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I started my conversion today, taking a Grommes Little Jewel mono Hi-Fi amp from the late 1950's and making it into a guitar amp. Doing this in stages and so far not doing a copy of any particular circuit, the idea is to have this as an ongoing learning experience.
Stage one was to replace the power supply caps. I had between 5 and 6 volts of AC ripple, and replaced the main cap can (which was a 40/30/10/20) with a 40/20/20/20. The second cap can was a 10/100, which I replaced with a 20/20, didn't use the second 20 but put in a separate 100uF since it was a cathode cap in the phase inverter, not a high voltage power supply cap. That got me a working amp with very low hum, way too much bass gain, and quite brittle - what I expected and hoped for, since there are no cathode bypass caps in this stock.
Next stage was to replace the input section, which had a multi-switch with phono equalization networks, with a straight in input through a 47K series resistor. I redid the first two gain stages with no particular frequency shaping networks - 100K anode resistors, 1K cathode with 20uF cathode bypass on both of the two gain stages, kind of standard following a Fender-ish layout. I used a 47K series resistor to the first plate, because that's what I had handy. I used .02uF coupling caps.
The result of this is an amp that works, but has too much gain. The tone controls work as intended, but don't suit guitar at all - 18dB of bass boost at 50 hz, and 15dB of treble cut at 5Khz. I will probably put in 1.5K cathode resistors to drop the gain a bit, put the volume control between the first two stages rather that at the input of the PI where it is now, and put in another type of tone stack. It also has a paraphase PI, which may get turned into a long-tail. We'll see...
So far so good, anyway. This amp is all point to point wired, and the factory wiring is impeccable, absolutely straight dressed runs, and components installed very neatly. I'll try to get a pic. A review of a Grommes Little Jewel kit is here: http://www.grommesprecision.com/grom...ewelreview.PDF
thanks, Brian
Stage one was to replace the power supply caps. I had between 5 and 6 volts of AC ripple, and replaced the main cap can (which was a 40/30/10/20) with a 40/20/20/20. The second cap can was a 10/100, which I replaced with a 20/20, didn't use the second 20 but put in a separate 100uF since it was a cathode cap in the phase inverter, not a high voltage power supply cap. That got me a working amp with very low hum, way too much bass gain, and quite brittle - what I expected and hoped for, since there are no cathode bypass caps in this stock.
Next stage was to replace the input section, which had a multi-switch with phono equalization networks, with a straight in input through a 47K series resistor. I redid the first two gain stages with no particular frequency shaping networks - 100K anode resistors, 1K cathode with 20uF cathode bypass on both of the two gain stages, kind of standard following a Fender-ish layout. I used a 47K series resistor to the first plate, because that's what I had handy. I used .02uF coupling caps.
The result of this is an amp that works, but has too much gain. The tone controls work as intended, but don't suit guitar at all - 18dB of bass boost at 50 hz, and 15dB of treble cut at 5Khz. I will probably put in 1.5K cathode resistors to drop the gain a bit, put the volume control between the first two stages rather that at the input of the PI where it is now, and put in another type of tone stack. It also has a paraphase PI, which may get turned into a long-tail. We'll see...
So far so good, anyway. This amp is all point to point wired, and the factory wiring is impeccable, absolutely straight dressed runs, and components installed very neatly. I'll try to get a pic. A review of a Grommes Little Jewel kit is here: http://www.grommesprecision.com/grom...ewelreview.PDF
thanks, Brian
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