This set doesn't have a power transformer. It's series string.... I still have the concern for the wattage. I didn't realize you were saying baxically the same thing as I. But, I still doubt it will be as low as 30 ohms..In any case, there is the surge factor... I prefer to start high, and, work my way down.. Perhaps ten watts is plenty, but, Tim is afraid to even try that. He is afraid he will dammage the set. I tried telling him that wouldn't happen, the worst that could happen is the resistor prematurely fail.. I can understand why he refuses to spend sixty dollars on twenty watt resistors, but, I tried even telling him to put resistors in parallel to get the desired wattage...
Bill Cahill

Even if it is a series string, the filaments do not come into play - they are a separate leg/circuit. The total load coming from the doubler cannot exceed the ampere rating of the original seleniums, which Tim stated was about 320mA.

Second, the voltage drop across the selenium is rarely above 5V, and with the 7/10s of a volt that a silicon diode drops, you only have to drop the additional 4.3V or so. You aren't dropping tens or hundreds of extra volts, so the resistance and wattage are going to be low.

I stand by the calculations - two previous posters also weighed in, saying 25 watts is overkill - and I too have never used more than a ten watt resistor. In any case, it is imperative that the set be fused, and adding a B+ fuse would also be recommended.

60 bucks for a 20 watt resistor(s)? Where? Three 5 watt resistors could be had for a buck-thirty plus shipping.

I too can sense Tim's confusion -and a good reference for replacing seleniums is nowhere on the web. It's all Ohms Law and Kirchoffs Law, but no one on the web applies either to a selenium replacement scenario. I guess someday they'll all be replaced, making this thread obsolete.

Cheers,

The set is fused, so no worries there. The 60 bucks was for buying a dozen 25 watt resistors to experiment with, at $4-$6 each. Bill and I started our conversation on this a couple days before I started posting in this thread.

I really appreciate everyone weighing in on this thread, and will be ordering parts today. I'd rather do this job completely and correctly, and I'm thankful for the guidance here :)

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,

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.

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? :smoke:

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,

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

Cheers,

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?

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,

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

Took me about a half hours' worth of racking my brain to trace everything out.....turns out that on this, M2 is on the left, and M1 is on the right.....but I guess it depends on how you're looking at the chassis.

Just one last question. I want to put the dropping resistor in between the filter choke and the resistor junction, and not between the diodes and the choke? If that's the case, I might want to mount that remotely with another Dale resistor when it's finished, as that's a REALLY tight junction.

Before or after the choke is fine, it's all one series DC path. C2A and C2B, along with the choke form a Pi Filter, and additional DC resistance shouldn't be a problem. Technically, you could even raise the value of R91, the fusistor, but best to leave that alone.

Cheers,