Build my amp, works fine and then fuse in mains lead blows. Turns out the cheap Chinese 5AR4 arced.
Put in the spare TAD 5AR4 and all powers up normal except that the B+ is now instead of 370V about 270V and the TAD gets very hot....
Check the PSU design in Duncan Amp's PSU designer and no design faults detectable. Researched internet and discover lots of complaints about modern day 5AR4, seems that at this moment only the Sovtek is reliable. Order a couple of new ones.
So far it looks like all problems are resolved however how to prevent / minimise the chance of happening again?
A few modifications have been done:
- Add a UF4007 with a 1nF 3kV capacitor parallel in series with each diode
- Reduce the filter input capacitance (before the choke) to 40uF
- Being aware of today's "low ESR" add an inrush resistor in series with the input capacitance (minimizes the start up current)
- Being aware of the limitations on the currents an electrolytic capacitor can handle use 4 * 10uF capacitors and make sure they are not getting too hot (although under the chassis I have created a ventilation draft across them)
Especially the inrush limiter makes a big difference on the start up current when there is downstream (after the choke) large capacitance.
AM
Put in the spare TAD 5AR4 and all powers up normal except that the B+ is now instead of 370V about 270V and the TAD gets very hot....
Check the PSU design in Duncan Amp's PSU designer and no design faults detectable. Researched internet and discover lots of complaints about modern day 5AR4, seems that at this moment only the Sovtek is reliable. Order a couple of new ones.
So far it looks like all problems are resolved however how to prevent / minimise the chance of happening again?
A few modifications have been done:
- Add a UF4007 with a 1nF 3kV capacitor parallel in series with each diode
- Reduce the filter input capacitance (before the choke) to 40uF
- Being aware of today's "low ESR" add an inrush resistor in series with the input capacitance (minimizes the start up current)
- Being aware of the limitations on the currents an electrolytic capacitor can handle use 4 * 10uF capacitors and make sure they are not getting too hot (although under the chassis I have created a ventilation draft across them)
Especially the inrush limiter makes a big difference on the start up current when there is downstream (after the choke) large capacitance.
AM
All your notes appear relevant and are good measures in my book. The only thing I might add would be to ensure correct operation of the amp. In other words, never apply power to a hot amplifier. Always allow all the tubes (and IRCL devices) to cool completely before re-applying the power. Ensure this either through procedure and discipline, or use relays and/or timers.
Hammond practice
My Hammond H182 organ has 50 uf filter caps, but the "400 ohm" choke comes first right off the cathode. The 5AR4's look like they lasted about 30 years, from the old ones left in the boxes in the back. The organs were produced in 1968. I replaced the electrolytic with 50 uf 1000VDC film motor caps (genelec), with extremely low ESR and 14 ga wiring. So far, the new Sovtek 5AR4 is handling it fine.
So I don't burn out my precious HV transformer, I took a terminal header and between the transformer and the tube I put 1/2" of fine wire teased out of a 19 stand 20 gauge. You can't buy 200 ma 600 VAC rated fuses, nor 1/4x1-1/4 fuse holders in that voltage. Hopefully the single strand will melt before the transformer guts if the tube shorts out. Geoelectro that owns an organ service puts 1/4x1-1/4 fuses in the primary of the HV transformer of tube organs he services, sized just big enough to blow if the rectifier has a problem. This is much smaller than the fuse that would be required to run the tonewheel generator motor.
My Hammond H182 organ has 50 uf filter caps, but the "400 ohm" choke comes first right off the cathode. The 5AR4's look like they lasted about 30 years, from the old ones left in the boxes in the back. The organs were produced in 1968. I replaced the electrolytic with 50 uf 1000VDC film motor caps (genelec), with extremely low ESR and 14 ga wiring. So far, the new Sovtek 5AR4 is handling it fine.
So I don't burn out my precious HV transformer, I took a terminal header and between the transformer and the tube I put 1/2" of fine wire teased out of a 19 stand 20 gauge. You can't buy 200 ma 600 VAC rated fuses, nor 1/4x1-1/4 fuse holders in that voltage. Hopefully the single strand will melt before the transformer guts if the tube shorts out. Geoelectro that owns an organ service puts 1/4x1-1/4 fuses in the primary of the HV transformer of tube organs he services, sized just big enough to blow if the rectifier has a problem. This is much smaller than the fuse that would be required to run the tonewheel generator motor.
Thanks for mentioning that - I do this through procedure and it has become second nature so I do not even think conciously about it. But for others that may be new to valve technology this may be worth mentioning since it is likely that they are not used to doing it in that way.
