Converting to 6SN7 from extended

[Nutube]

My extended has three 9 pin sockets- actually it is a unit Doc build
I am making a pair of conversion socket for the 6SN7 to 9 pin.

1) I was planing to use a 12au7 for the 9 pin regulator middle position. is that ok? or should I go ahead with converting that as well to 6SN7. I am hoping you would say yes as it is a PITA building those conversion adapters
In an extended FP3 is it the goal to use all 3 of the  same tube ?- Or is it because their heaters will be the same value? Or is it a sonic thing

2) Doc Posted these figures
0.3 amps 1.37 ohms
0.6 amps 0.62 ohms (as used in stock Foreplay III)
0.9 amps 0.35 ohms (close to the upgraded Foreplay III)
1.2 amps 0.22 ohms
1.5 amps 0.15 ohms
1.8 amps 0.10 ohms (as used in the S.E.X. amp)
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Two 6SN7s in a stock FP-III want 1.2 amps; three in an upgraded FP-III will want 1.8 amps.

My current Xircon resister is 62 ohms I believe
If I am using two 6SN7 and one 12au7 in the middle; what should my resister value be?
"Two 6SN7s in a stock FP-III want 1.2 amps; three in an upgraded FP-III will want 1.8 amps. "
When it is stated that 2 6SN7 wants 1.2 amps- how do I compute for the resultant resister? to exchange from the 62 ohm?
What is the formula for using the 1.2 amps to determine the reisiter change needed
Thanks again

[Paul Birkeland]

The 12AU7 in the regulator position is fine, don't bother changing to a 6SN7 unless you want to for aesthetics.  The heater current will be:  .6A+.6A for the 6SN7's and .3A for the 12AU7, so

1.5 amps 0.15 ohms

[Nutube]

Hey PB
My existing parallel resistors are 1.2 ohm plus 0.62 ohm;  Is it co incidental that they add up to 1.8?
I understand that my existing three  12au7 will need 1.8 amps and it happens to have a parallel total of 1.8 ohms

you wrote that I need 1.5 amps 0.15ohms
so do I need a O.15 ohms; 5 watt resisters inplace on my existing pair?

[Paul Birkeland]

Right now you have a 1.2 and a .62 in parallel, which gives you about a .4 ohm resistor total.  Try a .2 ohm and a 1 ohm in parallel and see how that goes. 

[Nutube]

Thanks PB

"Right now you have a 1.2 and a .62 in parallel, which gives you about a .4 ohm resistor total.  Try a .2 ohm and a 1 ohm in parallel and see how that goes. "

I think your answers 0.4 ohm and your new values 0.2 and 1.0 help me reverse calculate and I discovered
that the formula should be
V/Rr( where lower case r= resultant) = V/R1 + V/R2

6.3/R = 6.3/0.2 + 6.3/1.0
6.3/Rr =31.5+ 6.3
6.3/Rr = 37.8
6.3  = 37.8R
6.3/37.8 = Rr
0.1666 = Rr

Thanks for your help appreciate it .

[Paul Birkeland]

I think your answers 0.4 ohm and your new values 0.2 and 1.0 help me reverse calculate and I discovered
that the formula should be
V/Rr( where lower case r= resultant) = V/R1 + V/R2
Thanks for your help appreciate it .

That's what DIY is all about!

[pro_crip]

I was reading Morgan Jones this morning, the section on distortion, and he lumped the 6J5GT in with the rest of the *SN7's. Did I misinterpret something or is that correct. Being I got 6 Pinnacle 6J5GT's (for shipping only as I recall) lying around, I wanted to build his phono stage (p.568) which never got built because going from schematic to circuit scared(s) the bejeezus outta me. So I was wondering if I could build my FPIII with those pinnacles? Thanks

Rich

[Paul Birkeland]

You could build a Foreplay III with 6J5's, but you would need six 6J5's to get the job done.  If you can spring for an Extended Foreplay III and a pair of 15k Magnequest B7's, PM me and I can help you make a kickass preamp out of those bits, or you could opt to build your own gigantic chassis.

[pro_crip]

The only thing I'd need would be the iron and over at MQ they're only $99 a pop for the nickel version (no idea what that means). The extended FPIII and 6 Pinnacle 6j5's I already have. Let me think about this overnight, it's an intriguing idea. Thanks

Rich

[Paul Birkeland]

With the Magnequest iron, you could do a preamp with two 6J5's and a 6SN7.  I was trying to keep you from having to make a new chassis.  The circuit would change a little bit (simpler circuit) due to the addition of the output transformers.