Hi everyone.
As far as I know, arcing is caused by a high current through the rectifier tube, which leads to a high anode-cathode voltage drop.
If this is so, will arcing happend if using a 250 Vrms secondary? Even if the full voltage is across the tube, would (in theory) 250 Vrms (352 V-pk) be enough to make a rectifier arc? Let's take a GZ34, compare to its PIV of 1700 V, those 352 V seem nothing to worry about.
Maybe there're other reasons for arcing appart from high anode-cathode potential, like insufficient emission.
Or maybe I'm confusing apples and oranges 🙂
I appreciate any observation on the matter. Thanks!
As far as I know, arcing is caused by a high current through the rectifier tube, which leads to a high anode-cathode voltage drop.
If this is so, will arcing happend if using a 250 Vrms secondary? Even if the full voltage is across the tube, would (in theory) 250 Vrms (352 V-pk) be enough to make a rectifier arc? Let's take a GZ34, compare to its PIV of 1700 V, those 352 V seem nothing to worry about.
Maybe there're other reasons for arcing appart from high anode-cathode potential, like insufficient emission.
Or maybe I'm confusing apples and oranges 🙂
I appreciate any observation on the matter. Thanks!
Too large of a filter cap can cause arcing. Some tubes do arc more easily than others. Those flat top 5R4 tubes with the thick bases were the worst.
Yes, a big cap leads to higher surge and ripple currents. What I was asking for is the reason why, no really what makes it happend.
I don't know about that. Many times I've helped people with arcing Chinese and Russian ST bottle 5U4 "equivalents". Installing either a U.S. made OS 5U4GB or current production ElectroHarmonix (EH) 5U4GB solves the problem. The 1st filter cap. was within documented limits. Those ST bottle 5U4 "equivalents" are CRAP.
AFAIK, "potato masher" 5R4s do fine, when the 1st filter cap. is <= 15 μF.
An internal short circuit in the rectifier.
Exceeding the peak voltage of the capacitors with no load causing a short circuit in the filter capacitor --1.414 x peak = 1.41 X 250=352.5V
Once a rectifier arcs it is wise to replace it due to deposits on the elements leading to future short circuits and/or reduction in emission due to loss of emissive surface area.
Exceeding the peak voltage of the capacitors with no load causing a short circuit in the filter capacitor --1.414 x peak = 1.41 X 250=352.5V
Once a rectifier arcs it is wise to replace it due to deposits on the elements leading to future short circuits and/or reduction in emission due to loss of emissive surface area.
Trying to draw excessive current from a tube rectifier will overtax the emission capability of the tube cathode and cause the voltage drop to rise sharply. It may also cause cathode hot spots.
A well made rectifier tube can arc for two reasons, exceeding the Peak Inverse Voltage rating, or the peak forward current rating. Exceeding either one of these will invite a flashover.
Notice the words "well made rectifier tube." For a rectifier tube to live a long life the cathode coating must be of a uniform thickness over its entire surface. Any thin spots will heat up fastest at a time when the filter caps are empty and demanding a fill-up. The peak current density in the small thin spot will exceed it's emission capability causing a flashover at that spot. This is not unlike secondary breakdown in a semiconductor. Once a tube has arced in this manner it is usually bad forever. It will continue to arc at that spot on every power up.
The spacing between the cathode and plate must be uniform over the entire surfaces of both. If there is a spot where the spacing is smaller than the rest of the tube a larger than normal percentage of the tube's current flow will be concentrated in one area causing a flashover even when the tube is operated well within its ratings. These may arc and recover if the bad spot is killed. Old 5U4's can spark and keep on working. Once a 5AR4 sparks it is usually worthless.
There was a period from 2008 to 2011 when all three of the major new production tube makers cranked out some pretty poor quality 5AR4's and many flashed over on first power up.
You can help out these kinds of tubes by adding a 1N4007 or UF4007 in series with each plate. and an Inrush Current Limiter like a CL140 in series with the transformer CT. A larger IRL like a CL90 in series with the line power helps too.
