ST-70 Power supply questions

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ST-70 Power supply questions

Postby franciscano » Sun Oct 30, 2016 5:57 am

Hi, I'm in the process of building a st-70 amp. I'm building the power supply on pcb, and i would like to use solid state rectifiers.

Has anybody tried to use a soft start delay on B+? using a mosfet?

I've found the circuit outlined below on a forum i do not remember where. So i have incorporated the design on to my schematic.

Would the mosfet circuit affect the function of the amp in a bad way?

Any thoughts regarding the circuit would be very helpful.

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Thanks,
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Re: ST-70 Power supply questions

Postby DeathRex » Sun Oct 30, 2016 6:22 pm

I have a version of the soft start circuit, but haven't tried it. I found it on DIYAudio. With 400B+ and silicone rectifiers you're going to have a lot of B+. Why have a sag resistor, it will lower the B+, but that baby is gonna get hot.
About the only reason to have a soft start, is to be able to use 450V capacitors. How much B+ are you shooting for?

Another way of doing it, I did once, is using a small tube rectifier to provide voltage to the FET, like a EZ80, EZ81, 6DW4, or such. Then the voltage will come up as fast as the rest of the tubes.
You should have a separate 6V filament for it, with a B+ of 400 or more. You could also use a 5Y4 or so with a 5V filament.

It would be great to use a pesky 6AL5, but the voltage is a bit too much.
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Re: ST-70 Power supply questions

Postby Geek » Sun Oct 30, 2016 9:51 pm

Hi,

Please don't cross-post the same question to many forums. Real bad netiquette.

I'll give you the same answer I did at DIYAP...

I experimented with it when developing my array of drivers and ST-70 tweaks and came to the conclusion it's completely unneccessary.

The peak power through the MOSFET can be very high, especially if the unit was on, turned off, then on again. Use a linear-rated transistor with a huge SOA and bypass all them switching guys, like ESBC series and stuff.

That IRF840 will fail from surges under certain conditions (like clicking the amp off, then back on again).... sent a few to the smokehouse that way, LOL!

That 33uF cap is a wee small and I'd swap the positions of the 3K3 and 1M resistors. If you want a soft-start, do a nice 10-second curve on turn-on. That would simulate say, a GZ34 warmup.

Cheers!
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Re: ST-70 Power supply questions

Postby franciscano » Mon Oct 31, 2016 2:57 am

DeathRex wrote:I have a version of the soft start circuit, but haven't tried it. I found it on DIYAudio. With 400B+ and silicone rectifiers you're going to have a lot of B+. Why have a sag resistor, it will lower the B+, but that baby is gonna get hot.
About the only reason to have a soft start, is to be able to use 450V capacitors. How much B+ are you shooting for?

Another way of doing it, I did once, is using a small tube rectifier to provide voltage to the FET, like a EZ80, EZ81, 6DW4, or such. Then the voltage will come up as fast as the rest of the tubes.
You should have a separate 6V filament for it, with a B+ of 400 or more. You could also use a 5Y4 or so with a 5V filament.

It would be great to use a pesky 6AL5, but the voltage is a bit too much.


I'm shooting for B+ of 480-490V, i will be using kt88 as output tube. I thought of using a sag resistor to mimic the tube rectifier sag. I don't know if i'm going to use it or not, maybe test it with and without, and see if there is any sonic difference.

I'm going to order a custom toroidal transformer, i would like to cut the 5v winding, and the heat from the rectifier tube.
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Re: ST-70 Power supply questions

Postby franciscano » Mon Oct 31, 2016 3:11 am

Geek wrote:Hi,
Please don't cross-post the same question to many forums. Real bad netiquette.


Hi, sorry, i was thinking different forums, different peoples, more answers :).

Geek wrote:That IRF840 will fail from surges under certain conditions (like clicking the amp off, then back on again).... sent a few to the smokehouse that way, LOL!


