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Starved-heater operation
Starved-heater operation
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Old 19th January 2019, 10:26 AM   #1
audiostrat is offline audiostrat  Greece
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Default Starved-heater operation

Just wanted to post a question about an unusual mode of operation for a preamp tube.

I own a Vox Bulldog Distortion pedal, which uses a 12AU7 tube in the circuit for some "tube" tone. The fun fact of the application is that the heaters are constantly operated at 3.3VDC, rather than the 6.3V nominal heater voltage. So, heaters are starved.

I have searched for, but not found any info on the following:

(1) Do the characteristic curves change in that mode of operation? I assume that starving the heaters would result in reduced emission, thus less anode current per anode voltage. Is there any info that shows the change?

(2) Does this practice affect the tube's life in any way? The circuit operates at +-6VDC, so I suppose the anode wattage is safely within limits.
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Old 19th January 2019, 11:29 AM   #2
DF96 is offline DF96  England
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Yes, the curves change. You are unlikely to find curves for such a low heater voltage.

Difficult to say what the effect on valve life would be. Low heater voltage can be a problem because the protective cathode space charge will be weak, but with such a low anode voltage too I suspect it won't do too much harm.
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Old 19th January 2019, 12:06 PM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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The life of the valve is not unduly changed as the anode current is very small. The low heater voltage encourages distortion and compression, which is what it is for.
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Old 19th January 2019, 12:54 PM   #4
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by audiostrat View Post
Just wanted to post a question about an unusual mode of operation for a preamp tube.

I own a Vox Bulldog Distortion pedal, which uses a 12AU7 tube in the circuit for some "tube" tone. The fun fact of the application is that the heaters are constantly operated at 3.3VDC, rather than the 6.3V nominal heater voltage. So, heaters are starved.

I have searched for, but not found any info on the following:

(1) Do the characteristic curves change in that mode of operation? I assume that starving the heaters would result in reduced emission, thus less anode current per anode voltage. Is there any info that shows the change?

(2) Does this practice affect the tube's life in any way? The circuit operates at +-6VDC, so I suppose the anode wattage is safely within limits.
Donīt worry about characteristic curves (thereīs none) nor tube life or sound, itīs a scam.
The tube does nothing, itīs just a marketing gimmick.

The distortion pedal functional block is solid state; or worse, it might be some kind of DSP simulating everything.

There is also a similar scam, the Takamine "Cool Tube" Acoustic Guitar preamp, same claims.

VOX uses the Valve Reactor circuit in some power amps, but there tubes do a useful job, filaments are properly heated and plates get 40/60V DC or so, which allow *some* current to pass ... but with 6V +V and cold filaments? .... forget it.
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Old 19th January 2019, 08:59 PM   #5
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by JMFahey View Post
There is also a similar scam, the Takamine "Cool Tube" Acoustic Guitar preamp, same claims.
One of my guitars has that preamp in it. I bought the guitar for three reasons: 1) Because it is a steel-string guitar with a wide 1 7/8 nut for finger-picking, and that nut width is almost as rare as hen's teeth (2) It is very well made and has excellent playability and acceptable tone and 3) Plugged-in, it sounds very much better than most piezo-pickup acoustic-electrics.

I have always been skeptical about what, if anything, the "Cool Tube" preamp actually does. I suspect the tube might actually do something useful, namely, soften harsh transients from the piezo element. But I don't know for sure.

But whether the tube itself is a gimmick or not, Takamine got the design of their "Palathetic" piezo pickup and/or preamp right: it has virtually none of that piezo harshness or "quack" that is so common in acoustic-electric guitars.


-Gnobuddy
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Old 19th January 2019, 11:02 PM   #6
PRR is offline PRR  United States
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The tube may last forever; or die tomorrow from bad assembly.

12AU7 is about as cheap as it gets.

If it sounds good, why worry?

If it does not sound good, sell it on. It is an attractive box and many seem to change hands online.
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Old 20th January 2019, 12:16 AM   #7
Hearinspace is offline Hearinspace  Canada
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Starved-heater operation
"Owner's Manual Many thanks for adding the VOX COOLTRON Bulldog Distortion /Big Ben Overdrive / Brit Booster pedal to your rig. Our main goal is to provide you with the most toneful products anywhere and we believe you'll enjoy using these pedals as much as we enjoyed designing them! What is COOLTRON?

COOLTRON is the name that we have given to a new way of using preamp type tubes at low voltage. In our case we chose a 12AU7. Tubes are normally used at high voltages, and this is the recognised way of using them, but it negates their use in battery-powered devices such as guitar effects pedals which is what COOLTRON is! Historically, attempts have been made to use tubes at low voltages, usually termed as being used in "starvation" mode. Unfortunately, these circuits have severe problems in the tube functioning in an acceptable manner. One problem is being able to bias the tube to a satisfactory operating condition. Another problem is that the necessary heater (filament) current is too high to provide adequate bat- tery life which we all know would be annoying when you have to change batter- ies every ten minutes during a gig.

