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4.4.135.93
In Reply to: Filament voltage ? posted by shif on August 25, 2004 at 20:11:50:
This won't answer your question completely or probably satisfactorily for that matter. But, just went through this myself, having 5.7 volts on the 12AX7 filaments when line voltage drops to 115 volts.
Checking specs on the 12AX7 (RCA), filament voltage is spec'd at 6.3 volts (parallel) +/- 20%. 20% makes a little over 5 volts the bottom acceptable so 5.7 volts is well within allowable spec for the tube.
Now, what it does to the sonics in general I have no idea. But, from the allowable +/- 20% I would think that the tube should within it's normal lifespan. In my specific case (phono stage) I was informed that voltage variances higher or lower than the nominal 6.3 volts do affect the RIAA eq. The more the variance the more the EQ is affected. I can't hear a difference between 5.7 volts when the line volatge drops out a bit vs. 6.3 volts when line AC is up at 120 volts so who knows ------Hope that's a start anyway.
Follow Ups:
Yup, I've read your thread on the subject in the Hagtech Audio Circle.
I've also posted this on the diyAudio board.It's easy to change the R in the C-R-C circuit to get the heater voltage up and that's what I'll do this weekend. My B+ voltage is also below spec but that's another matter.
-Cornet enthusiast
So then your line voltage is lower than 120v as mine is (during high demand)? If that's the only "problem", you'll probably find that all voltages are right on design when the line voltage is at 120V (or is it 115 for your Hammond 370 --- one poster mentioned a variation between the two trannys but I can't recall the specifics. I'm sure you read it. Also, Jim jumped in with some insight after he came back from vacation so maybe check the thread again if you haven't since Sunday or Monday.I opted to leave it be as I discovered that voltage varied throught the day and was really only fell at the 115V mark and below during hot mid-to-late afternoons when the demand was high (probably everybody cranking on the AC). Dropping R values would only serve to create voltages in the opposite direction in the morning, evening and nightime hours when my line voltage was back up to 120 and a bit beyond.
Maybe consider tracking line voltage and associated Cornet voltages at intervals throughout the day and night for a couple-few days and see whether you'd end up in the same situation as myself.
If you are going to drop R value to bring up the filament voltages, maybe it would be better to adjust other R values to deal with the other lowered voltages as well. I believe that Jim jumped in late with some insight on this as well.Please post here or at Audio Circles if you do drop values and have some input on any audible changes due to the voltage change. I can't be sure at all that I detect any difference myself when listening to the Analog system when my line voltage is "normal" or low.
Good Luck
Filament voltage update:I changed out a resistor (R21) and now measure 6.2 volts on the heaters.
Just listened to three hours of great LP music. I definitely have increased gain in my phono stage now since I need to turn down the volume on my preamp to maintain similar spl. Overall there is a greater weight to the music and better balance top to bottom. The RIAA function may be more accurate with the gain tubes functioning near their rated heater voltage. My phono stage is delivering much better “punch” to the music (for lack of a better word, you know what I mean).Now I'll start experimenting with various cartridge loading resistor values...
-S
Thanks for the update.
The RIAA is defintely more accurate at spec'd filament volatge of 6.3 volts, this according to Jim himself.
My cornet runs to a hybrid 100 wpc integrated with much more power than I need so I didn't look for a change in the volume knob position when I knew my voltages were down compared to up as it always seems to be situated between 1/4 and 1/3 of the way up and 1/3 is definitely past moderate levels in the small room. I'm sure that gain would go up some as well. I have to admit though, I can't hear any difference between when the voltages are up at 6.3 (120V line voltage) or down around 5.7 (115V line voltage).You were using the Hammond 370 though, yes? What was R21's original value in your Cornet?
Yes, I'm using the 370BX now since my 270BX smoked and died.
I had only changed out the tranny and was enjoying the music daily, when I finally got around to measuring the voltage at various points in the circuit. (Cornet Classic)R21 was 2.4 ohms which yielded 5.77v. It is now 1.8 ohms and yields 6.15v.
Next I plan to change out R2 to get the B+ back up to spec. Jim's comments in the AudioCircle dated 8-23-03 indicate dropping R2 from 6.8K to 3.9K when using the 370BX. I will buy several resistors from 3.3K to 4.7K so I can experiment.
Another evening of spinning vinyl has confirmed my original sense of greater dynamics. The proof is in the position of the volume knob.
Later last evening I put a 10K in parallel with R14 to yield about 8.3K cartridge loading. So far I like the sound. I plan to further reduce R14 and discover the best value for my situation. I'm using a medium output moving coil cartridge, Benz ACE.
Regards,
S
Good deal. R21 value was supposed to be dropped as well with the 370 so you've taken care of that and should be right on spec when you take care of the B+ with R2.Regards
Such a drop in filament voltage will change the tube parameters VERY noticeably.Whether it will make the circuit misbehave, and how it will affect the sound no one will be able to tell you.
Who know, you might actually love that sound better.
The only way to find out is to experiment yourself... that is usually VERY easy to do using a variac or some other means of changing that voltage.
Only then you will know, as it all depends on the product, the system, and your taste.
Generally a slightly lower rather than higher voltage will extend the tube life.
Yes, the life of a tube can go up dramatically with low heater voltage - if the circuit is designed for it.On a recent consulting project I built a custom modem to work over a long and very old submarine cable. About a thousand miles with twenty some vacuum tube repeaters along the way. One end was cut at about 3000 meter depth about halfway between HI and CA with a seismometer attached. Anyway, the design of the repeaters was worth some study. Incredible 50's slide rule work. Anyway, since the cost of such a telephone cable is on the expensive side, the repeater amplifiers required long life from the tubes. It used an odd military pentode with the heaters running at 4.5V (below 6.3V nominal). I forget, but life goes up by some power ratio as the voltage drops. In short, a 10,000 hour tube can be run to 100,000 hours with the right design changes.
For audio, I wouldn't touch it.
jh
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