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Friday, March 20, 2009

D3a Experimenter's Printed Circuit Board

This project is a circuit board which, through various jumpers, can be configured for various circuits. Specifically, it is designed around the D3a vacuum tube, a high Gm, high mu pentode. Additionally, there are several other tubes such as the E180F and E810F that share the pin arrangement with the D3a allowing their utilization as well. 



It is assumed that people reading this and using this circuit board are experienced tube builders. Thus, this page will dispense with specific building advice. Below is a list of parts necessary to populate the board. There are a few suggestions for specific part values, particularly when there is really only one part that makes sense. But, in general, it is left to the builder to determine what parts to use. 

This project deals with high voltages that can and will kill you. Do not attempt to use this information unless you are qualified to work with high voltages. I am not responsible for your lack of knowledge or mistakes. Nor am I responsible for your utilization of my incorrect or misleading information. This is the internet, not an engineering text book.


Finally, while most tube circuits are potentially dangerous due to high voltages, due to the possibility of pins sticking up from this board some of which can be at dangerously high voltages even when not in use, and due to the hands-on nature of the project, please exercise extreme caution in using this board. Do not make changes or adjustments to the board while a power supply is connected. Moreover, even though the power supply has been removed, double check that there are no high voltages remaining on the board before touching it.  

For a generalized list of safety suggestions for working around high voltages, please see this thread.
 
If you have some of these boards and do not feel comfortable using them, please contact me in order to arrange to return them. 

Board Connections

This circuit board contains six external connections.
  • B+
    This is for connecting the main high voltage power supply.
  • G
    This is for connecting to the main power supply's ground or earth terminal. It is recommended that this terminal be connected to an external "star" ground point, and that all other ground connection be made to this external point.
  • C+
    This is for connecting a seperate screen grid supply. Note that the screen grid can also be fed in various ways via the B+ supply (see below).
  • IN
    This is the input for supplying an AC signal to the control grid. The signal's ground connection should be connected to G
  • OUT
    This is the output signal. This is always at a high voltage as the output is taken from the tube's anode. Use a capacitor or some other means to block this voltage. Output ground should be connected to the G terminal.
  • B-
    This is for connecting a negative bias supply for the adjustable constant current based "fixed bias" setting.

Parts

Parts values will vary depending on tube, operating point, and circuit. However, here are some suggestions for getting started. All resistors, other than the plate loads and the SMD stoppers, are on 12mm grids.
  • Pins and Jumpers
    From Digikey, I picked up S1011E-36-ND for the pins and A26242-ND for the jumpers. There are lots of alternatives, and I do not see any voltage ratings on these jumpers, so you might choose something else. These, however, have little handles on them making them easy to move, and they are pretty cheap.
  • RA_P and RA_T
    These are the resistor loads for Pentode and Triode, respectively. I probably could have made them a little larger ... It appears that in most instances you will want 5W parts here, and the only 5W resistors I can find that seem to fit are the Mills wirewound available at Soniccraft, Percy, PartsConnexion, etc. Other options are paralleling resistors, or some creative lead bending.
  • RSTOP, RSA, RSB, R8
    These are all stopper resistors.
  • R1, R2
    These are in series and set the current from the plate load CCS. I typicall put a 10R in for R1 for safety and a 200R trimmer in R2 for setting current.
  • RGL, RBA
    RGL is the grid leak and connects to ground. RBA is the grid leak when using battery biasing of the grid and connects between the battery and the grid.
  • R5
    This part connects the screen grid to B+ for triode operation.
  • R6
    This is a current limiter for the screen grid for pentode operation.
  • R7, R8
    R8 sets the current for using CCS based fixed bias of the grid. R7 adjusts the bias.
  • R3
    This is the cathode bias resistor.
  • R4, R10
    These are 0805 sized plate and cathode stoppers.
  • C1
    Cathode bypass. 7.5mm pins, 16mm diameter. 1000u Silmic from Digikey should fit.
  • C2
    This is the input capacitor. There are spots for a 22.5mm box type cap, or up to 30mm axial. Ignore the "+" sign -- use a non-polar.
  • C3
    This is the screen grid supressor. At some point I did a calculation and figured I wanted at least 3.3u here (larger than for most pentodes). A Sonicap Gen I 400V from SonicCraft should fit and are reasonably priced. Solens and whatnot will fit, too. 35mm grid.
  • ICA, ICB, ICB1
    These are all either 10M45's or DN2540 depletion mode mosfets.
  • LED
    Standard 3mm LED
  • Z1-3
    DO-41 Zeners. Jumper the spots if you use less than 3.

