Tube DIY Asylum

Do It Yourself (DIY) paradise for tube and SET project builders.

Return to Tube DIY Asylum

Message Sort: Post Order or Asylum Reverse Threaded

What capacitors to buy to refurbish old RCA amp?

198.233.141.11
Posted on February 9, 2016 at 16:58:34
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 Figured it's time to refurbish my old RCA mono amplifiers (MI-12188A). These are rather large 70W PPP 807 amplifiers, with a big choke input power supply, and regulated screen supply.

Right now the coupling capacitors are a motley assortment of originals and replacements. The replacements are more or less whatever I had on hand when the originals failed.. I've never been one to fix something which isn't broken, but would like to "normalize" the amplifiers so they're both the same.

Now, I don't really want to use either orange drops, or the ever present yellow film caps. Each amp needs four 0.1uf capacitors, three at 400V and one at 200V. I also need one 0.5uf at 1000V, and a couple of fairly low value electrolytics. I'd also like to try and get "made in USA" parts if at all possible, it seems a shame to use China parts in 65 year old US equipment. So for the electrolytics I guess Sprague Atoms are the only real option.

I'm also on the fence about replacing the filter capacitors - 8uf 1000V rectangular can paper in (probably PCB) oil capacitors. They seem to be working fine.. no heating and no hum in the audio. I don't own a capacitor tester which is capable of putting high voltage on them, but maybe there is some sort of pass/fail test I can use to make absolutely sure they're okay.

What sort of capacitors do you guys use in vintage sound equipment which you care about?

 

Hide full thread outline!
    ...
RE: What capacitors to buy to refurbish old RCA amp?, posted on February 9, 2016 at 17:13:15
FenderLover

photo Ampwithnewcapsandresistors_zps0bb150b3.jpg

For vintage Fender amps, I've been using Yellow or Red Jupiter caps. Yellow for blackface and silverface era amps (1960-80s). And Red for woodie and tweed eras (1940-50s).

Red ones sound pretty close to original 1950s Astron caps. The Yellow ones have a tone similar to the original blue MOLDED caps.

Russian metal case K40Y-9 are excellent coupling caps, too.

As for 'lytics, I've stuck with Sprague Atoms or F&T. Need solid reliability in the PSU and bias circuits.

Edit: I agree with Vinnie, for 90% restos... OD are fine caps. Maybe a tad brighter tone than some of the boutique caps. But, these last forever. Very good quality. Some boutique caps have prematurely failed (leaked DC) on me.

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 9, 2016 at 17:15:25
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 What's wrong with orange drops? They are great general use caps. I refurbished a lot of vintage gear using them before I started screatch building. Obligato is making new oil caps, but I don't know if they have that high a voltage. I have used them in several of my projects with good results.

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 9, 2016 at 17:32:37
deafbykhorns
Audiophile

Posts: 1067
Location: Florida
Joined: October 17, 2003
 For the film try some Nichicon QXP
They look vintage and perform well. I think some of the McIntosh refurb guys use these

http://www.nichicon.co.jp/english/products/pdfs/e-xp.pdf

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 9, 2016 at 20:07:30
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 The key thing when using Orange Drops is avoiding all of the series, except 716P.

Mixing cap. types up lets you hear the amp, not the capacitors. A combination of Soviet surplus K40 PIOs and 716Ps usually works out very well.

A lot of Chinese 'lytics are guano and I will not waste my time trying to find the few that might be OK. OTOH, made in Japan 'lytics by Panasonic and Nichicon are 1st rate and all too often are better than U.S.A. made stuff. CDE (Cornell Dubilier) makes quality parts, but they rarely are price competitive with Panasonic and Nichicon. If you can get appropriate parts rated for 105o C. service, pay the man his money!


Eli D.

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 9, 2016 at 20:33:16
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 If you are going to have a long term relationship with this amp I suggest you step up to some Solen or similar caps to replace the 'lytics. They perform way better than any electo cap. The original oil caps in the power supply will last forever if they have not been abused. The problem I don't like with the russky caps are that their leads are quite short. So, if your original caps need to span any distance they might not be able to reach. good soldering! dak

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 00:31:27
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005

"The oil caps in those are fine but you say they are 8uf and the schematic says 14uf.The russian K40y-9 or the k42y will work well for the .1s. The 30uf lytic I would use a poly film cap there as these will outperform an electrolytic by a lot..There are good lytics but for that cap I would use a 35uf at 500v.
This amp has no global feedback that I can see.I would bump the cap's value after the choke if it's only 8uf and personally,I would ditch the 5R4 in favor of Schotkeys.For a single 5R4,you may be close to the limit with four 807s and a 6sl7 under dynamic conditions.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 06:24:57
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004



Here Is the original RCA schematic - I was lucky to get the instructions with my amp. The one you have is based on reverse engineering, and isn't completely correct. There is global feedback here, it is just taken from the primary of the output transformer instead of the secondary. Enough to keep the damping factor up to about 2.5, which is no dynaco but it's not dissimilar to many triode amps. I put my 8 ohm speakers on the 4 ohm tap to help control them a bit further.

