Mid-30s amp, interesting power supply

[Diverted]

Just drew out this schematic this morning for a mid-30s amp I recently picked up. Has anyone ever seen a power supply with the FC going to ground? I haven't; pretty interesting.

[tubeswell]

yep

[sluckey]

And another...

https://sluckeyamps.com/rocky/rocky.pdf

[Diverted]

Sluckey, that's a slick looking amp, I may build it. I see the 5K resistor in place of the FC; what about the center tapped resistor? Seems like a hard part to find. What about two 820s in series, with the filter cap to ground between them?

[sluckey]

I would replace the center tapped resistor with two 750Ω/10W resistors. Here's that center tapped resistor...



The 5K/30W resistor is mounted on the side of the chassis.

[Diverted]

Thanks. Oh cool, a candohm. I see those all the time in 1930s radios, not so much in amps.

[Diverted]

I'm sure I drew out the schematic accurately, but can anyone verify that power supply? It seems legit to me based on what I saw tracing it out, previous posts here and the two schematics provided by commenters, but received this comment elsewhere and I was wondering what you all think:

"Something is a little fishy on that field coil. How does it survive shunting across the B+ DC reservoir rail as shown if it is a pure inductance? Impossibly light gauge wire to support a really large DCR and keep it from burning up and shorting out your supply? 😉 Of course that would reduce the magnetizing current to result with a wimpy magnet...are you sure that's how it is wired?"

[sluckey]

"Candohm"???  I was unfamiliar with the word, so I had to look it up. Seems they were a common failure item. Back in the '60s I restored a lot of antique radios, mostly consoles from the '40s and early '50s. I don't recall ever seeing a candohm resistor in any of that stuff.

This old 1954 vintage amplifier was removed from a non-working Rock-Ola model 1442 jukebox that was beyond restoration.

Learn something new every day. Wonder how long that memory will last. It's OK. Can't know what you forget.   

[Diverted]

Wow, it's a rare day indeed when I teach you something you don't already know! 
I have seen tons and tons of them and replaced them as a matter of course even if they weren't shorted to ground, which maybe 30 percent of the ones I saw were. I mostly collected and worked on Zeniths from the late 30s and every model from that time period used them ... other manufacturers not so much. Here's a typical one (far left) from a 1938 Zenith 12S267.

[sluckey]

received this comment elsewhere and I was wondering what you all think:

Don't worry about it. There were plenty of Field Coils wired exactly as you show. The commenter was concerned about it being a pure inductance and that's ridiculous. All field coils have a DCR.

Heck, my Rock-Ola had a DCR of 5000 ohms. 300V B+ and 5K equals 60mA and 18W, very believable. Look at the size of the PT on my amp. And it doesn't even have a 6.3V filament winding...

    https://sluckeyamps.com/rocky/rocky.htm

[Diverted]

I was wondering why the FC was such a large DCR (7.5K) when I first started looking. I'm used to seeing 500-1500 ohms. Original filter can rated for 350VDC, so B+ may be 330? and FC is probably seeing maybe 275-300 after the 200 ohm choke? At 300 there's 40ma.

[sluckey]

The lower DCR ones were usually wired in series with the B+ and served double duty as a filter choke. This was a very common configuration in smaller amps such as found in console/table radios. But a jukebox needs a big amp with a big speaker and big speakers need a big magnet (FC) and that requires a lot of current.

What is the project you're working on?

[Diverted]

It’s a mid-30s amp I picked up for a song. Couldn’t find schematic anywhere so drew up the one in this post. Single ended 6L6, so primitive there is no volume or tone control. Not as Spartan as my 1932 Rickenbacher… that one doesn’t even have an on/off. Plug and play!😂