AM
the 5ar4 with cap input must see two 75r to two 200r(Rt) from power trafo to protect ....
The Valve Wizard
http://www.drtube.com/datasheets/gz34-philips1970.pdf
The Valve Wizard
http://www.drtube.com/datasheets/gz34-philips1970.pdf
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Joined 2009
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I didn't realize tube rectifiers can short ? I thought there was a physical gap between the conductors. In my amp I fuse the B+ after the psu choke/cap filter (the fuse doesn't see the pulsed current flow but the dc average).
Thanks for mentioning this, this is another one that I forgot the mention (second nature again).
Measuring the DC resistance gives the ballpark figure (often on the high side) - one has to calculate the "reflected impedance" (this is easily done using PSU designer and otherwise one can use the Radio Handbook formula. With the "quality" of todays tubes it is better to go a bit above the recommended minimum value.
AM
If the anode gets hot enough as exhibited by red plating (e.g. cannot readiate enough of the heat as the result of too thin material being used) then it can short on the reverse peak. Similarly if the distance between the anode and cathode is not even then the reverse voltage capacity may not be enough.
If the valves are still hot and the capacitors are largely discharged then when applying power there is often a very large surge attempting to charge the capacitors in one cycle. This can have damaging effect, not only to the rectifier but also to the filter input capacitor and other parts down the line like the mains transformer or output transformer primary winding.
( How long does it take a tube amp to start working after applying power? That is how long it takes for the tube's cathodes to heat up
and it is a safe bet that it will take quite a bit longer for the cathodes to cool down. )
AM
If I understand correct from your description then this is an example of a "choke" input filter. This present a very "easy" load on the rectifier tubes , transformer and capacitors and itresults in a very long life for those components. But you'll have to over-dimension the choke considerably and make sure the choke is far away from the OPT as it will be working pretty hard. There are some guidelines but double the current rating starts to sound like the ballpark figure to aim for. (IIRC)
It offers better voltage regulation too but unfortunately it does not give an as high output voltage as achievable with a capacitor input filter. I did look at this option but was not aware of the higher current rating requirement for the choke and already had ordered and recieved the parts when I discovered the current rating and higher mains TX HT voltage requirement.
AM
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Tube quality
Bigun, look at your fuse. I bet it says "250 VAC" or even "32 VDC". I'm pretty sure, because 600 VAC fuses I am familiar with start in the 1/2 dia by 2" long size, and the fuze holders are about 2 1/2" long. If the fuse is not rated to take out B+ voltage (my b+ is 375 VDC), then it won't break the arc many times. 1/4x1 1/4" fuseholders usually are made of black PVC plastic,which turns to carbon when arced over. They are also not rated for 375 VDC. The phenolic headers I'm using come from the bad old tube days and were actually used for mounting components on metal chassis in b+ circuits. So thought they dont say "600 VDC rated", the phenolic header technology has been proven. Now, electricity can arc about an inch per thousand volts in dry air. Good luck.
Geoelectro reports that he has replaced several power transformers in B/C3 Hammond organs that the rectifier shorted out and burned up the transformer. He has nothing nice to say about any modern rectifier. B3's are often gigged, which subjects them to shock when carrying. You wind two wires together, heat them red hot over and over, and see how straight they stay! 1960's western tubes were miracles of day to day quality control and fifty years of design improvement. The apparent design can be copied, but the production machinery is not available for copying. At least Soviet tubes bounced around in the back of a truck belonging to an important general, in recent times.
Bigun, look at your fuse. I bet it says "250 VAC" or even "32 VDC". I'm pretty sure, because 600 VAC fuses I am familiar with start in the 1/2 dia by 2" long size, and the fuze holders are about 2 1/2" long. If the fuse is not rated to take out B+ voltage (my b+ is 375 VDC), then it won't break the arc many times. 1/4x1 1/4" fuseholders usually are made of black PVC plastic,which turns to carbon when arced over. They are also not rated for 375 VDC. The phenolic headers I'm using come from the bad old tube days and were actually used for mounting components on metal chassis in b+ circuits. So thought they dont say "600 VDC rated", the phenolic header technology has been proven. Now, electricity can arc about an inch per thousand volts in dry air. Good luck.
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DOn't know where you got the inch/thousand volts from but imho it looks to me to be extreem.
High voltage variable capacitors as used in HAM radio and other transmitters with tubes running 2.5kV and more have a rule of thumb. The plate spacing has to be at least 1 mm for every kV, to be on the safe side use 1.5mm for every kV.
