4-40 Push Pull Tube Monoblocks

Mark IV of my tube monoblock design could be called a step backwards, but I deliberately designed these to be simpler and cleaner while still offering decent power and virtually the same sonic character of my bigger 6-70 Mark IIIs.

These were built as a commission for a customer who didn’t need all the power in the world. I decided a pair of KT88s in push-pull, cathode bias, 500V B+, and 3.3K output transformers would do the trick, aiming for a phased plasma rifle in the 40 watt range. Sorry, just what you see pal.

The input and driver stage are identical to the 6-70s. It works extremely well so wasn’t going to mess with success. Part of my secret sauce is having an AC balance pot in the driver to minimize distortion. I would strongly recommend including this in any design provided you have access to a distortion analyzer to set it correctly.

KT88s with cathode bias have a max grid leak resistor of 220K. If you ran fixed bias this is halved to 100K, which is about the limit of what the 6SL7 driver can handle (in the 6-70s with parallel EL34s it’s 2x 220Ks = 110K load). I chose cathode bias both for simplicity and for the additional loading of the driver; the results are very good and distortion is still very low so it’s not like you are crippling the thing by skipping fixed bias.

I happened to have a spare set of 100W 3.3K Edcor transformers handy so I worked around these. These don’t make anywhere near 100W but the extra iron doesn’t hurt, and makes for dead flat response way down into the sub basement. 3.3K is a little low for a pair of KT88s BUT the customer is running 8 ohm speakers, so the recommended configuration is to hook up an 8 ohm load to the 4 ohm tap which makes for 6.6K primary impedance. If you are buying new OPTs I’d recommend 5K to 8K primary.

The results are as follows:

8 ohm load to 4 ohm tap max power 40w at 1%

8 ohm to 8 ohm 30w at 1%

8 ohm to 8 ohm 50w at 5%

1W 0.3%

10W 0.2%

The 8 to 8 3.3K had higher distortion across the board as expected, but surprisingly the distortion curve past 1% is pretty gradual so you can stretch it to 5% and still be within the “hockey stick”. At 5% is when it shoots for the stars. But I only rate my amps at 1% max so it’s 40W when you use the 4 ohm tap.

Frequency response is identical to the 6-70s. 20Hz-45khz -1db and 20hz-62khz -3db. My analyzer only goes down to 20Hz so I cannot rate it accurately below this but it’s dead flat at 20.

As a rule I bypass my cathode resistors with box type polypropylene film capacitors. 100uf/63V film caps are readily available and work beautifully. They aren’t as cheap as standard electrolytics (around 20-30$ each) but in my opinion they are worth the extra bucks and you’ll never have to replace them. If your design needs a higher capacitance, just parallel them.

Great results with greatly reduced complexity vs my flagship 6-70s. I’m hard pressed to hear any difference vs the 6-70s, which surprises me given KT88s usually have a different character to EL34s. This customer will be very happy.