12AX7 in Foreplay 2?

[Cary]

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.

 

[corndog71]

It can with some part changes but you'll get more than enough gain from the stock 12AU7.

 

[Paul Joppa]

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.

 

[Grainger49]

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.

 

[Grainger49]

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

 

[Paul Joppa]

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.