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Solid state rectifiers

67.239.202.10
Posted on September 17, 2009 at 06:12:18
hartthethird
 I recently bought a pair of monoblocks on ebay.
These amps have mkIII power and output transformers, and triode electronics driver and cap boards. I failed to notice before purchave that the builder had used solid state rectifiers instead of the 5ar4 tube. The amps are beautifully built and have been trouble-free. I would like some feedback on the pros and cons of the different designs. The amps are built on home made chassis and don't have room to add the rectifier tubes.
Thanks,
Gene

RE: Solid state rectifiers, posted on September 19, 2009 at 16:49:32
dcriner
Audiophile

Posts: 79
Location: Chicago area - USA
Joined: September 11, 2005
 Some are concerned with the possibility of cathode stripping. The issue would be that a SS rectifier will produce high-voltage B+ before the other tubes' cathodes have warmed up. Some people contend that is bogus.

My advice is to not worry about it, especially if the amps have been operating OK. The only other alternative, it seems, is to worry about it.
Doug

RE: Solid state rectifiers, posted on September 20, 2009 at 12:17:31
xaudiomanx
Audiophile

Posts: 978
Joined: August 16, 2004
 Sorry, but I have to disagree. Cathode stripping could occur if the capacitance on the rectifier tube before the choke or any resistor is too great. Depending on the quality of the rectifier tube and the tube itself most would think about 50UF right on the B+ of the rectifier is about as high as you would want to go. Other rectifier tubes would have different ratings. Using a 5AR4 about 50UF is on the high side. 40UF is safe.

Now for the SS rectifier. The reason for making sure the caps can handle the extra voltage is because when you use a SS rectifier as compared to a tube version the B+ goes about 50-60VDC higher depending on the wall outlet voltage, and that's even more before the circuit starts drawing current. The caps voltage rating of the stock Dynaco ST-70 or MK III is about right there for the voltage the caps have to handle as these amps were designed around 117VAC.

Let's not forget that these amps were all designed to be used with 117VAC and the standard wall outlet voltage is now at about 122VAC give or take. That in itself makes the voltage goe higher and using a SS rectifier, even more.

Paul

RE: Solid state rectifiers, posted on September 19, 2009 at 05:08:17
xaudiomanx
Audiophile

Posts: 978
Joined: August 16, 2004
 Hi,

I would like to see a picture of those amps if you could send me a pciture of them. I think those were a pair that I originally built.

A SS rectifer is supposedly a faster recovery and faster clipping if pushed. A tube rectifier is slower ramp-up and supposedly softer clipping with slower recovery, giving it that more tube like sound. I haven't really noticed the difference between the two and I have built many different amps using both types of rectifiers, sometimes to the same design for comparison. Ususally to hear any difference I had to sit on the edge of my seat and grit my teeth, along with wiggleing my ears. But it wasn't until I wiggled my ears that I heards a difference but that might not be related to the different rectifiers.

The only drawback to using a SS rectifier in an amp is making sure the power supply caps can handle the extra voltage when using SS(about 50 or so extra volts).

Paul

RE: Solid state rectifiers, posted on September 21, 2009 at 05:40:18
hartthethird
 Hi Paul,
I don't have a digital camera but the amps I refer to are the ones you built.
I am in no way dissatisfied with them but was curious about possible sonic effects because obviously most of us are into the "tube sound" whatever that is.
I think Doug gave the best advice, I'll try to stop obsessing and enjoy the music.
Thanks to all,
Gene



RE: Solid state rectifiers, posted on September 22, 2009 at 05:16:26
xaudiomanx
Audiophile

Posts: 978
Joined: August 16, 2004
 If it's the amps I am thinking of(Mark III clones) they are quite good. They use Magnequest output trannies.

I have done many experiments with different types of rectifiers and to be honest, other than the voltage issues being increased and having to make capacitors handle the extra voltage, especially on warmup, I have never heard any differences between the two.

Is there a choke on that pair of amps? If there is, one mod that would be a huge benefit in terms of speed and just overall better sound, is to take the choke out of the position it's in and move it over to just filter the input circuit. Put a 100 ohm resistor in the choke's place it is now. I don't know the amount of capacitance in the input section but if it is over 50UF change them as well to no more than 50UF. Lower is better but you also don't want it to hum.

Another thing you can try is another decoupling stage.

After the first 100 ohm resistor where the B+ ties in put a 1K resistor(1 watt) with a small value cap(30-50UF) and then put the choke in line after the 1K resistor and cap. You will notice how quiet that amp will get but if it is quiet now, don't sweat it. But the biggest improvement will be moving that choke over. If it hums with a 100 ohm resistor try a 1K.

But as Doug said, "Enjoy it!"

Can you describe the amp to me? I built many amps for different people.

If you like those amps you should really give my new ones a listen. Email me personally and I'll send you pictures.(xaudiomanx@aol.com)

Paul

RE: Solid state rectifiers, posted on September 22, 2009 at 13:11:48
hartthethird
 I don't think my amps are the ones you're thinking of. Mine have Dynaco iron for power and output transformers. I probably won't perform any of the mods you're talking about 'cause I dont want to mess anything up. The amps do have chokes, made in Japan generics. I do appreciate your input on this stuff as I'm a relative novice.
Gene

RE: Solid state rectifiers, posted on September 22, 2009 at 15:08:18
xaudiomanx
Audiophile

Posts: 978
Joined: August 16, 2004
 The pair I am thinking of I sold to a person in Massachusetts. Those had Magnequest direct replacement trannies. They do look identical to the Dynaco but they are black and the leads are teflon insulation for each color. Are you the person with the biasing problem?

Paul

RE: Solid state rectifiers, posted on September 23, 2009 at 05:03:35
hartthethird
 No, that is not me. The amps I have are built on u-shaped steel chassis with wood sides to which angled pieces of aluminum were attached. The seller described them as a "'57 Chevy" look. The transformers are cloth lead. I live in Nevada.
Gene

RE: Solid state rectifiers, posted on September 23, 2009 at 14:32:18
xaudiomanx
Audiophile

Posts: 978
Joined: August 16, 2004
 No! Not my amps! Anyway, the mods I described would definitely be a major improvement. Not the kind that is subtle. Very obvious!

Paul

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