Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: RobBozic on December 15, 2014, 04:42:55 am

Title: Pre PIMV gain loss?
Post by: RobBozic on December 15, 2014, 04:42:55 am

Hi,if you install a JCM 800 style MV in a JTM45 and turn it up to "10" , how much gain do you lose with the 1meg pot?






Thanks in advance
Rob

Title: Re: Pre PIMV gain loss?
Post by: terminalgs on December 15, 2014, 11:03:25 am

Quote from: RobBozic on December 15, 2014, 04:42:55 am

Hi,if you install a JCM 800 style MV in a JTM45 and turn it up to "10" , how much gain do you lose with the 1meg pot?

If you do it like this JCM800 4001 MV:


(http://i.imgur.com/x3onbfq.png)


With the tandem 1M pots turned all the way up, the 10K R7 & R6 resistors form an AC voltage divider than attenuates nearly nothing when tandem-pots set to 10.  Maybe 1%.


I don't think the diodes parallel to R6 and R7 (D1 and D2) effect this very much.  On the top of the sine wave, the diodes are forward biased and have a very low impedance, on the bottom of the sine wave, the impedance of the diode is very large,  but the 10K resistor's impedance is 10K,, so I don't think the diodes effect attenuation when tandem pot is at 10.  attenuation would be between 0% and 1%.


anyone have a different take?

Title: Re: Pre PIMV gain loss?
Post by: sluckey on December 15, 2014, 11:16:06 am

I think he is referring to the JCM-800 2204 circuit that has a pre PI MV. The MV pot is between the treble wiper and the PI input cap.

I don't think you will see any difference in gain with the pot on 10. If you actually have a JTM-45 just tack a 1M resistor from Treble pot wiper to ground and you'll know for sure.