SDS Labs Capacitor Upgrade Board

[guitarpsych]

Has anyone tried the SDS Labs Capacitor Upgrade Board?

Here is the description I've seen: "These Power Supply boards replace the original can capacitors in Dynaco ST70. Fit on standoffs under the output transformers, no cutting or drilling required to install, just build, bolt in, hook up the wires and go!"

Any problems or issues? Is it difficult to install? Worth installing if you need to replace the can capacitor anyway?

[TomMcNally]

The SDS boards are pretty painless ... follow the directions and move the wires over one at a time. If you have never done this kind of thing before it might be a little more complicated - but there are lotsa guys on here who can help you figure it out ... make sure you can take some nice closeup pics and let us take a look. I've used the SDS boards on my Dynaco MK-III Poseidon clones, they were so easy I don't remember much about them ...

[mesherm]

Cons:
It is a tight squeeze to shoehorn one in. Only Panasonic caps are short enough.
Bias circuit needs to be removed.
Choke leads will probably need to be extended since they have to reach further.
Uses surface mount resistors on underside so surface mount soldering experience helpfull if building it yourself.

Pros:
Increases the capacitance a lot.
Bias circuit onboard and improved.


So I guess it comes down to how comfortable you feel with a soldering iron and wire strippers. If your can cap is NG but the bias circuit components are ok you might consider just replacing it with an 80/40/30/20 unit and wiring it 30-choke-80-40-20.
If the bias components are old and need replacing also then the SDS board might be for you.

[WA4SWJ]

I agree. I installed one in my old ST-70 amp and it works great. You must remove the bias circuit for it to fit and you'll have to jockey wires around some. But it works just fine and the amp sounds great. In my opinion, removing the GZ-34 in an ST-70 is a good thing since it is running at its max limit - especially if you have the newly manufactured tubes. It also removes the rectifier filament current from the power transformer helping it to run a little cooler. Others may disagree and that's fine, but I think solid state rectifiers and the larger filter caps make an amp better. They just seem to have more punch (if that's an audiophile term). I have both types and I feel my amps with solid state rectifiers sound just a bit better.

If you're interested I can send a picture separately.

[TomMcNally]

By the way - there is another board you may see referred to as the "CChong" ... he has one on eBay now ...

http://cgi.ebay.com/NEW-CAP-BOARD-FOR-DYNACO-DYNAKIT-ST-70_W0QQitemZ260100846468QQcategoryZ71545QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

These are pre-assembled and very nice quality.

[EWBrown]

One trick, if you are undecided whether solid state or tube rescifiers are better, is to take an old dead octal tube, and carefully remove the base (it should at least have pins2, 4, 6 and 8 on it, some may not have all 8 pins). solder in two SS rectifiers, anodes to pins 4 and 6, cathodes joined together and connected to pin 8. Pin 2 isn't used with SS, it's the other side of the 5V filament winding. uF4007s are pushing their maximum safe performance envelope here, look for devices with at least 1200V PIV and 2 to 3 A current capability. If you settle with SS rectifiers, it's a good idea to install an inrush current limiter into the primary circuit, just to soften the initial power-up "impact".

Now it's a simple matter of being able to painlessly swap between rectifier types.

Note: with the SS rectifiers, the B+ voltage will be higher, so biasing will have to be checked and reset as required.


I've used both tube and SS rect, each has its merits and de-merits.

One long listening session, over at a friend's house, we were pushing my ST70 to its maximim, with every kind of music including some hard rock/heavy metal (Nickleback at just below clipping levels) and the JJ 5AR4 stood up just fine, no fireworks... Since then, I changed from Svetlana EL34s to JJ KT-77s, and I like the difference. That particular ST70 has the C CHong PSU cap board, Roy Mottram's VTA-70 driver board (got it before the Red Board was born) , the Triode PA-060 power trannie, new C354, and the OPTs were NOS Dynaco "house numbered" for Hammond Organ (?) equivalents to the normal A470s. I replaced the stock speaker terminal strips with heavy duty dual binding posts, they fit nicely into the slots, and once tightened down, they don't slide around.

/ed B in NH

[EdStiles]

Another almost painless option is to get a Weber Copper Cap rectifier. These are designed as a solid state replacement for a GZ34 having the proper voltage "sag" and an inrush limiter. My personal recommendation would be the WZ-68 which mimics two 5AR4s in a single tube like structure. I recommend this because a single 5AR4 is running close to max in a Stereo 70.

Copper Caps rectifiers look like a copper "tube" and install just like a GZ34 so they are easy to check out. My only recommendation would be to add a current inrush limiter such as a CL-90 ( or whatever the recommended size is for the ST-70) to the transformer primary to get a nice "slow" start when turned on.

The Weber Copper Caps cost about $22 + shipping

http://www.webervst.com/ccap.html for more info.

Ed

[Brinkman]

Thanks Ed. I have been using the WZ34 Weber copper top and had no idea that I could substitute it.

I also am using the SDS capacitance board. I have Roy Mottram's board assembled (in a modified form, per instructions of Kegger from Audiokarma), but have not installed it because the installation instructions are in reference to a stock ST70, not one with wires and bias supply all rerouted to an SDS cap board. I am just too ignorant and inexperienced with electronics to feel comfortable further modifying my ST70's circuits without assistance. Just goes to show you can assemble an ST70 from scratch and still have no idea how what you've built functions.

If anyone can tell me if the Roy Mottram board is fine to use with the SDS cap board in an otherwise unmodified ST70, please let me know.

In the meanwhile, I'll be brushing up on my algebra so I can enroll in electrical engineering at my local community college this fall. I think I've caught a bug.

[EWBrown]

I have used the Roy Mottram VTA-70 board along with the C Chong cap board (can't vouch for the SDF board)in an otherwise "nearly stock" ST-70 - see my earlier posting. Works and sounds great! The only original Dynaco part is the chassis and the OPTs (house-numbered Dynaco OPTs for another vendor) , everything else is of new manufacturing, Triode PA-060 PT and C354 choke, VTA_70 board, Chong Board, etc. The ST-70 is "stock" in that there are no "fancy" mods or upgrades. I started out with JJ EL34s, later changed to JJ KT-77s, which required only a minor bias "tweak". Tube rectifier, JJ 5AR4 / GZ34. So far, it has not gone pyrotechnic on me, and it's hard some hard usage! :o Yellow_Light_Colorz_PDT_11

/ed B in NH

[eef]

I just installed the SDS cap board in my ST-70, and it sounds great. I checked the bias when it was sitting on my workbench, and got it to 1.56. Just rechecked it in the stereo cabinet, and it was low. It won't go much above 1.4 volts before it runs out of potentiometer.

Then I noticed some people suggest running a low bias to extend tube life. As an experiment I set it to 1.25V on each side, and it still sounds pretty good to me. Looks like the real problem is running it too high.

Any suggestions? The SDS instructions suggest making sure that Rcath is connected to pin 8 on the 5AR4. It seems to me that it would not be coming up to even 1.4 if I had a wire missing.

One final thought, I had upgraded the bias supply once before, so now I have two diodes in series coming on the bias tap.

Thanks!