• 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.

use double triode as rectifier

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Hard work done this day. Bigger SMPS into play and everything in a case with the ECC82 on top. Now it's a wait for the 'bended pins' phenomenon to appear. Could be a matter of days or years. In time I will tell (and put another 'sucker' to the test haha gnegne) :rolleyes:.
 

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Yes. Using the control grid as part of the anode might lead to grid overheating. Fortunately the 12AU7/ECC82 is based on 6C4/EC90 which is a power triode and so has a fairly robust grid.

Yes, I'm wondering about long term consequences of grid being at 100+ volts. What could possibly happen? Anybody experienced a broken grid?

If grid is just 'broken' like in a lightbulb, I probably wouldn't notice because the grid has no purpose in this circuit. Right?

If grid is short-circuited, I could have a dead tube and will notice. Right? How big are chances the one or the other will happen? Could the transformer be damaged in some way? Or any serious effects?
 

PRR

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> grid to the cathode will work in most

Grid at cathode potential will *hold back* the electrons.

Grid at plate potential will give MUCH lower (about Mu times lower) forward resistance.

You surely want the lower resistance.

Find any small triode with Positive grid curves. 6J6 is one. Plot attached. P-G strap is 30X lower forward resistance than K-G strap.

If running more than a couple mA diode current, put a few-K Ohms between plate and grid so grid current will be only enough to pull the grid up, not melt it.
 

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It will hold back and as far as pure rectifier duty is concerned the grid tied to plate is the way to go, however, if you want a safety rectifier that will saturate instead of arcing when shorted and don't need heavy current the grid to cathode can work too. I love the idea of the grid resistor to restrict grid current. Let it go positive to suck electrons off the cathode but don't let it do all of the actual work.


12AU7 has a 600V peak negative grid pulse rating. Those are usually socket limits so it should be just fine at 100Vac.
 
Stress on the heater-cathode can be reduced by elevating the heater to say 50% of output DCV. Elevating heater using a resistor divider (decoupled) will also provide some protection if heater-cathode arcs or shorts, compared to if heater was solidly grounded (as per a CT to ground).

You can think of the grid stopper in the same manner as when pentodes are used in triode mode output stages and the screen is connected to anode (but preferably through a screen stopper).

You can also measure the rms voltage across the grid stopper, as a way of checking the grid current level. There is no design centre value for grid current, only for anode current - so there is some risk there.

You find it instructive to set up PSUD2 to cross check the peak plate current you are likely to need for a 6mA load current. Just looking at the 12AU7 curves, the saturation resistance of the plate is over 300 ohm. The peak plate current is going to be above 30mA, so there is a lot of drop.
 
Hard work done this day. Bigger SMPS into play and everything in a case with the ECC82 on top. Now it's a wait for the 'bended pins' phenomenon to appear. Could be a matter of days or years. In time I will tell (and put another 'sucker' to the test haha gnegne) :rolleyes:.

The power supply with ECC82 as double diode (grid to anode) is working for more than 100 hours without interruption and without problems now. I'm very happy with the sound and believe it is an upgrade soundwise vis-a-vis the solid state version. But if you don't buy that, it's nice to see it glowing in the dark anyway.

I'm thinking about building another version where I put octal, noval and B7 sockets on top (facilitated by 12V and/or 6V for heaters) so that I can try 12H6 (or 6H6 for that matter) on octal-socket, 6AL5 aka EAA91 on B7-socket and of course the tube where the discussion was all about ECC82. Just to play with and learn about the differences. I have found some used copies of EAA91 in my attic-garbage can. And have ordered some 12H6 nos locally for cheap.

But to be honest, where I hoped for the better by changing solid state to tube (ECC82), now I don't have any expectations 12H6 or EAA91 will be an upgrade.
 
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