The amplifier design below deals with high voltages — LETHAL voltages. If you are not comfortable or qualified to deal with these potentially deadly voltages, please do not attempt to build this circuit. Proceed at your own risk, expense, and responsibility.
This driver board is optimized for a 300B tube. However, with a few minor tweaks, it can be used to drive many other tubes as well. 2A3 and 45 immediately spring to mind. With a separate B+ supply for the output tube, 801A and 211 in class A1 could be driven by this board as well.
This 4.0×6.4 inch (10×16 cm) circuit board has been optimized for signal integrity. The purpose of this is to ensure that every amplifier built on this board will exhibit the least amount of hum and be the least susceptible to electromagnetic interference as possible. The board is accompanied by 39 pages of design documentation, including the full schematics, bill-of-materials, and assembly and adjustment guide.
The circuit topology for the DG300B Driver Circuit is shown below.
The input transformer provides conversion from the differential input to the single ended topology of the rest of the amplifier. The differential input provides many advantages. Most prominently, the differential input moves any ground loop on the input side of the amplifier outside the signal path. The result is a dramatic reduction in mains hum and a great improvement in sound quality. That said, some may wish to save the expense of the input transformers, in which case the circuit has provisions for capacitive coupling as well.
A common cathode stage provides the voltage gain of the driver circuit. This stage uses a Constant Current Source (CCS) anode load and LED biasing (zener biasing optional), as this provides the lowest THD and maximum gain possible in this type of gain stage. The common cathode stage has relatively high output impedance, hence, isn’t suitable for driving a 300B tube directly. To provide a good, low-impedance drive for the 300B, a cathode follower is used. In addition to providing a low drive impedance, the cathode follower ensures that the coupling capacitor to the input stage remains fully charged at all times. This eliminates blocking distortion (what guitar players refer to as “farting out”). In addition, the combination of a grounded cathode stage and cathode follower stage has the added benefit of providing distortion cancellation. This is because the two stages create similar distortion components, but those of the grounded cathode stage are in opposite phase to those of the cathode follower stage. It seems counter-intuitive, but slightly increasing the THD of the cathode follower actually reduces the THD of the two stages combined. The circuit has been optimized for the lowest total THD possible with the 6N6P tube. The end result is a sonically transparent driver circuit that really lets the 300B shine.
Driver Tube Options
The circuit is designed for use with four different driver tubes; Russian 6N6P, JJ ECC99, 12BH7A, and the ECC88-family including E88CC, ECC88, PCC88, 6922, 6DJ8, and 6N23P. With some creative wiring, a quartet of d3A configured as triodes can be used as well. However, do note that changing from one tube to the other does require a few component changes and wire link options to be moved, as the tubes are biased slightly differently and have different pinouts for the heaters.
Output Tube Options
The DG300B has been optimized for use with the 300B output tube. This driver circuit is equally well suited to drive the 2A3 or 45 tubes. All that is required is a couple of resistor changes. These tubes provide lower output power and would be well suited with high-efficiency full-range drivers, headphones, etc.
I chose the JJ brand of 300B tubes as they are in current production and available through AES (https://www.tubesandmore.com/search/node/300b) at reasonable prices. They have proven to be very rugged tubes. Their bias points seem to settle within a couple of hours of use and at least mine have remained stable through years of daily use.
There are many options for output transformers. Select output transformers that meet the following specifications.
|Output Power||Min. 15 W|
|Primary Voltage Swing (not commonly specified)||Min. 270 V RMS|
|Primary Impedance||5 kΩ|
|Primary Current||100 mA|
|Secondary Impedance||Match the impedance of your speakers|
If you’re price conscious, Edcor (www.edcorusa.com) CXSE25-4-5K (5 kΩ : 4 Ω) or CXSE25-8-5K (5 kΩ : 8 Ω) is probably your best option. The James (https://jianshin.myweb.hinet.net) JS-6113HS looks like it would be a good fit as well. Both the Edcor and the James transformer will set you back about $80/each.
One Electron makes a nicely specified and well suited output transformer as well, the UBT-2. It is available from Antique Electronic Supply for about $110/each.
Increasing the OPT budget to about $160/each lands you in Lundahl (www.lundahl.se) territory. The LL1623/120mA appears to be a good candidate, though with 5.6 kΩ primary impedance, the max output power will be ever so slightly lower.
In the $250 category, you’ll find Electra-Print (www.electra-print.com). When dealing with Electra-Print, it is important to be concise and to-the-point. The output transformers shown in the amp on the front page are specified as “15 W, 100 mA, 5 kΩ primary, 4 Ω and 8 Ω secondary, copper for 300B”. A less expensive option is to use an OPT with a single secondary. A 6 Ω secondary will work reasonably well with both 4 Ω and 8 Ω speakers, but it is a compromise.
Personally, I find the Edcor CXSE25-series to be absolutely incredible good value for the money. The Electra-Print transformers do clean up the sound a bit, but are also three times as expensive. I have no personal experience with the James or Lundahl transformers.