Parallel valve rectifiers.

[bikerhifinut]

Just been checking my stocks of valves.
I have a number of EZ81 mostly NOS rectifiers, and a few EZ80 also.
I noticed the price of GZ34's is quite high now, even the chinese ones are generally £15 or so and forget a cheap Used Mullard or USA 5AR4 at any sort of resonable price as a rule.
So I thought how about 2 x EZ81 in parallel when I want to supply more than 1 pair of EL84 for instance.
Heater draw roughly the same, 2A instead of 1.9A.
So is the accepted convention to strap the anodes together and treat as 2 single diodes with the cathodes strapped together in a FW rectifier? Or just parallel the valves electrode for electrode?
My instinct is to go with the former on the basis that if one valve fails, you dont stress the survivor with twice its working current and all you lose is half the phase.

Andy.

[ITAM805]

The Klemt M40 did it with 2 EZ80/81's running four EL84's at a claimed 40W

[bikerhifinut]

I see that paralleled them electrode by electrode.
Interesting.
I have designs in the archives from Kit amps using a pair of 5U4 where the valves are strapped as single diodes in a FW rectifier, so both strategems have been used commercially it would seem.
I think I'll go with my gut feeling and if I do this I'll Strap them as individual single diodes, obviously commoned at the cathodes on the positive output. That way if a valve fails the other one doesn't suddenly get a double load. Although I guess the amp will just try to pull all its current through one Half wave rectifier?

Thanks.

A.

[FERNSEH]

I think that's the manner the two U14 rectifiers are wired in the pre-war Marconi 702 TV and also the HMV 800 radiogram.

DFWB.

[ukcol]

Same with the Murphy A40C. Personally I don't like it because if one valve fails completely (open circuit heater say) then the PSU runs half wave with an increased stress on the mains transformer.

[TrevorG3VLF]

My Heathkit transmitter runs two 5R4GY in parallel, so will run on one valve. I have made a single solid state rectifier to replace both valves but have not tried it yet.

[Leon Crampin]

I remember seeing a recommendation by a valve maker to parallel the valves pin to pin in these applications - so each valve sees the full secondary voltage between its two anodes. The theory was that a valve maker would match the two diodes within a given device, and so connecting them pin to pin would give the best current sharing.

I have my doubts, having tested many rectifiers on my valve tester over the years - matching seems to be absent, especially after a few hundred hours' use and new devices from the likes of MOV can be pretty hopeless.

My preferred solution would be to adopt the "Murphy A40C" connection, but also to add current sharing resistors in each anode. Their value would depend on transformer winding resistances (hot), transformer voltage and the desired output voltage on load, but taking steps to balance diode currents is always beneficial to valve life.

An example of good design (although perhaps not with the best choice of rectifier) is the 4-anode U801 used in the Ekco T217 TV. This is wired as a half-wave rectifier and has a load sharing resistor for each of the 4 anodes.

Ekco were good designers and specified their components properly.

Leon.

[bikerhifinut]

Thanks, lots of insight here.
I had another idea a bit out of the box.
How about using each EZ81 for one channel of a stereo EL84 amp? Ok double the amount of PSU caps and resistors etc, but at our amateur level its not a great extra cost, and you get round the issues of balance and stress if a valve fails?
Got thinking as I have some 100mA 10H chokes and separate rails would mean I could use them.

Andy.

[kalee20]

That's best! Better separation of channels. Plus if a rectifier valve fails you know about it - and the other doesn't do twice the work.

[Boater Sam]

No reason not to parallel them up, but load sharing resistors on each anode are a must, 82 to 100 ohm will do. Sensible to fit a fuse too, you will have quite a massive overload capability.

[beamcurrent]

I have a PSU for one of my ex broadcast cameras that has 6 (yes six) GZ34 or eqv. in parallel. it makes 250v DC at 1 amp.

http://www.tvcameramuseum.org/marconi/mk3/mk3-5.htm

Anyway back to the subject. you should have load sharing resistors in each section, one per valve (in the cathode) in the normal full wave rectifier circuit. 47-100 ohms good. if you strap the anodes, the two anodes of one valve, 2 per valve in the anode connections. conveniently you can measure the voltage across them to get an idea of the individual currents.

[bikerhifinut]

All good info.
The type of application I am thinking of would typically be a stereo amplifier, and my instinct would be to use each EZ81 to separate the PSU positive HT rails. In that case I wouldn't need to worry about balancing resistors.
Just need to dig out the Mullard Valve data book I have and check out the voltage limits if I intend to run more than 300V.
As stated before, it seems that EZ81's are easier to find either new or NOS and are usually quite cheap, whereas GZ34's are a bit expensive for new projects if Mullard or other good older makes are wanted.
The chinese stuff is ok I reckon as long as you dont use em in anything more demanding than a Leak stereo20.
Also I have a bit of a guilt trip at times if i need to use rarer valves in a new project. I'd rather look at current production if possible.

A.

[Skywave]

Just been looking at the prices for 5U4G rect. valves. How much? Good grief! Are the electrodes gold-plated?

Al.

[Herald1360]

Made out of unobtanium is the problem. Gold is easy! There's probably no sweet spot between the price paid by a few audiophiles fighting over even fewer valves and making a profit building new ones, though. Mass and professional markets just use silicon.

[bikerhifinut]

Originally Posted by Skywave

Just been looking at the prices for 5U4G rect. valves. How much?

Plenty of cheap 5U4G if you avoid the NOS western market.
Not sure if allowed to post ebay links so all i can say is you can get 2 russian NOS 5U4G for £5.99 + 3.50 [ostage = £9.49. that's pretty darn cheap I'd say.
Places like Tube Shop Will do you a Chinese version for about £15.
They are ok in most applications.
The Russian ex military stuff should be ok too, and so far there doesn't seem to be a shortage yet.

As to using silicon instead, That's the route i will go in future for scratch builds. And not just because it's cheaper by a Country Mile.

It's just that I do have a stock of EZ80 and EZ81 and I like to use what I have.

A.

[Leon Crampin]

If you can energise the heaters (3x6.3V), a pair of PY82s will out-perform most 5V rectifiers - for peanuts.

EY82s are uncommon, but their characteristics are very similar to a parallel connected EZ81.

There are anode voltage limitations on these devices, however.

Leon.

[G6Tanuki]

I sadly foresee a time when 1970's colour-TV's become worth more for their rectifiers [and their PL519s] than for anything else.

[bikerhifinut]

I don't think its that bad.
Most of the mainstream valve audio seems to use current production Valves as far as I can see.