The diodes add some margin to the tube's actual PIV capability. Their low voltage drop adds nothing to the tube's total drop, so their effects are not "heard."
The CL140 is a 55 ohm resistor when cold. This slows the initial current hit to the rectifier as it is warming up, avoiding mane flashovers in non uniformly made tubes. It becomes a 5 ohm resistor after a few seconds, minimizing voltage drop.
The CL90 on the line input slows the initial surge drawn by the whole amp when full voltage is applied to cold tube heaters, slowing the start up of the whole amp by a few seconds.
Notice the words "well made rectifier tube." For a rectifier tube to live a long life the cathode coating must be of a uniform thickness over its entire surface. Any thin spots will heat up fastest at a time when the filter caps are empty and demanding a fill-up. The peak current density in the small thin spot will exceed it's emission capability causing a flashover at that spot. This is not unlike secondary breakdown in a semiconductor. Once a tube has arced in this manner it is usually bad forever. It will continue to arc at that spot on every power up.
The spacing between the cathode and plate must be uniform over the entire surfaces of both. If there is a spot where the spacing is smaller than the rest of the tube a larger than normal percentage of the tube's current flow will be concentrated in one area causing a flashover even when the tube is operated well within its ratings. These may arc and recover if the bad spot is killed. Old 5U4's can spark and keep on working. Once a 5AR4 sparks it is usually worthless.
There was a period from 2008 to 2011 when all three of the major new production tube makers cranked out some pretty poor quality 5AR4's and many flashed over on first power up.
You can help out these kinds of tubes by adding a 1N4007 or UF4007 in series with each plate. and an Inrush Current Limiter like a CL140 in series with the transformer CT. A larger IRL like a CL90 in series with the line power helps too.
The diodes add some margin to the tube's actual PIV capability. Their low voltage drop adds nothing to the tube's total drop, so their effects are not "heard."
The CL140 is a 55 ohm resistor when cold. This slows the initial current hit to the rectifier as it is warming up, avoiding mane flashovers in non uniformly made tubes. It becomes a 5 ohm resistor after a few seconds, minimizing voltage drop.
The CL90 on the line input slows the initial surge drawn by the whole amp when full voltage is applied to cold tube heaters, slowing the start up of the whole amp by a few seconds.
In short:
High current causes hot spots on cathode surfaces, some cathode material evaporates and forms a highly conductive gas, a plasma. So part of your vacuum tube is temporarily turned into a gas discharge tube.
High current causes hot spots on cathode surfaces, some cathode material evaporates and forms a highly conductive gas, a plasma. So part of your vacuum tube is temporarily turned into a gas discharge tube.
Most tube rectifiers include a maximum capacitance spec.
But do not forget, they also include a series resistance spec, along with that capacitance.
DCR of Primary x step up ratio; plus DCR of the portion of the secondary that drives one plate.
Get that wrong, and a good rectifier may struggle too.
Make it struggle very badly, and get an ARC.
If the inrush current exceeds the maximum, and if the coating is uneven (thanks to Tubelab_com for mentioning the uneven coating),
You may have a problem. Stack 2 problems together, and there you go.
But do not forget, they also include a series resistance spec, along with that capacitance.
DCR of Primary x step up ratio; plus DCR of the portion of the secondary that drives one plate.
Get that wrong, and a good rectifier may struggle too.
Make it struggle very badly, and get an ARC.
If the inrush current exceeds the maximum, and if the coating is uneven (thanks to Tubelab_com for mentioning the uneven coating),
You may have a problem. Stack 2 problems together, and there you go.
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A good recipe to trigger an arc in a rectifier tube is to have it work into a large reservoir capacitor with too small a current limiting resistor, which may not cause obvious trouble during normal operation, but ...
