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Old 30th January 2009, 05:27 AM   #11
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Quote:
Originally posted by Nihilist
Thanks for all the info guys.



I've tried a 5Y3 rectifier with stock 80ufd can cap. -VS-. a SS rectifier and 900ufd capacitance.

I can't see (hear ?) any reason why I would want to go back to a tube rectifier.

With a SS rectifier I have a higher B+ , more power supply filtering, and tighter , more solid bass. Why would I use a tube rectifier ? What is the benefit ?
All your observations are correct. By all means, use a SS rectifier if you find it superior. I don't understand why tube rectifier is popular in DIY Hi-Fi equipment. For guitar amplifiers the drop of voltage in rectifier tube under load causes "sag" compression that is often a desired effect for guitar sound, but this is not beneficial for natural sound reproduction.

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I have a hard time believing that the capacitance used in most tube gear is satisfactory. IME , 4700ufd is really about the minimum needed to get rid of AC ripple . I know this flys in the face of all conventional tube designs , but it is my experience . As I've said, my experience is mainly with SS , but I can't imagine tubes are any less susceptible to AC hum.
Ripple is smaller when current is smaller, if capacitance stays the same. Tubes use higher voltages and because of that draw less current for the same input power, therefore same % of ripple can be achieved with a smaller capacitor. Better power supplies usually use CLC filters though. Also in case of pentodes and tetrodes, there can be quite high ripple in anodes if screen grid voltage is well regulated and this is a different than in solid state devices that are all like triodes.
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Old 30th January 2009, 09:38 AM   #12
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Quote:
Originally posted by Nihilist
I've tried a 5Y3 rectifier with stock 80ufd can cap. -VS-. a SS rectifier and 900ufd capacitance.

I can't see (hear ?) any reason why I would want to go back to a tube rectifier.

With a SS rectifier I have a higher B+ , more power supply filtering, and tighter , more solid bass. Why would I use a tube rectifier ? What is the benefit ?
The only benefits to using a hollow state diode are the delay in HV due to the need for the cathode to warm up. This gives the other VTs time to heat up. A SS power supply comes up within a second or two, and can overvolt other components, especially where DC coupling is used. Of course, the solution is to separate the heater and HV power supplies. Turn on the heater power, let the VTs heat up, and then switch on the HV.

The other benefit is with guitar amps. VT-based power supplies have worse regulation, and this can give a compressive effect as the finals pull down the HV. As the HV comes back, this can give "sustain" to the notes played.

For HiFi, SS is definitely better. The only reason I used a hollow state power supply was I had a power xfmr with the required current rating, but it was a 650Vct secondary that overvolted badly with silicon diodes. Using a 5U4GB instead dumped off enough extra voltage to get that down to where it needed to be (458Vdc with silicon, to 350Vdc with the 5U4GB, between its forward drop and the reduced size of the input resevoir capacitor.) It scores glowey bottle coolness points, but otherwise silicon would have been better.

Quote:
That being said, perhaps I should try a different tube rectifier than the 5Y3. Perhaps an indirectly heated rectifier , as I just can't get my head around the AC (filament power) and DC (B+ power) being together but supposedly seperate.
Simply the result of floating the heater winding: the AC rides on the DC instead of being referenced to ground.

Quote:
I have a hard time believing that the capacitance used in most tube gear is satisfactory. IME , 4700ufd is really about the minimum needed to get rid of AC ripple . I know this flys in the face of all conventional tube designs , but it is my experience . As I've said, my experience is mainly with SS , but I can't imagine tubes are any less susceptible to AC hum.
It isn't. Since VTs are high voltage, low current critters, a hollow state diode can't source the currents needed to charge up big capacitors. That puts a definite limit on the size of the resevoir capacitor. To get rid of what would otherwise be inacceptable residual AC, use an LPF, either an LC or RC, to attenuate it to acceptable levels.

This should be done anyway, and 4700uF is overkill. Even though it's NBD to find silicon diodes that could handle the resulting Isurge, power xfmrs weren't designed for that, and certainly not NOS. Such power xfmrs like the Hammond "Classic" series weren't either. Even though your silicon diode won't mind an Isurge of 10A or more, your power xfmr just might mind that a whole lot. Even with silicon, you're probably limited to an input capacitor of ~47uF anyway. At these low currents, filtering is NBD anyway. I haven't had any noise issues in my designs.
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Old 30th January 2009, 04:52 PM   #13
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Thanks again gents ,


Seems as if quite a few people vouch for both SS and Tube. I think I'll be sticking with SS rectifiers. I've yet to see why I wouldn't.


The reference to 4700ufd for power supply filtering is due to my personal experience with SS gear. Seems as if anything less than that just doesn't get rid of ALL the hum.

For tubes, it may be way overkill. I dunno.

For what it's worth, I've been running my vintage Magnavox amps with DIY SS rectification wired so that the B+ is running directly into 720ufd worth of capacitance before loading into the OPT.

Been runnning this way for a lil over a year now. No worries !

I've been told by Magnavox afficianados that my trannies should have smoked long ago


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