I've used the IRF840 as a point of start. And i was thinking of using something with higher voltage rating, someone recommended QP8N80C, it is rated for 800v.

I think the 30 seconds delay circuit will offer some protection against turning the amp off and on again, as the timer will reset itself.

Do you think the mosfet will affect in anyway the sound ? Should i bypass the mosfet with a relay after the turn on curve?
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Re: ST-70 Power supply questions

Postby WA4SWJ » Mon Oct 31, 2016 6:26 am

Before I say anything else, I have to say that I have not tried schemes like this so take the following with a grain of salt.

First, why do you want to complicate things like this? Just a whole passel (great southern term) of additional potential failure modes with dubious sonic results.
Second, what benefit would be derived from this? I don't see any reason to do this honestly, unless of course you want to regulate plate voltage for some reason and this is not the way to do that of course.
Third, in my experience with my ST-70 clone, lower power supply impedance results in better bass response. Some time ago I replaced my GZ34 power supply tube, which kept failing, with solid state rectifiers and have had no more problems and much better bass (I'm talking speed and transient response, not lower frequency response). Anything added to the power supply doesn't help in that regard. Granted, when the gate drive to the MOSFET is fully ramped up, the Rdson of the MOSFET will be a fraction of one ohm, but again, why complicate things? I just don't think sonic benefits will be the result. If you're trying to avoid the mystical "cathode stripping", well there are some commentaries on here about that as well.

Of course, I could be wrong and since I haven't tried this stuff I have no direct listening experience. But I hope that what ever you decide to do that it does work and that you like it. Enjoyment is the word. If this approach makes you happy then awesome! No one can argue with that.

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Re: ST-70 Power supply questions

Postby franciscano » Mon Oct 31, 2016 11:21 am

WA4SWJ wrote:Before I say anything else, I have to say that I have not tried schemes like this so take the following with a grain of salt.

First, why do you want to complicate things like this? Just a whole passel (great southern term) of additional potential failure modes with dubious sonic results.
Second, what benefit would be derived from this? I don't see any reason to do this honestly, unless of course you want to regulate plate voltage for some reason and this is not the way to do that of course.
Third, in my experience with my ST-70 clone, lower power supply impedance results in better bass response. Some time ago I replaced my GZ34 power supply tube, which kept failing, with solid state rectifiers and have had no more problems and much better bass (I'm talking speed and transient response, not lower frequency response). Anything added to the power supply doesn't help in that regard. Granted, when the gate drive to the MOSFET is fully ramped up, the Rdson of the MOSFET will be a fraction of one ohm, but again, why complicate things? I just don't think sonic benefits will be the result. If you're trying to avoid the mystical "cathode stripping", well there are some commentaries on here about that as well.

Of course, I could be wrong and since I haven't tried this stuff I have no direct listening experience. But I hope that what ever you decide to do that it does work and that you like it. Enjoyment is the word. If this approach makes you happy then awesome! No one can argue with that.

Regards,


Well i don't want to complicate things. I just want to use the circuit to slowly ramp up voltage, not for sonic benefits.

As this will be a from scratch build, and as i'm on a limited budget, i was thinking of protecting the tubes for longer life. :)

What i don't know, if it will work and what impact will have on the sound.
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Re: ST-70 Power supply questions

Postby Geek » Mon Oct 31, 2016 2:06 pm

franciscano wrote:Should i bypass the mosfet with a relay after the turn on curve?


Actually, a simpler solution used by commercial designers is a power resistor that gets bypassed by a relay after 2-5 seconds. But they put it on the primary side of the trasformer. It was used before the invention of NTC thermistors (inrush current limiters). A Pioneer beast I worked on had a 10R, 50W resistor in the primary that was bypassed by a relay after a few seconds. It requires the use of big Ohmite "Brown Devil" class of resistors as the cement ones will fuse (they don't like HUGE surge currents).