Happened to me once, PY88, 220 ohm current limiter, 150uF reservoir cap, 350V, no problem for many hours, damper diodes are tough ...
until I pulled a mains plug - oops wrong one - quickly plugged it in again. A flash in the PY88, a loud bang like a pistol shot, and the limiter resistor exploded into shrapnel.
What had happened ?
Well, when I inadvertently pulled the plug, the reservoir cap depleted rapidly, and was empty when the connection with mains was re-established. The filaments still hot, the rectifier tube conducting like mad, the inrush current into the cap in the 10s of amps, blew up the limiting resistor. I have to admit that it was 22ohm rather than the intended 220, and it was a carbon film, and it was a feable 1W ...
The rectifier survived, with just a brown spot on the inside of the glass, dampers are tough, did I already mention that ?
Happened to me once, PY88, 220 ohm current limiter, 150uF reservoir cap, 350V, no problem for many hours, damper diodes are tough ...
until I pulled a mains plug - oops wrong one - quickly plugged it in again. A flash in the PY88, a loud bang like a pistol shot, and the limiter resistor exploded into shrapnel.
What had happened ?
Well, when I inadvertently pulled the plug, the reservoir cap depleted rapidly, and was empty when the connection with mains was re-established. The filaments still hot, the rectifier tube conducting like mad, the inrush current into the cap in the 10s of amps, blew up the limiting resistor. I have to admit that it was 22ohm rather than the intended 220, and it was a carbon film, and it was a feable 1W ...
The rectifier survived, with just a brown spot on the inside of the glass, dampers are tough, did I already mention that ?
Thanks everyone.
I remember reading that before, but I didn't understand why a silicon diode helps in the forward conduction period. As silicon diode and vacuum rectifier are in series, the current is the same. If the "only" help is in the PIV, then it makes sense. Do anyone know if silicon diodes help in the forward conduction too?
I remember reading that before, but I didn't understand why a silicon diode helps in the forward conduction period. As silicon diode and vacuum rectifier are in series, the current is the same. If the "only" help is in the PIV, then it makes sense. Do anyone know if silicon diodes help in the forward conduction too?
No, a series diode only helps in blocking the reverse. Said that it helps when the tube rectifier has started to flash, the si diode will extinguish the flash when
voltage reverses. Without the diode the flash would increase when voltage
reverses.
As stated, it does not. Tube rectifiers arc over for two reasons, excess voltage in the reverse direction, excess current in the forward direction, or often a combination of both along with less than stellar alignment of the internal elements.
The silicon diodes increase the PIV rating of the diode / tube pair placing less voltage stress across the tube in the reverse direction.
Their low voltage drop adds nothing to the tube's total drop, so their effects are not "heard."
Many tube purists claim that adding silicon in the path will destroy the sound. Hence my comment about behavior in the forward direction.
Excess current in the forward direction is a critical issue as the tube is warming up. No cathode will have a perfectly uniform coating thickness, and emission will start first in the thinner areas. Once the cathode is hot enough for the entire surface to be emitting electrons the issue is less of a problem. The CL140 will limit the peak current during these few critical seconds of startup.
The SSE amplifier board that I sell was designed in 2006 and was available in 2007. In 2009 people began to see issues with 5AR4's flashing over on initial power up. Sometimes a new tube would flash over, but a second identical tube would work and live a long happy life.
At first this was just seen in JJ tubes. By 2010 JJ's were doing better, but Sovteks were turning into fireworks displays. There were random batches of bad Shuguang tubes as well. I added the diodes and CL140 to all SSE boards in 2010 and the sparking rectifier tubes went away. Nobody noticed "silicon poisoning" in the sound quality.
The SSE does run the rectifier tube pretty hard. It has a B+ voltage in the 425 to 450 volt range depending on transformer choice and line voltage. Total current draw can be 200 mA or more depending on tube choice and bias current. I run mine at about 220 mA with EH KT88's and an old Sylvania 5AR4. The same tubes have been in it since 2010, but it no longer sees daily use.