# HV Pulse amplifier help 6BK4

#### Qmavam11

If this is the wrong place, tell where I should post.
I'm building a HV pulse circuit for a specialized electrophoresis.
High Voltage, Low current, under 1ma.
I plan on using a 6BK4.
Here are the tube specs,
https://tubedata.tubes.se/sheets/127/6/6BK4.pdf

I have been given a schematic that I'm sure is a little unconventional,
but have respect for the designer, although he said, "it may need tweaking".
The design ties the grid at +6v and then pulls the cathode to 0V, to pinch it off.
I love the simplicity of the circuit, but will it work. Note: expecting to pulse 7kV or more!
I'll post the design and I asking for input about whether it is a viable design,
or what would you change. See attached file or dropbox picture.
https://www.dropbox.com/s/cxv0osk0z3yc7fo/Oil Water, Pulser Schematic.jpg?dl=0

Thanks, Qmavam11

#### Attachments

• Oil Water John's Pulser Schematic.jpg
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#### Qmavam11

Is there a 6BK4 spice model?

#### Potentiallyincorrect

At the least, you will need a capacitor to couple the output. 0, -7kV, 0 pulse.

#### Potentiallyincorrect

Know a little about electrophoresis. I suspect that the voltage pulse trailing edge will need to be nearly as fast as the leading edge. Might need active rather than resistor pull-up.

#### Koonw

Try this 6BK4 Sylvania model:

