Specifications

Once you have the amp working and sounding good, here are a few ideas to “shape” the

tone to suite your tastes.

1. Reduce low frequency muddiness when running the amp at high gain settings by

changing both C1 and C2 from .1ufd to .047ufd or .022ufd or .01ufd or even .0047ufd!

Make sure to change both equally.

2. Change both C8 and C9 like above. This modification will provide similar results to the

above mod but since this modification appears after the phase inverter, there will be a

different tonal response. Experiment with small changes to both stages vs. one large

change per each stage.

3. Install 33ufd/500v or 47ufd/500v caps in C12 and C13 positions - tightens power

supply and provides quicker dynamics.

4. Experiment with value of R9. Install a 1.5K resistor for better definition and controlled

gain when using a 12AX7 as input tube.

5. Experiment with the value of C3 and C6. By decreasing the values from 22ufd to

10ufd all the way to 1ufd or .47ufd, the gain response of these two stages can be

shaped to have reduced muddy low end gain and better defined mids and highs.

6. Pin 8 and pin 3 share a cathode bypass resistor and capacitor. By separating pin 3

and pin 8 from V1 and assigning each their own cathode circuit, additional tone

shaping can be achieved (a la Marshall). Start by soldering pin 8 to the C3 / R9 circuit

on the PC board. Solder the now empty pin 3 of V1 to a separate capacitor and

resistor pair. Good values to choose would be 2.7K ohms for the resistor and .68 ufd

for the capacitor. This gives a tight midrange response without being too bright.

7. Change out 6V6 tubes for EL84 tubes by fabricating your own cover plate adapter and

installing a 9 pin tube socket – allows installing EL84s vs. 6V6. I would encourage

retaining the tube rectifier when applying this modification for great “sag”.

8. Change out 6V6 with 5881. This is a great spongy tube that responds extremely well

when pushed hard. Because the 5881 consumes twice the filament current as

compared to the 6V6, it is recommended to use a solid state rectifier in place of the

tube rectifier when using 5881 tubes. By not using the tube rectifier, there is additional

power transformer over head. This overhead is needed for the increased filament

requirements of the 5881. The filter caps must be upgraded to +500V.

9. Install solid state rectifier in place of tube rectifier – tightens up dynamics and power

output. Filter caps MUST be upgraded to +500V types. Additionally, the 6V6 tubes

must be very good quality to withstand the higher plate voltages.

10.Run amp without negative feedback altogether by disconnecting feedback line from

speaker output jack – provides more overall gain and distortion with the amp volume at

“12”. Similar to setting the feedback adjustment fully counterclockwise.

11. Build an external speaker load box to lower the wattage going to the speaker. Great

for recording. And it only requires 2 resistors for the basic box (see Appendix E).

12.Wire the amp for fixed bias vs. the original cathode bias to get more power and

dynamics out of the amp. The transformers will run slightly cooler as well.

13. To decrease muddy response by lowering the value of C7 to .0022, .001, or 500pF.

Cool Modifications

34 TubeDepot.com