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Old 31st Dec 2019, 7:48 pm   #61
Bazz4CQJ
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Default Re: "Burning - in" Double Triodes

If you are "lashing" something together to do the tests, beware the risk of oscillation. When I was plotting lines for the 12AT7 (slightly higher Gm), the original rig would sometimes show odd readings and it was oscillating. The MkII rig had quite a few ferrite beads and the odd cap added to it.

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Old 31st Dec 2019, 8:07 pm   #62
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Default Re: "Burning - in" Double Triodes

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Originally Posted by Bazz4CQJ View Post
If you are "lashing" something together to do the tests, beware the risk of oscillation. When I was plotting lines for the 12AT7 (slightly higher Gm), the original rig would sometimes show odd readings and it was oscillating. The MkII rig had quite a few ferrite beads and the odd cap added to it.

B
Yes - always remember that, deep down, every circuit-with-gain wants to become an oscillator! If yours hasn't yet managed to do so, it's only a matter of time! Turn your back for a microsecond and the oscillations will build up...
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Old 31st Dec 2019, 8:16 pm   #63
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Default Re: "Burning - in" Double Triodes

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Originally Posted by GrimJosef View Post
If you take the grid positive of the cathode you run the risk of passing some grid current. In principle this might heat the grid, which would be A Bad Thing (the wires are physically fragile and not designed to disspiate much heat). I'd be surprised if you did any damage but you might want to put a current-limiting resistor in series with the grid supply.
Or, just pulse the grid supply, -10V for 50msec, and +what-you-want for 1msec, and use a 'scope to measure Ia (and maybe Ig) during that millisecond of interest. The 'rest' time will allow heat to dissipate.
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Old 31st Dec 2019, 8:49 pm   #64
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Default Re: "Burning - in" Double Triodes

OK thanks. I'll fit a bead inline with the grid bias resistor when taking the grid away from Vg=0.

So far I've inspected the (Chinese) 12AU7 from RS and I was surprised to see how flimsy the socket pins were. The last time I looked at a classic valve like this the pins were made of a very hard/stiff metal. On this valve these pins have a very shiny but slightly lumpy finish and the metal is quite soft. So I had to bend each leg to make it align with a 9 pin socket base as some pins weren't straight. They feel almost as pliable as copper wire.

I've also run the first VG=0 curve and it is fairly similar to the datasheet.

Anode
Voltage mA
5V 0.1mA
10V 0.378mA
15V 0.789mA
20V 1.29 mA
25V 1.836mA
30V 2.43mA
35V 3.06mA
40V 3.72mA
45V 4.43mA
50V 5.14mA
55V 5.9mA
60V 6.69mA
70V 8.349mA
80V 10.125mA
90V 11.96mA

I'll have a go at measuring the gm at some point but I want to do this at several anode voltages and currents.
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Old 2nd Jan 2020, 9:45 pm   #65
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Default Re: "Burning - in" Double Triodes

I had another play with this valve today. I tried measuring gm. I'm not sure what the correct method is but I held the anode at a fixed voltage and the cathode at 0V DC. I then wiggled the grid across 0V DC +/- 0.2V and measured the difference in anode current for this tiny voltage change at the grid.

I was a bit surprised by the results as the gm seems to be double the value on the datasheet. However, I'm measuring either side of Vg=0V rather than at a more typical point with a negative Vg and the datasheet curves suggest a different gm when tested like this.

I measured gm at low anode voltages because I wanted to analyse an oscillator that starts with a low anode voltage at Vg=0V. So I measured gm at anode voltages of 10V to 60V at Vg=0V. See the results below. Does this look about right?
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Old 2nd Jan 2020, 10:41 pm   #66
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Default Re: "Burning - in" Double Triodes

Gm increases somewhat with grid bias. In normal operation the grid will run a few volts negative of the cathode, and that's the point on the curve where the Gm value is specified.

Grid bias is usually arranged via a dropper resistor in the cathode. It wastes a bit of HT voltage but it provides a negative feedback effect (sshhh! don't let the audiophiles hear) that acts to stabilise the quiescent current. In old stuff, separate grid bias batteries were used and cathodes were the filaments and inevitably commoned at ground to isolate stages. Oscillators usually set their own grid bias by self rectification by the grid/cathode acting as a diode. This allows the Gm (=> gain) in an oscillator to soar to almost double, until oscillation gets going.
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Old 2nd Jan 2020, 10:43 pm   #67
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Default Re: "Burning - in" Double Triodes

When I did tests on ECC81's for use as a standard valve for my AVO VCM, I took the test parameters from the Avo databook and worked around those.
In this case, the VCM tests at Va=250 and Vg=-2 and Gm should be around 5.5mA/V

So with Va fixed at 250, I varied Vg from -2.2 though -1.8 while measuring Ia, and the plot typically looks like the one attached. In this particular case Gm was measured at 5.1 and 5.3 over two runs, averaging ~5.2mA/V.

