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Old 22nd Sep 2009, 6:06 pm   #1
YT2095UK
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Lightbulb OA2 parallel?

can OA2 voltage regs be wired in parallel to supply even Greater current at 150v?

I`v just built my 1`st ever Valve rectified PSU using a GZ34, and I thought it would be nice to stay clear of PN junctions entirely (so no Zenner shunts) to maintain the theme.
but since it will be a Bench PSU and therefore subject to iregular (ab)use, the 30ma afforded by a single OA2 is rather insufficient, esp since the OP is a good 100ma capable.
so I wondered if I had 4 of these and 4 of the OA3 (the 75v version) and a switch selector, I could have 75, 150 and 225 VAC @ 100ma with another 20ma leftover for good measure.

is it doable?
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Old 22nd Sep 2009, 6:16 pm   #2
Leon Crampin
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Default Re: OA2 parallel?

No. You can't shunt gas stabiliser tubes because the striking and maintaining voltages will not match, or stay matched. There would thus be no current sharing.

Use a single reference such as the 85A2 and make a series stabiliser - 2 EL86s in parallel (with sharing resistors) will do nicely and give far better regulation.

I presume you're after a DC output?

Leon.
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Old 22nd Sep 2009, 6:32 pm   #3
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Default Re: OA2 parallel?

I am indeed after DC out, and I do like your idea but only have EL84`s, and only 3 of them (one in use), however the case I plan on using will only accomodate One transformer, and I don`t want to be cutting into my 6.3v rail to supply the parts needed for the HT side, leaving me little or nothing spare.
I have about a dozen PL36`s and 504s however, I`m wondering now if I could possibly squeeze in a 24v transformer?

it`s a pity though, I do so like that Purple glow
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Old 22nd Sep 2009, 6:33 pm   #4
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Default Re: OA2 parallel?

There was a thread a while back which had several circuits for high voltage stabilsed power supplies using vacuum tecnology

https://www.vintage-radio.net/forum/...ad.php?t=43328

Keith
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Old 22nd Sep 2009, 6:50 pm   #5
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Default Re: OA2 parallel?

I have seen that thread, but am still somewhat constrained by having to use what I already have here and can maybe spend a few quid if I need to.
although to Look at some of the cct diags it wouldn`t be too hard to replace the valves with transistors and have them work just the same, it struck me that most PSU`s use emitter followers, and the outputs of these use the Cathode (also an emitter), so I`m Guessing that since I understand the transistor versions it shouldn`t be hard to transpose the technology over to Valves as I`m quite comfortable with both.
I guess my 4 OA2`s and 25W pygmy bulb shunt`s not going to do it *sigh*
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Old 22nd Sep 2009, 10:37 pm   #6
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Default Re: OA2 parallel?

Hi, gas regulators are very simple but inefficient as you are always drawing full load from the transformer. However short circuits across the output are easily tolerated.
PL36's would make excellent series regulators due to their high anode dissipation and cathode current rating, ifyou can find a suitable heater tx.
Transistor hv stabilised power supplies usually end in disaster. Finding suitable rated devices is one thing, accidentally shorting the output usually ends in destruction due to the low output impedance.
If you really must use gas filled v-r's then use 4 separate (equal value) anode dropper resistors, one for each tube with the cathodes commoned. Choose this resistance so the tube current does not exceed the maximum rating at no load. Connect the anode of a 1n4007 diode to each anode of the v-r tubes. Common the 4 cathodes and this is your output.
Rob.
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Old 23rd Sep 2009, 9:02 am   #7
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Default Re: OA2 parallel?

ah yes, I can see what your getting at with the diodes, without them I can only have 4 lots of 150v at 30ma,
so it seems either way, unless I use something like a PL36, P/N junctions are somewhat inescapable, and although I have plenty of rectifiers it would bring the valve count up to 9 and 4 bulbs!
it`s Almost tempting to make a bigger case and build this monster but we already have central heating here
I just found a large Grundig transformer with 21vac out, I wonder if that will be enough to fire up a pair of PL36`s?
it seems that the logic is inescapable, and that if I`d be comited to using 4 diodes and 3 VRs I may as well make those diodes a Triode and save a lot of work/parts.

Thanks for the help on this, I have a Much better idea of how these OA2`s work in circuit and how to go about employing them now, Cheers
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Old 23rd Sep 2009, 11:24 am   #8
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Default Re: OA2 parallel?

