Higher B+ is what the tube needs.
If you plan a mains transformer-based power supply, check out PS6 on tubecad.com. It comes with voltage quintupler, of course better ripple filter is needed.. or if you have already a working 12v PSU, get a boost converter on Ebay. One can give you 100v from 12v input. This would give more breathing space for the tube.
If you plan a mains transformer-based power supply, check out PS6 on tubecad.com. It comes with voltage quintupler, of course better ripple filter is needed.. or if you have already a working 12v PSU, get a boost converter on Ebay. One can give you 100v from 12v input. This would give more breathing space for the tube.
Forgive me for my ignorance. I am a newb. If the B+ voltage is increased, would any of the values have to change on any of the components other than the voltage ratings of the caps? How about the MOSFET?
120v would be better than 12v.
Yes, unfortunately some changes would be needed. Depending on how high you increase the B+, changes such as these might be in order:
1. Higher rating CCS (LM317 no longer applicable)
2. Higher rating output MOSFET or AC coupling
3. etc
Here's a bit of a thought process of how high the B+ should be if you want to keep most components as shown: LM317 is rated for maximum 40v difference between IN and OUT pin. Let's take a safe number of say 30 volts (a safety margin, as you would need to swing some voltage). Since OUT is grounded, that means IN should be at most 30v and the same voltage applies for MOSFET's source. The MOSFET's gate would then be around 34v and this is also the plate voltage for the 832A tube if you want to keep it DC-coupled (no capacitor between plate and MOSFET gate). Let's say 34v is half-way between the actual B+ and GND. So, B+ would be around 70v. With B+ this high, perhaps you can get the tube to idle at around 2mA. So, from here..
1. R5, R6 should changed to (70-34)/2mA = 18k
2. R1 replaced with 1k trimpot, adjust until plate voltage is around 34v.
3. Add some 12v zener across Gate and Stopper so that you don't exceed maximum allowed Vgs during tube warm-up.
Some other recommended changes:
3. R4 and R7 should be something around 220k - 1Meg
4. Add Grid Stopper resistor, something around 1-10k
5. Your output devices (MOSFET + LM317) is dissipating something around 70v/125mA = 8.75W per channel. This requires a rather large heatsink. If your chassis is aluminum/metal, perhaps you can use it as heatsink.
Yes, unfortunately some changes would be needed. Depending on how high you increase the B+, changes such as these might be in order:
1. Higher rating CCS (LM317 no longer applicable)
2. Higher rating output MOSFET or AC coupling
3. etc
Here's a bit of a thought process of how high the B+ should be if you want to keep most components as shown: LM317 is rated for maximum 40v difference between IN and OUT pin. Let's take a safe number of say 30 volts (a safety margin, as you would need to swing some voltage). Since OUT is grounded, that means IN should be at most 30v and the same voltage applies for MOSFET's source. The MOSFET's gate would then be around 34v and this is also the plate voltage for the 832A tube if you want to keep it DC-coupled (no capacitor between plate and MOSFET gate). Let's say 34v is half-way between the actual B+ and GND. So, B+ would be around 70v. With B+ this high, perhaps you can get the tube to idle at around 2mA. So, from here..
1. R5, R6 should changed to (70-34)/2mA = 18k
2. R1 replaced with 1k trimpot, adjust until plate voltage is around 34v.
3. Add some 12v zener across Gate and Stopper so that you don't exceed maximum allowed Vgs during tube warm-up.
Some other recommended changes:
3. R4 and R7 should be something around 220k - 1Meg
4. Add Grid Stopper resistor, something around 1-10k
5. Your output devices (MOSFET + LM317) is dissipating something around 70v/125mA = 8.75W per channel. This requires a rather large heatsink. If your chassis is aluminum/metal, perhaps you can use it as heatsink.
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Hi! Sorry for the late return, I got the circuit working on PCB, sounds really 'velvety' but it really shows the downsides of the design, I got a cool PCB made, but it's not working out of the box. Back on the project shelf for now...
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