But it isn't really needed. You will not extend tube life. IF it's the cathode-stripping myth you're concerned about, only directly-heated power tubes are affected.... and ones of the transmitter class, not audio. *maybe* an 845 will have its emission extened a couple months with one, but EL34's? T'aint happening.

So I have to echo the "overcomplicated" concerns.

Cheers!
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Re: ST-70 Power supply questions

Postby dcriner » Mon Oct 31, 2016 6:25 pm

I would install a CL-90 in series with the 120-V supply. Or, just forget about it, and not worry about it.
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Re: ST-70 Power supply questions

Postby Geek » Mon Oct 31, 2016 10:52 pm

dcriner wrote:I would install a CL-90 in series with the 120-V supply.


That would do great for the initial turn-on, but if warm, they take time to cool down.

franciscano - Take a look at the spec sheet of inrush limiters.
http://www.digikey.ca/product-search/en ... icl/656273
(or use the USA site)

If you want to go the ICL route, choose one rated for between 80-90% of your nominal current draw. If you choose one too large, they won't warm up and you'll have degraded performance. If you use one too small, it will make your room smell really bad :o)

Cheers!
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Re: ST-70 Power supply questions

Postby franciscano » Tue Nov 01, 2016 12:39 pm

Hi, from what i see on the digikey cl-90 is rated only 2A. Wouldn't this be very undersized ? I mean, the thermistor should be installed in the AC for the whole power transformer, right?

So, having 4 kt88 1.6A each heater, 3 driver tube 0.6A each. This will be about 8.2 + ~300ma for the rest, 8.5A. Or i'm missing something?

Another question, i was browsing the forum and found out about the damper diode 6CG3, anybody has any experience with such tube? I was thinking i can add the tube after the rectifiers to delay the B+.

You maybe wonder why i'm trying to complicate things with the mosfet and such. The reason i don't want to use a tube rectifier like gz34 or 5ar4, is because i want to have at least 300ma headroom, and they are not cheap.

It seams 6CG3 can do 350ma and it is about ~$5.

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Re: ST-70 Power supply questions

Postby franciscano » Tue Nov 01, 2016 12:48 pm

Geek wrote:
dcriner wrote:I would install a CL-90 in series with the 120-V supply.


That would do great for the initial turn-on, but if warm, they take time to cool down.

franciscano - Take a look at the spec sheet of inrush limiters.
http://www.digikey.ca/product-search/en ... icl/656273
(or use the USA site)

If you want to go the ICL route, choose one rated for between 80-90% of your nominal current draw. If you choose one too large, they won't warm up and you'll have degraded performance. If you use one too small, it will make your room smell really bad :o)

Cheers!


From my calculation i have a current draw of 8.5A, or at least i think so, i found B57364S100M rated for 7.5A so within the 80-90%. Again if i understood correct :)
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Re: ST-70 Power supply questions

Postby franciscano » Tue Nov 01, 2016 1:26 pm

OK, i think i was wrong on the power rating, i have to find out primary current rating. Which from my calculation is about 1.5A primary current.
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Re: ST-70 Power supply questions

Postby DeathRex » Tue Nov 01, 2016 2:41 pm

You can always use a CL80, or CL70, lower resistance, higher current, and a bit faster warming up. Your primary current should be a little over 2 amps.
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Re: ST-70 Power supply questions

Postby dcriner » Tue Nov 01, 2016 4:49 pm

I install a CL-90 in all my vintage tube-type radios and amplifiers. I have not experienced any noticeable degradation of performance. Many older tube-type electronics was designed for an a.c. powerline voltage of 115V. Nowadays, the modern a.c. line voltage is nominally 120V, but often even runs a couple of volts higher than that. So, if a CL-90 reduces the device's running voltage by a couple of volts, that's good.

Reducing the turn-on surge current is not a bad thing, whether you believe in cathode stripping or not. And, if you reduce the steady running a.c. voltage by a couple of volts, that will prolong the life of tube filaments/heaters.

I bought some CL-90s in bulk.
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