Brownface Super Bias circuit diode

[brwnfce]

Hello all,

New to the site, wondering if anyone can advise, going over a 1961 Super, can't find anything on the diode in the bias circuit, thought I had everything bought, caps, resistors but missed this, diode is marked F4. Should I replace this, can it be left alone, what would be a suitable replacement? Any help would be very welcome.

Cheers.

[martin manning]

I'm assuming you have a 6G4 or 6G4-A? If the one in the amp is working you can leave it, but some people would replace it with a modern rectifier for peace of mind. A 1N4007 would be fine for that.

[brwnfce]

Yes it's 64G-A, and it's working fine, I'll get a replacement on standby until needed, thanks for the information , much appreciated.

[brwnfce]

6G4-A!

[pdf64]

That diode is mission critical, some failure modes (eg gradual breakdown) might wreck the transformers.
For the sake of a horrid old (selenium?) diode, it seems somewhat reckless to leave it in

[sluckey]

For the sake of a horrid old (selenium?) diode, it seems somewhat reckless to leave it in

Fender did not use selenium in the 6??? brownface days.

[jabguit]

For the sake of a horrid old (selenium?) diode, it seems somewhat reckless to leave it in

Fender did not use selenium in the 6??? brownface days.

Selenium rectifiers were in the 5E6 layout from Fender. Were they abandoned in the 60s?

[martin manning]

Selenium rectifiers were in the 5E6 layout from Fender. Were they abandoned in the 60s?

Yes. When silicon rectifiers became available selenium rectifiers were replaced. You can see when that happened in the early Fender schematics, e.g. from 1959 5F6 to 1960 6G6 Bassman.

[sluckey]

Selenium rectifiers were in the 5E6 layout from Fender. Were they abandoned in the 60s?

Selenium was used in ALL 5??? series fixed bias amps. Silicon was used in ALL 6??? series (and later) fixed bias amps. 1960 seems to be the year that Fender transitioned from tweed 5??? series amps to brown 6??? series amps.

[martin manning]

1960 seems to be the year that Fender transitioned from tweed 5??? series amps to brown 6??? series amps.

Fender model code is: <Decade of 20th century, numeric><Circuit variation, alphabetic beginning with A><Amp model, numeric><Small circuit variation, dash+alphabetic beginning with A>

[martin manning]

Selenium rectifiers should definitely be replaced with silicon, since they can release toxic compounds when they fail. A silicon diode in a fixed bias supply is not under a lot of stress, but replacing it is cheap insurance against possible failure, further damage from which which may or may not be arrested by the mains fuse.

[brwnfce]

I forgot to ask, worst case scenario should the diode fail what would be the consequence?
Regards.

[brwnfce]

IMG_1015.jpeg

[martin manning]

I forgot to ask, worst case scenario should the diode fail what would be the consequence?

Nothing good. If it fails open, you lose the negative bias voltage and the power tube current will shoot up. If the mains fuse doesn't open, the power tubes will overheat, as will the output transformer, and one or both could be damaged. If it fails shorted, the bias is lost, but now you also have AC on the bias filter caps, which will cause them to fail.

The rectifier in the photo looks newer than 1960. Can you read the other numbers? The old Astron cap should be replaced.

[pdf64]

As mentioned, my view is that rather than a total failure of the bias supply diode, the worst case scenario would be its gradual breakdown. That would allow a bit of AC on to the reservoir ecap, which it would self rectify but become damagingly hot in the process.
The cap would gradually fail, thereby reducing the bias supply voltage from the AC peak to the AC half wave average, eg magnitude reduced about 30%?
The valves and PT would run hot but, because there was still some bias, the amp probably wouldn’t draw enough current to blow the mains fuse. And the whole thing would slowly bake itself into a smoking, stinking wreck.

Hence my view that any component involved in getting bias from the PT to the output valve’s grid is mission critical; including grid leaks and grid stoppers.