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Old 2nd Jan 2020, 11:48 pm   #68
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Default Re: "Burning - in" Double Triodes

Thanks for the replies. Sadly, I can't test at a high voltage like 250V as I don't have a PSU that goes that high. The nearest I have is a little boost converter in a box. It can be tweaked up to about 200V DC but I doubt it can deliver more than 2mA at 200V DC. It's original job was to act as the reverse bias in a PIN diode switch many years ago so it only needed to run at flea power.

I did try setting up the circuit as the 2MHz crystal oscillator in the TF144H and it worked fine. With an anode voltage set at 50V and 33k in the cathode it managed to develop 4Vrms across a 2MHz crystal. This was with the lowest ESR (2MHz) crystal I could find here.

My VNA measured about -1400R in series with 18pF at 2MHz when looking into the grid of the oscillator with the crystal removed and 12pF and 270pF caps in the feedback as per the TF144H. The 18pF VNA figure is a fair bit higher than I was expecting and I think the pin to pin capacitance of the valve base affects the result here. The dielectric of the material in the base is unknown but if the material is something like bakelite then the dielectric constant could be about 5.

I measured 1.6pF just looking into the isolated grid pin of the base with the valve and other components removed. If I put these numbers for gm and the capacitances into a model along with the valve's internal capactances I predict about 16.8pF in series with -1900R at 2MHz. This is fairly close to reality but I'm used to measuring tiny semiconductors and I'm used to seeing extremely good agreement with models. So I'm a bit disappointed it wasn't closer.
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Old 3rd Jan 2020, 12:07 am   #69
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Default Re: "Burning - in" Double Triodes

Looking at the 12AU7 datasheet it is possible to guess the gm at different anode voltages and currents. Assuming I'm reading the graph below correctly, the slope of the red line is just under 1.5mA/V at 5mA Ia and 250V anode V. If I guess at the slope of the 50V response as per the blue curve at the 5mA Ia point and then elongate this line downwards I can measure the slope and it looks to be about 2.5mA/V. I measured 2.8mA/V which seems reasonable.

I should also point out that this Chinese 12AU7 from RS has already 'aged' a tiny amount because I now get a slightly different response for anode current vs anode voltage at Vg=0V. I'll formally remeasure this at some point but the valve appears to have lost a tiny amount of emission after a few hours' operation.
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Old 3rd Jan 2020, 12:51 am   #70
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Default Re: "Burning - in" Double Triodes

Hope this isn't boring or OTT but I extracted various VNA models looking into the grid with the crystal removed. I took S11 data files at anode voltages starting at 5V through to 60V.

One interesting thing I spotted when I post processed the VNA data was that it predicted the oscillator would start up even with an anode voltage of just +7.5V at the anode and this is with 33k in the cathode and the VG=0V bias of the TF144H. This was based on having a crystal with a very low ESR <100R at 2MHz.

This seemed crazy for a valve that typically runs at 200V but I refitted the crystal and the minimum voltage for a genuine off/on startup was just 8.2V at the anode! This was in very good agreement with the VNA model taken at 7.5V! I only casually measured the ESR of this crystal when I looked at it a few days ago and I think it was something like 90R at 2MHz.

But the main thing is that it really can start up even at 8.2V at the anode if the crystal ESR is this low. A typical 2MHz crystal might have an ESR of 250R at 2MHz and I have a bag of 2MHz crystals (stamped CQ) that have an ESR of over 500R at 2MHz. So there can be a huge variation depending on the crystal manufacturer.

90R ESR seems very low (to me at least) for a 2MHz crystal but then I rarely measure crystals that are this low in frequency. This crystal has ACT 03 2.000MHz stamped on it and I only have one of these.
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Old 3rd Jan 2020, 2:13 am   #71
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Default Re: "Burning - in" Double Triodes

Quote:
Yes - always remember that, deep down, every circuit-with-gain wants to become an oscillator! If yours hasn't yet managed to do so, it's only a matter of time! Turn your back for a microsecond and the oscillations will build up...
With this in mind, and a slowly improving model (complete with base pin capacitances) I had a look at the grid impedance with the oscillator components removed. So now I just had 33k in the cathode and a 1Meg resistor from cathode to grid giving Vg=0 bias at a 60V HT at the anode.

