SS Power Rectification and Filter Capacitance Idea ?

[hilldweller]

If I was to go with solid state rectification and increase filter capacitance, Would it be a good idea to remove the rectifier tube and socket, fill the hole with another new higher value multi cap can and then paralel the two multi cap cans together for double the capacitance. It should work and look good too. What are the thoughts on this? Yellow_Light_Colorz_PDT_20

[Sal Brisindi]

Being the expert that I am on the Dynaco ST-70 since I recapped 2 of them (I'm kidding about being an expert) I did notice a difference between my 2 St-70's, one using the stock capacitance (30/20/20/20uf) in the replacement quad capacitor and the other using the upgraded quad cap (40/80/30/20uf). The difference was only when the ST-70 was at the maximum volume with some techno music my son had that has a lot of bass. The smaller cap ST-70 bass distorted a little bit where as the upgraded cap did not. Both my ST-70's have 5AR4's in them.

I personally like the 5AR4 in it but you do save 10 watts of heat with the diodes. The only thing with diodes is when you first turn on the amp the B+ might exceed the capacitor ratings of 525 volts from what I read.

Regards,
Sal Brisindi

[hilldweller]

Being the expert that I am on the Dynaco ST-70 since I recapped 2 of them (I'm kidding about being an expert) I did notice a difference between my 2 St-70's, one using the stock capacitance (30/20/20/20uf) in the replacement quad capacitor and the other using the upgraded quad cap (40/80/30/20uf). The difference was only when the ST-70 was at the maximum volume with some techno music my son had that has a lot of bass. The smaller cap ST-70 bass distorted a little bit where as the upgraded cap did not. Both my ST-70's have 5AR4's in them.

I personally like the 5AR4 in it but you do save 10 watts of heat with the diodes. The only thing with diodes is when you first turn on the amp the B+ might exceed the capacitor ratings of 525 volts from what I read.

Regards,
Sal Brisindi

Thanks Sal, I will be using a standby switch, current inrush limiter, and a Weber S.S. coper Tube Rectifier that emulates the operation of a tube rectifier. If you don't know about thse weber copper tube rects., See the link below.
:)
http://www.webervst.com/ccap.html

[Bob01605]

In relation to what Sal mentioned above. The 40, 80, 30, 20 cap is a good upgrade for many older ST-70's - especially those that hum a little or create a situation in which the fuse blows but only after a period of time - a few seconds to a few minutes. If a fuse blows instantly then you may have a bad power tranny or a wiring dead short somewhere. Also, if that quad cap is "hot" to the touch after 15 minutes or so of play time and not just "warm" then one or more sections is starting to go.

Another thing to check out on a suspect cap is to measure the resistance of each section to ground. (the outer metal case of the cap). A new cap will ususally measure over 1 meg on each section but as a cap wears out the resistance to ground will start to drop. If you are only getting 5000 or 10000 ohms on a section then that section is weak and the whole cap should be replaced.

Sometimes as a stop gap measure you can disconnect that section and put in a single electrolytic cap of maybe 47 Mfd @ 525 volts or higher in it's place just to keep the amp playing. Connect everything that was connected to the bad section to the positive end of the single cap and then ground the other end of this single cap.

As to adding capacitance just to "see" how the increased capacitance will affect overall sound - Try adding the positive end of a 47 Mfd (@525 volts or higher) cap to the section of the quad cap that has the two red wires and one choke lead. That's one of the "20" sections and the one that faces the back of the amp. This is the section that powers the output transformers. Dynaco got away with just 20 Mfd here probably because vinyl records don't have as wide a dynamic range as modern digital sources like CD's. This is a good "tweak" if you notice a crackling sound on peaks when playing your 70 at high volumes.

Bob Latino