Hello all, I have a question related to my current build. Since I am using precious gu81m tubes and do not wish to see them prematurely depart this world, I'd like to implement a safety device.
I have separate boards and circuis for all voltages on board (bias, screen, anode for drivers) but I would like to have a safety device which will take out the B+ on the gu81m (1600v) if bias to mosfet stage/gu81m suddenly goes out.
Can I use a thyratron? I found almost nothing on operation but I gathered a firing voltage is necessary to have conduction from cathode to anode. Can I assume the absence of grid voltage (ie bias) will shut the tube off?
Please understand my concern
I can find and implement a thyratron for little effort and money so that side of thing is not a problem.
Also other questions:
1. the thyratron will behave like a standard gas rectifier with DC flowing through the filament transformer's CT? This would be DC however not rectified AC. Is it possible to saturate the heater core?
2. If the above has a positive answer can I solve the problem by taking the "processed" DC from the heater wires rather than through the heater transformer's center tap?
3. Are there sonic effects caused by thyratrons in the path of the b+?
Thanks
I have separate boards and circuis for all voltages on board (bias, screen, anode for drivers) but I would like to have a safety device which will take out the B+ on the gu81m (1600v) if bias to mosfet stage/gu81m suddenly goes out.
Can I use a thyratron? I found almost nothing on operation but I gathered a firing voltage is necessary to have conduction from cathode to anode. Can I assume the absence of grid voltage (ie bias) will shut the tube off?
Please understand my concern
I can find and implement a thyratron for little effort and money so that side of thing is not a problem.
Also other questions:
1. the thyratron will behave like a standard gas rectifier with DC flowing through the filament transformer's CT? This would be DC however not rectified AC. Is it possible to saturate the heater core?
2. If the above has a positive answer can I solve the problem by taking the "processed" DC from the heater wires rather than through the heater transformer's center tap?
3. Are there sonic effects caused by thyratrons in the path of the b+?
Thanks
I thought the state was voltage dependent hence the use in pulse equipment....
Just out of curiosity....what would be the effect of a thyratron on the b+ as opposed to an all ss powersupply? I don't mean as a rectifier but rather after the caps as a sound "colouring" device maybe?
Inviato dal mio GT-I5800 usando Tapatalk
Just out of curiosity....what would be the effect of a thyratron on the b+ as opposed to an all ss powersupply? I don't mean as a rectifier but rather after the caps as a sound "colouring" device maybe?
Inviato dal mio GT-I5800 usando Tapatalk
In pulse equipment, the "downside" of the pulse causes the thyratron to switch off. Some pulse forming networks employ a second "de-q'ing" thyratron to effectively short the input to ground to shut it off. You could use it as a crowbar, however, I doubt your rectifiers/ transformer will love you for long.
No idea about audio use.
No idea about audio use.
I hope you're designing in many many safety devices! This project has got hairs on it if it is being used as a learning curve
I hope you're designing in many many safety devices! This project has got hairs on it if it is being used as a learning curve
You are right! Many safety devces are in place.
1. Full grounding of chassis and boards
2. fuse on each transformer 220v line
3. separate transformers for LV, HV, filament supplies
4. 20kv insulation on all HV (>500v) lines
5. plexiglass protection panes to create internal layers inside the amp
6. oversized caps for all boards
7. separate voltage sources (+600,+1600,+300+500-300+150v)
8. at least 2 cap stages for every board (in case a cap stage blows)
9. Teflon cap machined to cover all exposed HV leads
10. sofstart, timers ecc (to avoid nasty arching with the huge rectifiers)
what I wanted was to protect the tubes themselves from bias failure.
In B+ line..
When bias falls away, GU81 will pull too much current (for a short period, wich it can handle) and fuse will blow.
I don't know much about the GU81M but I have learned a few things with big TV tubes. A fuse on the transformer secondary, or primary will remove the energy source when an overload occurs. It does nothing about the energy stored in the capacitor banks after the rectifier. This energy is sufficient to blow the strap connecting the cathode to the base pin in half in some of my amps. A fuse on the DC side of the power supply will become a fireball in the event of a short. Been there blown that!
I would look into some type of circuit that constantly measures the bias supply voltage and kills the B+ supply in the event of bias loss. In fact I would look at a power sequencer that fires up the heaters and the bias supplies, then the plate supplies and finally the screen supply. I tend toward PIC chips for this sort of thing, but since you are from Italy the word that comes to mind is ARDUINO.
I would look into some type of circuit that constantly measures the bias supply voltage and kills the B+ supply in the event of bias loss. In fact I would look at a power sequencer that fires up the heaters and the bias supplies, then the plate supplies and finally the screen supply. I tend toward PIC chips for this sort of thing, but since you are from Italy the word that comes to mind is ARDUINO.
elegant solution tubelab.com!
I wonder if a small metal enclosure around the fuse on the DC side of the cap banks will protect the internals from the fuse fireballing?
The sequenced fireup is unfortunately based on the separation of HV and LV I have.
I am waiting for a 2500v custom wound toroidal for HV. All heaters and misc. voltages will be separate and rely on other transformers (some of which are visibile in my pics).
Right now...i have all filaments on, delay 5 minutes, and then all the HV voltages on through a softstart.
