HH Bass Machine

[Ted]

I have in front of me an HH Bass Machine 250W MOSFET amp with a burnt out mains transformer.

No schematic as yet. Genuine spare parts probably very unlikely...

The transformer appears to have 45-0-45 secondary windings fused at 6.3A.

It also has a 90V winding fused at 100mA. Before I start pulling it apart, does anybody have the faintest idea what this might be for?

[Enzo]

A 90v winding? My first three guesses would be for a preamp tube power supply.

[Ted]

A 90v winding? My first three guesses would be for a preamp tube power supply.

My first guess too. But no. It's an all semiconductor amp.

It looks as though the 90V does go to the preamp however...

Its got a phaser and a graphic built into the preamp. My second guess was a Magnatone-style varistor circuit to create phase shift. There are a couple of hefty power transistors around the phaser circuit on the pre-amp.

[Fragger]

These people may be helpful, it`s hard to say?

MAJ Electronic (scroll down)

[Ted]

Yeah, I know about them. I mailed them yesterday and am awaiting a response.
But thanks.

[Ted]

As suspected, nothing available off the shelf.

I can get a 500VA 45-045 toroid easily enough. For the 90V supply - supposing I just wrapped some 16/0.2 around it - obviously it would need some experimentation, but I seem to remember toroids being about 2V/turn. it is only 100mA after all.

From looking at the schematic from MAJ, that 90V is half-wave rectified to give a 110V DC rail. The front end of the preamp runs from +/-15V and has bipolar transistors.
The 110V is zenered down to 47V to run the the back end of the preamp which uses FETS.

[J M Fahey]

Well, it's reasonable to feed single rail FETs with >40V DC, to roughly match Op Amps running from +/-15V.
So if that's the only use for that extra winding and it complicates your sourcing a replacement, I'd use the regular 40+40 VAC (or thereabouts) you'll get , which means ~60/65V rails for the power amp, and get my +47V from there, with a simple Zener+resistor.
No *real* need for winding an extra secondary.

I guess HH *already* had that transformer on stock ad, of course used the "spare" winding ... or maybe it was there to feed the electroluminiscent panel .... but if you don't have one, why worry?.
Good luck.

[Ted]

Well, it's reasonable to feed single rail FETs with >40V DC, to roughly match Op Amps running from +/-15V.
So if that's the only use for that extra winding and it complicates your sourcing a replacement, I'd use the regular 40+40 VAC (or thereabouts) you'll get , which means ~60/65V rails for the power amp, and get my +47V from there, with a simple Zener+resistor.
No *real* need for winding an extra secondary.

I guess HH *already* had that transformer on stock ad, of course used the "spare" winding ... or maybe it was there to feed the electroluminiscent panel .... but if you don't have one, why worry?.
Good luck.

I was having those very same thoughts. I wondered about the electroluminescent panel as well - this amp doesn't seem to have one.

[Ted]

Seems fine running from the main 62V rail. I had to change the zener dropper resistors on the pre-amp boards but that was all.

Wouldn't be the first time that production expediency has been mistaken for design genius.