cathode battery mod

[Guitarist]

Has anyone tried this mod? http://www.audiokarma.org/forums/showthread.php?t=47389


These guys are touting significant improvement over extra parallel caps...and "NiMH will recharge themselves in circuit while the amp is on." Huh?

At 1.2 Volts would a 100K plate resistor still be a good value for a 12ay7 or an 12ax7?

On the subject of batteries, I wonder how long 4 AAs would last to run the filaments. Twice as long if wired for 12 volts, I think.

To me, the power supply is the biggest drag to making tube preamps. I found some discount 12V 23a batteries that with 12 or so would run the plates for years I'm sure. It would be sweet to have a Marshall preamp in a standalone pedal

[Alex R]

12ax7 filaments draw 300mA at 6.3v, 150mA at 12v.

Decent AA batteries have about 1.8 amp-hours each at this sort of current drain. However with batteries in series I reckon you will only get the same amp-hours as one battery; you need them in parallel to multiply amp-hours. I conjecture that this is because the internal resistances of the batteries mounts as you put them in series, making you pay with current loss for the increased voltage in very linear proportions.

Thus supplying 12 volts at half the current won't give more hours, as the wattage doubles. Not sure how you'd get 12 volts out of four batteries anyhow

Actually I think four AAs would soon drop below 6v with that kind of load (but that's an instinct). Going over the voltage is not good for filaments, so using five isn't a good idea either. You might need some form of regulation. However that would draw a bit of current...

Don't know about the cathode batteries. Interesting idea. I guess the presence of that voltage holds the bias independently of the battery's internal resistance...? Someone here will have tried it I'm sure.

My rule of thumb on plate resistors is that so long as they drop the supply voltage by very roughly 50%, giving room to swing the signal, that's ok. In reality however 100K plate resistors rarely seem to produce that much voltage drop, hence the 220K-gives-more-gain idea I think. I doubt whether the value is very critical, as it's the negative bias volts produced at the grid that controls the triode's conduction, not the drop across the plate (or, I guess, the cathode) resistor, and 1.2 volts at the cathode is very normal indeed.

[Alex R]

...I had a look at the audiokarma thread, and noted the following phrases:

"night and day difference"

"highs are so much cleaner"

"soundstaging and imaging just seemed to open up by a wide margin"

"like a curtain was drawn back"

"allowing more music out"

Pure poetry, isn't it? Doesn't mean it's not a good idea though. Doesn't mean it is either. Actually, what does it mean?

[Old Tele man]

...it's going "back" to the battery-powered days, when three separate batteries were used in a radio: A-battery, B-battery and C-battery.

[Gregg]

Hi,

I have the preamp tubes in one of my HiFi amps running like that. I'm using only rechargeable batteries there. I tried regular batteries and after couple of hours they explode.

[Bruce / Mission Amps]

...I had a look at the audiokarma thread, and noted the following phrases:

"night and day difference"

"highs are so much cleaner"

"soundstaging and imaging just seemed to open up by a wide margin"

"like a curtain was drawn back"

"allowing more music out"

Pure poetry, isn't it? Doesn't mean it's not a good idea though. Doesn't mean it is either. Actually, what does it mean?

To me it means more Net driven BS... try this, use a normal biasing resistor and a gigantic capacitor value instead...
I see lots of cheap caps at 16v and 10000's of uFs! Use one of those in it and after a few seconds of tube warm up, it will probably charge up and act like a battery too.
******************
If you want to lower the ESR a lot, strap a .22uF to .68uF@200v plastic cap across the electrolytic cap.

[loudthud]

Many of the small caps used for cathode bypass have a pretty high ESR. It's so high, manufacturers are embarrassed to spec it. A battery OTOH is probably below 1 ohm. I'll bet there is an audible difference with program material where phase distortion is important.

Try grid leak or fixed bias and just ground the cathode.

ESR = (damping factor) * X(c) at frequency of measurement

Xicon 22uF 10V axial lists Df=.2 @ 120Hz
Xc=60
ESR=12 OHMS

[Guitarist]

Well I like the notion and will try it. I suppose AA or even AAA rechargables will work for the cathode supply just fine, it is rather unorthodox but - Let The Ears Decide.

