Recapping Altec 525A

[thecr4ne]

I've got 4x Altec 525As (M20/M30) that I'm working on cleaning up. 2 are still fully stock. 2 have been worked on at some point, but I'd like to just make them all the same. I'm new to the art of cap selection so I'd love any feedback you may have.

I'm looking at CE MFG's 20/20/20/20 cans...not cheap, but easy.

I got Sprague Atoms (orange...I guess slightly older than the blue?) for the 3000uf caps will have to figure out a mounting scheme.

Leaning toward Solen "fast" polypropylene for the output cap. Solens are 5.1uf, originals are 5uf. Close enough?

Then there's the mystery cap.

.25uf 400V Hopkins Eng. Co.
Anybody know what type this is? Thoughts on good replacement, or if it's even worth replacing?

Other items I might check off the list. Adding diodes to or in place of the old rectifiers, grounded IEC socket pigtails for the power cable. Maybe a Pigtail for the output.

I'm also considering replacing the old cloth cables for the mics with newer foil shielded 5+ground conducter wire, though I'll test the existing cables once the recap is done before making a decision.

I'd also like opinions on the transformers. I have 3x 4665s, and I also have a bunch of Triad HS-1s I might try as well. Or is there anyone out there that likes these with the resistor jumper instead of the Transformers? Pros/Cons?

Anything else I should think about?

Here's the schematic for reference.

 

[rmburrow]

5.1 uF is adequate.  Remember the cathode of the 5840 is elevated to HT so you need a cap rated for 350 volts or so; the voltage across the cap is full HT until the tube heats and draws current.  If you use a output transformer, you don't want the capacitor to short out and put DC on the primary and ruin the transformer.  Use a good quality capacitor for C2.  Have you tried using a M30 or M20 mic on these supplies before working on them?

 

[thecr4ne]

Original C2 is rated 200VDC, under-spec apparently. Turns out I can get the 400V caps cheaper than the 250V anyway. 

I tried them with the 165 (M20) but got no audio. Didn't mix and match every mic/psu combination. Could be bad cables, could be bad tubes, could be the guts. Recap will be necessary sooner or later considering their age, may as well happen now. Power cables were all pretty thrashed and have been removed already.

Transformers test good.

Thoughts on C1 type?

 

[ln76d]

C2 could be even 1uF. If you want close then 5.1uF is definately ok but 4.7uF would be also enough.
C1 220nF-470nF range film cap. Find just something which will fit physically.

 

[thecr4ne]

What difference results from increasing/decreasing the value of C2? C1?
(I already ordered the 5.1uf caps for C2, and plan to keep to the spec'd values, with higher voltage rating)

 

[thecr4ne]

Solen's arrived (C2) all look good. As did the 3000uf Spragues (C3 and C4). Date codes indicate they were made in '75 and '76 (USA, Orange) so I spent the weekend reforming them. Since they're rated 25v I was able to do this with a handful of wall warts, breadboard, multimeter and a 200uA ammeter I was planning to re-purpose as a VU meter in another project. Leakage is down under 10uA each, which is far lower than most charts I've seen indicate for a cap of this spec. After searching for a source for some decent polypropylene caps for C1 and hitting the ridiculous shipping cost brick wall every time, I'll probable just get the same series Solen caps as C2, as I can get them in .22uf 630V from the same supplier as the CE MFG 20/20/20/20uF cans for less than I'd pay on shipping for cheaper caps elsewhere. Awaiting some cap clamps for the spragues and the next sale either on ebay or the suppliers site to order the rest.

 

[thecr4ne]

Amplifiedparts.com is doing 20% off all capacitors till Nov 13 so I just ordered the 20/20/20/20uf cans and the .22uf solens for C1

 

[thecr4ne]

The Solen caps have been installed, and I'm working on the electolytics now. Power cables were removed and replaced with pigtails with Male IEC connectors.

My Spragues turned out to not be in that great a shape after all after some more thorough testing. I got a bunch of Mallory 3000 uFs, which are likely of the same era (~70s) as the spragues, but they're actually within tolerance, and are a direct replacement physically for the existing mounts. (one of which I broke removing the old cap so yay).

I got diodes but have yet to replace the selenium rectifiers.

I am now up to 5x of these, with 4x 4665's. I also finally got a 29B cardioid capsule, which has some gunk in the threads. I will likely order some new cabling (redco tube mic cable is about $1.50/ft) and will add the resistor to the one that ends up with the 29B once I confirm everything is working otherwise.

Next time I have some time to focus on them I'll try to get a little further along.

 

[duantro]

How's your progress?  I've got one of these that isn't supplying the 200v on pin 6, need to pop it open and see what gives.

 

[thecr4ne]

Haven't worked on them in a while. Lacking funds and my "workshop" also happens to be a guest bedroom, and we've had guests lately.

I'm awaiting delivery of some 3000uf caps. Not happy with the measurements I'm getting from the Spragues or the Mallory's I've tried reforming, so I ordered some ELNA's off ebay (the price was right). Assuming they're in spec, they'll get swapped in. Also need to decide how I want to mount them and the diodes that will replace the selenium rectifiers. I just need to order terminal strips, but the little things seem to be disproportionately expensive, or shipping kills...you know.

I got a cheapo decade board from ebay to help dial in the value of the resistor I'll need to keep the voltage right once the rectifier is replaced.

That's where things stand currently. Not to mention the thousand other projects currently ongoing, some audio, some not.

 

[thecr4ne]

Today I finally got around to finishing one of these (I have 6 total now). Had an oops moment (forgot to solder a ground) but otherwise things went well.

