Thanks George.
Have you considered doing something yourself for the basement?
Like a light positive pressure, or enhanced ventilation, or vent the bottom of the basement, between the ground and the vapour barrier?
Maybe it can be of any help, but once a customer asked us to install an ozone generator to keep a cheese room clean from external bacteria.
There was an Ozoner sensor in the room connected to an electrical lockable door. Automatically the room was ozonized at the end of the day and vented early morning, to be able to work in the room at 8:00 AM.
Something like this could help monitor the radon when still upstairs and start venting before entering the basement: https://www.airthings.com/it/wave-radon
Back on topic:
I've seen you recommended those as well: https://www.mouser.it/ProductDetail/576-0251001.MRT1L
Have you considered doing something yourself for the basement?
Like a light positive pressure, or enhanced ventilation, or vent the bottom of the basement, between the ground and the vapour barrier?
Maybe it can be of any help, but once a customer asked us to install an ozone generator to keep a cheese room clean from external bacteria.
There was an Ozoner sensor in the room connected to an electrical lockable door. Automatically the room was ozonized at the end of the day and vented early morning, to be able to work in the room at 8:00 AM.
Something like this could help monitor the radon when still upstairs and start venting before entering the basement: https://www.airthings.com/it/wave-radon
Back on topic:
I've seen you recommended those as well: https://www.mouser.it/ProductDetail/576-0251001.MRT1L
Last edited:
Could this be a solution on top of the amp? http://nofancomputer.com
It is designed for CPUs dissipating up to 100 W, and it is not bad looking at all on top of the amp instead of the tube (seems a toroid):
Last edited:
I often fuse things to prevent frying and expensive or irreplaceable OPT in the event of a component failure.
You must observe the ratings of any fuse, and respect them, or at least fully understand the consequences of violating them before doing so.
Let's start with the garden variety 1/4 inch glass enclosed fuse that's about an inch long. They are often rated for 125 or even 250 volts AC, but only 25 or 32 volts DC. Why? What happens if I put one of these in the plate or B+ lead of an OPT and a tube arcs?
A simple fuse will contain a sacrificial piece of metal that will melt into an open quickly. Any arc that tries to start will die out during the next zero crossing of the AC supply if the ratings of the fuse are respected. There is no zero crossing of a DC source so the arc can continue to grow. If the DC supply can generate sufficient current the arc will grow large enough and fast enough to ionize the metal / air vapor that was generated by blowing the metal link in the fuse. This will generate a HUGE amount of pressure inside the fuse either blowing one of the end caps off or causing the whole fuse to explode. I had a few fuses violently explode on me before I found out about this failure mechanism.
If you choose to fuse a tube amp always place the fuse in the cathode circuit and size the bypass capacitor large enough to withstand the voltage required to fully cutoff the tube assuming that the full B+ voltage is on the plate. I have used the little green fuses on vacuum tube UNSET boards and have seen one fuse blow saving the tube and all other components when an FQP9P25 failed to a short in the cathode circuit due to a stupid mistake while attempting to draw composite curves with power supplies.
My FETSET breadboard had a 1 amp fuse in series with the drain lead of the P-fet, and I was running it on a current limited bench type power supply when a screwdriver slip caused two fets to fail to a short without blowing the fuse.
Silicon fails faster than a fuse can blow most of the time, so fusing a FETSET may be futile, but probably still a good idea if a non limited power supply is used. Understand that fuses can fail violently when they blow, but so can a plastic cased mosfet if the power supply has sufficient stored energy. Any fuse not rated for high voltage DC should not be used in the B+ circuit of a tube amp due to the risk of explosion. Tiny glass or ceramic shards flying at high velocity can take out an eye very easily.
My old school HP648B power supply is a 50/60 Hz switcher. It has 1000 uF of capacitance across the output terminals internally. The current limiter takes hundreds of mS to react and by then at 600+ volts your test circuit is GONE! I had two 6HJ5 vacuum tubes EXPLODE during curve tracing experiments. One simply scattered glass all over the bench, so I fused the cathode of the second tube and tried again. This time the tube violently exploded as did the fuse and the 1 ohm sense resistor in the cathode circuit.
I find that these 1 ohm Dale 2 watt "flameproof" resistors will also work like a fuse in the event of a tube arc, but like most everything else they are out of stock for the next year or so.
https://www.digikey.com/en/products/detail/vishay-dale/cpf21r0000fkee6/13996238
You must observe the ratings of any fuse, and respect them, or at least fully understand the consequences of violating them before doing so.
Let's start with the garden variety 1/4 inch glass enclosed fuse that's about an inch long. They are often rated for 125 or even 250 volts AC, but only 25 or 32 volts DC. Why? What happens if I put one of these in the plate or B+ lead of an OPT and a tube arcs?