The COOLTRON system gets over both these problems and provides true tube sonic performance at a very low voltage and at a reduced heater current – ideal for inclusion into the aforementioned guitar effects pedals – and therefore can give these pedals a vast improvement by giving valuable tube sonic performance. Basic Operation The COOLTRON circuit basically operates by using two very special circuits:
A) To achieve the required biasing function, a unique circuit is used that provides the right conditions between the anode (plate) of the tube, and the grid. In general terms this circuit is called a "servo" circuit, and replaces the normal self-biasing networks that would normally be used. Although not the same circuit, this servo theory is used many times in high-end tube hi-fi, to achieve controlled stable parameters.
B) The heaters are provided with a special power supply that provides a low voltage and low current supply to the heater elements in the tube. Due to the servo-biased tube circuit providing the correct operating conditions for the tube, the tube now functions as it would if run at a higher voltage – i.e. the signal excursions can be operated linearly, pro- viding balanced clipping conditions, and with the right harmonic & distortion characteristics – even when running from a B+ of only 6 volts. Both of these conditions are virtually impossible to achieve using nor- mal "starvation" methods. Due to the tube now running at such a reduced supply level, the anode current is much, much smaller than normal. This means that the amount of heat required at the cathode to achieve sufficient cathode current emission is much smaller – hence the ability to run the heaters at a lower level. Therefore both these points combined means that we have a usable tube circuit running off a battery power supply – battery life is approximately 16 hours from 4 x AA size batteries – but also the tube runs a lot cooler – hence the name "COOLTRON" "
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Old 20th January 2019, 01:25 AM   #8
audiostrat is offline audiostrat  Greece
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I have read the description of the pedal, but since it doesn't state anything about tube life span I posted.

JMFahey, I have a schematic of this pedal that I found floating around the net years ago, and it depicts the use of a 12AU7 in series with the signal. Yes, the clipping comes from some 3mm leds, but the advertising does not state the clipping comes from the tube - a "sonic performance of the tube" is mentioned.

The following two tests could show that the tube really is in the signal chain, giving some "character":

(1) I installed an smd switch in the pedal that allows you to choose one of the two triodes in the 12AU7 package (only one is used). Changing the triode used resulted in less volume and a bit less sparkle, but even if only volume was lost (and "sparkle" was a placebo due to the volume loss) the signal is passed along the tube - so no scam here.

(2) I have built (and happily using for a compact pedalboard) the overdrive of the 1st channel, using the same schematic I have but only omitting the tube. The two effects sound different, the tube-driven being a bit more compressed and mid-focused. Small differences, not anything great, but definitely noticeable. I really didn't think in the first place that the tube would matter, but maybe it does a little. (maybe stands for "provided I have the correct schematic"). With some mods the pedal without the tube is a great effect for my taste though, proving to me that 90% of the sound is indeed NOT the tube. I wouldn't disagree on that.

I just think that I can now manage use the treble control more, which was always a problem with that pedal everywhere I used it. So I thought that maybe the tube is dying, and posted. I have been using it for 10 years now, it has undergone lots of hours of playing. Maybe the best answer is to buy a replacement tube and check, but a conversation is always worth it.
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Old 20th January 2019, 06:04 AM   #9
Gnobuddy is offline Gnobuddy  Canada
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This thread jogged my memory of something I stumbled across some years ago - a 12AU7 based guitar overdrive pedal that came to be known as the "Matsumin Valvecaster".

I chased down a schematic (attached), which shows the heater wired for 12.6 volt operation, but run off 9 volts (only about 70% of rated heater voltage.) The same 9 volts is also the B+ for the anodes.

So is the 12AU7 just a useless gimmick? Well, in this particular schematic, there are no solid-state amplifying or clipping devices at all - no FETs, diodes, or transistors. The two sections of the 12AU7 are both wired as common-cathode gain stages, one feeding the other. So any signal that actually emerges from the circuit has definitely gone through both triodes.

The next obvious questions are, does any signal emerge, and if so, what if anything are the triodes doing? This video clip will let you answer those for yourself: YouTube

Between the Valvecaster schematic and that video (there are plenty more on You Tube), it is now clear that yes, a 12AU7 can actually work with 75% of normal heater voltage and only 9 volts B+.

That makes it a lot easier for me to believe that the same valve can also actually work - i.e. amplify and distort - with only 50% of normal heater voltage and 6 volts B+, as in the VOX Bulldog and Takamine Cool Tube preamps.

I don't own a single 12AU7, otherwise, I would be tempted to try and measure (by hand) a few characteristic curves with only 6 or 9 volts B+ and the heaters operated at 50% and 75% of normal voltage.

But even without actual curves, we can deduce from Ohm's law, the 9-volt supply, and the 220k and 100k anode resistors in the Valvecaster, that quiescent anode currents are in the region of a few tens of micro amperes. In other words, about a hundred to a thousand times lower than the typical anode currents for the same valve run at a more conventional operating point.


-Gnobuddy
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Old 20th January 2019, 08:28 AM   #10
JMFahey is offline JMFahey  Argentina
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Quote:
I have a schematic of this pedal that I found floating around the net years ago
Thanks.
Would you please post it?
Quote:
the clipping comes from some 3mm leds
I expected something like that.
Hey, even in some Marshall Valvestate mains fed preamps, with 12AX7 fed proper 6.3V per filament and around 200V +V , they *still* clip with a couple red Leds (or a 5 diode network), so the tube is there more to be able to be mentioned in the brochure than anything else.
Quote:
a 12AU7 based guitar overdrive pedal that came to be known as the "Matsumin Valvecaster".
Thatīs about the absolute minimum filament temperature at which some emission is achieved.
Given that cathode emission is roughly proportional to the third power of temperature, which in due time is roughly proportional to filament voltage squared, tubes are usually specīd within 10% of suggested filament voltage and acceptable emission is supposed within thatbrange.
Given the high exponent order of the emission function, halving filament voltage which in principle does not look like "that much", ends being up a very big deal.
And then we have very low voltage to attract those very few electrons floating around; building any gain stage around 3.3V filament and 6V +V supply looks very iffy.
If we get the VOX schematic I can build a gain stage, even a "Matsumin" and check what it can actually do.
FWIW both BK Butlerīs "Blue Tube" and "Mini Boogee" pedals which use reduced filament voltage and low +V apply *positive* bias to grids to at least get some electrons doing useful duty.
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