Preface

The following circuit pretty much resembles a Fender Champ type AA764, with a few noteworthy exceptions:

• Instead of noval ECC83/12AX7, octal 6SL7 was used in the front end and driver stages,
• Instead of a 6V6 beam power tube, paralleled sections of an octal 6SN7 small signal tube were used,
• An additional "gain boost" switch was introduced. Well, actually it is a "gain reduction" switch (see below).

So, basically we have an all-octal, all triode variant of the Fender Champ here. Deviations from the original are rather small, since - who would have wondered - almost half a century of improving this circuit at the Fender factory doesnīt leave much to do.

The schematic

PSU board layout

Some notes on the circuit

12AX7, 6SL7 and relative stage gain

• Under the actual loading conditions (tone stack and grid leak) both stages with both tube types 6SL7 or ECC83 actually will deliver quite the same stange gain of about A=40..44 times (for each stage). Gain differences are utterly negligible up and into the grid leak resistor of the powerstage . If you donīt believe this, just measure it yourself.
• The original Fender Champ type AA764 power stage uses a 6V6 beam power tube, biased at about -20V, which will need an input swing of up to 40 V_peak-peak to be driven to full output power. Even worse, there is a feedback loop from the power stage back to the driver stage cathode, that effectively lowers the gain of the driver stage by the actual ammount of loop feedback injected by this loop.
• Now, the power stage of our circuit variant uses paralleled 6SN7 sections, biased at about -10,5V, which will need only about 21 V_peak-peak to be driven to full output power (which is 1/2 the voltage swing the 6V6 needs even without loop FB). Moreover, there is no loop feedback from the power stage to the driver stage, hence there is no gain reduction at the driver stage.

In short, the 6SL7 front end and driver stages, have more than double the relative gain feeding the 6SN7 power stage, than those ECC83 based front end and driver stages feeding a 6V6 beam power tube stage like with the original Champ type AA764 circuit.

The "gain boost" switch

Opening the switch, so that the driver stage cathode resistor is not bypassed, actually lowers the gain of this stage to an overall and relative ammount of gain that makes it comparable to the Fender AA764 circuit again.

Essentially, calling it either a "gain boost" or a "gain reduction" switch is moot, of couse - it is just a matter of personal preferences and music style

The output transformer

Since the combined iddle current of the paralleled 6SN7 sections power stage is rather low (about 15-16mA in total), it was not necessary to use output iron designed for 6V6 or EL84/6BQ5 power class tubes. Instead, an output transformer of considerably smaller core size could be used. In the end, a suitable unit from a german Nordmende table top radio was chosen and bought via Ebay for less than a dozen Euros.

One small problem showed up with this OPT: While the printing on this unit says it has a reflected impedance of 7kΩ, I actually measured it to be more in 5-5,5 kΩ range. This would be too low a reflected impedance for the paralleled 6SN7 sections to keep them happy. Fortunately this OPT features both 4Ω and 8Ω taps at the secondary, so the nominal 8Ω speaker was wired to the 4Ω tap of the transformer to give a reflected impedance of 10-11kΩ, which is ideal for the paralleled section 6SN7 flea power stage - voila!

Further usage ideas

At about 1 W_RMS output power, the 6SN7 anode voltage swings from about 130V to about 425V, which covers a total swing of almost 300 V_peak-peak. Thinking about it, this little amp circuit would happily excite even the most demanding monster power SE tubes used as "afterburners", as long they donīt need grid current drive (class A2 operation).

Considering this, and how good this small amp circuit sounds, this could lead to a most interesting project - use the amp circuit as is at home and for practice, and kick in the "BIG FINALS" (a couple of 300Bs in parallel would do nicely) when on stage ...

In such a setup, the 1 Watt output should be kept working into a dummy load instead of a speaker. Alternatively, if the flea-power output is not wanted or needed, the small OPT could be replaced by a suitable choke. But that somewhat would compromise the idea of a "complete amp within an amp", of course.