Valve tester HT PSU puzzler

[Diabolical Artificer]

I've just built another HT PSU for the valve tester I built some time ago, as I wasn't happy with the simple design that I used. It used a IRF730 mosfet with a transistor current regulator - very simple but as it had no protection, I blew a few mosfets and transistors.

So I built another PSU from a design in The Art of Electronics. After a few niggles it ran well. So we have two seperate PSU's on separate PCB's fed from one tfmr, both are adjustable from 0 - 300/340v. They are supplied from a 275VA transformer that I wound myself with two secondarys overwound to give 250mA at 300v. So in theory I should have two PSU's capable of 100ma @ 300v DC at least, one for the anode, one for g2. Both tied to a common ground.

However, when testing it out today and previously, I noticed that when testing a big valve like a for instance a 1625 valve,with heater voltage on for a few seconds, then winding each supply up from 0v, when both reached about 150v the anode supply starts going down quite sharply. This happens the other way round too. On second thought this happens on all pentodes.

I thought I'd wound the tfmr wrongly, so I found a big tfmr - 240v secondary 250VA - and powered the second g2 supply from that. Same thing happened. This is with - 15v bias applied to g1, so the valve isn't pulling any where near it's full potential anode current.

These are plugged into a heavy duty extention lead, so it's not that, the heater PSU isn't effected which one might expect if it were a mains lead issue.

Something rings a bell as regards how g2 is connected in amplifiers, EG g2 usually has a resistor of a 100 ohms or so with a cap decoupling it to ground. This isn't practical in this case though. I must be missing something obvious.

Andy.

[PJL]

Remember that g2 behaves as a grid and increasing the voltage will increase the anode current. Are you measuring current when you do this?

[ms660]

Not sure if I've read the post right but sounds like poor regulation.

Valve going into heavy oscillation?

Maybe try some fixed resistance loads across the PSU instead of a valve and see what the response is to them.

Lawrence.

[kalee20]

Agree PJL!

With the g2 supply powered from a separate transformer, you have got just about 100% independence, so it MUST be the valve that is clobbering the supply. Measuring the anode current will provide the key!

[Silicon]

A 3,000 Ohm load (30W) on each power supply should give you a linear relationship between output voltage and output current.

If the current limiter is coming into effect at less than 100mA you won,t get to 300V.

[TrevorG3VLF]

Originally Posted by pwdrive

Valve going into heavy oscillation?

1625 = 807?
I have an old AVO tester and tried to test a 6146. The valve went into oscillation, it tripped the tester and my radio was blanked out. I haven't tried to test an agile transmitter valve since.

Trevor

[G8HQP Dave]

Parasitic oscillation is likely. You need stoppers on all grids, and possibly the anode too.

[ms660]

And possibly a bead or two.

Lawrence.

[Boater Sam]

1623 aka 807 with 12.6v heater. I can't test these either on my homebrew tester. I can get over 100mA anode current with other valves but have oscillation problems with 807s and 1625s and sometimes with EL34s. I have tried 100 ohm stoppers, sometimes they work but not always.

[Diabolical Artificer]

Thanks chaps. Yes am monitoring A I it only gets to about 10mA before things go pear shaped. I wondered if it could be oscillating having read about it in the Sussex thread. Will find a load to test it - keep meaning to build a hefty DC load.

I'm aware that the anode and g2 complement each other and in an amp one goes up, the other down. The anode supply should be good 200mA at least and the g2 supply should cope with what little current g2 is pulling.

Will also try some beads and a 100 ohm R on anode (as per 807 datasheet) and g2, meter g2 and scope the anode etc, and report back.

Andy.

[Craig Sawyers]

Why not waft an oscilloscope probe around the valve - if it is hooting, you'll pick it up easily with an open probe.

It is an interesting exercise to try that with an energy saving light bulb.

[Diabolical Artificer]

Have scoped the anode, didn't think to waft it in it's direction. Anyhoo, it looks like its oscillating - see pics. Have tried a 100 ohm on the top cap - a little change, but no fix. Put one on the patch bay pin of g2 ( its unfeasable to put resistors on all valve base pins) no change, so have found some ferrite beads and will fit these. Do you have to put a turn or two around them or just fit them on the wire?

