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Old 4th Apr 2018, 11:43 pm   #21
Bazz4CQJ
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Default Re: Discussion of subcircuits for homebrew valve testers

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Originally Posted by Thatvalveguy View Post
Tube testing itself is not an accurate buisness, due to cathode interactions. most tubes that have been unused for some time will change their readings in the time following the first test, this is due to the composition of the barium surface layer changing.
I guess that it is quite common for some new (old stock) valves to "burn" in appreciably over the first few hours of use. I once burned in an ECC81 over ~24 hours on the 163 as I wanted to use it as a standardised valve. I was a bit surprised to see that one of the two triodes changed appreciably more than the other during that process, highlighting the fact that that way they were made gave rise to significant variations, even with two identical valves in the same envelope. I recently tested an Acorn valve (out of its WWII box) and the Ia on that cycled up and down (by a small but visible amount) for several hours before settling down.

When you start looking at the papers which were published (not the mention the papers that were never published), the enormous effort put into developing and refining valves in the last century is something we no longer appreciate. You could probably fill a modest library with that stuff.

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Old 5th Apr 2018, 8:53 pm   #22
Ed_Dinning
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Default Re: Discussion of subcircuits for homebrew valve testers

Hi Gents, the Philips Technical Library has many books on valve construction, use and applications. It is a great resource as well as having a great many details of valve aging processes and their foibles.
It does however take a lot of research through many volumes to find the gems of information.

Ed
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Old 6th Apr 2018, 12:22 am   #23
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Default Re: Discussion of subcircuits for homebrew valve testers

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Hickock testers[/B]
The hickock units, well their bridge? approach is a nice idea. However it cannot be calibrated by a voltmeter.
Yes it can, except for just one adjustment, the sensitivity of the meter bridge, which is done with their method simply by applying an AC source (which is checked by an AC voltmeter) via a resistor to unbalance the bridge, or with my method using a current sink, which is itself calibrated by a voltmeter, so it could be included inside the unit.

The Hickok TV10 has excellent meter marking in uMhos and scale switching options and is better made than many supposed laboratory grade lab instruments I have seen, so you will be hard pressed to beat it, but it is a good challenge that is for sure.

The TV7 is not quite as good in this respect due to the meter and it was designed more to help technicians in finding serviceable parts. As were some of the domestic style Hickok testers with good/bad pass fail meters which are not laboratory instruments, not to be confused with a TV-10.

The Hickok tester with the additional metering is the TV2.

Last edited by Argus25; 6th Apr 2018 at 12:30 am.
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Old 7th Apr 2018, 2:38 pm   #24
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Default Re: Discussion of subcircuits for homebrew valve testers

I never questioned the quality or the user friendliness of the Hickock units, just that their patented approach means that its not as straightforward as a pure DC tester.

Work was started on an electrical prototype for the grid regulator and current measurement unit, the functionality of the grid regulator already being verified in a different prototype.
The other protorype was unsatisfactory, in therms of leakage, power busses ran parallel to the grid current sense circuit, so to en order to avoid allot of problems with leakage a new prototype was needed.

When final electrical testing is complete the last step is to simulate a shorts from G2 to G1 to see if the circuit survives reverse polarity.

One goal was to get the entire bandpass filter, grid regulator oscillator and current sense circuit on one PCB, considering the limited space of a 100x160mm PCB SMD becomes ever more tempting.

The floating current measurement uses an 1k resistor that gives an burden voltage of 200mV full scale, i think i need to take a good look at the RC filter on the input, to avoid the A meter being influenced by the grid signal applied by the transconductance measurement 1400Hz signal.

Heres some photo's of the first and second prototype, and mechanical prototype.
The mechanical proto has a 10 turn pot for setting the grid voltage, 10 turn pot for calibrating the Gm measurement. One display displays the grid voltage, the other displays either grid current or GM. the aluminium was botched slightly.
The switch switches the bandpass filter between CAL and measurement mode. Because experience shows that contacs become bad after some years all the measurement circuits will use Reed relays for the signals.

