Friend of Leo's
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- Dec 11, 2009
- Bloomington, MN
Love Harvards - I'm sure this will be fantastic
My name is King Fan and I have a new addiction, simulating these 10,000 maniac bias circuits. Here's the nice solution @Ten Over mentioned above -- as always, if I did it right.
As he predicted, the bias range runs about –19 to -35V. These sims aren't perfect -- but a range anything like that should work well.
The one you made for me the other day saved me tons of time.
I'm am also now hooked on these; as much as I like clipping in different values and experimenting, the simulator is just great.
Yeah, 6.8K. Typing with one hand post shoulder surgery... ;-)Thanks for the info. Was that a 68k or a 6.8k before the diode?
I didn't make it through all the responses, so I may be repeating someone else here - but I prefer a 10k bias pot over the larger ones like 50k. It's much easier to fine tune the voltage on a smaller value pot. Both rotate 300 degrees, but the 50k is offering 5x the total resistance. Once you land on your fixed resistor value, I've never had one yet that couldn't go from probably 45% to 75% bias adjustment on the 10k pot. The "safe zone" of adjustment is usually a little less than half of the full range of the pot. I even started using a dry erase marker to put dots on the pot so that I know where to keep things. And with dry erase, you can wipe it off and do it again with a tube change.
About 23mA.Just out of curiosity, what did your plate dissipation come in at the -27?
It’s quiet and clean all the way up the dial and sounds very nice.
Well, I finished my fine tuning of the bias (at least for now). I’ve settled on a pretty cool number of 59% (see pics). It sounds good to my ears. I’ll play this for a while and adjust again if I feel like it needs it.If your tubes are dissipating 7.3W, that's about 60% of 12W, a traditional 'safe' max for the 6V6, and 52% of 14W, which many people now take to be a 'useful and safe' max for 6V6s. Since your target is roughly 50-70%, you're cool, but not too cool. And that's cool.
You probably know this, but just as we don't bias to a specific plate current, we definitely don't bias to a specific bias voltage. Especially a Fender vintage bias voltage. Your -27V isn't doing badly, but it wasn't your target, cuz each tube or tube pair will require or react to different values differently.
Hmm, clean is nice, but is that the sweet spot? *Now* you actually start to use your adjustable bias. Since you're at the low ~50% end of the 14W range, adjust bias "by ear" -- play an E chord several times, maybe part of a simple scale, and adjust bias. Play again. Listen, measure current and *plate voltage*, calculate % max dissipation, and keep going up until you reach 70% or so; then come back down until your ears tell you you've found your best tone.
Let us know if that doesn’t makes sense; depends on how you learned to bias. I use Rob's calculator; extra easy if you have plate current... but do recheck plate voltage, which changes with bias.