A friend was not satisfied with the reliability and sound of his Pignose G40 amp, which he'd originally bought for his harmonica, but now his wife was using it on fiddle (and hence wanted a clean yet warm tone). He was prepared to trash it and buy a different amp, but decided instead to let me modify the heck out of it for approximately the same cash that a good new amp would have cost. I provided this report to him after completing the modifications:
Goal: make this amp sound good in general, and work well for a fiddle with piezo bridge pickup in particular.
Schematic Reference: "Pignose G40V Schematic / Designed by Dennis Kager for Pignose Industries" Available at: http://www.blueguitar.org/new/schem/misc_amp/pignose_g40v.gif
I changed the above schematic to reflect my modifications, and you can click here to see this modified schematic. Please use your browser's "back" button to return to this text.
Condition of amp before repairs and modification: The 6L6 power tubes are loose in their bases due to degradation of the glue holding the bulb to the base; sockets are ceramic and look good. Amp sounds shrill, distorted, weak and intermittent--all at the same time. On examination of main filter capacitors, the bottom (closest to ground) 100 microfarad/400V unit at "Va" node has leaked electrolyte and is functionally compromised. The mounting plate for the 10-inch speaker is attached to the cabinet by four screws and is not a perfect fit; at least one screw is threaded into splintered wood parts.
Modification Overview: With a budget of $150 allotted to parts, my strategy is: (1) Replace the stock speaker, output transformer, and all vacuum tubes with higher-quality components. (2) Provide an adjustable bias control for the power tube section. (3) Reduce preamp gain and optimize signal path and grounding as appropriate. (4) Buttress the mounting of the speaker plate to the cabinet.
Adjustable Bias Control: Refer to the "before" and "after" schematics. I doubled the value of C1 to 0.1 microfarad, which increased 60-Hz AC coupling to the bias power supply and made Vf at the top of R2 -62V DC rather than -43V DC. Then I replaced R2 with a series of resistors containing a 10-turn trimpot. Resistances in this network sum to the original value of R1 and allow bias settings spanning -47 to -36 V at the wiper of the trimpot.
Reducing gain and cleaning up signal path: I replaced all heater wiring with twisted pairs to reduce hum, and replaced the two major chassis ground lugs after scraping paint from the chassis for better contact. To reduce preamp gain, I eliminated the second stage of amplification (handled by V1B in the "before" schematic). I instead hooked the two triodes of V1 together in parallel and used them like V1A in the "before" schematic. While I kept the plate load resistor (R13) the same, I adjusted the cathode resistor (R11) empirically for best performance. I added a 10K resistor at the grid to promote stability (among guitar amps the G40V is exceptional in hooking the pickup directly to the grid). The 0.022-microfarad capacitor (C10) coupling the signal to the top of the volume control network is replaced with the 0.047-microfarad unit formerly used as C9 (in the eliminated triode circuit). I also eliminated C13 from the circuit, increasing the high-frequency response. Overall, these changes drop the gain by about 20 dB, increase both low- and high-end frequency response, and invert the signal relative to the stock design. The latter is compensated at the primary of the output transformer, which I wired such that voltages changing in a positive direction at the amp's input jack correspond to the speaker pushing air out towards the listener.
Vanity mods and mechanical strengthening: I replaced the stock red LED with a "super-bright" yellow LED with a clear lens, and removed the Pignose logo from the speaker grill. Using 90-degree angle-aluminum stock and screws, I secured the speaker mounting plate to the cabinet much more robustly than it was before. I added a cable clamp for the AC line cord, to help the chassis strain relief and train the cord towards the bottom of cabinet for coil-up and storage. I had to re-mount the whole amp chassis about 1/2-inch further back in the cabinet to accommodate the larger output transformer. The bottom of the front panel is now flush with the speaker grill.
Comments: The Pignose is designed to be a hot-running amp due to the close spacing of the tubes and inefficient power transformer. I'm a bit concerned about the long-term viability of the stock power transformer. My mods have not added to the loads on this transformer, and the stock fuse values hold. Mounting the chassis further back in the cabinet has probably helped with heat exchange, and the heat you feel when standing over the amp indicates a good draft. When replacing the metal grill in back, make sure the ventilation slots angle UP from the tubes (the opposite of what may look most aesthetically pleasing). The slats are for convective air flow--not to keep rain out. I would keep amplifier duty cycles less than 2 hours and leave it off when not in use (same for an unmodified Pignose).
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