Brownface Super Bias circuit diode

[martin manning]

I don't know if that diode is original, but it is an early plastic package. I recall seeing something similar, probably in the late '60's. Metal can "top hat" style rectifiers were the usual type seen in Fender amps, I believe.

Seems like you have the right plan, replace only parts that are bad, out of spec, or safety critical. Do you have it working now? Some of the old resistors may be noisy (need to listen), and some of the film caps could be leaky at working voltages. You'll need to check for DC voltage where it shouldn't be, and be aware of possible leakage through the fiber circuit board.

[brwnfce]

Thanks again, will do. Cheers.

[brwnfce]

Sorry, no not working yet, got to get a current limiter sorted out.

[brwnfce]

Current limiter not so easy to procure here in the UK, Kendrick amps make one, looks the business, but it's $139 before shipping and our wonderful 20-23% sales tax and I'd still have to find a 120v 100w tungsten filament lamp. Is there another method I could use?

[dgrainger]

I wired one up from hardware store parts a few years ago. Hit google for "light bulb current limiter"

[brwnfce]

HI, yes I hear you on that, but the US plugs /recepticles are not readily available here from what I can see.

[martin manning]

Perhaps you can find incandescents made for some specialty purpose. A couple of these in parallel would be fine: https://www.thelightbulb.co.uk/40watt-s ... pliance-2/

[pdf64]

Old independent hardware / discount / clearance shops in run down areas used to be good for filament bulbs.
Also ask around friends and family, someone’s going to have spare incandescent or halogen bulbs at the back of a dusty shelf.
Ideally build up a few wattages, eg 40, 60 & 100W.
Or a 40 and a 60W could be run in parallel.
Then get appropriate lampholder/s, switches, sockets and a box.
The safest lamp holders are a bayonet type that have a built in isolation of the lamp terminals when there’s no lamp fitted. So it’s worth getting that type , then use adapters if the bulbs you get happen to be edison screw types.
I used an IEC male inlet, IEC female outlet and a regular switched 3pin socket outlet.
A 20mm fuse holder and a toggle switch to bypass the LBL. You may need a bit of terminal block, to help keep the neutrals and earth wires grouped together.
You’ll probably be able to get everything from CPC,

[brwnfce]

Thanks I'll try them.

[R.G.]

I would vote for replacing that diode with a 1N4007 too. As noted, the bias supply is critical. Other things can die from a loss of the bias supply.

On a more heretical note, I'm also in favor of putting a silicon diode in series with both cathodes of your rectifier tube. You can get 1kV/6A diodes for under US$0.50 each. One of these in series with each leg of the PT into the rectifier tube will mean that if the rectifier tube shorts, you still won't put AC onto your filter caps, killing them. In fact, if the rectifier tube shorts, you might not even notice; the amp will keep playing, just with a modestly higher B+.

[CraigGa]

Current limiter not so easy to procure here in the UK, Kendrick amps make one, looks the business, but it's $139 before shipping and our wonderful 20-23% sales tax and I'd still have to find a 120v 100w tungsten filament lamp. Is there another method I could use?

Why do you need US parts? I made one for not much more than a fiver with parts from Wilco

Craig

[brwnfce]

Just now at the powering up via the current limiter stage, posting shots of the Bias circuit and 3 prong power connections before doing so.

[martin manning]

Just now at the powering up via the current limiter stage, posting shots of the Bias circuit and 3 prong power connections before doing so.

Connections look ok on the primary side of the power transformer, but it doesn't look like the AC inlet strain relief is fully engaged, and you should feed more cable through, so that an inch or so of the outer cover is inside the chassis. Heat shrink is better than electrical tape, or, you could use one of the ground switch lugs for a tie point and eliminate that spliced connection. The earth ground lead is best secured to a dedicated bolt, but that means you'd have to drill a new hole, and I can see where people are reluctant to do that to a vintage amp. The bias board looks ok, can't see the rest of the circuit.

[martin manning]

...Oops

[brwnfce]

Copy that on the power cord, outer casing is caught right on the grommet, was a struggle using the original, the splice is twisted horizontally, soldered,tube shrinked and taped. Apologies to the purists but I spliced new components in, J hooked then soldered, I'm not confident enough in my soldering skill level to start messing behind the boards. I'll revisit the power cord but the amp will not be getting gigged or moved at all for now. Used the Sozo for both cosmetic reasons and as they were recommended in those positions by the person who rebuilt a Super Reverb for me. I already had the F&T electrolytics.