Tube DIY Asylum: BDT Preamp Schematics by John Swenson

Here are the schematics for the BDT preamp.

There is one tube per channel, the 6AR8 Beam Deflection Tube. It works using electric field deflection of an electron beam, similar to the CRT in a scope. The beam gets pushed back and forth between two plates, forming a balanced output.

The outputs go through a custom transformer from Dave Slagle designed for this circuit. The output can be wired up in several ways. The COMs can be wired together and connected to ground, or left separate. You can use one output for SE or both for balanced.

If you look at the spec sheet for the tube the plate curves look perfectly horizontal, this is true with respect to the GRID, but NOT for the deflectors. In order to get horizontal plate curves for the deflectors the plates need to be cascoded. This could be done with triodes or pentodes, but I'm lazy and did it with MOSFETs. Previous experiments showed that this should wok very well. It did. The bias for the cascode is provided from a LND150 cascode CCS across resistors. (as in the Gary Pimm SBCCS) The bias voltage can drive many cascodes so you only need one for as many channels as you have.

The accelerators are driven with 250V and the transformer primary (B+) gets 130V. The B+ can be anywhere from 24mA per tube to less than 1mA dependant on the volume control setting.

The HV supplies are simple C filters driving a regulator. The regulator is based on the infamous Gary Pimm SBCCS circuit I like so much. Its exactly the same except for one extra LND150 in the reference circuit which works as an error amplifier. I've tried many different regulator circuit including shunt types and this was the best sounding. I've tried this in several different designs and it has worked very well. It can easily be scaled up to handle high currents so you can even use it for power amps.

There are two supplies, one for 130V and one for 250V. They are the same except for R3, see the schematic for the two values. The exact voltages are not critical, if they are off a little it will still work fine. The pots help take care of the manufacturing tolerances in the MOSFETS.

The heaters take 6.3V DC (it hums too much with AC), I just used a LM340 type regulator for this. I'll leave that as an exercise for the reader. Pin 5 of the tube MUST be grounded. It connects to a number of internal structures in the tube that have to grounded, as well as the heater. (I think they ran out of pins in the 9 pin base)

I used the Antec AN-1T250 toroidal transformer, with the primaries configured for 240V but actually running at 120v. I stacked the HV secondaries, the bottom driving the 130V supply and both driving the 250V.

I built my very first quick and dirty version of this design many years ago with a 9V battery for the volume control and have never changed it since. So this version is STILL run from a 9V battery. I should run it from a supply but I haven't bothered yet. The battery works great, I have to change it every couple months, but that hasn't been a problem yet.

Currently I'm playing around with different circuits for implementing a balance control. Since the volume is DC controlled adding balance does not harm anything. I haven't yet come up with one I like yet so I didn't put it in the schematic.

The BDT is very sensitive to magnetic fields, hence the rather extensive efforts at keeping down fields from the power supply (which is why there are no chokes) and careful shielding. Its not an insurmountable problem, but it has to be taken into account.

One more thing, because its DC volume control, its easy to build a remote volume. A pot on the end of a wire works for a wired remote. I'm working on a remote that has volume on both the front panel and remote. It uses rotary encoders that send pulses which get counted by a FPGA which drive a DAC which generates the control voltages. There is no free running clock, the pulses from the encoders do all the counting etc, when the knobs are not being turned all the signals are DC. I haven't decided if I'm going to display the volume with Nixie tubes or a magic eye tube. Maybe Nixies for the volume and magic eye for the balance?

John S.

BDT Preamp Schematics (Open in New Window)