Selenium rectifiers

[Findm-Keepm]

Originally Posted by Electronic M

Option B would be to take 7 modern diodes and string them in series as 7*0.7 = 4.9 which should be close enough to 5 to not matter much, and possibly cheaper than a dropping resistor.

He's got two selenium rectifiers in a voltage doubler configuration - best not to use series diodes in his application but they are fine in a straight half-wave configuration.

Of note, I had fun experimenting with an old GE Selenium today - put 40V from one of my Lambda supplies on the anode, and hung a 120 ohm 10 watt resistor off the cathode to ground - got 353 mA through the selenium, and had a volt and a half of loss through the selenium. My guess is they get leaky, and drop less across them as they age. I've got a coffee can full of Sarkes (ST logo) seleniums in the shop at home, and will try one or two later tonight. I think my Sorensen DCR40-40 is up to the task - just gotta get the right load without exceeding the limits of the selenium rectifier, as my shop at home is quite small and I don't want/need a stinkfest again. I had a Silvertone portable give up it's flyback in there last May, and it still smells of burnt flyback.

Cheers,

[Kamakiri]

Brian, if you'd be so kind, please review with me where I'm at, and the next step in the process.....when doing something that I've never done, I like to be very clear on what I'm doing.

Here's where I'm at right now.....

I have in my possession, the following parts (in quantities more than I need I assume):

(4) 1N5408 diodes, rated at 3 amps.

(8) 39 ohm 5 watt resistors

As shown, I've removed the selenium rectifiers, and installed two terminal strips, marked with the respective polarities. The underneath shot is just showing my work.

The selenium rectifiers that were in the set had a part number of 13-84630-2, and were rated 450 mA. Replaced at one time, I assume, with a heavier duty part.

[Bill Cahill]

Now, why do I have a feeling you will burn up that resistor????

[Kamakiri]

Originally Posted by Bill Cahill

Now, why do I have a feeling you will burn up that resistor????

Bill, I've got a lot of hours in working on this set and I'm trying to do something that I've never done before.

That said, mind elaborating?

[Findm-Keepm]

Tim,

Attached is a simplified schematic of what you are doing - I drew it from memory - if you want a marked up copy of the Sams, I can scan that tomorrow, noon-ish. The Sams is out in my shop, and I am in for the evening.

As you can see, the silicon 1n5408's are simply installed where the selenium was removed - cathode (banded end) to the "+" connection, and anode to the "-" connection. Off of the cathode of the output (rightmost) rectifier, we add in a series resistance. At 39 ohms/5W, you are good for up to about 12V of voltage drop across the resistor. I LOVE those RH-series Dale resistors you've chosen, as they make for easy mounting, and the chassis can sink some of the heat. Some purists want only axial or old ceramic resistors in their sets.


Sams (IIRC) says the B+ should be about 195V for your set.

If the B+ is low, (lower by 5-8 volts or more) add some Parallel resistance to bring it up. I don't think that will be the case, as 17 to 75 ohms is my experience for most series resistors in tube sets.

If the B+ is high, (higher than 5 volts or more) add some Series Resistance. Doing so will not only drop more voltage, it will also decrease the power dissipated by the (each) resistor.

Your "fusistor" is the 4.7 ohm 15 Watt resistor under the chassis in one of your photos.

Hope this helps - I'm up for another hour or so - I have to take my daughter to school in the morning, so an early evening for me.

Cheers,

[Findm-Keepm]

I re-read what I wrote above, and I wasn't clear that each selenium rectifier is replaced by one 1N5408.

Cheers,

[Kamakiri]

Originally Posted by Findm-Keepm

I LOVE those RH-series Dale resistors you've chosen, as they make for easy mounting, and the chassis can sink some of the heat. Some purists want only axial or old ceramic resistors in their sets.

That to me was an obvious choice in this case. That resistor replaced a resistor that used to be in the lower right corner of the second picture. There was so much heat there that it ended up burning the terminals right off the phenolic strip, and the old resistor was just dangling. I also upped the rating on it, and used a 25 watt instead of a 15 IIRC.

I fully believe in over-building a circuit, especially when it's apparent that the way it was made just wasn't good enough over the long haul

I'll be doing this job today, after all the housework I just ran out of steam last night....

At the risk of sounding like an idiot, B+ is checked across the first filter cap, yes?

[Findm-Keepm]

B+ is measured at the point I marked on the schematic. B+ refers to the main DC supply in most sets - it's a carry over from the old battery days when they had A and B batteries - A Batteries were for filaments and low voltage bias, and B batteries (45V, 67.5V and 90V) were for the plate and other high voltage needs.

I marked up the schematic for you - hope it helps. Start with a 39 ohm 5W resistor and measure your B+, and follow my previous advice if it is high or low.

The Sams should carry you forth from here.

Cheers,

[Electronic M]

Originally Posted by Bill Cahill

Now, why do I have a feeling you will burn up that resistor????

Those metal cased types don't burn they explode....Don't ask me how I know.