FPIII experimenters ?

[iain42]

A friend has  a stash of 76 and 56 tubes that he would let me have. Has anyone tried these tubes with the FPIII? Are these good tubes to use? The tubes look really cool but looks aren't everything.

Any info appreciated

Thanks

[Grainger49]

I swapped my 12AT7 tubes in my Paramours for 76 drivers.  The voltages were good for a drop in on the B+ and a cathode resistor change to get the operating point right.  Remember that the 76 is a single triode the 12AU7 is a dual triode.  You will have only one stage of amplification.  I'll dig up some information from PJ that will help you with the operating point.

[iain42]

I swapped my 12AT7 tubes in my Paramours for 76 drivers.  The voltages were good for a drop in on the B+ and a cathode resistor change to get the operating point right.  Remember that the 76 is a single triode the 12AU7 is a dual triode.  You will have only one stage of amplification.  I'll dig up some information from PJ that will help you with the operating point.

Thanks!

[Grainger49]

From PJ’s post on setting bias on the 76 tube for a Paramour, it assumes there is a C4S feeding the tube: 

You need three numbers: the C4S input voltage (normally around 300 volts), the plate voltage with no signal (normally around 200 volts), and the "saturation" voltage (plate voltage with the cathode resistor shorted out - normally 50-100 volts). As long as the middle one is 60 volts or more away from the other two, you can drive the 2A3 to full power and you're good to go.

Again, the 76 is only one triode and the FP III uses one triode for the VA stage and one triode for the CF stage.  You will lose one of these.  Also, the Paramour has about 380V going into the C4S.  The FP III voltage checks show some high voltages on Terminals 10 and 25.  I bet those are before the OD3 regulator.  Someone else will need to help you there.

One last thing, the 76 conversion required a new cathode resistor, different than the 12AT7.  I bought a 2600 ohm resistor.

[iain42]

From PJ’s post on setting bias on the 76 tube for a Paramour, it assumes there is a C4S feeding the tube: 

You need three numbers: the C4S input voltage (normally around 300 volts), the plate voltage with no signal (normally around 200 volts), and the "saturation" voltage (plate voltage with the cathode resistor shorted out - normally 50-100 volts). As long as the middle one is 60 volts or more away from the other two, you can drive the 2A3 to full power and you're good to go.

Again, the 76 is only one triode and the FP III uses one triode for the VA stage and one triode for the CF stage.  You will lose one of these.  Also, the Paramour has about 380V going into the C4S.  The FP III voltage checks show some high voltages on Terminals 10 and 25.  I bet those are before the OD3 regulator.  Someone else will need to help you there.

One last thing, the 76 conversion required a new cathode resistor, different than the 12AT7.  I bought a 2600 ohm resistor.

This sounds a little more complicated than I expected as usual :) How about using 4 76 tubes so nothing is lost? I also have a great  stash of 6SN7 tubes. That seems a little more straight forward.

[Grainger49]

6SN7s are drop in replacements when you change out the socket.  By that I mean no resistor values need to be changed, but you would want to replace some when doing the conversion.  And that is the tube that was probably in mind when the FP III tube socket holes were made large enough for octal tubes.

As for using 4 - 76 tubes, that would look cool!  I don't know what the FP III power supply can handle.  Hopefully Doc or PJ or PB will answer.

[iain42]

6SN7s are drop in replacements when you change out the socket.  By that I mean resistors need to be changed.  And that is the tube that was probably in mind when the FP III tube socket holes were made large enough for octal tubes.

As for using 4 - 76 tubes, that would look cool!  I don't know what the FP III power supply can handle.  Hopefully Doc or PJ or PB will answer.

I love the look of triodes in the morning. 4 76's and an 0D3 would look wicked cool.

This might be a common question but what are the other tube drop in replacements that just require resistor changes?

[Grainger49]

  .  .  .  This might be a common question but what are the other tube drop in replacements that just require resistor changes?

I think there is a post already, maybe a sticky, that addresses other tubes.  It is a carryover from the old forum.

[Caucasian Blackplate]

You can indeed build the FP-3 circuit with the 27/37/56/76.  I built a 27 Foreplay a long time ago that I still have, but it requires a 2.5v transformer with quite a bit of secondary current.  The PT3 in the FP-III is not up to it.   There is indeed a post that has been put up here (maybe in legacy products?) that refers to the variants that can be run. 
-Paul