Removed the selenium rectifier and replaced with 2 diodes on a terminal strip. also replaced the diode on the board.

Electrolytics replaced with CE mfg for the quad, and Pioneer/Elna's for the 3000ufs.  The other two caps replaced with Solen fast caps. The 5.1uf cap was too fat to go where the original was so I swapped sides for that and the .22uf. The 3000uf's are a lot smaller than the original cans, so I just looped the leads through the slots on the phenolic mounting plates (managed to keep them intact) and looped the leads to hold them in place and to use as solder lugs. I might re-think this going forward as it's not pretty, but it works well enough for now.

I also re-cabled one of my 165 mic bodies, which had a cut end. Redco was cool enough to send me a 2ft sample piece of their TGS-M7 tube mic cable. The outer jacket was thicker than the original wire, which made it a little tough to deal with on the mic body end, but it all worked out well enough without having to butcher anything. Would definitely use this cable should I re-cable the others.

It sounds great! Nice and quiet, and good clean sound. I tested 1 165 mic body (omni capsule, re-cabled) and 2 175 mic body's (cardioid, original cables) I also tested a third cardioid capsule, which sadly seems to have issues, but the others all worked. I tested with the resistor jumper in place of the transformer, just in case. Will try with the 4665 later.

Not sure how soon I'll have time to take care of the rest, but I'm glad to know it's not for nothing. I still need to poke around and make sure all the voltages are dialed in, and make any necessary adjustments. Pics below.

 

[Ricardus]

A few years ago I found a company that will make you any custom can capacitor you need. I'm building a 1966 Heathkit AA32 tube integrated amp and needed a new can, and they made it for me.

https://hayseedhamfest.com/

 

[thecr4ne]

Nice, thanks for sharing. I'll look into them.

 

[thecr4ne]

I had enough parts on hand to do 4 of these, and I got through them all today. Once they were all together, I went through and checked voltages with both the 165 and 175 mics. As expected, replacing the diodes with new ones put the  B+ voltage too high. Heater voltage was right on. I lifted one side of the diode for the high voltage and put a decade board in line to drop the voltage.  2.8Kohms seems to be the magic value for all 4. Nice that it was consistent.  I don't have any on hand so I'll scrounge some from a tech at work on Monday and put them in when I get a chance.  I decided I'm happy enough with my rudimentary capacitor mounting scheme.  Hayseed Hamfest quoted $40 for 3000uf cans. I can't afford that so I'm sticking with the Elna's,

Happy to  finally be making progress. Hope to wrap these four before Thanksgiving and see about ordering parts for the last two after.

 

[thecr4ne]

I was able to scrounge up some 27K resistors that measured closer to 28K and got them installed inline with the B+ diodes. Voltages are closer to the spec, within 2 or 3 volts, varies a bit by mic, capsule and psu, but they're all close enough to spec. I'm calling these done, with the exception of re-cabling, which I'd like to do still.

 

[andYz00m]

How do they sound? Im about to service a pair of these that are both completely stock. I didnt think about the B+ shifting with modern diodes replacing the rectifier but that makes sense.

Did you replace tubes at all?

 

[thecr4ne]

How do they sound? Im about to service a pair of these that are both completely stock. I didnt think about the B+ shifting with modern diodes replacing the rectifier but that makes sense.

Did you replace tubes at all?

So, I tested all of these with one mic, (the rest need re-cabling) and multiple capsules. It all sounded clean to me, haven't really put them into heavy use yet so I can't give critical listening answer to "how do they sound" I actually have another of these I plan to recap, and a bunch of mics to re-cable so I'll update here when I actually get to it.

I haven't changed the tubes, luckily the tubes in all of mine work fine. That said, I did buy a box of 200 NOS 5840W tubes, so I should be good for a while. Though I recently read somewhere that the 5840 and 5840W are slightly different so I need to make sure I get everything wired correctly for them.

 

[andYz00m]

So, I tested all of these with one mic, (the rest need re-cabling) and multiple capsules. It all sounded clean to me, haven't really put them into heavy use yet so I can't give critical listening answer to "how do they sound" I actually have another of these I plan to recap, and a bunch of mics to re-cable so I'll update here when I actually get to it.

I haven't changed the tubes, luckily the tubes in all of mine work fine. That said, I did buy a box of 200 NOS 5840W tubes, so I should be good for a while. Though I recently read somewhere that the 5840 and 5840W are slightly different so I need to make sure I get everything wired correctly for them.

Nice! Talking to Bowie, he suggests that about 1 in 3 5840s are low enough noise for mic quality.

I have two of these mics I’m restoring and one has a sylvania tube and the other has an original GE. The one with the GE is darker and lower output and the one with the sylvania is brighter and higher output but has some noise (sounds like filter hum).

We’ll see!

 

[andYz00m]

Another question, given the cathode follower circuit, it can handle pretty high spl but curious about the capsule.

Ive used these on electric guitar cab and they sound awesome but im curious if anyone knows the actual SPL handling of the capsule and if its mechanically fragile.

 

[kingkorg]

Another question, given the cathode follower circuit, it can handle pretty high spl but curious about the capsule.

Ive used these on electric guitar cab and they sound awesome but im curious if anyone knows the actual SPL handling of the capsule and if its mechanically fragile.

This will depend on specific capsule, but they can take quite a lot SPL. They are sensitive when it comes to very low frequency bursts like a hole of kick drum, you don't have that issue with guitar cab. I don't think you can damage typical condenser capsule with a guitar cab.