Code:
``````**** 6BK4 ** Advanced Grid Current **********************************
* Created on 05/15/2023 22:11 using paint_kit.jar 3.1
* www.dmitrynizh.com/tubeparams_image.htm
* Plate Curves image file: 6bk4.png
* Data source link: Sylvania 6bk4c
*----------------------------------------------------------------------------------
.SUBCKT 6BK4 1 2 3 ; Plate Grid Cathode
+ PARAMS: CCG=2.6P  CGP=1P CCP=1P
+ MU=2103.1 KG1=16349.49 KP=40249.51 KVB=2504.16 VCT=0.95 EX=1.4
+ VGOFF=-0.6 IGA=3.25E-4 IGB=0.18 IGC=5.76 IGEX=1.46
* Vp_MAX=25000 Ip_MAX=2.5 Vg_step=2 Vg_start=0 Vg_count=7
* Rp=4000 Vg_ac=55 P_max=40 Vg_qui=-48 Vp_qui=300
* X_MIN=104 Y_MIN=62 X_SIZE=560 Y_SIZE=446 FSZ_X=1260 FSZ_Y=727 XYGrid=true
* showLoadLine=n showIp=y isDHT=n isPP=n isAsymPP=n showDissipLimit=y
* showIg1=y gridLevel2=y isInputSnapped=n
* XYProjections=n harmonicPlot=n dissipPlot=n
*----------------------------------------------------------------------------------
E1 7 0 VALUE={V(1,3)/KP*LOG(1+EXP(KP*(1/MU+(VCT+V(2,3))/SQRT(KVB+V(1,3)*V(1,3)))))}
RE1 7 0 1G  ; TO AVOID FLOATING NODES
G1 1 3 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))/KG1}
RCP 1 3 1G   ; TO AVOID FLOATING NODES
C1 2 3 {CCG} ; CATHODE-GRID
C2 2 1 {CGP} ; GRID=PLATE
C3 1 3 {CCP} ; CATHODE-PLATE
RE2 2 0 1G
EGC 8 0 VALUE={V(2,3)-VGOFF} ; POSITIVE GRID THRESHOLD
GG 2 3 VALUE={(IGA+IGB/(IGC+V(1,3)))*(MU/KG1)*(PWR(V(8),IGEX)+PWRS(V(8),IGEX))}
.ENDS
*\$``````

#### Attachments

• 6BK4 paint.png
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• 6BK4 transfer plot sch.png
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• 6BK4 transfer comp.png
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• 6BK4 transfer.png
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#### Qmavam11

Thanks, for that Koonw,
I haven't used LTspice for many years, I kinda gave it up because I was lacking knowledge on what I was doing.
This is the circuit I'm looking at.
If anyone can take the time to model this, you would have my undying gratitude!
Thanks, Mikek

#### Qmavam11

At the least, you will need a capacitor to couple the output. 0, -7kV, 0 pulse.
I'm not sure why I would need a cap to couple to the vessel. I want a DC pulse on the vessel, also the vessel is a small has a small capacitance, 18pf to possibly somewhat larger, (still experimenting)
Know a little about electrophoresis. I suspect that the voltage pulse trailing edge will need to be nearly as fast as the leading edge. Might need active rather than resistor pull-up.
I also know little about electrphoresis! I don't know if fast edges are even important, or they might be of extreme importance. On the other hand there is work being done with fullwave rectified 60Hz DC, i.e. no filter capacitor. It is all a series of experiments to learn.
Thanks, Mikek

#### Koonw

6BK4 pulse amplifier simulations, cap vs direct coupled.

#### Attachments

• Pulse Amplifier Sim.png
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#### Qmavam11

I was told you need to add the heater. Just passing it along, I don't know.

#### Koonw

I think he probally means the cathode to heater voltage. According to specs, it's 400V -ve and not recommended +ve. When output of about 4Kv, cathode has about 200V pulse which is still within heater to cathode voltage as heater is grounded. If not certain alway keep the heater floating, grounded via a cap if hums are detected. The cathode can be shunt with suitable resistor for saftey reason and to reduce heater to cathode voltage.

#### Attachments

• Pulse Amplifier Sim 2.png
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#### Qmavam11

Thank you Koonw,
Our load is a very low capacitance 18pf to maybe 100pf, over 200MΩ parallel resistance, the limit of our measurement equipment. Actually thru a glass tube wall. (still getting data on the
I don't think we want C4, as we want a DC pulse.
I'm surmising that RK is there to make LTspice work or are you suggesting that be installed?
It has been suggested to put a15V zener across the Fet source and drain, to protect it.
Thank you, Mikek

#### Qmavam11

Sorry, (still getting data on how our load may vary with different samples)

#### Koonw

Loading output with 25p cap turns the waveform into a triangular one, 200Meg has little effect on the waveform. The purpose of Rk is the same as Zener 15V, while the formal clamps the voltage across source and drain to 17V and Zener to 15V.

#### Attachments

• Pulse Amplifier Sim 3.png
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• Pulse Amplifier Sim 3-2.png
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• Pulse Amplifier Sim 3-3.png
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Last edited:

#### Qmavam11

In sim 3-3, I can live with the green waveform, but I don't know what the represents.
What parameters does that have?

Does that green waveform rise more slowly with a 10MΩ R3?
At this point, we don't know how rise time affects our experiment.
We are moving water bubbles in an oil/water emulsion, it takes time to get them moving.
It may turn out that a fast rise time is just wasted, because it's over before the bubble starts moving, yet to be known!
Thanks

#### Qmavam11

BTW, the frequency can be slowed way, way down. We are thinking 3Hz to 50Hz.
Different solutions may require different frequency and/or duty cycle.
Rise and fall times will probably be much harder to see at the low frequencies. :-(

Thanks, Mikek

#### Koonw

The green one is when the load is 1p, is in there for comparision of how the waveform changed with caps value, all other params stay the same. Zener 15V will clamp the output to some maximium value when the level not long raised, but resistor does not.

#### Koonw

Because this is direct coupled, so 3-50Hz is not a problem. It may require 2 tubes to share the top load and stay in safe operating points.

#### Attachments

• Pulse Amplifier 3 Hz Sim 3-5.png
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• Pulse Amplifier 50 Hz Sim 3-4.png
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#### Qmavam11

Hi Koonw,
I sent, what I thought was a PM, haven't got a response, so don't know where it went.
Any chance you can send me the latest Ltspice file?
Thanks, Mikek
nodddjunk@Knology.net remove the ddd

#### Koonw

I just responded to your PM.