Paramour 6CG7 conversion ?

[kuramura]

Hi
I have recently rewired heaters in my Foreplay c4s from 12au7 tubes to 6CG7 and I love the sound. That made me wonder if I could use 6CG7 in Paramour c4s. If so, should I change c4s resistor values or make any other alternations beside heaters? Both Foreplay and Paramour are in the same chassis and heaters are dc powered so additional current is no problem.
Thanks,

[Paul Joppa]

You would have to cang the cathode bias arrangement to allow for the 70% reduction in gain.

Which version of Paramour do you have?

[kuramura]

Thank you for your reply Paul.
I think it's Paramour 1, on manual says "copyright 2000" and Paramour C4S upgrade manual "copyright 2001"
The cathode resistor is 499 ohm and it says 4.5mA so I should increase the value of that resistor to get 1.35 mA ?  What about C4S boards?
Thanks,

[Paul Birkeland]

With a CCS, the cathode resistor only sets the bias voltage, not the current.

The big unanswered question is why you would want to lower the gain so significantly.  The amplifier will be somewhat unusable with a 6CG7 driver.

To make the amplifier usable, you would want to set the 6CG7 bias voltage to be pretty high, as you will need considerable drive voltage to obtain adequate output power. 

-PB

[Paul Joppa]

Leave the two 499 ohm resistors on the C4S board - no need to change it.

A bias resistor of around 1200 ohms is needed - you want to see 175-200v at the driver plate. If you use a trimpot - say, 2000 ohms - you can adjust it for tube drift.

The low sensitivity is compensated by the high gain of the Foreplay, so in your case I would not worry about that.

[kuramura]

Thank you for pointing out gain difference. From what I've read 6sl7 would be closer match to 12at7, would I have to make any changes beside sockets and proper  heater voltage to use this tube? Also, Paramour C4s uses only one half of 12at7 per channel and other half is grounded, should I also ground unused half of 6sl7? I have triodes in 12at7 connected in "mirror image" so I can use whole tube when change from one channel to another.

[kuramura]

Thank you for you reply Paul
I've posted before reading your answer. I will try to use 6CG7 after making proper adjustment before going to octal sockets.

[Paul Joppa]

Yeah, posting too fast.  :^)  The 6SL7 is not well suited to driving a 2A3 - the plate resistance is too high.

[kuramura]

I've  changed  bias  resistor to 1200 ohm but now cathode voltage is 4.8 vdc. Is that a problem?

[kuramura]

I'will get a trim pot.
I have 220uf 25v silmic II across Rk.
So help me understand it - The only voltage I should worry about ( in driver with c4s board ) is plate 175-200 ?

Thanks.

[Paul Birkeland]

I'will get a trim pot.
I have 220uf 25v silmic II across Rk.
So help me understand it - The only voltage I should worry about ( in driver with c4s board ) is plate 175-200 ?

I apologize, I didn't see that this was for a Paramour.

The 4.8V of input bias voltage shouldn't be a problem if the plate voltage is still around 200V. 

-PB

[Paul Joppa]

I'will get a trim pot.
I have 220uf 25v silmic II across Rk.
So help me understand it - The only voltage I should worry about ( in driver with c4s board ) is plate 175-200 ?

Thanks.

175-200v is the ideal, though I would be happy with anything in the range 150-225v.

There is no technical need to bypass the cathode resistor. I suggest you listen to it both ways, after allowing some break-in time for the cap. Personal direct knowledge is so much better than taking the advice of some doofus like me on the internet!

[kuramura]

Thank you for valuable information Paul.
Since I'm running 6CG7 in Foreplay II (I think) with c4s with power voltage of about 157dc, all resistor values in circuit and c4s boards are as per manual, I have separate trafo to produce 6vdc for heathers and another for 157dc, do you think I would gain anything sonically if the voltage in Foreplay was increased ? If so, what would have to be changed in circuit?
 Thanks,

[Paul Joppa]

I don't think there are sonic advantages to increasing the high voltage supply in a Foreplay as long as it is used as a preamp - it already has plenty of headroom.

To change the voltage would mean re-designing it - changing the resistances and the capacitor voltage ratings, and replacing the cathode bias LED with something that has a different voltage drop. Not a simple exercise.