Vintage JMI AC30 - Convert added Top Boost to Non-Top Boost

[drpicklo]

Thanks in advance for your replies!

I have a copper panel AC30 with the "added top boost" in the back. I actually prefer the brilliant channel in the non-top boost amps. Does anyone know how to convert back to the non-top boost? I've reviewed a few schematics but I really don't want to kill this amp. I've done some builds and pedals, so I am handy with the iron. Perhaps someone who has converted one to a top-boost can talk to their experience and potential "re-engineering" back to the non TB.

 

[Cirrus]

The top boost module goes between the first valve stage and the phase inverter. So you'd want to break the connection from the top boost volume knob to the grid of V2. Then you'd want to break the connection between the (220k?) resistor at the end of the top boost tone stack and the coupling cap into the phase inverter circuit.

Then connect the top boost volume knob straight to the phase inverter's input coupling cap.

That pretty much covers it I think. I guess you couple "power down" the top boost circuit by disconnecting its filter cap (~30uf iirc) from the H+ line too.

I think the cathode resistor/ bypass cap arrangement of V1 can be variable too. Some amps share a 1.5k resistor and 22uf cap between both halves of the valve, others have separate resistors and caps which gives an opportunity to roll off some gain at the bass end on the brilliant channel.

 

[Masco]

Then connect the top boost volume knob straight to the phase inverter's input coupling cap.

Output of the Brilliant volume pot wiper through the 220k ohm mixing resistor to the input coupling cap of the PI. Don't forget the mixing resistor or you'll get a lot of bleed through from the other channel. You'll also have to remove the 100pF cap across the volume pot. Disconnecting the TB module filter cap won't disconnect power to the unit because the other end is just connected to ground. You have to find the V+ feed coming from the 22k resistor from the tag bds. and disconnect it. I've always seen 1k5 resistors on the cathode of V1, even if they are treble versions bypassed w/ a 100n cap. You could always voice differently by splitting the cathodes and using 2k7 Rk's bypassed w/ you fave bypass cap value(or none at all). YMMV.
If you'd rather keep the TB unit connected, you could easily convert the Normal channel to a NTB Brilliant channel by simply replacing the 47n cap w/ a 500p coupling cap. Just a thought.

 

[drpicklo]

Would it be easier to convert the Normal channel to the non-TB brilliant?

You guys are awesome - thanks for the fast responses! If I only had that JMJ 30

 

[Cirrus]

Sure sounds that way!

 

[drpicklo]

Is it just a few caps?

 

[Cirrus]

If you'd rather keep the TB unit connected, you could easily convert the Normal channel to a NTB Brilliant channel by simply replacing the 47n cap w/ a 500p coupling cap. Just a thought.

Voilà!

 

[drpicklo]

Is this the correct cap near the normal volume?