I don't do technogarble well but..."A capacitor can be modeled as an ideal capacitor in series with a resistor and an inductor. The resistor's value is the ESR"

Alex: so does this mean 4 AAs in series will last 6 hours. 1.8 aH/.300 ? Not that great but workable. % volts are fine for these tubes, may be a little quieter, though.

Greg: this is likely an overkill application to do with a guitar amp but we'll give it a shot. You've tried it, how does it sound with your audio system? It survived the explosion, great!

Bruce: bowing, bowing: I suppose a MASSIVE cap is easy to swap in, many have settled on <5 UF for these as the flab is terrible otherwise. Who cares about warm/charge up time.

Loudthud: Your the best, man. Would you do a YouTube show on tube amps? You always have some wise angled comment that causes posters to stop and rethink their biasing for a bit. As well, 1 in a 100 look cool as they are even without doing or saying anything...still you know what you're talking about (unlike me here in the creative dept. but I love blowing people away with tone machines especially in small packages). So fun.

This might be good in a 12AY7 EH tube preamp. Eh? Ahh?

I wonder how this battery approach would work with cathode bias on the output tubes. Assuming rechargeables are available at that voltage. Might have a great effect on the transition out of Class A.

[Bruce / Mission Amps]

If you are willing to go to this kind of experimenting extremes then why not ground the preamp tube's cathodes and hold the grid resistors off ground with some negative Vdc .... a little -1 to -4v bias supply, make it variable and thrill yourself... don't forget to bock the negative bias DC voltage with a coupling cap to or from a volume pot!
A baby bias supply could be made up with a little 1, 2 or 5 watt 1.5v to 3v zener diode in the high voltage secondary center tap lead.
Or a string of 1N4007s and tap where you want the -.6-.7v drops to add up.
Or just steal some -Vdc from the bias supply rail.
If you have one, steer the -Vdc through a final high resistance voltage divider with a 22uF@225v cap across the last grounded bias supply resistor.
Take the -1.5v to -3v voltage from the negative lead of the cap and bingo...
That should do the same thing as a battery and you have no silly battery life to worry about.
I could see that sounding interesting and cheap as $1.00 in parts.

[Alex R]

Alex: so does this mean 4 AAs in series will last 6 hours. 1.8 aH/.300 ? Not that great but workable. % volts are fine for these tubes, may be a little quieter, though.

Yes, that's what the figures imply. There will be an initial drop in voltage, then a fairly steady period for a while, then another sharper decline. I'd probably want to set it up as an experiment one day with a vm hooked up and plot the voltage curve as the batteries died, then you know what you're dealing with.

Check out the figures for fatter batteries.

[Wild Bill]

If you are willing to go to this kind of experimenting extremes then why not ground the preamp tube's cathodes and hold the grid resistors off ground with some negative Vdc .... a little -1 to -4v bias supply, make it variable and thrill yourself... don't forget to bock the negative bias DC voltage with a coupling cap to or from a volume pot!
A baby bias supply could be made up with a little 1, 2 or 5 watt 1.5v to 3v zener diode in the high voltage secondary center tap lead.
Or a string of 1N4007s and tap where you want the -.6-.7v drops to add up.
Or just steal some -Vdc from the bias supply rail.
If you have one, steer the -Vdc through a final high resistance voltage divider with a 22uF@225v cap across the last grounded bias supply resistor.
Take the -1.5v to -3v voltage from the negative lead of the cap and bingo...
That should do the same thing as a battery and you have no silly battery life to worry about.
I could see that sounding interesting and cheap as $1.00 in parts.

Ah but Bruce, that sounds complicated! Besides, batteries have more mojo. Especially if you can find some of those old button cell bias batteries from those 1930 circuits on Ebay! If you soak them overnight in the right brand of beer (the brand is dependent on the type of music you intend to play) you can get them to work for a few hours at a time. If this gets too expensive perhaps second-hand beer would work just as well!

How've you been? I've been away this past year as time allowed me pretty well only one board to hang out at and there was a new Canadian board that had no techies so I decided to help out. It's worked out well and attracted enough other techies that I don't feel the need to spend all my time there anymore. There's always someone to help a newbie now.

Here's the link:

http://www.guitarscanada.com/Board/

Lovely thread in the "Amps and Cabs" forum about how "Re-Tolexing Improves Tone!"" that might give you a chuckle.

You never did tell me what's a good brand of mountain beer from your neighbourhood for me to try...

Wild Bill