Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Topic started by: Cary on October 09, 2010, 09:49:25 AM
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Can the 12AX7 be used in a Foreplay 2? If so what mods have to be done. I was cleaning out my workshop and found an unassembled one that I had forgotten about.
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It can with some part changes but you'll get more than enough gain from the stock 12AU7.
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If you search the old forum (on the Asylum site) you'll find several threads on this. Bottom line, you can plug them in, you'll get sound out, it won't hurt anything, and some people even liked the sound. But the operating point is all wrong, the cathode follower draws a good bit of grid current, and distortion is likely to be high.
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IIRC, I cut and pasted here that post where you give heater currents, plate resistors, cathode resistors that you had posted on the old board.
I have no idea where it is now.
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Found it, I have aligned the columns as best I could:
This subject comes up periodically, and I promised a week or two ago to post some numbers. (Took a while, I've had the flu.) Below are the circuit changes to duplicate the Foreplay normalized operating points with other tubes.
Note the last column, which is an estimate of the power transformer volt-amps needed. The stock transformer is probably capable of no more than 7 volt-amps rms; you will notice that very few tubes require that little, so most will need a larger transformer. A good candidate would be the Hammond 369AX, which has universal primaries and 2A filament power.
In the table, Ib is the plate current in amps, Rk is the cathode resistor in ohms, and RL is the plate load resistor in ohms.
Voltage amplifier Cathode follower
Tube Ib Rk RL Ib Rk VA
5670 0.001253 581 81,403 0.002777 17,938 6.82
5687 0.004283 350 23,816 0.009493 5,451 19.60
5965 0.001149 472 88,774 0.002547 19,380 7.88
7044 0.004182 290 24,388 0.009270 5,505 19.41
12AT7 0.000511 8331 99,803 0.001132 43,359 4.76
12AU7 0.000839 1788 121,561 0.001860 27,825 5.39
12AV7 0.001302 478 78,336 0.002886 17,170 8.18
12AX7 0.000107 2376 950,329 0.000238 204,443 3.98
12AY7 0.000308 2069 331,075 0.000683 72,624 4.37
12BH7 0.001404 1101 72,648 0.003112 16,665 10.26
E80CC 0.000750 1259 136,000 0.001662 30,294 9.00
6DJ8 0.003497 221 29,169 0.007751 6,442 11.34
6N1P 0.000860 899 118,631 0.001906 26,200 9.21
6N30P 0.012082 141 8,442 0.026780 1,951 33.71
6SL7 0.000131 2776 777,152 0.000291 168,127 4.03
6SN7 0.000906 1407 112,544 0.002009 25,481 9.31
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Wow - a blast from the past alright! I think those are to match the original Foreplay I and II operating voltages, and ignore possible grid current issues. Note the 12AX7 VA stage calls for a plate current of only 100 microamps! I would not advise these operating points today; I really think you want at least 3-4mA current, especially in the CF stage, and anything with a mu over 25 is likely to run into grid current nonlinearities at the Foreplay voltages.