The model predicted significant negative resistance over a huge bandwidth in series with a few pF. So I measured the real circuit with the VNA with these changes included. It agreed quite well with negative resistance occurring all the way up to 150MHz. See the model vs real 12AU7 in the diagram below. The agreement is quite reasonable I think.

You can see the red traces show 'negative' resistance up to about 150MHz and this in series with several pF. So the only thing missing at the grid is a few hundred nH of test probe inductance that is AC grounded at the far end and the tube should oscillate at VHF.

I think my little £30 nanovna from China could also make these negative resistance measurements quite well too so there is no longer any need for any exotic lab gear to do these basic tests...
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Old 3rd Jan 2020, 2:15 am   #72
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Default Re: "Burning - in" Double Triodes

I can probably dig a few high quality HC6U crystals out of the drawer around 2MHz (likely to have 2.15MHz and 2.35) if you need some.

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Old 3rd Jan 2020, 2:58 am   #73
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Default Re: "Burning - in" Double Triodes

Thanks! I may actually have some 1.8MHz crystals in that package in the loft. Presumably these will have low ESR and I could measure them if I can find them..

I'm afraid I couldn't resist turning the test circuit into a VHF oscillator and the VNA model at 60V HT predicts instability at 127MHz if 246nH is placed in series with an ATC100B 150pF DC blocking cap at the grid. This 150pF cap will be a low inductance DC block and I chose 246nH as I have a stash of Coilcraft 132 Maxi spring inductors here. The Coilcraft data shows that the 15 turn Maxi inductor maintains close to 246nH up beyond 150MHz so is an ideal candidate for testing in the circuit as it should give a result close to the model.

https://www.coilcraft.com/maxi.cfm

The VNA model predicts oscillation at 127.5MHz as in the plot below and when I fitted the Coilcraft 246nH and ATC150pF parts to the 12AU7 it oscillated at 130MHz when sniffed with an H field probe and my old HP8566B spectrum analyser. This result was a bit more reassuring and it is close enough to prove the VNA S11 model is OK I think...

I'm used to applying this stuff to tiny semiconductors so this is a new experience for me. At least I can now say I've done some design and analysis with valves

These do seem to be a bit flighty especially when fitted to a valve base and I suspect that great care must be needed when designing amplifiers using these old valves. I think the stray capacitance of the base can play a role here and it shouldn't be ignored for some designs...
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Old 3rd Jan 2020, 7:29 am   #74
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Default Re: "Burning - in" Double Triodes

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These do seem to be a bit flighty especially when fitted to a valve base and I suspect that great care must be needed when designing amplifiers using these old valves. I think the stray capacitance of the base can play a role here and it shouldn't be ignored for some designs...
That's a fair assessment. Add in the preference in valve circles for circuitry with relatively little feedback, and you can see how device characteristics take on great importance.

In valve development, a lot of work went into trying to reduce the effective L of cathode connections to get the things to give gain to higher frequencies. The history of the EF50 makes interesting reading, not just the technical aspects, but getting the things and all evidence of their development out of Holland only just ahead of the nazis. Another area of careful RF development was in the bases for things like the 4CX250B which needed integrated ceramic capacitors in the base itself like a feedthrough structure to decouple g2 and keep the frisky thing stable. Precursors of techniques needed with RF power transistors.

I'll have a dig in the crystals drawers on Monday night. What range are you interested in?

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Old 3rd Jan 2020, 8:12 am   #75
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Default Re: "Burning - in" Double Triodes

There are many subtle processes that combine and present themselves as a grid leak current. Some of those processes, like gettering of stray gas, take a long time to settle. That is one dominant aspect of valve performance used for VTVM's and electrometers for example.

For low power audio valves, about the only concern I can think of is if a stage uses grid-leak bias - which is either a vintage input stage, or microphone amp type applications. In that situation, it could well be worth running in the valve so that it settles on an acceptable anode idle voltage, rather than do a quick 5 minute check and think nothing will noticeably change.

For high power audio valves, I'd expect the likelihood of a grid leak issue to arise (as an infant mortality) is greater than for low power parts, as internal temperatures and masses are greater. So whether that is seen as 'running in', or 'weeding out', it has the same affect of minimising collateral damage to other parts like transformers.

The 'go to' technical reference for me is the RCA 1962 Electron Tube Design technical compendium (shown in the tubebooks.org list as 1963).
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Old 3rd Jan 2020, 10:14 am   #76
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Default Re: "Burning - in" Double Triodes

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The 'go to' technical reference for me is the RCA 1962 Electron Tube Design technical compendium (shown in the tubebooks.org list as 1963).
Couldn't find it at tubebooks but it's also available on line here https://archive.org/details/RCA_1962...on_Tube_Design - all 951 pages of it, and CD copies are available. Looks extremely interesting.

