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Old 10th February 2013, 04:30 PM   #51
MelB is offline MelB  Canada
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I have tried a few different brands of these things and the GE Brand are by far the toughest. Their CL-80 works great for power amps drawing 1 to 2 amps continuous (digikey KC008L).
http://www.ge-mcs.com/download/tempe...20-325B-LR.pdf
I have had similarly rated ones from Cantherm fail. After a month or so they crack and open up. Maybe I have too much capacitance in the B+ to fill up, but it does not seem to bother the GE ones.
http://www.cantherm.com/products/inr...therm_mf72.pdf
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Old 10th February 2013, 10:21 PM   #52
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Quote:
Originally Posted by Leakstereo20 View Post
Wavebourn, you have made several tubeamplifiers for PA according to your site.

Soft-start for longer lifetime of capacitors and tubes is one issue, "overload" is another.

For home hifi it would be nice if the amplifier shuts down (or partially down) by overload ....may be depending on the amount of mA measured across the cathode resistor? ...like a DC sensor for solid state amplifiers.

Do you have any suggestions to a simple tubeamp overload protection, which does not affect the sound? ....in several tube designs with good reputation old tubes "runs" with bias and leaves us a red glowing plate and a tube melt down.
Can we do anything to avoid this?
It is what you can find in my Pyramid design: LED senses current drawn by screen grids and controls input attenuator LDR and "Overload" signal. No clipping, just hard optical compression when overloaded.

It DOES affect the sound, since it is compressed, but you SEE bright red LED when it happens and can turn the volume down.
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Old 10th February 2013, 10:26 PM   #53
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Quote:
Originally Posted by Leakstereo20 View Post
Thank you very much Wavebourn.
The circuit looks great. I like the fet regulated heater supply.
Could you say something about the delay-time? The +B 300V from the gate of the fet?
Time constant depends on capacitors in power supply reflected by transformer ratios and current limiting resistor in the primary. It is _not_ a time delay, and I don't advocate any time delays in tube amplifiers that don't use Eimac transmitting tubes in factory specified regimes.
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Old 10th February 2013, 10:36 PM   #54
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Quote:
Originally Posted by hpeter View Post
dont understand why tubes (as being recommended in general), should be heated first and later anode supply turned on
tube heaters when cold emits no electrons, so leaving them connected to circuit under full voltage cannot make them break.
Just piece of metals in vacuum
In my opinion, leaving everything normally connected is better than heating them up and later turning anode supply on. Risk of current spike through tubes

Vacuum rectif. has softstart "built-in", but the voltage sag is not nice.
You are absolutely right. Some tubes like transmitting and generating tubes were used in regimes that would cause huge current density from parts of cathode that start emitting before the whole cathode is hot enough that damages cathodes tearing parts from them off. Particularly, in the radar station of the anti-aircraft system we learned in the university as the part of compulsory additional defense education, such measures were implemented, as well as measures to prevent anode voltage from switching on until waveguide commutator is completely switched. But it is a different story: marketing people like to invent some features of their production to demonstrate how special it is, even when such features are not needed absolutely. Even harmful: abrupt appearance of anode voltage when cathodes are hot stresses tubes causing current in uncharged coupling capacitors between anodes and grids that causes positive voltages on grids, currents through grids, and high anode currents.
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Old 10th February 2013, 10:42 PM   #55
Jaimo is offline Jaimo  Canada
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Quote:
Originally Posted by Yvesm View Post
This happens here too !
Use a self excited relay so that power is not reapplied before YOU decide

Yves.
I built a latching relay into a Alesis Powertrip 8 but added an second tweak to the circuit - I installed a push to make switch in series with the coil and used a time delay relay - one has to hold down the pushbutton for 15 seconds before the relay energizes. This is my way of preventing casual visitors from turning on my system and my preamp, DAC's and source devices can be correctly sequenced in the event of a brown out (thuds be gone!). This is my little secret and helps keep my teenagers buddies away from my stereo.
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Old 11th February 2013, 02:41 AM   #56
wa2ise is offline wa2ise  United States
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I've taken out of dead PC power supplies the thermistor they use on the line side to limit inrush current. PC power supplies are rated around 100 to 250W, which is similar to the power consumption of tube amps. Even this crude engineering seems to work fine. The thermistor is placed on one of the primary leads of the power transformer. Done this in a SCA35 amp, now I don't hear the momentary loud transformer buzz upon turning it on I used to hear before adding the thermistor, which means there is reduced inrush current happening. And it's lasted years.
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Old 11th February 2013, 06:36 AM   #57
tomchr is offline tomchr  United States
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I use a CL-90 (90 ohm at 25 C) NTC in the softstart for my 300B amp supply. You can find the schematic about half way down this page: Neurochrome.com : : Audio : 300B SET Amp – Power Supply

~Tom
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Old 11th February 2013, 08:46 PM   #58
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I've decided to keep it simple. I'm just going to change the 'standby' switch on the guitar amp to a DPDT and have the extra throw bypass two CL-80 in series, so they're out of circuit and cool in normal operation. I'll just have to remember to leave it in standby a little longer than before. And I'll resize the drain-down safety resistors across the caps so that they drain slower than the CL-80s cool, so there's no big problems if there's a short power interruption. Then I'll see whether I can downsize the fuse or change to a fast-blow.
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Old 11th February 2013, 08:54 PM   #59
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Quote:
Originally Posted by roline View Post
I use a CL80 on the 110 side of the transformer ... Then C-DIODE-C-L-C. The diode bridge feeds the first cap, Now I place a damper diode as a pass device to another cap that feeds the subsequent filter and B+ bias string.
Ah, you're using a rectifier tube as a soft-start device.

In a CLC I'm considering throwing in a SS diode between the + side of the first cap and the inductor, and maybe another between the inductor and the second cap. I have two concerns: 1) that it doesn't drop the voltge too much 2) that it doesn't raise the voltage either.
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Old 12th February 2013, 08:32 PM   #60
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Originally Posted by Wavebourn View Post
It is _not_ a time delay, and I don't advocate any time delays in tube amplifiers that don't use Eimac transmitting tubes in factory specified regimes.
Wavebourn, thanks for the reply.
Without being a tube expert, I have been of the opinion that a slow charge of electrolytes along with pre-heating of the glow would extend the lifespan of the electrolytes and the tubes.
But I understand that you are of a different opinion.

rgds. Kim
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