Bias selenium rectifier replacement

[olddawg]

What are you using the amp for? Guitar? Has the preamp been modified. While you are putting in diodes you may consider converting it to a SS bridge rectifier. PAs were usually designed for clean overhead. Might even make it more usable. And you could save your NOS tube for something more appropriate.

[Dave H]

While you are putting in diodes you may consider converting it to a SS bridge rectifier.

Did you mean a two diode full wave rectifier (not a bridge)?

[tboy]

I recall the early days of Ted Weber's Bullettin Board. Maybe someone from back then is still around here!

The history of this board goes back further than Ted's. He was an early member of our community and checked in on us nearly every day until his death. There are still a few of us around who have been here since 'the old days.'

[lo-fi]

What are you using the amp for? Guitar? Has the preamp been modified. While you are putting in diodes you may consider converting it to a SS bridge rectifier. PAs were usually designed for clean overhead. Might even make it more usable. And you could save your NOS tube for something more appropriate.

I'd like to use it for guitar. I knew those Dukane 1U460s are good platforms for building a 5F6-A Bassman. Anyway, I don't want to disassemble an old amp to build another Bassman clone.
Actually, as I plugged a guitar in the PA I noticed it's not bad at all stock. It sounded loud and warm into a 4x10 Matchless cab (I play classic blues and I fall in love with every obscure amp made in the 50s!).
The tone stack is basically a James and I liked the idea of having an alternative to Fender mid scooped tone stack. Mic channels have cascade gain stages. Both trannies look beefy. Given that you will need just one channel there are a lot of spare tubes to use for vibrato and, eventually, a spring reverb. I agree, the preamp might need some mod but it already sounds better than many amps I've heard. Then I'd add a cathode/fixed bias switch and a raw control (to reduce tone stack signal for more gain). I've already replaced the 12au7 with a 12at7 in the PI position.
A 3-prong cord was a welcome upgrade even though the amp was really quiet anyway.
Installing a SS rectifier was an option but I usually like tube rectifiers' sag the best. I love old school tones and SS rectifier at times makes it all a bit too stiff for me.
So, I won't give up on that old PA. Hopefully it's just a faulty tube rectifier.
I understand the importance of a light bulb meter when modifying the power section of an amp. I'll build it as soon as I can locate some old tungsten lamps (nowadays they're all LED...).

Last but not least...yes, no matter the physics and chemistry you studied in your life: you cannot say you know atoms until you don't discover the existence of...morons!

[olddawg]

Did you mean a two diode full wave rectifier (not a bridge)?

Whatever works... you may not need a tube rectifier. It will also raise your B+ too. Cost really nothing to try.

[Dave H]

There are still a few of us around who have been here since 'the old days.'

And I'm one of them. I was young back then. I'm older than that now
It was in the days of Windows 95 but now we have the much improved Windows 10

And thanks tboy for putting up with us for all those years.

[Enzo]

Maybe not a real early member, but I recall the old Ampage.

[lo-fi]

And I'm one of them. I was young back then. I'm older than that now
It was in the days of Windows 95 but now we have the much improved Windows 10

And thanks tboy for putting up with us for all those years.

We're all older, my friend! But your words reminded me one of my favourites Dylan's songs:

"...I was so much older then, I’m younger than that now"
(My Back Pages - Bob Dylan)

That's a better way to take it, isn't it?

We all miss Ted Weber! He was a real friend of music and musicians. Always inspired and always inspiring. Too soon gone.

Yes, Win95 and the basic Bullettin Board user interface (before web marketing, massive advertising, social madness hit the Internet). I recall Mark Norwine, Andy Ruhl, John Stokes, and many many others. Tboy of course! It may sound nostalgic but that was definitely fun.

[Helmholtz]

Whatever works... you may not need a tube rectifier. It will also raise your B+ too. Cost really nothing to try.

Provided the filter caps can take the increased voltages.

[lo-fi]

Just a quick (and final) update about the issue.
It took me some weeks to find some spare time to build a light bulb limiter (thanks patlaw for the good advice I've always overlooked before!), but I finally did.

I turned the amp on using the limiter after having installed all the stock tubes. Don't ask me why but everything worked out just fine. Both 5U4GB rectifier and old RCA 6L6GC power tubes were not blown. Bias voltage was correct and power tubes' plate current (transformer shunt method check) had the value I expected without the crazy oscillation I had read soon after diode proper installation.

The only possible reason, according to my far from being perfect knowledge, is the fact I used a couple of quite old NOS electrolytic caps (20 years old, never used before) with no smooth forming. Probably that first rough power supply feed caused some temporary anomaly.

Now everything is working fine, even without the light bulb limiter.
So, that first mistake in diode installation was not fatal for any component. Some good luck always helps!

Thanks everybody for your precious support.