Recapping high voltage amps

[spacelabstudio]

Hi, a friend of mine during his set last night had his Sunn Sceptre head suddenly drop in volume.  He ended up having to borrow another amp to finish his set.  I offered to take his amp home to take a look.

I don't really suspect the filter caps in his immediate problem, but I also suspect he still has the original 40 year old caps in there so it's probably not a bad idea to go ahead and recap anyway.  I'll pop it open a little later this morning, but for now I'm staring at the schematic and see the rectifier is allegedly putting out 500V they have the filter caps rated at 550V.  I can't find an 550V rated filter caps, but I can find a ton of 500V caps.

My question is how do you guys usually approach this?  Throw in 500V caps and hope for the best?  Or start stacking? 

I guess I'll know in a few hours how much room inside the chassis I have to work with.

Thanks,
Chris

[DummyLoad]

start stacking

-or-

http://www.tubesandmore.com/products/C-EC4020X2-525

http://www.tubesandmore.com/products/C-EC30-20X3-525  <-- i believe that this may be an exact replacement part.

http://www.tubesandmore.com/products/C-EC40X3-525

http://www.tubesandmore.com/products/C-EC80-40-30-20

ya the price... makes ya wanna $#!t egg-rolls, huh?

--DL

[HotBluePlates]

Be very careful if you try to stack can caps. If the original amp wasn't done this way, you either need very careful planning or shouldn't do it (or do it with cans alone).

I have some tube-based Hewlett Packard test equipment, and in some cases, the supply voltage exceeded 400-500v. However, can caps were used, and some needed to be stacked in series. For the ones that the negative end was grounded, a metal mounting plate was used for the twist-lock caps (you'll see what I mean with the Sunn). For the other ones, the can is the negative end and is at high voltage. These use a phenolic mounting plate to insulate the from the chassis, and a full-length, thick cardboard tube covering the can.

The cardboard is essential because it insulates the floating can so you don't come in contact with the live can. I can't say I ever seen those for sale, and I reused the ones existing in the particular test item when I recapped.

Of course, you probably don't have room to punch/drill more holes for more cans. Instead, add individual filter caps inside the chassis as the elevated cap of the stacked pair.

[spacelabstudio]

start stacking

-or-

http://www.tubesandmore.com/products/C-EC4020X2-525

http://www.tubesandmore.com/products/C-EC30-20X3-525  <-- i believe that this may be an exact replacement part.

http://www.tubesandmore.com/products/C-EC40X3-525

http://www.tubesandmore.com/products/C-EC80-40-30-20

ya the price... makes ya wanna $#!t egg-rolls, huh?

--DL

Wow, I think those are all likely candidates for a replacement part.  Thanks!  Pricewise, they're cheaper than buying the individual caps of comparable specs.

Chris

[sluckey]

When I restored my Sceptre I used two each 20µF/600V Sprague Atoms and one multi-can 20x20x20x30µF/525V. AES has it all.

http://home.comcast.net/~seluckey/amps/sunn/sunn.htm

[DummyLoad]

I can't say I ever seen those for sale, and I reused the ones existing

i do as well. i salvage them off eqpt. as well if i catch it. i have a accumulated a small stash of them. 

--DL

[DummyLoad]

multi-can 20x20x20x30µF/525V

same can used in the dynaco stereo 70 amp. i need to finish restoring mine...   

--DL

[stingray_65]

but for now I'm staring at the schematic and see the rectifier is allegedly putting out 500V they have the filter caps rated at 550V.  I can't find an 550V rated filter caps, but I can find a ton of 500V caps.

As you stated "allegedly". measure your voltage first before buying caps.
That 500V stated on the schemo is probably supplied with 110 or 115VAC mains, todays wall voltages can be quite higher.

Ray

[kagliostro]

One of my friends used a piece of PCB without copper and a big shrink tube to isolate a floating can cap

-----

an example of phenolic isolator



K

[spacelabstudio]

When I restored my Sceptre I used two each 20µF/600V Sprague Atoms and one multi-can 20x20x20x30µF/525V. AES has it all.

http://home.comcast.net/~seluckey/amps/sunn/sunn.htm

Nice.  Do you prefer the 6550s to the KT88s?  This head actually has an SS rectifier, so I guess I need to keep digging to see if there's a schematic for this version.  If not, the schematic I have is probably close enough.

Chris

[sluckey]

I've only had 6550s in mine. Is your ss rectifier sitting in an octal socket? I wouldn't worry too much about a schematic. I found two versions, a later model with a mid boost switch and an earlier one without mid boost. Neither schematic was an exact match to mine as far as the filter caps are concerned. Just do a one for one changeout if you are gonna do a full cap job. It ain't cheap. Notice that two of the caps in the multi-can are connected together?

[spacelabstudio]

I've only had 6550s in mine. Is your ss rectifier sitting in an octal socket? I wouldn't worry too much about a schematic. I found two versions, a later model with a mid boost switch and an earlier one without mid boost. Neither schematic was an exact match to mine as far as the filter caps are concerned. Just do a one for one changeout if you are gonna do a full cap job. It ain't cheap. Notice that two of the caps in the multi-can are connected together?

