Beam Deflection Tube Limiter

[Michael Koster]

Some time back there was a post about using a beam deflection tube as a gain element, and it reminded me of a project I've been contemplating. I'm thinking about a compressor/limiter based on a BDT.

I looked at the characteristics of a few BDTs, and the 6ME8 stood out as allowing a pretty large linear deflection voltage swing, which is desirable in a limiter input circuit. A lot of limiters use a step-down input in order to handle large signal input.

I could have audio signals coming in at +30 dbu or more, which is about 70 volts peak-to-peak. this is of course the whole reason for having a limiter, other than they're sometimes abused in the recording process to make things sound LOUDER...

Looking at the attached curves, the deflection voltage could swing 40 volts each way, in a diffamp giving > +30 dbu, before going into the nonlinear part of the transfer function.

Looking at the ratio of deflection voltage to plate current differential, using the deflection electrodes as a differential signal grid gives a Gm of approximately 175 to 35 uMhos over the range of -6 to -9V on grid1.

Grid 1 will be used to control the gain over about a 12-14db ratio.

It turns out that the 6ME8 is basically a pentode with respect to grid 1, so that made me wonder how to structure the anode circuit...

 

[Michael Koster]

The 6EM8 is a beam deflection tube, therefore the control grid G1 sets the anode current and the deflection "plates" steer the beam to one anode or the other, the total current of the 2 anodes remaining constant.

With respect to the G1, it the tube has a pentode characteristic (see attachment) and in the chosen operating range is quite linear over anode voltage. This implies that as the anode voltages change wrt one another as signal is output, the total current will remain constant. I can use that knowledge later...

It looks to me like a purely resistive load would give a fairly linear output, especially if the topology is kept balanced differential throughout.

One problem this limiter will have in common with so-called "variable mu" limiters is thet as the control voltage increases there is a common mode output current step that can cause an annoying audible "thump" when the limiter is used at fast attack times.

In this circuit the common mode current over the range of CV is about 6 mA, and the signal current is about 7 mA, so I conclude a simple resistor load on each anode is going to be trouble. I think I'll start with a common mode choke like a LL1667, with a resistor load chosen to give a 1:1 signal ratio from deflection plates to anodes when the DC anode current is about 8 mA per anode.

Look, the sidechain amplifier only needs to swing 3V into the control grid of a pentode! Is this too good to be true?

 

[smoking-amp]

Ah Ha! I think I see the Differential Gyrator coming over the horizen here.

One item of caution on these tubes to keep in mind is that they are quite sensitive to magnetic fields. I found that demagnetizing them out of the box was helpful in getting symmetrical curves. They seem to use magnetic parts inside. The radio Hams used magnetic shields around them too for their SSB converters to keep out hum.
Also, another effect on performance was the common mode voltage on the deflectors, some tube types seem more sensitive to this effect. I haven't really used the 6ME8 much, so I don't know its peculiarities.

Don

 

[Michael Koster]

So, here's a simplified schematic. The input is transformer coupled with a signal balance network to null out the f2 distortion due to unequal signal swing through the signal path top vs bottom.

Then cap coupled to a DC balancing network to set the quiescent current balance between the 2 anodes by tweaking the deflection voltages.

The gain control voltage from the sidechain drives the control grid of the 6ME8 through a cathode current servo. Remember that the signal doesn't change the cathode current, just steers the beam back and forth, so this trick should linearize the gain response to a 2-8V control voltage input.

Back to the signal path, the BDT anodes are loaded by a common mode choke that should provide the first rejection of the control voltage induced common mode thump signal while passing the audio at a level determined bt the BDT current swing and the load resistance.

This couples to the output stage which can provide controllable gain through the "plate feedback". The load resistor also has an adjustable center tap so that the CV thump can be balanced out and rejected by the diffamp output stage. The cathodes of the output stage are AC coupled but DC separate, allowing perfect DC balance and AC current sharing.

The output stage gain should be adjustable between 1x and 3x (0-10db) which is in the loop and will set the compressor's small-signal gain.

The output signal is taken back to the sidechain amp where it is rectified and processed for threshold, attack, release settings and applied to the 6ME8 current servo.

All of the balance adjustments make it look complicated, but I think they'll be necessary for good performance. One characteristic of this limiter may be an increase in f2 distortion as the Gain Reduction (GR) kicks in ;-)

Cheers,

Michael

 

[Michael Koster]

Ah Ha! I think I see the Differential Gyrator coming over the horizen here.

One item of caution on these tubes to keep in mind is that they are quite sensitive to magnetic fields. I found that demagnetizing them out of the box was helpful in getting symmetrical curves. They seem to use magnetic parts inside. The radio Hams used magnetic shields around them too for their SSB converters to keep out hum.
Also, another effect on performance was the common mode voltage on the deflectors, some tube types seem more sensitive to this effect. I haven't really used the 6ME8 much, so I don't know its peculiarities.

