# 12ax7 – Heater Voltage Affecting Bias Point?

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#### SlunkBoy

For the desired plate current in my 12ax7 circuit, I found I have to adjust the grid voltage significantly more negative than predicted by the curves (-2.8 Vdc as opposed to -2.1 Vdc). After puzzling over the circuit for a while, I noticed that my filament voltage was around 13.7 Vdc. Is it possible for a modest deviation like this to disrupt the operating point of the tube so drastically? I discovered the issue too late last night to make any adjustments and I can't think of any other variable that could account for the mismatch between prediction and measurement. The power supply is sufficiently stiff and the tube only has a few hours on it. Any suggestions would be greatly appreciated.

#### GoatGuy

Perhaps, but not terribly likely.

If you insert a series voltage-drop resistor ranging from 6.8 Ω to 7.5 Ω (½ watt), you will drop your 13.7 to 12.6 VACRMS.

⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅

PS — remember that's for ONE tube, only! If you have a pair of 12AX7's, you'd need a resistor of ½ that 6.8–7.5 Ω value. And so on.

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#### SlunkBoy

Understood – but I'm sure you've been called away from your hobby before by other responsibilities and been left with a gnawing question. Thanks for responding.

#### SlunkBoy

Responding to myself – this vintage exchange between Jeb-D and Brian Beck seems to support this as a possibility:

Jeb-D:

"The filament voltage meters 5.7V, which is the minimum voltage for the tube being used. It's not a directly heated triode. Does any one know if this will raise the Rp of the tube? Opposed to running it at 6.3V. It seems to bias at a significantly more positive grid voltage; for a given amount of current, when compared to the curves. I've tried with 5 tubes and all act the same way. Anyone else noticed this, or am I just trippin'?"

Brian Beck:

"Generally, yes. Starving a tube's heater will lower cathode emission, which will lower cathode current, which will reduce transconductance, gm. Since mu is more dependent on mechanical design and relative electrode dimensions, and since rp = mu/gm, then rp will increase as gm drops. There are complicating factors, which I'm sure others will bring up, but that's the gist of it."

I'll play with it tonight and report back for posterity.

#### trobbins

Increasing heater voltage will increase heater power dissipation and hence cathode temperature. That will likely cause some increase in anode current, as you are approaching +10% of nominal heater voltage.

Do you have a circuit that "must" operate at a specific anode current? A typical well designed amplifier circuit can cope with such a wide range of operating conditions.

How are you measuring grid voltage, and how is the grid bias achieved, and have you tube rolled to get a feel for how operating conditions change for say 10 different tubes?

#### nigelwright7557

Its maybe just manufacturing tolerance.
But its easy to guess, changing the heater voltage will prove one way or the other.

#### JonSnell Electronic

A few years ago, like 50 or so, we had a 'computer' error that took some finding. It was an all valve machine using ECC81s, (12AT7 for our American friends), as switches.
The issue was instability and the cause was excessive heater voltage. The boffins of the day, sadly all passed years ago, from Mullard, told us that if the heater gets too hot, the control grid also heats and can emit electrons as it reaches a dull red, as a direct result of excess heat. This in turn affects the electron flow to the anode producing unwanted results including a shifted bias point which was the cause of the 'computer' failure at the time.

#### GoatGuy

The boffins of the day, sadly all passed years ago, from Mullard, told us that if the heater gets too hot, the control grid also heats and can emit electrons as it reaches a dull red, as a direct result of excess heat. This in turn affects the electron flow to the anode producing unwanted results including a shifted bias point which was the cause of the 'computer' failure at the time.

Fabulous.

As I'm entering my 7th decade, I too am struck by the undeniable fact that whatever-all electronic engineering “stuff” I've accumulated … is someday, probably not far off, going “bye-bye”. Not a single 'kid' of my children's or grandchildren's generation has shown even a remote interest in the actually rather straight forward, but still also complicated math findings of the electrical engineering world. Not a one.

Maybe tho', this isn't much different from growing up as a “brainiac” in the 1960s. There weren't many boys interested in electronic things. Oh, plenty knew about points, dwell, ignition coils and condensers. Some knew something about relays, switches, household power, transformers and maybe even diodes and resistors. But electronics? Rare. Even then.

My school only had single-room-single-grade-level classes, about 35 kids as a pod, going from first to eighth grade together. In my class, 2 of us. In the class ahead fo us, 3. In the class behind, 1. So, 6 of us heading to the much larger high school. Once in HS tho', I found ONE additional guy from all the feeder schools, interested in things-electronic. Only 1. So, 7 of us …

When my 3 kids went to-and-thru grade school, there were none. In high school, exactly 1 girl and 1 boy turned out to be interested in things electronic. The girl went on to become a medical doctor. The guy became an Air Force pilot.

Just saying… I don't see a large body of up-and-coming kids taking on the mantle of “re-learning all these fascinating rules and findings”. I just don't.

And frankly, maybe it doesn't matter.

Because we are FAR, FAR from the consumer-oriented world of bespoke built electronics. Heck… at The Wife's bidding, I just took apart a double sub-bass speaker plus 5 surround-speaker setup, Yamaha 7 channel amplifier, fabulous cables, all the rest, and gave it away for free on Craig's List. Why?

