Fargen Mini Plex + Mighty Plex gain & tone mods

[BENJAMIN FARGEN]

Hi All,
I have had many requests for ways to add gain and brighten up older mini plex models.....here you go:

adding gain to dual input newer models:

change the first stage plate resistor from 100k to 220k
change the slope resistor from 56K to 33K

adding gain to older single input models:

change input grid resistor from 68K to 33k-10K
change the first stage plate resistor from 100k to 220k
change the slope resistor from 56K to 33K

adding more highs to older single input models:

add 500pf across volume pot.
change 270pf cap that feeds the 250K treble pot to 500pf.

DISCLAIMER NOTE:TUBE AMPS CARRY LETHAL VOLTAGES AND CAN KILL YOU IF YOU DO NOT KNOW WHAT YOU ARE DOING. IF YOU DO NOT UNDERSTAND THESE TERMS OR HAVE NOT WORKED ON TUBE AMPS BEFORE...TAKE IT TO A QUALIFIED TECH TO HAVE THESE MODS DONE. PLEASE DO NOT E-MAIL OR CALL FARGEN AMPS FOR TECHNICAL ASSISTANCE ON THESE MODS.....WE JUST DON'T HAVE THE MANPOWER OR TIME.

Enjoy!
BF

[Axemeister]

Hi Ben,

I am awaiting receipt of a MiniPlex that was built in April of '04.

Does this have these updated mods included at this time of production?

Thanks

[BENJAMIN FARGEN]

Originally posted by Axemeister
Hi Ben,

I am awaiting receipt of a MiniPlex that was built in April of '04.

Does this have these updated mods included at this time of production?

Thanks

Yes... all MP amps from 04' include these mods as stock circuitry now.

BF

[scottl]

Ben,

I was wondering, if you raise the plate resistor to 220k without raising the cathode resistor, wouldn't that raise the voltage dramatically? How would the tone change if you raised the cathode resistor to 3.3k? I know raising the plate raises gain, but how about raising plate and cathode together??

Of course, maybe I am off base since I am unfamiliar with your circuit.

Thanks!

Scott

[BENJAMIN FARGEN]

Originally posted by scottl
[QUOTE]I]I was wondering, if you raise the plate resistor to 220k without raising the cathode resistor, wouldn't that raise the voltage dramatically?

No.... the preamp plate voltage actually goes down when you increase the size of the plate resistor.... not up. With the cathode resistor remaining the same.....the voltage drop across the plate resistor affects the internal AC resistance of the tube and creates more available gain.

How would the tone change if you raised the cathode resistor to 3.3k? I know raising the plate raises gain, but how about raising plate and cathode together??

When you increase the cathode resistor you are decreasing the transconductance and the plate resistance increases.....reduced plate current creates a smaller voltage drop across the plate resistor and reduces the available swing for output voltage (ie more gain)

[scottl]

Hi Ben,

Have you seen this page?

http://www.tone-lizard.com/Mods_and_Odds.htm

They have a table that explains the effects of changing these resistors. Unless I am reading it wrong, they are contradicting you by saying you get less gain by increasing the cathode resistor.

My brain hurts now!

Btw, I had breakfast at Namm with your Fargen Amp guy.... Can't remember his name! But cool dude!

Someday I'll play that Bordeaux or maybe the new VOS...

[BENJAMIN FARGEN]

Scott,

I think you are reading his chart wrong........

This info is pulled directly from the site you linked which is basically exactly what I listed but in less technical fashion.
Increase value of R1(plate resistor) (typ 100K) Increases gain, to a point.

Increase Value R2 cathode resistor (typ 1.5K) Lowers gain/increase bias.

The ultimate tone by Kevin O Conner is a good reference for understanding this type of info
Take care,
BF

[rick66]

after sending out an email to ben, i just now realize he didn't want anyone emailing him directly on these mods. so, i'll ask the tech people here:

my question is does the voltage rating on the cap matter for the 500pF recommendations ben listed above? my cap are rated 100V. thanks for the help.

(and sorry ben for the email)

[BENJAMIN FARGEN]

Originally posted by rick66
after sending out an email to ben, i just now realize he didn't want anyone emailing him directly on these mods. so, i'll ask the tech people here:

my question is does the voltage rating on the cap matter for the 500pF recommendations ben listed above? my cap are rated 100V. thanks for the help.

(and sorry ben for the email)

No worries:>)

Yes & No..... your100V 500PF cap will work fine accross the volume pot...there is no high voltage accross the Volume pot

As for the swapping the 270PF out for a 500PF you will need at least a 300VDC cap...silver mica 500VDC-1KV can be found at most electronics stores and would be the best

BF

[rick66]

thanks for the reply, ben. i'm having trouble finding a silver mica cap you suggested for the swap of the 270 pF. thanks for the clarification.

[aroman]

Originally posted by BENJAMIN FARGEN


adding gain to dual input newer models:

change the first stage plate resistor from 100k to 220k
change the slope resistor from 56K to 33K




If I tell that to an Amp tech will he be able to find the first stage resistor and slope resistor without a circuit diagram ?

How much should I pay an Amp tech to do something like that ?

[BENJAMIN FARGEN]

Originally posted by aroman
If I tell that to an Amp tech will he be able to find the first stage resistor and slope resistor without a circuit diagram ?

How much should I pay an Amp tech to do something like that ?

Hi,

I would say no more than a 1 hr bench fee and the parts cost should be under $5.

Thanks,
BF

[calieng]

Can anyone offer some suggestions...

I did the mods on a single input model and now it is too bright...

Should I adjust the 500pF bright cap value (maybe 120pf)??? Or the 270pF cap I changed on the board to a 500pF that feeds the treble pot??? Or just remove the bright cap and leave the 500pF on the board alone???

I want to get about halfway between how the amp was and how it sounds now.

Thanks for any help.

[BENJAMIN FARGEN]

Originally posted by calieng
Can anyone offer some suggestions...

I did the mods on a single input model and now it is too bright...

Should I adjust the 500pF bright cap value (maybe 120pf)??? Or the 270pF cap I changed on the board to a 500pF that feeds the treble pot??? Or just remove the bright cap and leave the 500pF on the board alone???

I want to get about halfway between how the amp was and how it sounds now.

Thanks for any help.

Remove the bright cap accross the volume first.

Regards,
BF

[calieng]

...yep...that's right on the money.

Thanks Ben!

[BENJAMIN FARGEN]

Originally posted by calieng
...yep...that's right on the money.

Thanks Ben!

No problem....glad I could help.

BF