

bobdod

CH46E wrote: »

Haha, that mundorf is huge!

So, this may be a sacrilegious question, but what about replacing the 55uF caps with new electrolytic caps? I know films are "better" but the factory got good sound with electrolytics, and today's caps are likely better. Just wondering if this is an option to consider.

westmassguy

bobdod wrote: »

That's some very tidy looking work @westmassguy . Those Sonicaps are just too much $$ haha... About $600 in caps by the look of it.

Thanks. They're worth it, especially on the RTA-12s. They were Polk's best non-SDA, audiophile speaker back then, perhaps ever.

CH46E

bobdod wrote: »

CH46E wrote: »

Haha, that mundorf is huge!

So, this may be a sacrilegious question, but what about replacing the 55uF caps with new electrolytic caps? I know films are "better" but the factory got good sound with electrolytics, and today's caps are likely better. Just wondering if this is an option to consider.

Yep, I did not understand that a 350v 56uf cap as as big around as a coke can. That was my bad.

Someone else will have to comment about a electrolytic cap. I did use several in my Infinity speaker build for the bass section only. Seems to be just fine. I was dealing with values as high as 1,600uf so film was just out of the question cost wise.

pitdogg2

westmassguy wrote: »

bobdod wrote: »

That's some very tidy looking work @westmassguy . Those Sonicaps are just too much $$ haha... About $600 in caps by the look of it.

Thanks. They're worth it, especially on the RTA-12s. They were Polk's best non-SDA, audiophile speaker back then, perhaps ever.

They're worth it on all Polks. No they are not cheap but they sound so incredible.
I've never once wished I went less expensive.

CH46E

I think that 55if cap runs in series. Someone else please chime in if I'm incorrect but I was under the impression that series caps will have a more profound affect on the sound. So you may not want to go too cheap on that 55uf.

The 55 is in series right, not parallel ??

westmassguy

CH46E wrote: »

I think that 55if cap runs in series. Someone else please chime in if I'm incorrect but I was under the impression that series caps will have a more profound affect on the sound. So you may not want to go too cheap on that 55uf.

The 55 is in series right, not parallel ??

The 55uf is in parallel with one of the 4.4uf caps, but in series with the other 4.4uf.
Lot's of frequency shaping going on there.

CH46E

westmassguy wrote: »

CH46E wrote: »

I think that 55if cap runs in series. Someone else please chime in if I'm incorrect but I was under the impression that series caps will have a more profound affect on the sound. So you may not want to go too cheap on that 55uf.

The 55 is in series right, not parallel ??

The 55uf is in parallel with one of the 4.4uf caps, but in series with the other 4.4uf.
Lot's of frequency shaping going on there.

Yup, ok. I was not 100% sure by just looking at the schematic. They are still a little confusing for me.

Thanks!

bobdod

Looking at available values, there are almost no options for 4.4uF. Should I consider a 4.7uF option like a claritycap SA or CSA, or are 4.4uF Sonicaps the only option?

pitdogg2

4.7 is within spec

westmassguy

bobdod wrote: »

Looking at available values, there are almost no options for 4.4uF. Should I consider a 4.7uF option like a claritycap SA or CSA, or are 4.4uF Sonicaps the only option?

2.2uf X 2 in parallel

bobdod

Thanks to everyone for the feedback. I've decided to spend a little more, and will be going with the following for caps:
4x 4.7uF ClarityCap CSA ($41)
2x 56uF Jantzen Standard Z ($92)
8x 27uF Jantzen Cross Cap ($76)

I am also getting 7.5ohm 5W Mills resistors, hurricane nuts, and damping (not BH5) for behind the drivers.

Total upgrade cost will be approximately $250. I am looking forward to getting these completed and burned in.

pitdogg2

I'd buy the 12 watt versions of resistors, cheap insurance. We've all moved up in wattage but keep the same ohm value

CH46E

bobdod wrote: »

Thanks to everyone for the feedback. I've decided to spend a little more, and will be going with the following for caps:
4x 4.7uF ClarityCap CSA ($41)
2x 56uF Jantzen Standard Z ($92)
8x 27uF Jantzen Cross Cap ($76)

I am also getting 7.5ohm 5W Mills resistors, hurricane nuts, and damping (not BH5) for behind the drivers.

Total upgrade cost will be approximately $250. I am looking forward to getting these completed and burned in.

That is $250 well spent. You'll love how they will disappear in a room. The imaging will be spooky good. You will swear (if you have one) the center channel is on.