• 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.

866a Min operating voltages

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I have searched every where to find out how lo a voltage you can run 866a's at with no luck.

I would like to use a pair of 866a's in a classA2 811a amp.
yes they are overkill but they do look really good and do have some regulation. This appears to be a good thing from the general consensus around the forum. The required plate voltage for 811a's in class A2 appears to be around 450 volts.

Cheers.

:scratch:
 
I am entirely confident that you will experience no problems whatsoever below 500 volts that you also will not experience above 500 volts (up to the PRV limit). These should even work just fine at 120 volt mains voltages.

Just make sure you always allow full recommended cathode warmup time before applying plate supply. An automatic time delay would be a good idea here. Maybe a thermal TD relay tube.
 
I use the 866A in the 4 main power supplies of my 6C33CB Circlotron amp without any problems. Tubes run with 240 V. Forward drop in the tubes is between 12-15 Volts independent of the current through the tube. They work perfectly, no problems at all with rectifier noise - they are completely silent.

Uli's Circlotron

Give them sufficiently time to preheat - I give 5 minutes, also for the big triodes to heat up.

Good success!

Uli
 
Thats one good looking amp.

Whats the life span of the 866a or are you still on your first set?
And did you load them up with a choke before the filter cap. (I
read this some where it doesnt seen completely right)

My plan is to use a Small cap-Choke Large cap to filter the HT.
And how do you go with the warmup procedure.
Im planning to use a standby switch system like a guitar amp.

What do you reckon?

:scratch:
 
Yes, since all gas discharges show an exponentially increasing current after ignition, be it a mercury rectifier, a neon glow lamp or the fluorescent tubes in the office, current increase must be limited by some external means - be it by some resistances or dynamically by using a choke.

Since using 4 chokes in a stereo circlotron amp with its 4 separate plate supplies is far beyond feasibility, I choose to use two separate transformers for heater and plate/bias supply, with using one AC choke in series with the primary winding (yes, on the mains side) of the plate supply transformer. Thus adapting the current limitation principle used with every fluorescent tube in this world.....

I am using already such an AC choke in an amp I built 20 years ago, a little standard 2x EL 36 PP-amp with a Raytheon 1006 plate rectifier (and 330 uF caps just behind). This tube has run since then for more than 10000 hours and is still ok, so I choose this principle also for this big amp.

It runs now since 2 years almost every day, the tubes show so far no significant signs of ageing.

To avoid still more words, I show you a diagram of the power supply - just a simple quick-and-dirty sketch....

Here is my amp seen from above with the big toroidal choke and its magnetic shield,

and this ist the sketch of the power supply....

Since I am an amateur and since I have no means to exactly calculate such a choke, I did it by trial and error.... In my old amp I used a 150 W industrial mercury arc lamp choke as rectifier input choke, and in this big amp now I took a 250 VA toroidal core (former mains xfmr with all coils removed) and put on it a (former) 24 V and a 12 V coil - just to have some variation possibilities. Finally I came to using the 24 V coil - the entire coil certainly gives better limitation results, but voltages start to drop significantly on high load.

The small rf choke in the plate lines of the 866A is to reduce rf and ignition noise, and the 22 Ohms - 47 uF - 22 ohms - 470 uF chain allows for good smoothing with moderate input currents.
 
If the MVR is being used far below its normal capability, you can use a cap input, though reliability may suffer. It can be up to all of 4uF or so.

It is possible to run a stereo circlotron run off two supplies (a conventional stereo amp needing only 1), though channel separation may suffer. One approach for the channel problem is split supplies. ASCII doodle below illustrates:

......RC-
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......RC-
 
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