Because she was tired of it not working with the Roku (or Amazon Fire…) “stick” on the TV.

Hence why .. “maybe it doesn't matter”.

⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅

#### TheGimp

I'm looking back on 7 decades and one year away fro starting my 8th, I remember no other students interested in electronics while I was in school. My HS graduating class was 280, so probably 1000 students in the school.

My son has a passing interest in electronics, and just graduated with a BS in Biomedical Electronic Technology. However he is interested in working with Raspberry Pi to build a quadrocopter. No interest in audio.

#### kodabmx

I'm only looking back on 4 decades... I would run sound from the TV by optical into a cheap 5 dollar DAC into that sound system That's how I have TV sound on my tubes these days

My lover is younger than I am but not really much of a technogeek like me.

I did get him to build an LM386 amp kit once, and he can solder pretty well, but if it blew up, he'd have no clue how to fix it. Now it collects dust because it's a hiss machine.

#### JonSnell Electronic

I owned, until I retired a few years ago, a large electronics repair workshop. I have four boys all of whom spent time in the workshop after school and none of them were remotely interested in taking on the business, so I sold it and retired. The eldest was interested in computing and now works for Microsoft on the help desk from home. The next one came on technical courses with me to Sanyo, Grundig, Philips etc and now sells fasteners. The next one is a musician and the last one runs an online advertising company.

Shame really.

#### kodabmx

One of the last (there are two that I know of left in the city of Toronto) surplus electronics parts stores just "scrapped" 4 tonnes of transformers because "nobody builds anything anymore"...

Another shame.

#### GoatGuy

Darn… I started the fugue on the downbeat, so it seems. Lads… and except for Mona hereabouts, I'd say 99% “lads”, let's just face “the music”. We're the last of the buggy-whip and cotton-gin tweaking fanatics. Consider that this weblog is potentially open to several billion viewers, and yet those of us who regularly opine number in the hundreds. Maybe. Maybe only numbered in scores. By the stats, there are likely 10:1 to 50:1 readers who might even hungrily eat up the topics, posts, and highly contested findings. Who knows. I hope so.

But even then 2,000,000,000+ potential viewers … divided by … four score posters times five score silent regulars, times maybe a half score more occasional lurkers … is only 2,000,000,000 / ( 4 × 20 • 5 × 20 • ½ × 20 ) → 1 in 25,000 people at this point. And lets face it, I was being generous.

I think about my tony little town of 80,000 and imagining that there are only 3 people in that town possibly interested in this level of build-it-yourself-AND-understand-it-too detail is somewhat “downbeat on the fugue”.

But then again, there are possibly just as few — or fewer! — blokes (mostly) that make custom orthodontistry doohickeys, or who custom machine hundred-ton chain sprockets for nuclear power plant rail cranes.

It just pains me to accept a scrapper chucking 4,000 kg of no-longer-desired-even-occasionally transformers. Because “no one builds anything anymore”.

Yep.

All I know is that I miss “Bear”.
Fat f••k that ran Mike Quinn's electronics surplus store…
Down in a quonset hut, at the Oakland (CA, USA) private airport.
Where you could get ANYTHING.
Including radium sources, x-ray tubes, nuclear weapons trigger tubes, and …
Well things that little boys should not play with.
Didn't prevent me from playing with them.
One bit.
AND I still have all my fingers and both eyes.

LOL!
⋅-=≡ GoatGuy ✓ ≡=-⋅

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#### SlunkBoy

I thought this thread was dead after the first day – what a delight to come back to useful responses and illuminating "greybeard" stories! In the interim, I was able to confirm that adjusting the heater voltage back towards the nominal value improved the gap between the predicted and measured performance – though I do attribute some of the variance to manufacturing inconsistencies.

If it makes any of the septuagenarians feel better about the state of things, I'm not quite forty and have been an electronics hobbyist for half that time. Regrettably, I don't know anyone in my cohort who is interested in electronics at the hardware level – but I assure you that plenty of them are doing equally impressive things with ones and zeros.

#### nigelwright7557

I'm looking back on 7 decades and one year away fro starting my 8th, I remember no other students interested in electronics while I was in school. My HS graduating class was 280, so probably 1000 students in the school.

My son has a passing interest in electronics, and just graduated with a BS in Biomedical Electronic Technology. However he is interested in working with Raspberry Pi to build a quadrocopter. No interest in audio.

My first valve project was a 12ax7 guitar pre amp.
Worked quite well after a few shocks !

I had a local man who fixed TVs/Radio's and he helped me quite a bit.
He had been in the RAF during WW2 which is where he learned.

#### trobbins

SlunkBoy, it appears you only have one 12AX7 to play with.

It is well worth appreciating that any bogey valve performance prediction based on datasheet values can have a large tolerance and still be 'nominal'. It is also worthwhile tube rolling to appreciate what practical variation could be being observed, and if your sample valve has some poor/degraded aspect like grid leakage current (although you can normally test for that in circuit by shorting the grid leak).

#### SlunkBoy

Hey, now – I have two tubes to work with! Exacerbating the problem is that I'm working with very low plate voltages. At any rate, I was able to get a measurable improvement by getting my heaters to behave.

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