JMP Superbass 100w non master model# 1992

[VortiganX]

I probably didnt do a full conversion but I followed as best I could. I will add the pics when I can.

I added .68 cap on V2

I replaced 2 caps on V1 with .22 caps

and I replaced the cap on the presence with a .68 (not sure on this one.)

I do not know what 100k feedback on the 4 ohm tap means so I have not done anything there. If anyone has any suggestions it would be much appreciated.

I still need to get the right tubes and bias it to 25ma.

with the couple things that I have done I get plenty of distortion. I love it.

[neikeel]

I hope you mean 0.022uF caps on V1?

The NFB is the negative feedback loop, early amps used a 27k resistor on the 16ohm tap of the OT to give more NFB to keep the power section under control, a bit later 47k on the 8ohm tap was used and later still 100k on the 4ohm tap which gives a lot less NFB. Thi scan make the amp feel quite loose and trashy (particularly if you have boosted the upper mids with a 0.68uF cap on V2a and added a 0.68uF cap on the presence pot) but it might give you the tones you are after

[VortiganX]

hah yes I meant .022!

thanks for all the info. It is much appreciated. Ill get a 100k resistor and put it on there soon.

[VortiganX]

O.o


o.O


so.....

do i need a 1 watt 100k ohm carbon film resistor

or a 2 watt ?

[neikeel]

For the NFB? A 0.5w is usual, can be cc, cf, mo or mf. Just needs to have long enough leads to reach the turrets!

Re: bias

The figure of 25mA means nothing on its own

Correct bias will depend on the tubes and the plate voltage at idle with the bias chosen (usually the one that sounds best) not an arbritrary figure. Maximum safe bias current would be 70% of max dissipation at idle:

For an EL34 with 25w max = 17.5/plate voltage.

So if your plate voltage happened to be 700vdc then that would be the highest bias reading you could use.

If your amp has a more typical plate voltage of 480vdc then 36mA would be the hottest you could go. Coincidentally if you have 480v on the plates a figure of 25mA would be biased at 48% of maximum dissipation which is quite cold. Of course if that sounds good to you by all means use it as your tubes should last!