CCS

The main CCS is made from two depletion mode mosfets.

This CCS can be used as a plate load, or as a supply for the Screen grid acting in concert with the Zener diodes to form a shunt regulator. IXYS 10M45's or DN2540's in the TO-220 package will both work fine for the ICs. RSA and RSB are both stoppers and should be about 1K, and the combination of R1 and R2 set the current. It is a good idea to use something small for R1 (~10 ohms) and use R2 to adjust the current. For speciics, consult the relevant datasheets.
The grid bias CCS uses a single 10M45 for ICB1 (I have no idea why it is called that.) R8 is a stopper and should be 1K. R9 sets the current. Again, consult the datasheet for appropriate values.

Bias Options
  • Connect IA to IB
  • Connect K to KA
  • Connect BY to BYA
  • Connect G to GA
  • Connect IA to IB
  • Connect G to GA
  • Connect K to KB
  • Connect IA to IB
  • Connect K to KA
  • Connect G to GA
  • Connect G to GB
  • Connect the negative terminal of a battery to BA-
  • Connect the positive terminal of a battery to BA+
  • Connect K to KC
  • (note: be sure IA is not connected to IB)
  • Connect IA to IB
  • Connect G to GA
  • Connect the positive terminal of a battery to K
  • Connect the negative terminal of a battery to KC
  • (note: battery must be rechargeable)
  • Connect K to KC
  • Connect G to GC
  • Connect a negative voltage supply to B- and adjust R7 for appropriate grid bias
Triode Options
  • Connect PC to P
  • Connect G2A to G2 (G2 is the center pin of the cluster)
  • Connect B1 to B1A
  • Connect O to OA
  • Connect PC to P
  • Connect G2A to G2 (G2 is the center pin of the cluster)
  • Connect B1 to B1A
  • Connect O to OB
  • Connect B2 to B2A
  • Connect P to PA
  • Connect G2A to G2 (G2 is the center pin of the cluster)
  • Connect O to OA
  • Connect the choke or transformer directly to P
  • Connect power to B+
  • Either connect power to the other side of the choke/transformer, or connect the other side of the choke/transformer to B1
  • Connect G2A to G2 (G2 is the center pin of the cluster)
  • For choke loads, connect O to OA
Screen Grid Bias Options (Pentode)
  • Connect G2A to G2 (G2 is the center pin of the cluster)
  • Connect Z to ZA
  • Connect B1A to B1
  • Connect G2D to G2 (G2 is the center pin of the cluster) Adjust the current from the CCS for Screen grid rather than plate levels.
  • Connect Z to ZA
  • Connect Z to ZA
  • Connect G2C to G2 (G2 is the center pin of the cluster)
  • Connect a power supply to C+. The C+ power supply should be current limited in some way external to the board.
  • Connect G2B to G2 (G2 is the center pin of the cluster)
  • Connect G2C to G2 (G2 is the center pin of the cluster)
  • Connect a power supply to C+. The C+ power supply should be regulated or current limited in some way external to the board.
Screen Grid Bypass Options (Pentode)
  • Connect CS to CSA
  • Connect C to CA
  • Connect CS to CSA
  • Connect C to CB
Suppressor Grid Options
  • Drat -- looks like a bug. SGA connects to B+ rather than the plate like it should. To work around this, either run an insulated jumper wire from SG to G2A (R5 will be in series with the connection to the plate), or cut the trace to SGA with a XActo and solder a wire from SGA to either the bottom of P or OA. You can also solder the wire from SGA to pin 7 of the tube, but this will eliminate stopper R4 which may or may not matter. Finally, when you solder in R5, leave the lead that is close to G2A slightly long and connect a short wire from SGA to the R5 lead.
  • Connect SG to SGC
  • Connect SG to SGB

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