Rectifier to diode/resistor help please.

[garrard.mike]

I have to replace the selenium rectifier in my one valve wonder.
No schematic available, no manufacture ID
cheap as chips amp.

I understand the theory of replacing a selenium with a diode and resistor but I don't know how to work out the value of the resistor, or which diode to buy.

I have watched just about every youtube video on the subject and read threads here, but everyone seems to know the voltages of the HT to be able to then work out the value of the dropping resistor in series with the diode.

I have no information about my amp so I am wondering the best approach to working out the value of the resistor and the diode to choose.

Markings on the rectifier are:
E250
C50
Tc06e 11/16

The rectifier can be seen in the photo below.

If you can guide me as to what I need to do and buy to do this job, I would be most grateful.

I am replacing the paper caps and filter caps first (help asked for in another thread about the caps) and then the rectifier.

I have good electrical skills but lack the theory, so I would be grateful for help if you have the time. many thanks

[Station X]

Let's assume the selenium rectifier is working?

Measure the HT voltage. Use trial and error to determine a dropper resistor value which gives approximately the same HT voltage. Far quicker than doing a load of calculations.

[garrard.mike]

when you say measure the HT voltage, I can do that, but at which point in the circuit am I measuring, and what should I set my meter to?

The rectifier is working, the amp is working (just) so measuring voltages is something I can safely do.

I don't have a stock of resistors to put in place and measure again until I get it right, so again, I am blinded by mathematics to work things out.

I can easily find the HT voltage if I know which point to measure it at.

I still need to know which diode to buy as well.

many thanks

[Station X]

It's usual to measure the HT voltage at the smoothing capacitor. That is to say at the tag of the capacitor can which is connected to the valve. Measure the DC voltage between here and the chassis.

Sorry, but I can't help with the maths.

This thread may prove useful:-

https://www.vintage-radio.net/forum/...ad.php?t=57388

[paulsherwin]

The value of the resistor isn't critical. It needs to be rated 2W or better. If you need to buy something, use 200 or 220 ohms, 3-5W.

Any silicon diode rated at 400V 1A or better is suitable. The 1N4007 is as good a choice as any.

[garrard.mike]

very many thanks
most helpful, thank you for the replies.

[David G4EBT]

The attached sketches might help.

The first sketch shows where you should apply your test prods with the meter on high volts AC range to check the AC input to the rectifier, and with the meter of high DC Volts range to check the half-wave rectified unsmoothed DC output from the rectifier. If the selenium rectifier is healthy, the DC Voltage will be a bit higher than the AC. EG 150V AC input would most likely be 160V DC output. If it's much lower than the input, it's on its last legs.

The second pic explains what the reservoir and smoothing capacitors do, and the waveform of a half-wave rectifier.

If the rectifier is duff, you can replace it with a 1N4007 silicon rectifier, which will give an appreciably higher rectified DC Voltage, (EG 150V AC in might be 180V DC out), so will need a series resistor to drop that voltage to the desired voltage for the circuit in question. The value of the resistor can be calculated by dividing the current (in Amps, though it will be much less than an Amp: eg .015A would be 15mA) that the circuit will draw into the voltage you wish to drop. But you won't know what current your little amplifier will be drawing, so rather than get too scientific, start with say a 1K resistor, and see if that drops the Voltage too much or not enough, then select an alternative value accordingly. As to the Wattage, again, you'd need to use Ohms Law: Watts = Amps x Volts, but if you use a 2Watt resistor that should be fine.

Hope that helps a bit.

Be careful when measuring high voltages - ride at your own risk!

Good luck with it.

[garrard.mike]

David, I really appreciate your help.
I shall read up and digest.
Very many thanks indeed.
I have good safety knowledge so I'm happy in that aspect.
I have good understanding of the circuit and components, I just fall flat on my face with numbers.
I've had a soldering iron in my hand for 50 years, so no worries there either.

[ms660]

Working out the wattage by measuring the current through the resistor using an ordinary meter or arriving at that current by way of the load current will give you the wrong wattage figure, that's because the load never stops drawing current and the rectifier only conducts for part of the +ve half cycle.

Lawrence.

[trobbins]

It could be copper oxide disk based diode.
Can you confirm it only has two terminals?

[garrard.mike]

Hello trobbins
I can confirm it has two terminals.
Half wave rectifier. (if that is the right term for it)

I have ordered a diode and resistor as advised further back in the thread.

