Hi guys, I'm planning on re-using this little amp extracted from a 60's turntable as a guitar amplifier, obviously the first part of the circuit should be changed from piezoelectric pickup to the impedance of the guitar and adjust the eq., that's where I mostly need help, also what about the output transformer, I have a few others slightly bigger in size and similar impedance's would that make any difference? perhaps for 2-3W max.
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extracted as it is.
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You have 100 V.? marked on the schematic. Add up the voltages of the 150 mA. series heater string and you get 97.6 V.
Ceramic carts. produce substantial fractions of 1 V. as their O/P level. A guitar pickup's O/P is more in line with the mV. O/P of a mag. cartridge. You need additional gain.
Dave pointed you in the proper direction. Get rid of the 35W4 1/2 wave rectifier and use a bridge of 4X UF4007s to create the "raw" B+. Eliminating the vacuum rectifier makes room for another 150 mA. heater small signal tube. Install a 12AX7 and you'll have all the necessary additional gain. I leave the simple calculations for the value and wattage rating of the dropping resistor that must be added to the 150 mA. string to make the voltage come out right to you. If you get a resistance between available values, take the higher of the 2.
Ceramic carts. produce substantial fractions of 1 V. as their O/P level. A guitar pickup's O/P is more in line with the mV. O/P of a mag. cartridge. You need additional gain.
Dave pointed you in the proper direction. Get rid of the 35W4 1/2 wave rectifier and use a bridge of 4X UF4007s to create the "raw" B+. Eliminating the vacuum rectifier makes room for another 150 mA. heater small signal tube. Install a 12AX7 and you'll have all the necessary additional gain. I leave the simple calculations for the value and wattage rating of the dropping resistor that must be added to the 150 mA. string to make the voltage come out right to you. If you get a resistance between available values, take the higher of the 2.
You have a 12AT6 which is 1/2 of a 5751, about 70% gain of a 12AX7. I have a small guitar amp with both triodes of a 12AX7 feeding a 50C5, I think you will find yourself short of gain. You could get yourself a 12AU6 and rewire the 12AT6 socket for it and end up with a lot more gain. Lots of schematics out there with this lineup. As an example,
http://dogbarkmusic.com/Harm/harmony.pdf
What I would do though is to use your tone pot as a volume control going directly to the 50C5 leaving out the 250k. I would then use your 1M pot with a 500pFand 5nF capacitor as a Tweed tone control across the volume rather than C4.
Could also use a cathode biased 12AU6 rather than grid leak for more headroom.
http://dogbarkmusic.com/Harm/harmony.pdf
What I would do though is to use your tone pot as a volume control going directly to the 50C5 leaving out the 250k. I would then use your 1M pot with a 500pFand 5nF capacitor as a Tweed tone control across the volume rather than C4.
Could also use a cathode biased 12AU6 rather than grid leak for more headroom.
Per forum directive this thread has been moved to Instruments & Amps.
Hi, I was playing with it a bit today, and as you said, the radio triode doesn't cut it simply. Even using a guitar pre-amp wasn't enough. The 12AU6 is interesting I would have to get one and try it just to not do the obvious and replace the first stage with 12ax7 triodes and try something different.
I already got rid of the tone pot and tried something with the volume pot, but always before the triode. I think I will follow your advice though.
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