Old Gibson Stereo Amp Cap Question

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sr1200

Well-known member
Joined
Dec 6, 2010
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Location
Long Island, NY USA
Schematic: https://files.mycloud.com/home.php?brand=webfiles&seuuid=33161158458ec7f148b52dcfd183719b&name=ga78

This amp had been sitting in my mothers basement for what has to be over 40 or 50 years now.  It has no tubes in it, but doesn't look too dried out.  I'd like to replace the caps, but not quite sure which ones to replace.  There is a large cap that looks like a stick of dynamite in the middle that I had a replacement custom built (it says 20+10+10M on it with 450WVDC (working volts DC, I dont see a VSP rating on it as some of the others have)) but what i received has 3 160vdc ratings for the 3 values.  Do i add that together or is the whole thing supposed to be 450 for each value.  I can't seem to find it on the schematic, since i can't really tell what its actually connected to (until i drop the chassis).

In addition there are a pair of 20u 25wvdc 40vsp which look like they might be for the heaters... they KIND OF look like electrolytics, but not exactly.  I ASSume it wouldn't matter much if this is just to support the heaters if I use an electrolytic.

Thanks
 
Look in the power section.  You see 20mfd choke 20 mfd.  You need a can cap with a 450V rating, sadly, what you have won't work. 
Having never seen that particular amp guts, I can't advise w/ 100% certainty on anything unless there were clear pictures.

Several of the triodes (plate/anode grid cathode) have their cathodes fully bypassed with 20 mfd 25v caps. you should replace all of these and the amp will really come back to life.  Much more clear etc.

The heaters are AC, there are no caps in that circuit.

Andy
 
As an alternative to the stick o dynamite cap, if you can't source one, I say find a quality can cap that serves your needs and install it...Super nicely  ;D
or get some tag strips and individual axial e-lytics and find a space to wire them in nicely.

I'd be happy to help with anything else, I am a tube amp junkie, but would need to see good pics if you needed some specific guidance.

Take care!
 
Appreciate the response, would be glad to get some pics together. There are a 5 of the sprague 20uf you spoke of on the turret board (6 on the schematic?).  Ill grab pics in the morning.  Thanks again!
 
As an aside.  For anyone trying to decipher old gear and stereos and whatnots, Check this Gibson scheme out!  It is a good primer on plug and play shenanigans, d/t the separate power chassis.  Don't let it intimidate you :).  It's a heater winding and a B+/HT winding. solid state rectified with an extra diode on each side for back up and a choke.
I'd be interested to hear from the tube masters re: screen resistors etc.

I swore to restore my friends super blingy fully featured stereo that he burned out, omg that scheme (capitol records model 936 (see attachment) does a number on my brain!)
 

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sr1200 said:
Appreciate the response, would be glad to get some pics together. There are a 5 of the sprague 20uf you spoke of on the turret board (6 on the schematic?).  Ill grab pics in the morning.  Thanks again!
My pleasure, you're welcome! Please don't rush any photos on my account.  The advice I gave ya will be a good start and if you start doing some research, I think you can tackle this easily. Hopefully it doesn't have any issues.  When it's all up and running, carefully test vs. the voltage chart (one hand behind your back) and report any issues.  I would be curious to know the heater voltage when it's fully loaded and at nominal modern US Wall AC.  I find it very telling and often worrisome  :-[
 
replace the pwr supply diodes with 1N4007,  probably only need two with the newer parts,

rip out the preamp tube sockets and wire up new sockets up with new parts, you need new plate resistors because the old ones will crackle and they probably read 110K to 120K from age, use 1 watt instead of 1/2 watt,  and those .02 coupling caps s/b replaced as they will start to leak after you rebuild the pwr supply, they are probably either CDC Tiger Caps or Sprague Black Beauties, both which leak like crazy due to the chemicals inside, split one open to see for yourself, so by the time you wire in new plate R's and caps, you have pretty  much stressed the old sockets to the point where they would need replacement anyway, so why not do it now. it actually takes less time to wire clean sockets than to suck solder and wrestle out parts with needle nose pliers, those old amps were wired by people who use to wire stuff for the military, so each part will be wrapped tight with sailors knots,

if you want part of the vintage aging, use 120K instead of 100K, the new resistors will not generate noise but will have the mojo of aged resistors due to their increased value.