As for modifying to silicon rectifiers, there's a couple of things which persuade me against it. First, one pair of GE 5R4s has lasted 15 years - through three or four sets of 807s and they still test good and put out full voltage. Second, the power transformer puts out a huge voltage 755-0-755 and I would be afraid to lose the rectifier voltage drop.

Hum is cancelled by applying hum voltage to the input stage, a trick which works very well in practice, I can't hear anything even up close to apeakers.

So far the Russian capacitors look like they are the front runners for the 0.1s, I am going to get a set.

Sprague doesn't make electrolytically of the correct ratings, but those solens sure look interesting. Are they suitable for filtering?

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 07:19:21
krankkall
Audiophile

Posts: 296
Location: New Mexico
Joined: April 5, 2014
 To keep the sound as close to the 'original' as possible.....polyester.

CDE WMF, Mallory PVC, Mullard/Phillips Mustard, etc.....these are all polyester foil-n'-film caps.

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 07:32:42
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 I heard they moved production to China, and also, they have radial leads, designed for PC boards, instead of axial leads like something from 1951 would have. Otherwise I've heard only good things too, never any performance related complaints.

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 07:34:15
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 Those look like a good substitute for orange drops. I also repair old radios and may have to try some in that application too.

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 07:36:15
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 Those are all attractive looking options. My only worry with the Russian ones, is that they are already 25 years old. Maybe they can last a long time, but I have some experience repairing Soviet made equipment, and their electrolytics are always bad, almost as a rule. Hopefully the paper ones are better! They sure look cool though

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 07:39:17
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 Thank you for your experience! I too have had good luck with Panasonic capacitors, having used them in several restorations and construction projects. Actually I usually go with Panasonic as my first choice, but I want the RCAs to look as well as sound a bit closer to original.

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 07:40:21
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 Thank you for this, looks like Mouser has a good selection of CDE WMF. and unlike the Soviet caps these ones are not 25 years old NOS.

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 08:10:27
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 Long term relationship? I would definitely owe them some major alimony if I decided to move on.. I've had other amplifiers in and out of my system but prefer the sound and power of the RCAs.

I like the idea of solens or similar in place of electrolytics, if I did that it would mean the amplifier has no electrolytic capacitors at all.. that can't possibly be a bad thing.

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 08:56:50
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Yeah, at the rated 250 mA., the forward drop in a 5R4 is a whopping 67 V.

You could go SS B+ rectification, by using what I call a "cockeyed bridge". Use series wired pairs of UF4007s to connect the ends of the rectifier winding to ground. Snub each of those pairs with a very high WVDC 0.01 muF. cap. Take the "raw" B+ off the rectifier winding's CT through a 1200 PIV Schottky diode. Insert a CL-90 inrush current limiter between the Schottky diode's cathode and the PSU filter. The thermistor will do several good things, including a slight delay in B+ rise. Choke I/P filters generate nasty inductive kickback spikes that can destroy SS diodes. Either a high WVDC 0.01 muF. cap. at the filter's I/P or an appropriate MOV across the choke disposes of the problem. The OEM oil filled caps. will remain the PSU reservoir, but they will become part of a RC section that will follow a new stack of 2X 10 muF./450 WVDC 'lytics and voltage equalizing resistors. Ripple will be lower than OEM and that new R is how you compensate for the much lower forward drop in SS diodes. A "routine" inductive wirewound resistor is the type to use. Every last bit of additonal inductance contributes to the cause.

BTW, a "cockeyed bridge" is, in fact, just another FWCT arrangement.


Eli D.

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 09:13:08
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 The price difference is not that great. Also the size of the Solens are compact except for the large capacitance units. If your B+ is less than 500v, Panasonic makes the DC link which are rated at 500v. They come in rectangular packages. You can fit your own leads so that is a little more effort but that would allow you to use different types and gauges. They seem to have less of a break in period since they sound good right from the first note. cheers, Dak

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 09:47:05
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 The Soviet surplus K40 PIOs are hermetically sealed. Shelf life is a non-issue.