A simple fuse will contain a sacrificial piece of metal that will melt into an open quickly. Any arc that tries to start will die out during the next zero crossing of the AC supply if the ratings of the fuse are respected. There is no zero crossing of a DC source so the arc can continue to grow. If the DC supply can generate sufficient current the arc will grow large enough and fast enough to ionize the metal / air vapor that was generated by blowing the metal link in the fuse. This will generate a HUGE amount of pressure inside the fuse either blowing one of the end caps off or causing the whole fuse to explode. I had a few fuses violently explode on me before I found out about this failure mechanism.
If you choose to fuse a tube amp always place the fuse in the cathode circuit and size the bypass capacitor large enough to withstand the voltage required to fully cutoff the tube assuming that the full B+ voltage is on the plate. I have used the little green fuses on vacuum tube UNSET boards and have seen one fuse blow saving the tube and all other components when an FQP9P25 failed to a short in the cathode circuit due to a stupid mistake while attempting to draw composite curves with power supplies.
My FETSET breadboard had a 1 amp fuse in series with the drain lead of the P-fet, and I was running it on a current limited bench type power supply when a screwdriver slip caused two fets to fail to a short without blowing the fuse.
Silicon fails faster than a fuse can blow most of the time, so fusing a FETSET may be futile, but probably still a good idea if a non limited power supply is used. Understand that fuses can fail violently when they blow, but so can a plastic cased mosfet if the power supply has sufficient stored energy. Any fuse not rated for high voltage DC should not be used in the B+ circuit of a tube amp due to the risk of explosion. Tiny glass or ceramic shards flying at high velocity can take out an eye very easily.
My old school HP648B power supply is a 50/60 Hz switcher. It has 1000 uF of capacitance across the output terminals internally. The current limiter takes hundreds of mS to react and by then at 600+ volts your test circuit is GONE! I had two 6HJ5 vacuum tubes EXPLODE during curve tracing experiments. One simply scattered glass all over the bench, so I fused the cathode of the second tube and tried again. This time the tube violently exploded as did the fuse and the 1 ohm sense resistor in the cathode circuit.
I find that these 1 ohm Dale 2 watt "flameproof" resistors will also work like a fuse in the event of a tube arc, but like most everything else they are out of stock for the next year or so.
https://www.digikey.com/en/products/detail/vishay-dale/cpf21r0000fkee6/13996238
Attachments
Hi George,
I would like you to comment this evolution of your circuit: this afternoon I was walking in the snow and I had this idea.
So I took a piece of paper and sketched it. Now I just modded the previous schematic to get the idea run, and it does.
Basically it is now a single stage gain, because the nmos works with the highest possible gain.
The pmos has the purpose to deive the cathode of the nmos, and can be even smaller, because it gets few volts across drain-source.
The gate of the nmos is referenced to ground.
The purpose of U3 is to set a voltage divider from the output of the nmos equal to the input of the stage, so that you cancel the signal and only get the distortion at the output of the opamp U2. Then we amplify this error through the opamp and apply the feedback to the gate of the nmos.
U4 is used to set the Zout and the distortion of the stage by applying more or less feedback tot he gate of the nmos.
I want to have it as an external control like the volume, to adapt the amp to the speakers and the volume we listen at.
This way there's no need to have too much gain to drive the output stage, and you set the feedback by the gain of the opamp, without any loss of sensitivity of the stage.
Thank you in advance George!
Roberto
I would like you to comment this evolution of your circuit: this afternoon I was walking in the snow and I had this idea.
So I took a piece of paper and sketched it. Now I just modded the previous schematic to get the idea run, and it does.
Basically it is now a single stage gain, because the nmos works with the highest possible gain.
The pmos has the purpose to deive the cathode of the nmos, and can be even smaller, because it gets few volts across drain-source.
The gate of the nmos is referenced to ground.
The purpose of U3 is to set a voltage divider from the output of the nmos equal to the input of the stage, so that you cancel the signal and only get the distortion at the output of the opamp U2. Then we amplify this error through the opamp and apply the feedback to the gate of the nmos.
U4 is used to set the Zout and the distortion of the stage by applying more or less feedback tot he gate of the nmos.
I want to have it as an external control like the volume, to adapt the amp to the speakers and the volume we listen at.
This way there's no need to have too much gain to drive the output stage, and you set the feedback by the gain of the opamp, without any loss of sensitivity of the stage.
Thank you in advance George!
Roberto
Back on tube world, this is my take with the 12AT7 as driver and GU50 as output tube:
Having the pmosfet on the bottom of the GU50, I've used it to bootstrap R3 so that it becomes like a CCS for the driver.
It is more sandophobic-friendly having a bootstrapped CCS load instead of the depletion mosfet and it solves the actual issue of components shortage.
Having the pmosfet on the bottom of the GU50, I've used it to bootstrap R3 so that it becomes like a CCS for the driver.
It is more sandophobic-friendly having a bootstrapped CCS load instead of the depletion mosfet and it solves the actual issue of components shortage.