No, there is no socket for a rectifier.  It's two alarmingly small diodes up in the chassis.  This one has midboost.  12AT7 reverb driver instead of 12AU7.  The PI circuit is different from the schematic.  Voltages are uniformly high.  But now that I'm playing through it, I can't tell anything's wrong with it.  I'm gonna have the guy come in and tell me whether it's different from what he's used to.  Sounds pretty darn loud to me!

Chris

[SoundmasterG]

The Sunn bass amps tend to have a bit higher voltages than the guitar amps, but even so, most of the vintage Sunn amps have higher voltages than the 525v the replacement cap from AES/CE is rated for. Furthermore, they benefit from a little more filtering in the first two sections. You can save money but not time by using discrete caps in the amp in place of the can cap, and connect the first two sections in series with 220k 1w resistors in parallel with each cap. To do this correctly you also need some terminal strips and you have to drill a couple holes to safely mount the components. It is easier in the bass amps since they have more room, but you can make it work in the guitar amps too if you decide to do that appoach. Make sure to ground at the same point as before. Leave the can cap in place for cosmetics and to fill the hole. You also may need to extend a wire or two and use terminal strips for that too, depending on where you mount the parts. For the bass amps I generally go with two 220uF 350V caps in series for the first section and two 100uf 350V caps in series for the second section. That ends up being the equivilant of a 110uf 700V cap in the first spot and a 50uF 700V cap in the second spot. As can caps disappear everyone may end up having to use discrete caps in the future anyway.

Greg

[HotBluePlates]

... As can caps disappear everyone may end up having to use discrete caps in the future anyway.

They may never entirely go away.

The reason Antique Electronic Supply has them is primarily the many radios and boatanchor stuff out there that use them. And since they are pretty much the only source for caps that match the form factor and have the odd values that radios use, I'd bet they sell a lot of them.

Yes, if you have the space, you can do discrete caps using better and smaller parts.

[sluckey]

I'd love to see a pic of your Sceptre with SS rectifiers. Here's a pic showing the two I'm familiar with. I've also included a schematic of each showing the beefed up filter caps in the '71 Sceptre.

BTW, Triode USA sells a cap 'farm' board for Dynaco amps that can also be used in some Sunns.

[alerich]

Be very careful if you try to stack can caps. If the original amp wasn't done this way, you either need very careful planning or shouldn't do it (or do it with cans alone).

I have some tube-based Hewlett Packard test equipment, and in some cases, the supply voltage exceeded 400-500v. However, can caps were used, and some needed to be stacked in series. For the ones that the negative end was grounded, a metal mounting plate was used for the twist-lock caps (you'll see what I mean with the Sunn). For the other ones, the can is the negative end and is at high voltage. These use a phenolic mounting plate to insulate the from the chassis, and a full-length, thick cardboard tube covering the can.

The cardboard is essential because it insulates the floating can so you don't come in contact with the live can.

This is the arrangement in my Sovtek MIG60. It uses a stacked power supply (not simply stacking the caps for voltage ratings) with cap cans. The screen supply is tapped off of the "lower" cap and B+ is tapped off of the upper cap. The lower can in the arrangement actually has a rather sturdy plastic sleeve with end cap attached to it. These are oddball cap cans, too. It won't be fun to recap this thing when the time comes. I will likely opt for standard caps.

[Katie 77]

One of my friends used a piece of PCB without copper and a big shrink tube to isolate a floating can cap

I use large dia heatshrink on floating can, even if it's covered (AES,F&T, CEdist)...I've seen the clamps cut thru when overtightened

[spacelabstudio]

I'd love to see a pic of your Sceptre with SS rectifiers.

http://spacelabstudio.com/sunn_sceptre/

There are two (known) mods performed by a previous tech: addition of triode/pentode switch and another switch to increase the negative feedback resistance.

Chris

[DummyLoad]

nice pics - thanks for sharing.

almost unmolested - looks to be in near pristine condition. it probably could benefit from a cap job.

--DL

[sluckey]

Great pics! Thanks. Can you read the numbers on those diodes? Your filter cap setup is the same as my '71 model. Those axials are original. In fact, all the electrolytics under the hood are original. The can is probably original too. Can you get some voltage readings on the two 20µF/600v and all four caps in the can?

[spacelabstudio]

Great pics! Thanks. Can you read the numbers on those diodes? Your filter cap setup is the same as my '71 model. Those axials are original. In fact, all the electrolytics under the hood are original. The can is probably original too. Can you get some voltage readings on the two 20µF/600v and all four caps in the can?

Diodes are K-416.  I wrote down a few voltages the other night.  A is about 550V.  D is 355V. 

I'm going to have the guy come over and play it and confirm whether or not he thinks it's still running quiet.  I can't tell there's anything wrong with it.  Probably won't touch anything until I know there's something to fix.

I've done a few pre-emptive cap jobs and been left wondering whether it was really worth it, as I couldn't really tell any before/after difference.  I've started to adopt an attitude that I should be able to tell something is demonstrably broken or suboptimal before performing surgery.

Chris

[spacelabstudio]

So here's a funny story.  I can't figure out that there's anything wrong with his amp, so I have him come over to check it out, and tell him to bring his speaker cabinet.  As he's loading the speaker cabinet into his car he finally notices that the cone is completely shredded.  Wasn't the amp at all!  Boxed it back up and gave it back to him without having done a thing.  Interesting to peak inside, though.