Under the Red Regime, the citizenry got a very raw deal. OTOH, nothing was too good for the military.

Tube DIYers reap tremendous benefits from the collapse of Communism.


Eli D.

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 10:01:47
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 I ordered ten k40y-9 0,1uf 400V capacitors for the 0.1's, and two 0.5uf 1000V KBG-MN bathtub paper in oils for the 0.5uf capacitors.

They have the look I want and are exactly the value of originals. Not overly expensive either.

Now the two electrolytics. Mallory seems to have discontinued US production and sprague atoms are not available in a lot of values. Decisions, decisions

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 10:49:48
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 IIRC, Mallory is made in Mexico.

Look to Japanese Panasonic and Nichicon. If it must be U.S., look to CDE.


Eli D.

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 11:59:58
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 There is global feedback here, it is just taken from the primary of the output transformer instead of the secondary

I noticed now but I have never seen the GNFB come off the plate winding like that.I have seen local FB implemented like that.
.Most RCA amps did have two 5R4s,at my RCA theater amps do with 807s.

Here is the thing,the Schotkeys sound like a tube rectifier without the loss and they are quiet.I would work with Eli a bit on the 6SL7 driver stage because you can make a lot better amp with a more suitable driver stage.The 6SL7 is ok because your driving pure pentode but being you have four 807s and regulated screens,give those 807s some extra wheaties.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 12:24:45
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 The feedback configuration threw me for a loop at first too (no pun intended!) Especially the little trick they used to bias the first 6SL7 section properly. DC current flows through the feedback resistor, which would upset the bias of the first 6SL7 section - but rather than let that happen, they put the grid a bit positive with a voltage divider, while C2 makes sure it's still referenced to ground AC wise.

I think that from a conservative design standpoint, a second 5R4 would be a good idea.. but I have to say that in reality, it's really not necessary. At idle the amplifier draws 210mA, which approaches but does not exceed the 250mA limit of the tube. And I have used these amplifiers a LOT, I mean in the 20,000 -30,000 hour range since I got them.. and the USED GE 5R4GA's which came with the amp (and who knows how long they were used before I got them) are still good! And pretty hard use too, I used these to rock the house! When you have it really cranked the 0D3s dim a bit during transients.

I haven't noticed that the driver is inadequate... I suppose a more robust driver MAY be able to push the 807s into AB2, but that would probably expose power supply weakness. One possible area of concern is balance in the phase inverter, I do want to investigate that once all the capacitors are replaced. I will definitely make sure that the driver doesn't clip before the output stage does, but something tells me the 1950s powerhouse of engineering which was RCA wouldn't have let something out the door which was designed that badly!

Which model of RCA do you have? Are they the rack mount style ones with built in tube tester? I'd love to get my hands on a pair of those some day!

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 12:28:49
deafbykhorns
Audiophile

Posts: 1067
Location: Florida
Joined: October 17, 2003
 I have had many of these leaky brand new (NOS)

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 12:36:57
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 I have had many of these leaky brand new (NOS)

the paper in oil?
I plan on testing them each for leakage before use

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 12:49:52
deafbykhorns
Audiophile

Posts: 1067
Location: Florida
Joined: October 17, 2003
 Yes, russian PIO's
I have one right now in my older DIY amp squeaking after testing 3 yrs ago

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 13:06:33
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 My plan is to let them sit at 400VDC with a bench power supply and measure leakage with my Simpson 260 on the 50uA scale, before I go ahead and put them into the amplifiers. I don't have an actual capacitor leakage tester but I think this improvisation should be good enough to check things out.. got plenty of old wax caps and new orange drop/dip types to use as a baseline.

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 15:41:36
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005

I have several
I have a pair of 9358s,I have one of the 807 versions that you see in the photo,and i have four of the M-12182s which are the 6146 amps..I converted one pair to 6550s but I really think these amps have a lot more potential and I was see what they can do.
The 9358s and the 807 amp in the photo has round output transformers that are high frequency limited from what I saw on the sweep out of the amps.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 16:00:44
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004

Nice collection! I didn't realize the round output transformers were that limited, I've wanted something including them for a long time - my MI-12188a's are very wide range. I also have an MI-12202B which I have set up to do TV sound. It's very restricted above about 8k, but I like having an RCA amp from 1940 included in day to day life! If I see one cheap enough on eBay I want too get an appropriate MI series speaker to go with it.

