Fender Bassman 5F6A Repair

[Helmholtz]

With rectifier tube removed, I am seeing around 58 ohms and 62 ohms at pins 4 and 6 of the rectifier socket. Seeing about 120 ohms across those pins.

That's normal secondary winding DCR.
Replace the rectifier.

[mozz]

What is this bare wire ?

[pdf64]

That wire nut (on the incoming neutral?) has no place in a guitar amp.
How exactly is the chassis connected to the incoming safety ground green wire?

[misterc57]

Turned out to be the rectifier tube which tested good and was working a few days earlier, it is now shorted.

Green AC cord ground wire is soldered to a lug on the PT bolt/nut.

Black hot AC cord is soldered to fuse

White neutral AC cord is soldered to PT primary and wrapped by a wire nut and electrical tape

Borrowed a 5AR4 from another amp and swapped preamp tubes, all seems good. Most of the noise was at V2 with the original tubes.

Will be ordering replacement tubes for rectifier and preamp.

Going to remove connections from the selenium and use the plus tab as an attachment point for a 1N4007 and the other end of the 1N4007 to the board 15K

[mozz]

From the MEF archives. Let us know what brand it was Sovtek JJ? Make sure your fuse is correct.

"just connect a bleeder resistor across the standby to prevent the shock to the rectifier tube. It'll go a long way toward saving the rectifier."

[misterc57]

From the MEF archives. Let us know what brand it was Sovtek JJ? Make sure your fuse is correct.

"just connect a bleeder resistor across the standby to prevent the shock to the rectifier tube. It'll go a long way toward saving the rectifier."

Old rectifier is a GE Great Britain. 3A slow blow fuse

What value and wattage bleeder resistor across the standby? There was a .05-600 cap to ground on the rectifier side that was removed perhaps that was protecting the rectifier?

Thank you

[pdf64]

Old rectifier is a GE Great Britain. 3A slow blow fuse

What value and wattage bleeder resistor across the standby? There was a .05-600 cap to ground on the rectifier side that was removed perhaps that was protecting the rectifier?

Thank you

General Electric GE is the US company, the General Electric Company GEC is an unrelated UK company

I suspect that the 0.05uF unswitched reservoir cap might help to protect the rectifier from the extreme peak anode currents of that horrid hot switching standby.

It seems better to wire the standby permanently closed, as has no technical benefit, or at least advise the users to avoid ever using standby.

But as the use of standby is so ingrained with guitar users, a resistor across the standby switch could help to reduce the switching current surge, maybe 47k to 100k 3W minimum, rated for the Vpk, eg 500V. A decent MO with flameproof coating may be a good option.

Be aware that with the resistor, standby might not result in a complete mute. The higher the resistor value, the better the mute, but the switching surge may be higher.

The resistor idea has been documented by Merlin, though Fender used it in the 70s
http://www.valvewizard.co.uk/standby.html
https://el34world.com/charts/Schemat...nder_ps300.pdf

[Chuck H]

General Electric GE is the US company, the General Electric Company GEC is an unrelated UK company

Just an observance... Rebranded tubes aren't unheard of. Even between manufacturers. I haven't known GE to do this but my experience with their tubes is limited. I would think misterc57 is familiar with GE as a company and vintage tube producer. So maybe the tube IS a GE (General Electric, US) AND says "Great Britain" somewhere on the glass??? I don't think it's probable, but it wouldn't surprise me because I think it's possible.

[pdf64]

...
Green AC cord ground wire is soldered to a lug on the PT bolt/nut.
...

That's the worst place to fasten it.

[mozz]

To spot a GE tube sold by another vendor, I always look for the dots or a 188-3 . I'm thinking his bad tube was a older Euro tube mullard or such. They were usually more robust than anything you can buy today. I'd still check the filter caps.

[misterc57]

That's the worst place to fasten it.

Suggestions?

[misterc57]

Old rectifier

[g1]

AC cord earth is always supposed to have it's own dedicated fastener. When we were bringing things up to CSA spec., we were required to drill a hole in the chassis and install nut and bolt with appropriate lock washers for the lug on the AC cord earth wire.
Also, that wire is supposed to be left longer than hot and neutral, so it is the last thing connected if the cord gets ripped out of the chassis.

[misterc57]

Did some work today and catching up on these posts...

Installed a 1N4007 to bypass the selenium bias rectifier..

Seeing the following V DC values at the power tubes

pin 3 447
pin 4 446
pin 5 -51

Are we okay with these values?

Installed a new .047-600 cap at the Standby switch per schematic

Removed neutral wire nut install and now using the ground switch (two lugs jumped)

[mozz]

You put a 5ar4 out of a HP test equipment in there (expensive tube) , most likely a Mullard. I would be using a Sovtek or JJ until you have figured out there is no excessive current draw. How many watts draw at idle?