you can buy dual lytic caps which are nice, 20 uf at 450 VDC,  you could even get away with 350 after the 10K resistors but why chance it, with solid state rects, B+ will come up quicker than the pwr tubes pull down the voltage,  you might use a 500 volt rating for the first two caps as they are just as cheap as 450.

like Andy said, voltages will be high as this was meant for 110 VAC, heaters and B+ will be 10  percent higher, not a big deal as git amps only get used an average of 2 hours a week, unless you use it for an all day practice amp,

another worry is the dual outputs, the primary wire is probably getting pretty funky by now,
so yeah, lots of time and money to do it right, and you still need tubes,  since you have two, you can compare DCR to look for shorted turns, and check ratios if you have a sig gen.

take the knobs off and tighten the pots, they will be loose by now, this improves grounding, replace corroded hardware, spray them down and do the same with the input jacks,

sorry to be a buzz kill but old amps need a lot of TLC, not to mention the speaker,  :eek:
 
andyfromdenver said:
I'd be interested to hear from the tube masters re: screen resistors etc.
Well, they had only 8 pins available for the interconnect, so they had to have a single B+, so the choke had to handle all the current; alternatively, they could have had the choke mounted in the main chassis, that would have allowed a smaller choke, I don't know why they chose this option, designer's whim?..
No screen resistors is just penny-pinching, permitted because 6BQ screens can handle 330V, although datasheet says different. 6BQ5/EL84 have commonly been used with voltages much higher than specs.
 
Hopefully these links work:

Pics:
Left Board
https://files.mycloud.com/home.php?brand=webfiles&seuuid=e652efe325d2cff5d15fcea38d11c146&name=IMG_2601
Right Board
https://files.mycloud.com/home.php?brand=webfiles&seuuid=438f44ead744e1f4a9d02818ae33708f&name=IMG_2602
Extra Right Board
https://files.mycloud.com/home.php?brand=webfiles&seuuid=e747ec73f98a55ca4c01364bd1651880&name=IMG_2603
40 years of dust and transformers
https://files.mycloud.com/home.php?brand=webfiles&seuuid=7a99879b43326a75cb17fd73ae285521&name=IMG_2604
Speaker(Jensen)
https://files.mycloud.com/home.php?brand=webfiles&seuuid=6853297b84e11a4872c7138c1aec3e21&name=IMG_2605
Top Controls / inputs
https://files.mycloud.com/home.php?brand=webfiles&seuuid=86083277d47657fa8847b4c57d08a5f0&name=IMG_2606

My plan was to get some surround goop (dont know what its really called, but it looks kind of like diluted rubber cement) for the speaker surrounds (which feel like either old cloth or a really rough paper).  Replace the stick of dynamite since it looks like its about to fall apart.  I was going to replace the spragues, but the ceramic dics look a little hard to get to (could clip em out i suppose).  The sockets look to be in pretty good condition and grab a tube quite well.  Wasn't planning on changing the diodes but i suppose I should.  On the photos you can see some "home made" resistors which I've never seen on anything i've ever worked on. (hand painted stripes on a blob looking thing).  Didn't really think that i would have to replace the resistors, but I probably would have gone with 1/2 watt or 1 watt carbon since thats what looks like is in there now.  The mains power cord is dry rotted and doesn't have a ground (that was the only thing i originally wanted to replace in the PS chassis...)
In the first pic there are 2 0.03 mfd black caps that i can't see a max voltage on (it has a working voltage of 400v)
Are there any brands that I'd want to go with over others to keep the vibe similar.  I don't really mind if the unit doesn't sound original (wouldn't know anyway since ive never heard it) but I'd like it to be safe enough to use in the studio and not burn the place down if it sees some regular use.

One pic i didn't take was of the "wooden" footswitch.  There are no screws on it just what seems to be a material that might at one point have been rubber on the bottom.  Im thinking that it was just glued on there.  the switch is hard wired to the chassis.