I will measure the voltage going in and out of the rectifier to see what the difference is and make a call on whether or not to replace at this time, but it will need replacing, so probably best to sort it out while the record player in in peices.

again, thank you so much for the advice from everyone, I am really grateful for your time.

[paulsherwin]

You don't need to worry too much about the precise HT voltage so long as none of the component voltage ratings are exceeded. If you are changing all the caps then you are likely to use 400V or 630V types so there won't be a problem there.

I would certainly replace the selenium rectifier if carrying out any work on the electronics. As they age they drop more and more voltage and start to run hot. Occasionally they fail catastrophically emitting toxic and extremely smelly gas. The replacement diode and resistor cost very little and are good preventative maintenance.

[garrard.mike]

ha, yes I have seen on youtube and read here, the horror stories of selenium smoke, not something I want.
The amp is working but not great, and a complete re cap is on the horizon.

I have a diode on order along with a choice of a couple of resistors.

It's my plan to replace the three paper caps and fire it up and do some voltage measurements around the rectifer to see how it is doing, but ultimately it needs to go to a diode/resistor for the long term.

I am also planning to change the filter cap, two new caps are on order and the cap discharge lead I have made is waiting in the wings, as is the sling, to keep one arm behind my back!

thanks for your reply paulsherwin

[David G4EBT]

I think it's unlikely in a simple one-valve amplifier that it's a full wave rectifier, which would have four connections. Selenium half-wave rectifiers (usually 'Sentercel', made by ITT) were much favoured in simple mains powered equipment - Heathkit test gear used them extensively as it saved a valve, meaning less load on the mains transformer. They're often defective with the passage of time - still working, but providing much lower HT than desired.

Sometimes - as in the C3U Res/Cap Bridge they used a stick type - a Paxolin tube with small discs inside. Easy enough to update as a silicon rectifier and series resistor fit neatly inside when the discs have been disgorged. In the first pic below there's a Sentercel stick rectifier from a C3U that I restored, showing the discarded discs and the replacement rectifier and series resistor ready to be installed.

In the Heahtkit RF1U signal generator that I'm restoring just now, there's a square ITT Sentercel 'C2D' half-wave rectifier which - with 150V AC in - should give about 160V DC out, but it was well below that. I guess there are those who would disgorge the potted aluminium box and re-pot it with a silicon rectifier and series resistor, but I think to do that would be borderline OCD, so I contented myself by mounting the new components on a small tag-board, which in turn I mounted on a small aluminium bracket and used the same holes on the chassis to secure it as were used for the Sentercel. Just a safe and honest workmanlike repair which will be evident to any future owner.

The series resistor was selected on test. I started with 1K, which resulted in the HT being too low, and ended up with 560R, to give the necessary 150V DC. When passed through the existing 2k2 HT load resistor which forms part of the smoothing circuit, that gave the necessary 125V smoothed HT. Some pics below, and a sketch of the tag-board.

[garrard.mike]

really interesting David. Thanks for the photos.
On the phone to Cricklewood Electronics I talked about the diode resistor bit, they are sending me a choice of resistors to put in series with the diode, after I have done some measurements across the rectifier.

Once I change the three paper caps and replace the dodgy wiring with some new inter connect wire I have bought, I will be putting the meter across the selenium to see what I am getting either side of it and make a call on replacing it with the diode and resisitor.

Living in a small apartment with a flat faced poorly breathing cat, the last thing I need is selenium smoke with my mantovani!

[garrard.mike]

My little one valve wonder is probably very simple for you nice people.

To me it is a bit of a mathematical nightmare, but with the help of some very extensive reading in this forum, and a number of amazing you tube channels, I will get it playing again very soon.

I am very pleased with myself at stripping the changer back to nothing and re building it. It sings like a bird now. The crystal cart is still thumping out the tunes, I can't wait until the caps are changed and I can listen to some old records again...

I do have one of those modern Technics 1200 players, but I miss a Garrard auto changer and a valve amplifier banging out my childhood 1960s records, like my old Philco/Garrard used to when I was a kid buying sandie shaw records...

[G6Tanuki]

Don't fret too much over the value of the series-resistor: it's really only there to limit the initial switch-on surge.

[Remember that back when these sorts of players were made the mains voltage was often very variable so equipment was expected to tolerate voltages that would not be accepted today. I can remember seeing supposedly-230V supplies being anywhere between 200 and 260V depending on what the local electrically-heated Government grain-silo was doing!]

[garrard.mike]

Thank You G6Tanuki