811 vs 811A?

[Ryssen]

The same tube or?
572B vs 572 the same?

[jrdmedford]

Howdy. The answer to your questions depends on what
you are using them for: RF or Audio. Since you are here,
I will assume audio.

For audio applications, the 811/811A/572/572B are
all interchangeable with each other ; they have a mu=160 and 6.3VAC 4A filaments. The 572 series has much higher plate
dissipation capability than the 811 series due to the use of
graphite anode over a metal anode in the latter. For RF, the
subtle differences in interelectrode capacitance and the stray
inductances that are consequences of tube construction make
the 572 not necessarily interchangeable with 811.

These tubes are underutilized by the Single Ended community.
Lots of vintage stock are available and the Russians continue
to build these to service numerous RF industrial applications.

I built an 811A Class A2 amp (parafeed) that people really really loved. It's not that hard nor expensive. Either the 811A or
572B will serve you well in audio. Check out the link below
for an excellent starter project:

http://world.std.com/~doyle/811Aiter1.jpg


-- Jim

[Ryssen]

Thank´s for the detailed answer.

[w5jag]

572B is a lot more rugged, and generally is a drop in replacement for 811A. An exception to this would be the Svetlana 572B (the real Svetlana, now SED) which is sometimes not a drop in replacement in some circuits (Yaesu FL2100B comes to mind).

It's been a long time since I have seen an 811, I think the heat dissipating fins present on the 811A are absent on the 811.

As ^^^^ said above, for audio it probably does not matter. These tubes like around 2 kilovolts on the plate; you're probably not going to come close to pushing the limits on any of them at the typical voltages seen in audio projects! Four 572B's in parallel will loaf at 1 kilowatt for decades.

The thoriated tungsten filaments in these triodes make quite the light show.

[Original Burnedfingers]

The 811 amplifier looks interesting. What is the power output?

[jrdmedford]

Power is 9W ; about as much as a well designed 300B amp.
9W is plenty and you can drive alot of speakers with it.

About the operating point. It's pure Class A2. The 811A's
grid is always >0V and its always feeding current to the 6V6's
cathode. The 811A wants to be biased around +22V with 430V
on the anode,so a 6V6 fits the operating point nicely. At idle,
the 6V6 draws about 24mA off the 811A's power grid... Since
the bias point is 22V, you need to drive it up to 44V and down
close to 0V. A 6SN7 does this nicely.

You could pull this off with many other tubes..... With less B+,
lets say 360V and 90mA, the 811A's grid will need to be biased
at a lower voltage but will draw more grid current.... An ECL82
with its pentode stage as a cathode follower would very nicely do this!

-- Jim

[Original Burnedfingers]

Thanks Jim,

This one looks like a nice one to build.