Looks very cool, only i can hardly wire a plug let alone mod an amp. I really do respect anyone who can make head or tail of those diagrams, like i said in my Juke thread ,the diagrams look like prison break blue prints!!!
I wish i'd paid more attention in electronics in school. ----------
Ant138, I got a grade "U" (ungradeable)in physics at school! But if we had been building amps in class, then I probably wouldn't have spent so much time staring blankly out of the window!
As with anything, if you break down schems, bit by bit, they are easier to figure. Generally tubes have 3 components - a grid, a plate, a cathode.
Signal goes in on the grid (which will have a load resistor to ground), then comes out on the plate (which is connected to the high dc voltage by a "plate resistor" of 56K to 220K, usually 100K), then goes through a coupling cap (this lets AC signal pass but keeps dc off the next tube's grid, as this would screw up the bias) to the next stage/volume control/tone stack. The 3rd tube component is the cathode, usually just has a resistor to ground that "biases" the tube (this resistor may be bypassed by a cap to increase gain), setting the idle current, all current that passes through the tube must leave via the cathode so sometimes signal is taken from here too, such as in a "cathode follower" tonestack (see tweed bassman) or a "cathodyne phase inverter" (see 5E3 deluxe, 12AX7 next to the power tubes).
Power tubes usually have a 4th component, the "screen grid" which is connected to the high voltage supply via a resistor of 470ohms to 1kohm for 6V6/6L6/EL34/6550/KT88/KT90, or 100ohms+ for EL84. Sometimes power tubes are "cathode biased", as above via resistor, or more often "fixed bias" by grounding the cathode directly & applying a negative voltage to the grid to bias the tube. Power tube plates are normally connected to the output transformer primary winding, rather than a plate resistor, for their dc supply.
Rectifiers just have plates & cathodes, AC applied to the plates comes out as dc on the cathode & feeds the amp's power supply, or diodes are used instead.
Tubes all need a low voltage, high current "heater/filament" supply to work...these only need investigating if they go wrong.
2 types of electricity are primarily at work:
AC signal - starts at the mic, goes from the input jack to, preamp tubes, power tubes, output transformer, then the speaker & carries the sound of your harp. This is what your amp's power rating is, measured in, AC W RMS.
dc B+ voltage - starts at the power transformer, via the rectifier, power tubes to the preamp tubes (opposite way to the AC). Tubes need the dc to operate. dc W ratings relate to how much heat resistors & tubes can take before they die.
Even little tiny tube amps carry potentially lethal voltages, so google & always follow appropriate safety procedures.
Last Edited by on Apr 04, 2011 4:30 AM
Has anyone on here done this. I have decided to do option two, it looks the most simple. I see that the coupling caps are changed to .22uf, 3 resistors are changed, C3 & C4 are removed/disabled and a 1m resistor is stacked with R1 & R2? It looks like the bottom one is taken loose on the right side of the resistor, the top one is taken loose from the left. Both are stood up and the 1m resistor is dropped into the pc board where the top resistor was removed then the 3 loose ends soldered together. Am I right?
I picked up an Epi real cheap at GC last December. Anybody know of a good harp amp tech in greater Boston area who could do this type of mod? And what it would be worth?
Randy did the mods on mine, and I'm a very happy customer. I know this wouldn't be practical for across-the-ponders like Ant to do, with shipping and all, so find a good tech there and give him Randy's schems...you'll be glad you did. ---------- Todd
@ Lip Ripper I have not done the mods, but as an electronic savy kinda guy, let me add..... 1)I looked, but could not find C3, though I did find two marked C5 2)I would wager that the most important resistor to change is R13. That's the one that lowers the voltage on the pre-amp tubes. ---------- intermediate level (+) player per the Adam Gussow Scale, Started playing 2001