Either I retard the gu81m plate supply/screen by using a thyratron (any 2A unit ought to do it) by using the very same bias supply as a trigger or I use a PIC. My programming skills are nihil and I lack all necessary material...Also I would need to base arduino functioning on a relay of somekind which in turn brings me back to HV switches (thyratrons or equivalents).
I could use a thyratron for anode and screen supply for the gu81m. I predicted 500mA screen current and max 1.5A on anode plates (which is to say maximum humanly achievable conditions for my household). Any thyratron should do it.
This does put me in the odd predicament of finding yet more space in a cramped area but that can be dealt with.
I looked for HV switches (relays) and really could not find anything of value at a reasonable cost.
I wonder if a small metal enclosure around the fuse on the DC side of the cap banks will protect the internals from the fuse fireballing?
The sequenced fireup is unfortunately based on the separation of HV and LV I have.
I am waiting for a 2500v custom wound toroidal for HV. All heaters and misc. voltages will be separate and rely on other transformers (some of which are visibile in my pics).
Right now...i have all filaments on, delay 5 minutes, and then all the HV voltages on through a softstart.
Either I retard the gu81m plate supply/screen by using a thyratron (any 2A unit ought to do it) by using the very same bias supply as a trigger or I use a PIC. My programming skills are nihil and I lack all necessary material...Also I would need to base arduino functioning on a relay of somekind which in turn brings me back to HV switches (thyratrons or equivalents).
I could use a thyratron for anode and screen supply for the gu81m. I predicted 500mA screen current and max 1.5A on anode plates (which is to say maximum humanly achievable conditions for my household). Any thyratron should do it.
This does put me in the odd predicament of finding yet more space in a cramped area but that can be dealt with.
I looked for HV switches (relays) and really could not find anything of value at a reasonable cost.
@godfrey. Thanks. I am a true idiot for not having read wikipedia.
@Doz. Yes..I am interested in thyratrons for their switching abilities at high voltage. From your comments I got the initial idea was rubbish but tubelab's suggestion of changing the startup sequence would seem compatibile with thyratron use.
My doubts came from the notion of thyratrons in pulse equipment. I could not understand how the switching was done and thought of asking you guys for applications...
@Doz. Yes..I am interested in thyratrons for their switching abilities at high voltage. From your comments I got the initial idea was rubbish but tubelab's suggestion of changing the startup sequence would seem compatibile with thyratron use.
My doubts came from the notion of thyratrons in pulse equipment. I could not understand how the switching was done and thought of asking you guys for applications...
Ill keep that in mind! I still have a long way to go but I hope to have some ideas by the time the transformer arrives and connections will be made. I also must convert all HV to bridge hybrid (SS & MV) operation...the back panel is almost done.
I think I have found some krypton base tubes which could do the job...a small delaying tube and I could have a perfect delayed Anode/plate supply if this is what it takes to have perfect operation.
For the emergency fuse solution....could an encapsulated fuse be safe? I have plenty of teflon insulation, grommets and solutions to have a small "safe box" around the fuses.
I think I have found some krypton base tubes which could do the job...a small delaying tube and I could have a perfect delayed Anode/plate supply if this is what it takes to have perfect operation.
For the emergency fuse solution....could an encapsulated fuse be safe? I have plenty of teflon insulation, grommets and solutions to have a small "safe box" around the fuses.
The fuse must be rated for the voltage it will be used on otherwise it is a spark gap after it blows. Many fuses are not rated for DC use since the voltage is constant and an arc will not extinguish once it starts. Even on AC once the air inside the fuse ionizes the arc will not stop. If the metalization from the blown fuse link gets deposited on the glass and forms an arc path, the fuse explodes.
A thyratron becomes a low impedance once the gas inside ionizes. It will not turn off until the applied plate voltage drops below the sustaining value. Thyratrons are used to clamp (short) a DC voltage under fault conditions. They can also be used as a control element on AC voltages since the applied power drops to zero twice on each cycle. It may be possible to use a thyratron to turn the plate voltage on and off. You need to place it between the rectifier and the first filter cap. It will self extinguish twice each cycle.
My programming skills are pretty bad too, but the processor world doesn't get much easier than the Arduino. There is tons of pre written code available and pre built hardware is cheap. You still need to control realys for the big stuff. I have used Solid State Relays to control the primary side of a power transformer, even big ones. You can put a HV mosfet in between the HV center tap and ground on a medium sized tube power transformer and use it to delay the B+ or even bring it up slowly using PWM.
A thyratron becomes a low impedance once the gas inside ionizes. It will not turn off until the applied plate voltage drops below the sustaining value. Thyratrons are used to clamp (short) a DC voltage under fault conditions. They can also be used as a control element on AC voltages since the applied power drops to zero twice on each cycle. It may be possible to use a thyratron to turn the plate voltage on and off. You need to place it between the rectifier and the first filter cap. It will self extinguish twice each cycle.
My programming skills are pretty bad too, but the processor world doesn't get much easier than the Arduino. There is tons of pre written code available and pre built hardware is cheap. You still need to control realys for the big stuff. I have used Solid State Relays to control the primary side of a power transformer, even big ones. You can put a HV mosfet in between the HV center tap and ground on a medium sized tube power transformer and use it to delay the B+ or even bring it up slowly using PWM.
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