I use an regulator with an emitter follower scheme. Incorporating a diode, actually 2. That way I can vary voltage from 0-.9 volts. I use 2 in series to negate voltage drop due to voltage drop in transistor. They are beefy enough for a 4 pill, but I normally use 1 bias circuit per 2 pill section. Technically by Mototola specs, it should be per pill, but I'm happy with per 2 pill.
-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
I have done what you said, but trying running one of your amps and see what happens when you apply 18+ volts to the collector. I generally like to bias a 2 pill section at right around 225ma. Maybe 250ma max. at 12-15 volts, What I meant in earlier post, is that if you overvolt an amplifier like 18 heck I've done 19.5 Volts. At the lower voltage that you set bias point at, it will change with an increase in voltage on output. But yes you are right, a good stable bias circuit should remain stable and should only drop when it is tracking temp., and purposely trying to decrease forward bias so that thermal runaway is not encountered. Otherwise it will become a current hog quickly and that magic 10 ohm smoke will be seen. I did not explain myself correctly earlier. Sorry about that.
You might try using the lm-723 regulator that the motorola bias scheme uses as I did and a small variable resistor.
I use transistors minus one leg for thermal tracking on the heat sink by each transistor(pill) paralleled for more efficient tracking. I can find the actually tranny #'s if you like.
Forgot to mention the bias scheme I used was designed by bob85 and was directly modeled after a motorola bias scheme.
Here is the circuit in question and more on the subject via the reading...... http://www.worldwidedx.com/home-brew-mods/31641-switchable-biasing-bob85-style.html
Last edited:
Bob and I have discussed biasing quite in depth awhile back when I first got serious about properly biasing an amp. LOL. I can remember his first one or 2 runs with th 723 he wasn't happy with and at least my first few times experimenting with biasing was a learning lesson if nothing else. LOL
im around somewhere crusher,
about the regulator with pass transistor biasing, there was a post on here some time ago suggesting that only one diode was needed and i dont understand diodes in thermal tracking,
take a tip from me, use two diodes, one in contact with the heatsink very close to the finals or on the cap of the finals and the other in contact with the pass transistor,
using one diode dont work, using two diodes but only having one sensing heat works but drifts positive as the pass transistor heats up,
using two pass transistors one as the pass element and the base emitter junction of an identical device to thermal track the pass transistor works much better than a single diode,
check it out for yourself, you will see who does and dont understand vdrop in diodes and biasing, the fluke dont care much about opinions
about the regulator with pass transistor biasing, there was a post on here some time ago suggesting that only one diode was needed and i dont understand diodes in thermal tracking,
take a tip from me, use two diodes, one in contact with the heatsink very close to the finals or on the cap of the finals and the other in contact with the pass transistor,
using one diode dont work, using two diodes but only having one sensing heat works but drifts positive as the pass transistor heats up,
using two pass transistors one as the pass element and the base emitter junction of an identical device to thermal track the pass transistor works much better than a single diode,
check it out for yourself, you will see who does and dont understand vdrop in diodes and biasing, the fluke dont care much about opinions
Good to hear from ya bob, been a little while. I have been having good luck with 2 diodes. One diode is across pill and the other is in a TO-220AC case style mounted to heatsink next to transistor. I will have to find one laying around here and take a pic. Not as pretty as the pcb circuit, but it works well.
im around somewhere crusher,
about the regulator with pass transistor biasing, there was a post on here some time ago suggesting that only one diode was needed and i dont understand diodes in thermal tracking,
take a tip from me, use two diodes, one in contact with the heatsink very close to the finals or on the cap of the finals and the other in contact with the pass transistor,
using one diode dont work, using two diodes but only having one sensing heat works but drifts positive as the pass transistor heats up,
using two pass transistors one as the pass element and the base emitter junction of an identical device to thermal track the pass transistor works much better than a single diode,
check it out for yourself, you will see who does and dont understand vdrop in diodes and biasing, the fluke dont care much about opinions
This is the first I've heard on placing one of the diodes in contact with the pass transistor, pal is your friend bob.
Matter of fact, I got the emitter follower idea out of the Motorola book in one of the application notes. Might have been from one of heldges amp plans/diagrams. I just added a few things to basic design. I think his was for a very well regulated PS with no voltage drop under load. I actually use something lower than 12 volts with at least a 3 amp rating and higher wattage rating to account for heat due to much higher voltage drop. Actually if you look at some of the Joker Amplifers, that is were I got my idea/start from.
Where did they plans go? I clicked on the link and got a naked babe. Would rather have the plans!!!
the link still leads to the plans, look in the right hand top of the page, big button says download file, simply enter the "captcha" anti=spam code, click the button, then one more clock and download the files... the partially naked "babe" is advertising....