A surface arc more commonly occurs in equipment which is damp and I always use 2 mm for spacing to chassis etc for my B+ and plate wires. Never had an issue even when I had close to 100% humidity in the shack.
AM
I seem to recall being told something similar to the 1" per 1000v arc. However, that will only hold once the arc is initiated from hard contact with unlimited current availabilty.
I'm guessing that 19 strand, 20AWG wire is made out of nineteen strands of 32AWG wire. Geometrically, it looks approximately correct and vendors often mention "19/32 stranded 20AWG". Anyway, 32 gauge wire is rated to carry 0.53 amps. I'd assume the guys who wrote the standard picked a conservative point, and the wire won't fuse at 0.53 + 0.01 amps. In all likelihood, it melts at two, or three, or ten amps.
Still, your logic must be sound - your single strand of thin wire has got to be thinner than whatever was used to wind the transformer, right? And a chain is only as strong as its weakest link? Of course, your lone strand of thin wire is hanging out there in the air, easily shedding its copper melting heat. The thin wires of the PT secondary are all packed into a bobbin, covered in lacquer, with an endbell to boot. They probably get pretty hot with no way to cool off.
B+ fuse experiment
Appreciate some analysis of my B+ fuse experiment. It has been going on about 4 months, no shorted tubes or blown fine wires yet. If I blow a H182 Hammond power transformer, I have to go somewhere and move another 440 lb organ to my shop and tear it down- I really don't want to do that again. Even if the organ is free, the moving cost me $250 on the last one. Geoelectro's (of organforum) primary transformer fuse makes more sense, but the H182 has the 120VAC wiring all crammed together in a corner under the power transformer for short runs and less hum. Hammond did beautiful soldering of the 14 ga wire in and out to the amphenol connectors to the leslie etc. My 80w Weller iron won't pretend to solder 14 ga that beautifully, and I really don't want to buy a 300 w iron, nor do I believe my skills are anywhere as good as the Hammond ladies. Besides, I think they soldered the AC power before the transformer was installed. So I have the motor and transformer on the same fuse I added outside the chassis, which won't pretend to protect the power transformer from a shorted rectifier tube. The organ was manufactured with no fuse at all, which led to burned power wiring in the one I bought when the motor capacitor shorted out in 96. The factory tech replaced the capacitor, but left the hot wire with burned off insulation!@
Octal/noval sockets violate the 1000v/inch rule all the time, but they were not used in geophysics that went to the coast. In the seventies the oil exploration company I worked for was scrapping chassis that had tube power drivers for something- probably hydraulic solenoids, as they had 1500 hp vehicles that would pick up the vehicle on a hydraulic plate and vibrate against the ground in a swept frequency from 1/2 hz to 20 hz. The excursion of the plate was about an inch. The tubes they were using to drive that solenoid valve had plate caps!, not your standard 6CA7 or 6L6 or anything like that. So the B+ would not short out across the socket in the swamps, I assume.
Appreciate some analysis of my B+ fuse experiment. It has been going on about 4 months, no shorted tubes or blown fine wires yet. If I blow a H182 Hammond power transformer, I have to go somewhere and move another 440 lb organ to my shop and tear it down- I really don't want to do that again. Even if the organ is free, the moving cost me $250 on the last one. Geoelectro's (of organforum) primary transformer fuse makes more sense, but the H182 has the 120VAC wiring all crammed together in a corner under the power transformer for short runs and less hum. Hammond did beautiful soldering of the 14 ga wire in and out to the amphenol connectors to the leslie etc. My 80w Weller iron won't pretend to solder 14 ga that beautifully, and I really don't want to buy a 300 w iron, nor do I believe my skills are anywhere as good as the Hammond ladies. Besides, I think they soldered the AC power before the transformer was installed. So I have the motor and transformer on the same fuse I added outside the chassis, which won't pretend to protect the power transformer from a shorted rectifier tube. The organ was manufactured with no fuse at all, which led to burned power wiring in the one I bought when the motor capacitor shorted out in 96. The factory tech replaced the capacitor, but left the hot wire with burned off insulation!@
Octal/noval sockets violate the 1000v/inch rule all the time, but they were not used in geophysics that went to the coast. In the seventies the oil exploration company I worked for was scrapping chassis that had tube power drivers for something- probably hydraulic solenoids, as they had 1500 hp vehicles that would pick up the vehicle on a hydraulic plate and vibrate against the ground in a swept frequency from 1/2 hz to 20 hz. The excursion of the plate was about an inch. The tubes they were using to drive that solenoid valve had plate caps!, not your standard 6CA7 or 6L6 or anything like that. So the B+ would not short out across the socket in the swamps, I assume.
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