UPDATED 1/14/2011 - Push-Pull KT88 / 6L6GC / EL34 / 300B / 2A3 amp with "universal" driver PCB
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NEW 1/14/2011 - I have done a refresh of this board and am now offering them for sale on eBay. Changes were as follows:
Added an option to use the Lundahl LL1690 input transformer. You can also still install the Sowter transformers.
Updated the silkscreen and schematic to be more readable
Added additional mounting holes near the tube sockets to strengthen it when plugging in tubes (original mechanicals are still valid)
Made the coupling cap pad patterns fit a number of different styles of caps
Made the board RED!
Here is an updated PDF schematic, XLS parts list (BOM), and PDF BOM.
All of the other info below still applies.
This amp was designed mostly to work with KT88, 6L6, or EL34 tubes in either triode or ultralinear mode. I designed a driver PCB using two 6SN7's that can drive just most tubes, triode or pentode. The PCB also includes a tube-based power supply, using 5U4 or whatever tube you like, and snap-mount capacitors on the PCB.
For you skeptics... yes, this amp works well in both ultralinear and triode mode, using either KT88, 6L6GC, or EL34 tubes, WITHOUT CHANGING ANYTHING, even bias resistors. The bias points wind up being quite different, as you would expect. EL34's run close to their max dissipation limit, while KT88's loaf along. The EL34 has the lowest distortion, but I think the KT88 sounds the best.
Using KT88's, this amp is biased near class A and puts out around 25 watts.
The driver uses an input transformer to do the phase splitting (a Sowter type 3575-C, available form Sowter Transformers (www.sowter.co.uk) in the UK). It uses two differential amp stages after the transformer. NFB can be applied before the xfmr, though I never tried using it.
UPDATE - The new board can also accommodate a Lundahl input transformer (like the LL1690). You can wire the input as balanced or unbalanced.
For the amps I built, I used some military surplus power transformers and chokes, and some NOS Thordarson output transformers that I found on E-bay. The OPT's used were 30 watt units, 6600 ohms plate-plate. This is a good load for EL34's, KT88's, 6L6's, and the likes. It also will work with 300B and 6B4/2A3 triode tubes. You will get a bit more power out of 2A3/6B4 tubes by going to 5000 ohms plate-to-plate.
Any transformers will work. I really like the James transformers, which are very high quality for the money. You could also use Hammond iron, or other brands.
Some specs for the driver section:
@ B+ = 375V:
Vout max at clipping is 253V P-P (+/-126V)
Vout for THD=5% into 100k load is 70V RMS, or 197V P-P
THD @ 10V RMS out is ~0.24%
THD @ 20V RMS out is ~0.55%
Vout for input of 100mV (no negative feedback) is 10.77V RMS, or a gain of about 100
The output voltage is high enough that you can use this as a driver for even lower-mu triodes. It should work great with a 300B or 2A3, though I have yet to try it. Note you will likely have to adjust the B+ voltage and the cathode bias resistors for tubes other than the EL34, 6L6, or KT88!
Specs for the whole amplifier, with various tubes in ultralinear operation, 6600 ohms plate-plate:
| Svetlana 6L6GC Ultralinear | Sovtek KT88 Ultralinear | Electro-Harmonix EL34 Ultralinear |
| B+ = 380V, Ip = 70mA, Wp= 25W | B+ = 370V, Ip = 78mA, Wp= 26W | B+ = 382V, Ip = 67mA, Wp= 24W |
| 1W THD = 0.19% | 1W THD = 0.13% | 1W THD = 0.18% |
| 8W THD = 0.52% | 8W THD = 0.65% | 8W THD = 0.48% |
| 5% THD at 25.4W into 8 ohms | 5% THD at 26.1W into 8 ohms | 5% THD at 27.6W into 8 ohms |
| Vin = 260mV for max output | Vin = 230mV for max output | Vin = 270mV for max output |
Specs for the whole amplifier, with various tubes in triode operation, 6600 ohms plate-plate:
| Svetlana 6L6GC Triode | Sovtek KT88 Triode | Electro-Harmonix EL34 Triode |
| B+ = 380V, Ip = 70mA, Wp= 25W | B+ = 370V, Ip = 78mA, Wp= 26W | B+ = 382V, Ip = 67mA, Wp= 24W |
| 1W THD = 0.32% | 1W THD = 0.21% | 1W THD = 0.18% |
| 8W THD = 1.99% | 8W THD = 1.34% | 8W THD = 1.26% |
| 5% THD at 11.28W into 8 ohms | 5% THD at 17.22W into 8 ohms | 5% THD at 16.3W into 8 ohms |
| Vin = 260mV for max output | Vin = 230mV for max output | Vin = 270mV for max output |
Driver and power supply PCB (original version) Schematic (223k PDF file)
Here's how the PCB hooks up to the rest of the amp (click for full-size image):
The PCB is assembled with the tube sockets on the back side, and is then mounted inside a chassis with the tubes pointing up. Here is an un-dimensioned 1x template I used for the chassis. It should be scaled right if you print it on 11x17 paper.