Don

Hi Don,

A good differential gyrator could reject most of the thump.

magnetic sucsceptibility:
I can see it now, fiddling with the purity rings around the outside of the tube to get it sounding just right... Mumetal tube shields used to be made for these things.

Will it be hi-fi?

Michael

 

[smoking-amp]

Hi Michael,

Looking at the 6ME8 pentode curves, I see that plate current begins to drop off below 175 V. Not sure if the output will be dipping that low or not, but it will likely round off the nice linear modulation curves if it does. So might need a little more B+ on the LL1667. Also, I think there is a set of caps missing in the diagram between the LL1667 and the ECC99 grids.

Is this unit intended for recording chain use or playback chain use?

Another tube that might be of some interest (beside the other beam deflectors) is the 6LE8, which was also intended for color TV demodulation. It doesn't have the large +/- 40 V swing of the 6ME8 available on the differential g3 grids however. It's data sheet curves are set up differently from the 6ME8, but it does appear to be linear versus the g3 diffl. "deflection" for a certain range of load resistance. (around 13K) (a very dense g2 actually absorbs the g3 bounced electrons)

I use the 6LE8 with +12 V on g3 to make it into a nice pentode. But I think there may be some other "mods" that can be done with this interesting tube. It might be possible to lower g2 volts and then use g3 in differential +A2 mode for example. A more remote possibility might be to operate with the normal plates as "deflectors" and the g3's as plates.

Don

 

[Michael Koster]

Hi Don,

Thanks for the review and feedback!

Linear operating range:

Post #2 shows the proposed operating range of plate voltage vs current over G1 voltage in the red-line part of the curves. There is a little skew not shown due to voltage drop across the 1K2 DCR of the LL1667. From 2mA to 8mA there is 7.2V drop. I tried to stick to the flat part of the pentode curves so the output signal voltage of the variable gain stage would be entirely defined by the a-a load resistance and the current swing, which changes with G1, or more precisely in this circuit, cathode current.

Direct coupling:

I intend to direct couple the 6ME8 anodes to the ECC99 grids through the plate feedback network. Do you think the DC balance of the 6ME8 will be too unstable? I think for best operation, the circuit needs stable DC balance between the 6ME8 anodes anyway. The AC coupled CCS in the ECC99 tail should deal with a moderate amount of DC grid-grid drift. There is plenty of tail voltage as there is only 140V or so a-k across the ECC99 depending on the DCR of the OPT. Since I needed the +350 for the G3 acceleration voltage anyway I thought just build an unreg +400 or so and regulate the other +HV voltages down from that.

Target application:

This is intended to be a transparent "light-touch" compressor for recording chain use that one would go to for vocals, drum overheads, etc. that don't need to be "crushed". This is not a crusher by any means with only 12-14db gain reduction, maybe 18db if really stretched.

If it doesn't introduce too much distortion at low GR and in the +4 dbu signal range it might also be a good mastering compressor.

I do want a sidechain with attack, threshold, ratio, and release control. I think this has fast attack potential if the balance is stable enough to get a good dependable "CV null".

The ability to handle +30 db across the tube input terminals directly is a very useful property for these applications.

The 6LE8 is another interesting tube. I wonder if it splits the current between the two anodes the same way. I'll need to get the data sheet as no curves in the RC-30.

Michael

 

[smoking-amp]

6LE8 datasheet:
http://tubes.mkdw.net/sheets/135/6/6LE8.pdf

It's just a conventional mixer type tube essentially. But uses a split g3 grid(s) (one on each side of the cathode). The g3 grids are finely meshed wire to get good Gm. Instead of deflecting electrons from plate to plate, it returns them back to the g2. But ends up with a similar response to the beam deflectors (just wastes half the original cathode current to g2 when driven differentially).

The curves on the data sheet look different from the beam deflectors since they are not given at constant plate voltage, rather they are given like normal tubes. But what is important on these curves is the constant spacing of the curves when R loaded at about 13K (roughly a 45 degree load line on the chart at right angles to the curves) This makes them easy to use. Probably why the color TV manufacturers wanted such a tube.

Not shown on the curves is operation at + g3 voltages. At about +12V g3 the curves square up to very nice pentode curves. The finely meshed g3 and g2 make for quite high plate resistance.

The 7360 beam deflector tube is sorta half way between the 6LE8 and the other beam deflectors, with the plates off to one side from the cathode but with two grids between them to deflect. Current passes thru the grids there just like with the 6LE8, but instead of returning to g2, it crosses to the other plate.

I got a bunch of the 6LE8's when Antique Elec. had them on sale years ago at $0.50, so are throw away cheap to experiment with. Ebay has them for a couple $ usually. Nobody else wants them for anything.

Can at least use them for ganged volume controls if nothing else. They are less sensitive to magnetic fields too. I've used them with the screen feedback scheme (Mosfet follower off a CCS loaded plate and zener drop to the screen) and they are quite linear versus g1 drive. Just a tiny bit of 2nd harmonic, which can be adjusted with some plate loading R.

Don