EDit: doesn't seem that any of the sprague caps have voltages on them  i did see 20-6dc on them, but not sure if thats a part number or thats old school for 20 peak 6v working or something...

Everyones suggestions are greatly appreciated. 
 
abbey road d enfer said:
Well, they had only 8 pins available for the interconnect, so they had to have a single B+, so the choke had to handle all the current; alternatively, they could have had the choke mounted in the main chassis, that would have allowed a smaller choke, I don't know why they chose this option, designer's whim?..
No screen resistors is just penny-pinching, permitted because 6BQ screens can handle 330V, although datasheet says different. 6BQ5/EL84 have commonly been used with voltages much higher than specs.

Thank you for the input!

@OP, going to check out those pics.
I got to see and play a lot of the classics when i was working for my repair friend, but not any old Gibson stereo amps.

Are the transformers rusty?  does it seem like it lived in climate control for it's life? lastly, why are the tubes missing?

If the general condition is good, a meticulous and thorough restore would be a great project.

 
thought that might be point to point, good deal, no need to replace sockets, those 100K's look like they are 1 watt too,

so maybe just the filter caps and check leakage on coupling caps by looking at grid voltages,

 
sr1200 said:
My plan was to get some surround goop (dont know what its really called, but it looks kind of like diluted rubber cement) for the speaker surrounds (which feel like either old cloth or a really rough paper). 
That would be neoprene glue. Temporay fix for a rotten suspension. Be careful applying gently, since it rigidizes the suspension, so the diaphragm will break near. Ideally, they should be reconed; it's expensive, but worth it. I guess they're P10Q's; a pair in good condition can ask up to $900!


On the photos you can see some "home made" resistors which I've never seen on anything i've ever worked on. (hand painted stripes on a blob looking thing). 
Far as I can see, they're ceramic caps too.


In the first pic there are 2 0.03 mfd black caps that i can't see a max voltage on (it has a working voltage of 400v)
Since B+ is 330V, you don't need more than 400V working voltage here.


EDit: doesn't seem that any of the sprague caps have voltages on them  i did see 20-6dc on them, but not sure if thats a part number or thats old school for 20 peak 6v working or something...
Looks like the cathode decoupling caps; they don't need to be more than 6V, since the voltage there is about 1.5V.


Lastly, don't forget to disconnect the "meat-puppet killer" cap on the polarity switch.
 
looks like it is in really good condition.
once you vacuum out/spray off the dust, it's going to look pretty stinking good.
That looks well made, and physically pretty tight.

Per the scheme, you'll need four 20mfd high voltage caps.
maybe this would be a good all-in-one, and you get a slightly larger capacitance for the first one.
https://www.cedist.com/products/capacitor-ce-mfg-525v-30202020uf

I need to scrutinize what that section is on the side wall of the right. can you easily tell where it leads?

or, you may fly individual axial elytics closer to the plate load that they serve, that way you don't have to fit one big 4 cap unit somewheres.
 
sr1200 said:
The side board, best i can tell, is the 20u and 200ohm that fits between v4/5 and v7/8

ok, perhaps those weird fuzzy things are the resistors preceding the rectifiers and the two caps bookending the choke.
That's cool :). I'd make a new little board or use tag strips and refresh that section.
then you just need to add two more 20mfd 450 in place of the dynamite.
i can see the have the 2 x 10mfd sections paralleled.
should be an easy squeezy (Patrick Squeezy) and a can cap is inappropriate.
I think there is (and did CJ mention) a 2 section F&T cap. axial with one ground end and two + ends. that might be a good fit. would need to check diameters and such.

Take care!

 
andyfromdenver said:
ok, perhaps those weird fuzzy things are the resistors preceding the rectifiers and the two caps bookending the choke.
No, that can't be. There are only three connections coming from this board, and one clearly goes to chassis ground. It's definitely the end stage cathode res.
 
abbey road d enfer said:
No, that can't be. There are only three connections coming from this board, and one clearly goes to chassis ground. It's definitely the end stage cathode res.

oops I didn't look closely!  Thanks :)

aaaand it's in the main chassis...  ;D
double oops.
 

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