812A Build

[audiowize]

What a out using the 812a as a driver ? Overkill ? But somepeople use a 300b as their dac-outpit tube as well, soooo...mu of 29...high current and voltage capabable...no ?

You'll still have to deal with the grid current requirement of the 812 (the 300B doesn't have this issue).

 

[Blitz]

Well, if you go for 600V, 25mA, should be like -8V Ug, no ? Still grid current ?

 

[Miles Prower]

Well, if you go for 600V, 25mA, should be like -8V Ug, no ? Still grid current ?

That would be absurd. 600V for less than a watt of output. The 812 needs to be taken into Class *₂ to get any meaningful output and that always means grid current. These days, dealing with it is NBD since you can use a Source Follower to get that grid current with a Lo-Z source.

Power triodes that can stay in Class *₁ don't allow for much selection. Most are series pass devices and vertical deflection types that're in the same neighborhood as the 2A3.

 

[audiowize]

600V/25mA won't draw grid current at idle, but it also won't take much voltage in without drawing grid current and you'll be biased pretty close to cutoff too.

You can certainly make an audio amp out of these, you just need to address the grid current issue and allow for the grid to swing under 0V to get max power at most operating points.

 

[multi]

Unless you have Very insensitive speakers both the 300B and 812 would be too micro-phonic to use as a driver; The EL36 or 5881 driven on the screen make very good drivers for 845's and similar, very difficult to drive Tubes

 

[Blitz]

600V/25mA won't draw grid current at idle, but it also won't take much voltage in without drawing grid current and you'll be biased pretty close to cutoff too.

You can certainly make an audio amp out of these, you just need to address the grid current issue and allow for the grid to swing under 0V to get max power at most operating points.

Well, I have Rohrentester v9 here which is capable to measure g1-grid current.

I did just that: 600V Anode-Voltage, 45mA, Ug1=0V...grid-current: 0,0992 µA !!!

If this is a value we can trust this would mean that the 812A actually can be used nicely up to 0V on the grid...

At -5V /28mA@600V it has approx 7k8 rp...which is a bit better tah a 6sn7, but with a mu of 30 instead of 20.

If transformer-coupled as a driver tube, it could swing the 200V necessary for a 300B with 5Vp/10Vpp input signal.

This is not bad. Obviously it is not a very economical choice regarding power consumption, but you get these guys for 50$ nowadays, sometime less in RCA NOS quality...a TT-Triode with a gain of 30 and a rp of 7.8K...not bad actually, no ?