Can someone help me determine what rectifier to use?

[scatterbrain]

I am currently trying to build a musical instrument called a theremin. The project schematic is from the fifties, and calls for a hard to find power transformer. It is a full-wave center-tapped rectifier with the following specifications:
325-0-325 v. @ 100 ma.; 5 v. @ 2 amps; 6.3 v. @ 3 amps
I have found a similar part for sale online, but it is not exactly the same:
Secondary: 325-0-325 V, 150 mA
Filament winding 1: 5 V, 3 A
Filament winding 2: 6.3 V, 5 A
Mounting centers: 2.5" x 2.38" vertical mount
The dealer told me that it should suffice, but I am looking for a second opinion. Can anyone help me out?
thanks much,
Jordan

 

[tunedwolf]

It should be just as happy with the replacement transformer, however bear in mind that should it go faulty, it has the extra available current, and that also the offload voltages will be proportionally a little higher, so watch out for capacitor ratings etc. Check that your circuit doesn't call for a -40v biasing voltage from the transformer, because the replacement doesn't have a bias tap, also it might be wise to stick a couple of 1.5A diodes in series with the rectifier tube, just in case it fails...no point in torching your output transformer and tubes...also remember and fuse the power supply for the original 100mA, and not the 150mA of the replacement.
And for gawds sake, watch were you put you fingers even after it's no longer powered up, these things contain lethal voltages, and bite worse than a Terrier with a headache. Always discharge the main capacitors etc before you do any work on it. You might also want a VTVM handy for biasing up and testing etc

Hope this helps ya out a bit...

p.s...I forgot to actually answer your question....going on the filament voltages, I would say you are looking at a GZ34 or 5AR4 which is the direct equivalent as the rectifier...

 

[Nigel Goodwin]

Yes, the transformer is fine - you couldn't get much better!.

As for a rectifier?, it depends if you want to use an expensive valve one or not? - if not a couple of silicon rectifiers with surge limiting series resistors is fine. But bear in mind the capacitors will have to be rated to withstand the higher voltage.

 

[JimB]

Check that your circuit doesn't call for a -40v biasing voltage from the transformer, because the replacement doesn't have a bias tap

The original specification did not require a separate bias supply, so the new one will not need it either.

Always discharge the main capacitors etc before you do any work on it.

It is good practice to put bleeder resistors across the capacitors in PSUs for valve applications.
In this case I think 220k ohm 2w resistor across each high voltage capacitor would be good "insurance"

JimB

 

[Nigel Goodwin]

It is good practice to put bleeder resistors across the capacitors in PSUs for valve applications.
In this case I think 220k ohm 2w resistor across each high voltage capacitor would be good "insurance"

That's generally standard practice - however, I have known them go low resistance, probably the only time I've seen resistors go low in value!.

 

[tunedwolf]

Just because he didn't actually type it, doesn't mean that he didn't mean to, I'm merely making sure he double checks it before spending a wad on a transformer that is gonna be useless to his application.
Good practise it may be, however, I have yet to see any 50's, or indeed 60's valved audio gear with bleed resistors on the main smoothing block.

 

[Nigel Goodwin]

Good practise it may be, however, I have yet to see any 50's, or indeed 60's valved audio gear with bleed resistors on the main smoothing block.

They didn't care about the poor engineer back then!

 

[scatterbrain]

Thanks for all the great suggestions.
As an amatuer at electronics, and I am not quite sure how I would go about taking any of the recomended precautions. I am interested especially in knowing how I could power my instrument using silicon rectifiers.
Also, I would like to know how to add a bias tap.
If anyone could that would be greatly appreciated.
Thanks.

 

[tunedwolf]

it depends...if you require a -40-50v bias tap...it is normally wound as part of the transformer...there was an exception to this, and that was if it is cathode biased...you'll need to look to your proposed schematic to see which you require...

The silicon rectifiers go in place of the Tube rectifier...i.e one in each leg of the 325v windings...the 5v tap on the transformer is left unconnected, unless you have another use for it of course.

Beware though, that as it was designed for a Valved rectifier, there are two distinct disadvantages in using silicon rectifiers.

Some folk would say that a Valved rectifier will give the power supply a certain amount of sag that the silicon one won't, and while this is true, it can be replicated with the careful use of a couple of resistors, so it's not really a disadvantage.

The first and possibly the major disadvantage in using silicon recifiers is that the power supply for the anodes is there immediately upon power up, long before the cathodes have heated up, so there is a risk of cathode stripping, and it will certainly shorten the life of the valves if allowed to continue unhindered. It would therefore make sense to fit a standby switch, so that the anode voltage is not available until the rest of the Valves have heated and conduct properly.

The second disadvantage is that with silicon rectifiers being that much more efficient, the final output voltage will be appreciably higher. This often leads to component ratings requiring to be higher in order to accomodate them, and there is often a premium to be paid for it.

The Valved rectifier on the other hand, uses an indirectly heated cathode, which in essence will allow it to conduct slowly as it warms up, a natural timing process you might say, and because of this, the stress put on the circuit as a whole is brought up slowly, so there is no need for a standby switch.

It's horses for courses really, I prefer using a Valved rectifier in a circuit specifically designed for it.




hmm...precautions....

When taking measurements etc, connect your test equipment before you switch it on, and only ever have one hand anywhere inside it at a time when it's powered up, keep the other hand in your pocket.

It may also be prudent to have it running off a Mains Isolating Transformer when you're working on it. If you do make sure your test equipment is also running off the same Isolated supply, and bond the gnd points of all your test equipment and chassis of your build together. (Very Important - Not to electrical earth).

Make sure all your wiring is neat and tidy, and the insulation is rated for the task you are putting it too. Same goes for all of your components in the build.

The stress put on components at this temperature and voltage can change their nature from what you would normally expect in say a low voltage circuit. Don't use Vitreous Enamel power resistors for example.

Be absolutely certain of the ratings of your capacitors, make sure they are rated for a working voltage well above what you will be working them at.

Make yourself a couple of leads, say 14" long with croc clips at either end, and a Ceramic 100K 5W W/W resistor in the middle, you can then clip those on caps etc to discharge them, and leave em in place while you work on the circuit.

Make sure the chassis is properly earthed.

Make sure your design is well ventillated, heat builds up very quickly.

Never operate the equipment without a load on it's output, Valved equipment is generally forgiving of a short circuit, but not an open one.



Your enthusiasm is admirable, though your lack of experience is a worry for me. These old valved applications, were lethal in the wrong hands, so please be very very very careful.


These are the sort of questions you should be asking yourself...

Do I have the right sort of test equipment for this build ?

Do I have enough experience in the general electronics field to extend to this area ?

If something goes wrong, do I have enough life insurance ?

If the answer to any of those questions is no, then this really isn't a project for you at this time in your experience, because as keen as you are, it is beyond your skill level and you're only going to get hurt.

If you can, find an older engineer, someone with experience in working with Valve(Tube) applications. Maybe an old TV or Radio engineer, or an old Radio Ham in your area, there has to be someone you could turn to locally for help.
I bet they would be tickled pink to be asked to collaborate with you on this, and you will learn all sorts of stuff from them, not all related to just this project but in general, also practise and safety, that you just won't learn from any forum or book. This to me would be a major invaluable advantage to you, and an opportunity to make a new friend in the process.

I wish you well with it...