The 6146 amp I believe is the successor to my amplifiers - I think the substitution to 6550 was an excellent idea. If they have similar transformers I would agree there's tons of potential there, especially with four of them!


 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 10, 2016 at 16:17:18
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Here is the poly cap you can use in place of the 30uf lytic
On the driver,there are nice high impedance CCS or even an enhancement mode mosfets you can use that will give the 6SL7 an easier time of driving the outputs.I know it works by itself but keep in mind this is 1940s technology and they weren't using it for hifi.. You're allowing the 6SL7 to drive into a higher impedance load and it won't add a sand type sound as there is no gain but you can try a few things.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: You don't want the atoms, posted on February 10, 2016 at 18:37:19
Russ57
Audiophile

Posts: 3754
Location: South Florida
Joined: November 16, 2001
 Only good thing is they are hard to kill. Otherwise overpriced and don't sound very good. Elna, Nichon, Panasonic....all very much better choices (IMHO).

 

I just saw the amps like the ones you have., posted on February 10, 2016 at 18:45:03
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 They remind me if the 6146 amps but with 807s..I'm surprised they don't have a pair of 5R4s but you may be able to use a single GZ34 which is 250ma and lower loss. Those are nice amps.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

feedback, posted on February 10, 2016 at 19:40:58
FenderLover
 Mike, what are the two 47pico + 100k-ohm networks off the primary tap doing? Looks like they attach to one cathode of the 6SL7.


Thanks!

 

RE: I just saw the amps like the ones you have., posted on February 10, 2016 at 21:27:41
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004



The 5R4 is the only common rectifier capable of handling the voltage of the power transformer, which is really high due to the choke input filter. Here's a picture of them. The non matching plate caps are how they came, I'm hoping to some day find a second set of plastic ones.

 

RE: You don't want the atoms, posted on February 10, 2016 at 21:33:06
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 What do you think of the old standby Philips/BC components/Vishay capacitors?

The axial ones with the translucent blue plastic insulation, which have looked the same since the late 60s.

 

RE: I just saw the amps like the ones you have., posted on February 10, 2016 at 23:08:43
hennfarm
Audiophile

Posts: 535
Location: Oregon
Joined: October 8, 2008
 I have 6 black plastic plate caps that need a new home.....JH

 

RE: What capacitors to buy to refurbish old RCA amp?, posted on February 11, 2016 at 02:31:30
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 I guess I shouldn't be surprised as that has happened to so many things. The radial leads were nice and long, so i never had a problem with them. As you said, no performancde issues that I ever heard of.

 

RE: You don't want the atoms, posted on February 11, 2016 at 06:19:44
Jim McShane
Dealer

Posts: 5910
Location: Chicago
Joined: January 13, 2003
 I mostly agree with Russ - but there are some occasions where the Atom is helpful in achieving a certain tone. And they are axial, and there aren't a lot of axial caps around anymore.

 

RE: Try Illinois capacitors, posted on February 11, 2016 at 15:12:17
Russ57
Audiophile

Posts: 3754
Location: South Florida
Joined: November 16, 2001
 Jim should of had that:)

They make a 105C axial rated to 450VDC and the 85C ones are rated to 500VDC.

When it comes to aluminum electrolytics (which I try to avoid) I'd much rather have newer designs.....so I wouldn't use the old Phillips ones myself. Heck these days you can get high voltage, small size, non-electrolytics.

Do you "really" have to have axial? You can always add a terminal strip, etc.

 

Better rectifier tubes , posted on February 12, 2016 at 09:14:58
maxhifi
Audiophile

Posts: 584
Location: Alberta, Canada
Joined: August 4, 2004
 Was thinking about concerns regarding the ratings of the rectifier tube. Although the GE tubes in the amplifier continue to work well, I am thinking about what can be done to improve matters.

Looks like the military 5R4WGA-B-Y will drop in, and it is rated to 275mA and for higher voltage than the RCA original. I think I will pick up a couple of these, antique electronic supply has them for $9.95 each, they should keep me going more or less indefinitely.

I wonder if the Russians made a tube with same or higher ratings - although usually I wouldn't plan on using a Russian made tube this close to the edge and expect reliability!

 

RE: Better rectifier tubes , posted on February 12, 2016 at 11:58:16
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 The most rugged 5R4 variant I'm aware of is the so called "potato masher". That variant was used in early B-52 bomber avionics equipment. :>D


Eli D.

 

Page processed in 0.048 seconds.

[ Top Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]
[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]