Couldn't find any reference to hard copy versions; suspect they change hands for serious money!

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Old 3rd Jan 2020, 11:08 am   #77
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Quote:
it could well be worth running in the valve so that it settles on an acceptable anode idle voltage, rather than do a quick 5 minute check and think nothing will noticeably change.
When I first powered up this valve the initial test was to datalog it over 1000 seconds at Vg=0V bias at a fixed HT.

I did this at a set heater voltage (set by a lab PSU) and I logged the anode current using a lab DMM and a quick and dirty VB program via GPIB. The idea was to repeat the test some time later and look for differences.

I set the valve to be vertical in a crude jig and this new 12AU7 gave a gradual curve in anode current over the first minute or so as the heaters reached equilibrium and the anode current then settled to a flat line. However, at about 800 seconds the 12AU7 in my hastily fudged jig fell over and the valve tipped over to horizontal. This cause a permanent shift in anode current even when I set it vertical again. I was then able to change it a bit more by tapping the valve quite gently with a pencil. So this put things into perspective for me.

I'll dig out the log later today. The shift in current was quite subtle but easy to measure.

I also noted a subtle change in anode current when I re-ran the anode HT vs Vg=0V curve after I had been running the valve as a 2MHz oscillator. So my conclusion is that these aren't precision devices over time (or when subjected to vibration) so I think it is up to the designer to design circuits that can minimise the effects of all this. There will be ways to do this badly and ways to do it quite well I think.
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Old 3rd Jan 2020, 11:27 am   #78
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I'll have a dig in the crystals drawers on Monday night. What range are you interested in?
Thanks but don't rummage too long... I think I'm about done with the oscillator testing. The last thing of interest would be to see how much difference the crystal ESR has on the close in phase noise of the 2MHz oscillator. I have a 500R ESR crystal and an 80R ESR crystal here to compare and that is quite a change in ESR! However, I might need to do this at work with an E5052A SSA if my RTSA/SSA analyser here at home can't measure a difference.
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Old 3rd Jan 2020, 12:32 pm   #79
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Default Re: "Burning - in" Double Triodes

Here's the initial datalog of anode current vs time in seconds.

This wasn't the very first time the valve was powered. I did have to power it a few times to set it all up and check the heater operation but the valve was only operated for a few minutes before this 'start from cold' test.

I did let the heaters settle for several seconds as the PSU was in 500mA current limit for several seconds from a cold start and this is not in the log. I started logging once the heaters were out of current limit.

You can see where the valve tipped over and then when I started tapping it. The big negative spike must have captured the very peak of the microphony when tapping it with a pencil. But you can see the permanent change and I don't think this was anything to do with the pin connections at the valve base but I can't totally rule this out. I did softly wiggle the valve in the base and it seemed benign compared to the tapping with a pencil.

I haven't bothered to add titles to the graph but you can see it shows about 6.7mA anode current vs time in seconds.
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Old 3rd Jan 2020, 1:24 pm   #80
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If I repeat the test today, the anode current is down to 6.13mA. When new the valve showed 6.7mA under identical test conditions. I have used sold connections everywhere to make this a repeatable test. The current meter connections from the PSU are all made with decent 4mm connectors and I make sure the anode HT is kept at a constant 60.0V at the anode pin at the valve base with the other DMM. The valve is run with the cathode tied to 0V and the grid is fed via a 1Meg resistor.

Quote:
There are many subtle processes that combine and present themselves as a grid leak current.
Thanks. This stuff is all new to me and with this in mind I tried measuring the voltage at the grid with an ultra high Z DVM. I've got several DVMS like this and the highest impedance meter is many (many) Giga ohms when measuring small DC voltages. It will hold a DC voltage on the display for several minutes even after removing the probes!

So I assume I can estimate this grid leak current via the DC voltage seen across the 1Meg resistor.

The big surprise (for me at least) was that on all the high Z meters I saw -0.4V at the grid. I assumed it would be very close to 0V.

Whoops, maybe I should have measured this right at the start of all the tests as this could have been quite revealing over time.

-0.4V / 1Meg = -400nA grid current?

Note that when I touch the high Z DVM on the grid the anode current doesn't change it stays at about 6.132mA on the lab current meter. So I think this is a valid measurement and the meter isn't loading the circuit? If I use a regular 10Meg DVM I measure -0.367V at the grid and the current does change slightly.
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