First pic - timebase 50 Usec, x .5v div, so thats about 1.5v p-p amplitude. Second pic slower timebase setting, same x setting. One thing I've noticed is that my DMM with the range sw on mA shows a flat battery, it's brand new and the flat battery icon doesn't come on on the other ranges. I wonder if the oscillation is somehow affecting it.

Andy.

[ms660]

Try G1.

Lawrence.

[Diabolical Artificer]

Have put small ferrite beads on all of the wires to the valve base pins as nearer as possible to the valve. I've also put one on the lead of the 100 ohm R on the top cap. as well as that I've put a 1 meg R between the bias supply and ground on the patch panel banana jack.

We have some improvement in that I can now crank the anode up to 300v, however this is with the screen grid at 150v. As soon as I try and go above this, anode voltage drops off. So am going to knock up a bridge and 220u cap, power this from my variac and use this as g2/screen PSU. While this won't stop oscillation it might elliminate the present PSU as part of the problem.

When last tested with 150v on g2 and 300v on the anode, anode current was at 24mA. Could you expand on your comment please Kalee20 when you said a I is the key?

I have no hefty resistors to check regulation. I used a 150w light bulb before to try and measure total PSU current but this blew the mosfet pass tranny.

Thanks for your advice comments so far, Andy.

[kalee20]

Yes - measuring anode current is the key.

If it's only 24mA, then the current shouldn't be hitting current limit in the anode power supply. So the anode voltage shouldn't be collapsing. But it is. So therefore there must be more to it than steady 24mA DC. Oscillation with a peak current significantly higher than what's needed to invoke the current limit!

Grid and anode stoppers should help. I'd be tempted to try 4.7kΩ in series with g1 (connected as close as possible, 1/2" between the resistor body and the grid pin), and 0.1µF between cathode and the non-grid end of this resistor. 100Ω in g2 and anode - as G8HQP Dave has already suggested, and you have done, is also beneficial and may be essential.

[ms660]

Just for interest, Avo reduced the parasitic problem in one of their valve testers by making sure all the valve holder interconnect loops were of the same length and fitting ferrox cube beads from what I've read on page 7 of this manual:

https://frank.pocnet.net/instruments...160service.pdf

Lawrence.

[Craig Sawyers]

That is the reason that the layout of valve sockets on an AVO panel seem to have little logic to it. It is all to do with equalising the wiring loop lengths in the best possible way.

Craig

[TrevorG3VLF]

Just a thought, are the power supplies going into oscillation.
I presume they are stabilised.

[ionburn]

I don't normally work with voltages in hundreds of volts but I have found it useful to constantly monitor anode current with my AVO while at the same time using three other digital meters to monitor other voltages. If I come across something unusual then it tends to be easier to determine what is wrong. Low budget digital meters are readily available and tend to have 1 Mohm input so not too bad for valve use.

[Diabolical Artificer]

Thanks all, that's given me some more ideas and I'll give that a read Lawrence. Yesterday I tried using the variac and iso tfmr , output rectified and smoothed as a variable PSU. Same thing happened, but there was now 100hz ripple amplified on the anode, about 2v p-p.

I thought about putting a diode on the output of the PSU's as well as big toroid ferrite's on the AC powering the PSU's. I also found some RF chokes yesterday, and wondered if they would be of use. Just thinking out loud here.

These are the circuits I'm using for HT supply. Pic one shows the circuit I've just built, this is out of the Art of Electronics.I made one addition, a 10k 10 turn WW pot to fine adjust V is series with the 500k pot. They recommend putting a diode across, I presume in and out in case the supply falls below the OP, I havn't done this. I built this after I had problems with circuit 2 - second pic; it kept blowing mosfets. I had the same problem with this second circuit, hence building the one from TAOE. However circuit 2 seemed a bit stiffer. So yes, they should be regulated. I'll switch the meter measuring anode current to AC mA today, and put more meter's on other parts of the valve tester.

One last thing, after fitting the beads and grid R's, the trace on the scope is weaker, in that I have to turn the Intensity up. not sure if that is significant.

Andy.