The BNC connector is for feeding the measurement signal into the module, this to avoid running long traces to the rear connector, Keeping all the delicate circuits on a module that produces nearly no heat makes sense from a stability standpoint, as styroflex capacitors have a temperature coefficient of around 100ppm

A friend of mine is prototyping a 0-400v flyback module that operates from a 24V rail, perhaps a good idea to see if i can get a PCB to experiment with.
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Last edited by Thatvalveguy; 7th Apr 2018 at 2:52 pm.
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Old 7th Apr 2018, 3:44 pm   #25
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Default Re: Discussion of subcircuits for homebrew valve testers

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Because experience shows that contacs become bad after some years all the measurement circuits will use Reed relays for the signals.
That is a very wise idea.

Another great option there are the small metal cased hermetically sealed miniature relays from Teledyne , sometimes in similar sized cases to TO-5 transistors. They also come in latching varieties and like reed relays are long term reliable and the have low capacitances and many are double changeover types. I used some once to do the band switching for a homebrew shortwave radio and they have never given any trouble, unlike rotary switch contacts exposed to air & dust.
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Old 8th Apr 2018, 1:24 pm   #26
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Default Re: Discussion of subcircuits for homebrew valve testers

Today the current measurement was tested, it seems to work as intended, however the HCPL7800 seems to have quite a bit of output noise, so the ground side differential amplifier will need some RC filtering added like the datasheet suggested.

The most important test was to see if the circuit stayed stable once zeroed by the offset trim of the 411. which is the case, running for two hours the circuit has hardly drifted .1mV which equates to 0,0125A reading.

The circuit itself, perhaps its wise to change the sense resistor from 1K to 2K to get a 100A full scale reading, in this way the resolution is higher as most recieving tubes have less then 5A of negative grid current.

The 411 needs a negative supply to operate, i was thinking of assigning two pins of the header for external 5V 50mA floating supplies for the opamp and insulation amplifier, the negative with respects to output that is required for the opamp could be generated by including a 3V3 zener in the shunt regulator. The 411 need a maximum of 3.6mA supply current, running the shunt at approx 10mA this shouldn't be an issue. The CCS transistor at the bottom should always make sure there is a standing current. if the negative fails, the opamp is going to drive the noninverting input of the 7800 with approximately 1.8V, so better to add a diode from the input to ground, as 1.8V is cutting it close to the 2V input overload figure in the data sheet.

Also the input RC filter will have its -3Db point changed from approx 1Khz to about 200Hz.

I looked at the Thermal characteristics for the TO126 package transistors, those will need cooling at 100V 10mA bias, also that setting itself is beyond the SOA capabilities of the MJE340/350 pair. The 2SA1837/2SC4793 combination seems much better when it comes to safe operating area, but these are tricky to source. For a lower power unit, say 0 to -50V one can simply take BD139/140 and they wont need to be cooled. The junction to free air characteristics of the plastic TO126 package is about 100C/W
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Old 8th Apr 2018, 2:41 pm   #27
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Default Re: Discussion of subcircuits for homebrew valve testers

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For a lower power unit, say 0 to -50V one can simply take BD139/140 and they wont need to be cooled. The junction to free air characteristics of the plastic TO126 package is about 100C/W
You could just go for TO-220 cases and not bother with heat sinks, or easily attach one, like the MJE15030/31 complimentary transistor pair.There are also plenty of TIP series Darlingtons like TIP102/107 that would work if a Darlington was ok.
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Old 8th Apr 2018, 3:01 pm   #28
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Default Re: Discussion of subcircuits for homebrew valve testers

Thanks, i didnt think about that pair, they share pinouts with the Toshiba devices previously mentioned, i bought 100 pairs of the Toshiba's before they went unavailable at my supplier.
Current gain seems a bit low, but they will work.

Here's the changed circuit for the current measurement. Hand drawn of course.

The 47K/22n input RC filter largely rejects, the applied grid signal. From an AC standpoint the 47K is parallel to the 2K resistor because of the RC filtering.

The trim circuit for the LF 411 is not drawn, its the standard data sheet Trim circuit, but with a 50K pot. I was thinking of using a 2.5V shunt reference in the shunt regulator to supply the LF411 with -2.5V. Those IC's have much better temperature coefficient then zener diodes, any drift in the V- rail of the op amp is going to influence the offset voltage. LM336 or LM385 seems like a good candidate.

The oscillator section will have to drive about a 10K load, peanuts for the NE5332 that will buffer the oscillator.

The floating +5V supply for this circuit will have to supply about 20mA.

More to follow!
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Last edited by Thatvalveguy; 8th Apr 2018 at 3:09 pm.
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