The Amp Garage

Maybe I am late, btw Pedro should you check PI grid voltages again you should find them equal. This PI is a "differential amp" it amplifies differences between the two grids, it's idle condition is Vg1=Vg2.

5-10% difference in plate voltages is ok!

Teo

Usually my 12AX7s are EH except the OD tube which is a Sovtek LP, sometimes I try GT12AX7M (less gain but not a real improvement, true Mullard's are way better), or RFT as CL tube. Going to try JJ ECC803 soon, tried JJ ECC83S but don't like them. Power tubes are JJ 6L6GC but sometimes I used EH 5881...

The PI trimmer is there mainly to maximize the output power, you should watch on the scope and adjust to have the two halves of a sine wave clip symmetrical. I found that even with 100k plate resistor and 20k trimmer sometimes you reach trimmer's range limit without a perfectly simmetrical clipping;...

This mix design is somehow similar to mesa boogie usual parallel (mix) loop which I don't like so much. I read that it's actually a 10%-90% mix control although mods are available to make it more 0%-100%. I think you should better take a look at these: http://www.mif.pg.gda.pl/homepages/tom/files/FX...

Beware pedro, this has a huuuge impact on tone!

I suggest very low capacitance shieded cable from volume pot to v1b grid.

Teo

2) It controls how much of the output of the output of V2A is fed into V2B (same thing as the drive control). Dumble used several different names for these controls over the years. Actually the Ratio control is after V2B, it's the volume of the OD channel before it gets into the global master volum...

Disabling two out of four tubes doubles the output impedance of the amp while it also lowers the "open-loop gain" of the amp. You can mismatch the speakers (8 ohm speaker on 4 ohm output) to compensate the impedance but as already said it's usually not needed because you can hardly hear a difference...

Robben has changed a lot his playing style and live setup during recent years, now he uses his tele a lot and with his tele he almost never uses the OD and you can clearly recognize the power tubes overdrive with the boost engaged. So what he says in the interview is actually true. It is a completel...

I am running EH 6V6 at about 420 and they are fine, never tried higher voltages but with screen resitors 1k or more it shouldn't be an issue, Jim Kelly builds amp with much higher voltages on 6V6s.

Going to try JJ's too!

Teo

Try 4200-4000 (Fender bassman values) but also 3400 is good, if you go higher you have to increase also plate voltages otherwise you can't obtain 50w (rms) with just two 6L6.

I am actually using 5000 ohm and ˜410 volts on the plates under full load and power output is only 35w (rms) at clipping.

Teo