• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Grounded Grid Preamp Project And Possible Group PCB Buy

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#181
Re: A note of caution with the 12B4

Brian Beck said:
[B

There are at least three things that you can do to mitigate this potential problem: select for good 12B4s (they’re pretty cheap), run DC on the heater, and make sure that the heater-cathode DC voltage is small by elevating the DC heater supply if need be. [/B]
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it's better to elevate heating on some higher potential ,regarding to cathode.this is due to cathode-heater diode-like transfer characteristics,and when you elevate heting up,every possible AC modulation will transpose on horizontal part of transfer curve,and possibility of further amplification is much decreased.
say that 30 V more than cathode potential is good for start

I have somewhere some graphs regarding that,if further explanation is needed
 
#182
Agreed. Elevating the heater a bit will shut off the heater-cathode diode. But that isn’t the only effect I’m talking about here. You don't want to go too high with this voltage elevation for a tube with low insulation resistance, since then non-thermionic currents can still flow through the hot aluminum oxide heater insulator. I would just slightly elevate the supply to a 12B4, relative to the cathode, which would sit at about 17 volts above ground in the design I posted in the 12B4 thread. Where there is significant cathode signal swing (say in a cathode follower or the upper tube in a mu follower) I sometimes connect the negative side of a regulated floating DC heater supply to the cathode (via a 10K resistor to isolate capacitance). I also use split bobbin heater transformers to keep capacitance to ground low. Depending on the application, this can work well, because the heater supply always tracks the cathode, staying just positive of it.
 
#183
Brian Beck said:
Agreed. Elevating the heater a bit will shut off the heater-cathode diode. But that isn’t the only effect I’m talking about here. You don't want to go too high with this voltage elevation for a tube with low insulation resistance, since then non-thermionic currents can still flow through the hot aluminum oxide heater insulator. I would just slightly elevate the supply to a 12B4, relative to the cathode, which would sit at about 17 volts above ground in the design I posted in the 12B4 thread. Where there is significant cathode signal swing (say in a cathode follower or the upper tube in a mu follower) I sometimes connect the negative side of a regulated floating DC heater supply to the cathode (via a 10K resistor to isolate capacitance). I also use split bobbin heater transformers to keep capacitance to ground low. Depending on the application, this can work well, because the heater supply always tracks the cathode, staying just positive of it.
Click to expand...

Brian, when you connect the negative side of DC heater supply to cathode through a 10k, do you need a cap (like 20u) from this to ground, like commonly practiced when raising it via a voltage divider?
 
#184
After reading through this entire thread with building excitement, I was quite stunned to see it die! This pre seemed to meet my need of great sound with easy to obtain, and extremely low noise results. The 12B4 does not. Someone see this thing through!!!!!!
 
#186
pengboon said:


Brian, when you connect the negative side of DC heater supply to cathode through a 10k, do you need a cap (like 20u) from this to ground, like commonly practiced when raising it via a voltage divider?
Click to expand...

Pengboon, I just saw your question today (late July). Sorry, you'll have to whack me in the head to get my attention with so many threads to watch...(or PM me).

Anyway, no, you wouldn't want to do that since the large cap would cause the cathode to become loaded down with the 10K resistor because the heater side would be then be AC grounded. That would be a tough load. Just let the whole heater circuit float with the cathode signal. Since the heater is no longer connected to a voltage divider off the B+, there can be no B+ noise to worry about. Make sure that the capacitance to ground or to the primary from the heater supply secondary is very low. A split bobbin is best.
 
#187
OK Brian, will keep that in mind, I mean the cap.😀 Anyway, I have already implemented the voltage divider for the heaters and they work very well, and have very little noise, as I mentioned on my previous post. 🙂

p.s. we're going to have a little preamp friendly shootout this weekend just between some friends. I will have the 12B4 up against a Sonic Frontiers Anthem 1 (4x6922) and a Diva Classic 300 (3x6SN7) it'll be interesting how it turns out!
 
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