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w00t 1st December 2007 04:56 PM

Battery grid bias to fixed bias in Toccato?
Hi all,

With regards to Thorsten's Toccato preamp (, is there any way I can change the 3V grid bias batteries to fixed bias?

Search didnt turn up with anything with "toccato".


rdf 1st December 2007 08:11 PM

Quite a few ways, with no guarantees regarding the effect on sonics though. The simplest to me would be to replace the battery with a pair of LEDs in series, the type of LED chosen to provide the required 3 volt drop. Two reds are a good start. To get them up to the currrents required for proper operation connect the top of the LED stack to B+ with a two watt min (five would be better) resistor in the 65000 ohm range. That should bias the LEDs a ~ 4ma, enough given the low dynamic voltage on the input of the 5687 along with the high value 470K grid resistor equate to very low dynamic currents through the LEDs.

I take it this is about the inconvenience of batteries. Thorsten knows his stuff and used a battery for a reason, not a reason of course to avoid playing.

kevinkr 1st December 2007 10:48 PM

I would build it exactly as shown, even the small dynamic impedance of the led is going to affect the RP of the 6GK5 significantly and tweaking of the RIAA network to straighten that out will be required.

You can use a pair of plain old alkaline 1.5V AA or even AAA batteries to provide the grid bias - mount these in battery holders. Built as shown exactly 8 alkaline cells are required for a stereo pre-amp. Since they are not called on to deliver any current the life of the batteries will be equivalent to their shelf life at least. I wager they will last a minimum of 4 - 5 yrs in this application.

rdf 2nd December 2007 12:34 AM

Sorry w00t, there's an even better reason it won't work. My suggestion biases the grid positive rather then negative! DON'T DO IT! You would need an additional negative supply to correctly bias the 5687. Might not be worth the hassle.

Hi Kevin. My gaff probably gave the impression I was talking about LEDs as a cathode load. Re: dynamic impedances however, no way! A 5687 in that configuration has a gain of about 15 so in normal use you wouldn't expect more than a couple volts at its grid. The resulting dynamic current will be in the range of 4 uA through the 470k grid resistor and LEDS. Since the LED string is already biased at 4 ma, the current variation through it will be less than 0.1%. Operating well in their linear impedance range the resultant dynamic impedance variation will be an even much, much smaller percentage of the already small total value - say 40 ohms for two good reds? The actual impedance variation will be a tiny fraction of 0.1% of 40 ohms, all in series with 470kohm.

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