Reviving an AIMS Producer

AIMS was a maker of amps that was around for a short time in the seventies. It is said that it was started by a couple of refugees from Fender but I have no way of verifying that.

That said the VTB-120 is quite similar to a Fender Bassman with massive transformers and a pair of 6550s running at about 530 volts on the plates. I may have mentioned this amp before but it was recently acquired by a friend of mine and brought in for some needed maintenance.

Here are some pictures of what I found plus a shop sketch detailing what it was, what it was supposed to be and what it ended up being.

20 uf 600v capacitors are pretty hard to find and expensive to boot-the last one I bought was a Sprague and it ran me thirty dollars.

So what I ended up doing was some math and some research, and a liberal borrowing of  news from Fender schematics, and I ended up with two pairs of 100uf 350v caps in series with appropriate balance resistors-much better than the bodged 220uf axial snap-ins that are in the picture.

Calculating capacitor value is the opposite of calculating resistances.

With capacitors in parallel the capacitance is C1 + C2  and so on and the voltage rating does not change. When you place two capacitors in series as I did the sum is half the capacitance and twice the voltage rating-and you should have balance resistors. So, two 100uf 350v capacitors in series is equal to 50uf at 700v. You can calculate this by using this formula:

 Ct=C1xC2/C1+C2

He'd said it was making strange noises and that proved to be a loose phase inverter socket which I plan on repairing tomorrow. That and a bit of bias tweakage to get the level down a bit will be helpful as it is drawing slightly too much current for my taste..

More pics tomorrow.

New power supply configuration.