by EWBrown » Sun Jul 11, 2010 1:59 pm
With four 6L6GCs in parallel push pull, I'd SWAG that your amp can peak out around 100 watts. So the "dummy" resistor will have to be able to handle that amount. at a 4 ohm outout, this translates to 20V at 5 amps.
You would need a cluster of wirewound resistors to handle this, and the total power handling capability should be at least two or three times that amount
I'd use something like twenty-five 100 ohm, 10 watt wirewound resistors in parallel, that will give you a pretty much bullet proof 4 ohm dummy load. THese ceramic boxy WW resistors are fairly inexpensive, 5% or 10% tolerance is perfectly good, it doesn't have to be exactly 4.00000 ohms... With that quantity of resistors, any tolerance errors will effectively cance themselves l out, so the end result whould be very close to 4.0 ohms.
Don't stack them close together like bricks, leave at least 1/4 to 1/2 inch airspace between them, they can be arranged in a 5X5 array, or whatever format you prefer. Without any convective airflow, the "inside" resistors will get very hot.
Connect it to a length of zip cord, and terminate the end with the appropriate plug (usually a 1/4 inch standard mono plug) to match ouur amp's output. And, unplug the speaker(s).
The resistive dummy load will get fairly hot, so don't let it contact anything flammable or meltable, and protected from any accidental skin contact..
As far as fine-tuning for the "tone", all I can say is to experiment with the volume (and tone) controls on the guitar, individual channels and master volume last. You should be able to create a nearly unlimited variety of tones, funk, grundge, etc by trying various combinations of setings.
Distortion generated in the (12AX7 or 7025) preamp and driver stages will have a diferent tonal quality, than that which is mostly created in the output (12AT7 and 6L6GC) stages. The master volume control will allow you to vary the latter effects.
FWIW, 7025 is a lower noise version of a 12AX7, and are otherwise completely interchangeable. and the "European" ECC83s can also be used.
I never got good enough to be able to fully experiment with trying to achieve that "special" tone, I was too busy just trying to get the correct notes, and keep in correct tempo with the rhythm 8^0
Another "trick" to slightly lower the overall 7025 driver stage gain is to replace any 12AX7s or 7025s with 5751s, they have a slightly lower overall maximum gain, of 70, rather than 100, and they are pin compatible . That equates to taking about 3 dB of voltage gain off the driver stages.
Don't change the phase inverter tube (12AT7) , that stage is more critical to the tube operating parameters.
If you plan to use a separate solid state power amp, just be careful not to exceed its safe input signal voltage, or the SS amp will suddenly become an expensive smoke generator. Tubes are far more forgiving of "abuse" than are transistors. Tubes just moan and complain a lot, while transistors will die sudden, horrible, deaths when pushed too far.
/ed B
Real Radios Glow in the Dark