Once you have the amp working and sounding good, here are a few ideas to “shape” the
tone to suite your tastes.
1. Change both C1 and C2 to .01ufd or .0047ufd to reduce low frequency response at
high gain settings. Gives the amp the ability to “cut through” better.
2. Install 22ufd/450v caps in C7 and C8 positions - tightens power supply and provides
quicker dynamics.
3. Experiment with 22ufd/50v cap in C3 position - increases gain with it installed.
4. Change out 6V6 for an EL84 by fabricating your own cover plate adapter and installing
a 9 pin tube socket – allows installing EL84s vs. 6V6.
5. Change out 6V6 for 6AQ5/6005 by fabricating your own cover plate adapter and
installing a 7 pin socket – allows installation of 6AQ5s vs. 6V6.
6. Change out 6V6 with 5881. It is recommended to remove the rectifier tube and use a
solid state rectifier instead. In this way, the power transformer is not providing filament
current to a rectifier tube. This additional power transformer over head is needed for
the increased 6.3V filament requirements of the 5881. Additionally, the filter caps will
have to be upgraded to +500V.
7. Install solid state rectifier in place of tube rectifier – tightens up dynamics and power
output. Filter caps must be upgraded to +500V types.
8. Run this amp without negative feedback altogether by disconnecting the feedback line
from speaker output jack – provides much more overall gain and distortion with the
volume up. The Gibson GA5 is a nearly identical circuit to this tweed 5F1 without the
feedback.
9. Install a miniature toggle switch, SPDT between the speaker jack and the preamp tube
socket. This miniature switch can then be wired as an impedance selector for the 4
and 8 ohm taps from the output transformer. In this way, more cabinet choices
become available.
10. Adjust VR2 (negative feedback level adjustment) on the PC board for personal taste.
Fully counterclockwise (least amount of negative feedback) provides the most gain and
distortion with a strong midrange and thinner low end response. Fully clockwise (most
negative feedback) provides the least gain and distortion with the flattest frequency
response and best defined low end.
10
Cool Modifications
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