Foreplay III 6SN7 Conversion

[Grainger49]

I have been rummaging through the FP III information for the 6SN7.  The conversion mentions the dropping resistor for the heaters but no Plate or Cathode voltages.  

Are the 12AU7 and 6SN7 triodes equivalent enough that measuring the plate and cathode voltages after conversion are not necessary?

Resistor, 0.22 ohms, mentioned in reply #6 below with a quote from PJ.

 

[Paul Joppa]

The bogey 6SN7 is much closer to the bogey 12AU7 than the standard deviation between random samples of either tube.

For example, at the Foreplay operating point, I checked the GE data sheets which give detailed values for the various parameters. The 6SN7 has a mu of 20.2, rp of 9800 ohms; the 12AU7 is 19.0 and 9500 ohms. Transconductance is mu/rp, so 2060 micromhos vs. 2000.

 

[Grainger49]

Ok, Thanks!

(Three courses in statistics and I understand Paul's response.  Admittedly, they were in inferential statistics in the Psychology Dpt. but it still makes sense.)

 

[saildoctor]

Hi all, I recently ordered a Foreplay III with the extended upgrade and  I've been thinking I might want to build it using 6SN7's.  While I would think it prudent to make to unit as stock with the 12au7's to make sure I had all the bugs sorted out, i see that the extended power supply boards cover the sockets.  Is it a major deal to remove those if I wanted to change the 9-pin to octal sockets?  Thanks!

 

[Air]

The tack I would take if given this choice would be to order the Extended version of the Foreplay and to build it stock except for the upgrade Auricaps from the Extended version. After about a month of listening and break in I would do the conversion the 6SN7

 

[Paul Birkeland]

I almost agree with Chuck. Since the sockets would have to be yanked, and a fair bit of rewiring done, I would build the Foreplay III for 6SN7's right off the bat. Just be sure you are familiar with tube anatomy/pinouts.

The extended upgrade should be the absolute last step in the process. You don't want to have to remove all of that to replace sockets.

-Paul

 

[Grainger49]

Hi all, I recently ordered a Foreplay III with the extended upgrade and  I've been thinking I might want to build it using 6SN7's.  While I would think it prudent to make to unit as stock with the 12au7's to make sure I had all the bugs sorted out, i see that the extended power supply boards cover the sockets.  Is it a major deal to remove those if I wanted to change the 9-pin to octal sockets?  Thanks!

The only warning, and you probably already know, is you have to change the current/voltage adjusting resistor in the heater supply.  That and the pin outs are different.

Edit: From PJ on the old board on the 6SN7 FP III conversion, "The only change to the schematic is the resistor in the heater DC power supply, from 0.62 ohms stock to 0.22 ohms for two tubes (stock configuration) or 0.10 ohms for three tubes (upgrade configuration)."

 

[saildoctor]

Roger on all that info.  Thanks everybody!  I'll make it stock with 6SN7's, then install the extended.  Looks like i need to order me some sockets! One other question which i should have asked - the socket for the OD3 looks to be 8 pin - Is this the same octal type socket I would need if I were to make the shunt regulator in the extended a 6SN7?  Or would changing that from the 12au7 not be as straightforward as the others?

 

[Paul Birkeland]

Yes, you can reuse the 0D3 socket.  This is also made easier by how few connections there are to the 0D3.