Could this be a solution on top of the amp? http://nofancomputer.com
It is designed for CPUs dissipating up to 100 W, and it is not bad looking at all on top of the amp instead of the tube (seems a toroid):
View attachment 1151965
That thing looks amazing!
Does this explain the non-existence of a working LTSpice model of the 6JC6?
@Tubelab_com off topic, but have you ever applied positive feedback anode-to-pmos in a tube amp? By grid driving the tube we can autobias the tube referring the grid to ground.
I’m curious on the effects in a world where low DF and higher distortion at lower volumes are often a good starting point.
Thanks,
Roberto
I’m curious on the effects in a world where low DF and higher distortion at lower volumes are often a good starting point.
Thanks,
Roberto
@Tubelab_com I hope you will solve soon the issue with the lab.
@spiggs have you done some tests with low current drivers? I would like to know how low can I go with the driver current.
I was playing with a boostrapped concertina and I have to say it sounds quite well, so I came out with this solution where the load of the 12AX7 is bootstrapped by the pmos.
There are almost 1.5 mA through the 12AX7, and 27 pF of Crss of the pmos at 30 kHz need 158 uA @ 1 Wrms and 824 uA @ 23 Wrms.
The sensitivity of the amp is fine and there's plenty of power.
@spiggs have you done some tests with low current drivers? I would like to know how low can I go with the driver current.
I was playing with a boostrapped concertina and I have to say it sounds quite well, so I came out with this solution where the load of the 12AX7 is bootstrapped by the pmos.
There are almost 1.5 mA through the 12AX7, and 27 pF of Crss of the pmos at 30 kHz need 158 uA @ 1 Wrms and 824 uA @ 23 Wrms.
The sensitivity of the amp is fine and there's plenty of power.
I was thinking about the advantages of sharing the PSU node with output transformer and driver, by splitting the driver plate load and taking the feedback from the split point: there will be an optimum where we can cancel PSU hum by injecting part of it through the anode-to-grid feedback of the driver.
I have decided that my next HiFi amp build will be a two stage FETSET amp based on the working prototype seen in post #911 of the SCG thread in the Pass Labs forum. It just sounded too good not to build one. I have always wanted to revisit the class H SET that I built for a magazine contest back in 2007. I had a modified two stage SCG proto board running, but blew it up with a screwdriver slip.
Unfortunately the Radon Mitigation project in the lab has turned into a major mess so that nothing useful will be done in the foreseeable future. Every thing in a 2000 square foot basement must be moved at least twice so that the entire concrete floor can be sealed. The sealer produces an unplesant odor so work can only be done on days when all the house windows can be opened and fans can move air through the house. This morning it was 35 degrees F, about +1C.
Unfortunately the Radon Mitigation project in the lab has turned into a major mess so that nothing useful will be done in the foreseeable future. Every thing in a 2000 square foot basement must be moved at least twice so that the entire concrete floor can be sealed. The sealer produces an unplesant odor so work can only be done on days when all the house windows can be opened and fans can move air through the house. This morning it was 35 degrees F, about +1C.
Looking forward to see what you end up putting together zintolo after all the study and simulations. I wish I had your patience.
I currently have a couple SS driver stages to build that I want to try with the UNSET output stage. It will be a bit though as I have decided my 26HU5 UNSET really deserves a case worthy of moving into the living room. That along with some new speakers and finishing some other projects that have been sitting around partially assembled before I get back to playing with the other UNSET board.
Hey George. I just re-read your post #500 in this thread. Are you saying I can just stick a LND150 in place of the driver tube in the UNSET with the CCS mod and it will work? I have a few of those sitting in my parts bin.
I currently have a couple SS driver stages to build that I want to try with the UNSET output stage. It will be a bit though as I have decided my 26HU5 UNSET really deserves a case worthy of moving into the living room. That along with some new speakers and finishing some other projects that have been sitting around partially assembled before I get back to playing with the other UNSET board.
Hey George. I just re-read your post #500 in this thread. Are you saying I can just stick a LND150 in place of the driver tube in the UNSET with the CCS mod and it will work? I have a few of those sitting in my parts bin.
@Tubelab_com what is the maximum dissipated power you would suggest for the top mosfet?
I'm tempted to build the single stage FETSET with active feedback.
@spiggs I'm curious as well about those circuits, as they perform very well in simulation, and the single parts that I've tested perform very well.
I'm tempted to build the single stage FETSET with active feedback.
@spiggs I'm curious as well about those circuits, as they perform very well in simulation, and the single parts that I've tested perform very well.
George,
I am listening to the 300B TSE-II I built earlier this year. It’s delivering pure nectar right now.
I look forward with great interest to your FETSET!
In fact, I am also building a SCG preamp, as adding amps demands it it at this point.
But first, I’ll be building baffles.
I should also look for a job, but the other stuff is too much fun right now.