Hi, whilst the arrangement I described would work in theory it would have certain drawbacks. I won't go into these here (unless you really want me to).
As an aside, the available output "per tube" is only 25mA. From the data sheet, max tube current (continuous) is 30mA. This occurs when there is no external load. The minimum tube current to guarentee regulation is 5mA. Thus only 25mA can be "spared" for the load. Conversely the regulator tube can dissipate up to 25mA of "spare" output. As an example, say you have an oscillator circuit you wish to regulate that draws 40 - 60mA depending on conditions / loading etc. By adjusting the series dropper so that at 40mA load you have 30mA flowing through the tube and at 60mA load you have 10mA flowing through the tube, the operating conditions of the tube are met, and the output will be stabilised. The problem here is during initial switch on, the load draws no current until the heaters have warmed up. Hence 70mA will flow through the tube. Generally the tube will withstand this for a short time, you will see on the data sheet that an 0a2 will withstand 75mA initial current.
For an experimental unit you would be better off building a series regulator circuit. As I said Pl36's are ideal for this, though they really do need the full 25v heater voltage although you could probably get away with a 24v transformer. This winding should be fully isolated from the rest of the circuit and bonded to the ht output to reduce the heater cathode voltage.
of the series regulator.
Regards, Rob.
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Old 23rd Sep 2009, 12:38 pm   #9
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Default Re: OA2 parallel?

Don't forget that with a series regulator there is a minimum voltage drop across the series regulator valve that must be added to the maximum regulated output voltage required to determine the minimum voltage from the power transformer when rectified and filtered, this voltage will be measured at the valley not the peak in there is significant ripple. I like the GZ34 and the P/EL36, of the latter you might want to operate two in parallel as their average anode current is less than their peak and it also keeps their anode dissipation within reasonable bounds. You will obtain far better regulation if you add a voltage amplifier to the circuit such as an EF80/EF91/half of an ECC81/82/83 or a 6SN7.
All very non critical, must be thousands of examples around of practical circuits. Just ensure the heater to cathode maximum voltages are not exceeded.

It is worth the extra effort to add a couple of valves and end up with a really first grade unit for very little extra expense.
Victor.
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Old 23rd Sep 2009, 3:33 pm   #10
Leon Crampin
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Default Re: OA2 parallel?

The EL86 is a low impedance device, especially suited to this job.

Leon.
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Old 23rd Sep 2009, 4:30 pm   #11
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Default Re: OA2 parallel?

I am considering making a variable voltage power supply using silicon as this should be simpler and quicker to construct than a valved unit. My aim is up to 300V at 100mA. As a possible basis for a circuit I've looked at that in PW May 2007 which uses an IRF840 as a series control element. The circuit also has current limiting using a transistor to cut off the series FET as the current increases. This seems to be a simple approach and makes sense for me as I have all the bits I need.

I appreciate that valves are more forgiving and able to withstand abuse but has anyone any comment on the reliability of modern silicon devices for such purposes. I would be using devices very much over-rated for this application with a decent heatsink for the series control device. It would enable me to save my supply of 6080s, GZ34s etc. to maintain existing equipment.

Edward

Last edited by Brased; 23rd Sep 2009 at 4:58 pm.
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Old 23rd Sep 2009, 6:25 pm   #12
Leon Crampin
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Default Re: OA2 parallel?

There's no technical objection to using silicon - but there's a very practical one.

Valves don't fail short circuit - silicon does.

In any series regulator, a shorted pass device is the very last thing you need.

Leon.
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Old 23rd Sep 2009, 7:07 pm   #13
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Default Re: OA2 parallel?

I think I`m going to take the advice given here, bite the bullet and buy some EL86`s, it seems the "Audiphile" kings-new-clothes brigade haven`t yet forced the price up on these particular valves yet, and I have a nice transformer with a pretty hefty 6.3v rail to supply them, the extra 260VAC HT output on it I can use a with a voltage doubler for Oscilloscope CRT use.
giving me 6.3vac unused by anything internal, Variable HT @ 100ma, and a EHT lead for CRT use.

and maybe one day if I`m feeling particularly indulgent and bored, I may try the 4x OA2 idea out as well
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Old 24th Sep 2009, 7:29 am   #14
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Default Re: OA2 parallel?

If you'd like to try a couple of 807's as series pass valves I'd be happy to send you some FOC- I seem to have about a hundred or so NOS in stock

I can't even remember how I acquired them!

Regards

Philip
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Old 24th Sep 2009, 9:10 am   #15
YT2095UK
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Default Re: OA2 parallel?

Thanks, I accept! and will be happy to send the cash I had earmarked for the EL86`s.
I had a look at the specs for the 807`s and they look even better in terms of Anode current too.
I have no shortage of ecc triodes for the error amp side, so all in all, it looks like the makings of a decent power pack
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Old 24th Sep 2009, 11:40 am   #16
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Default Re: OA2 parallel?

You have a PM

Regards

Philip
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