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Just modified my st 70 to an individual bias pot per tube. Is there any way possible that splitting up the stock common bias arrangement could create an adverse effect to sonic character. It still sounds really good and the bias voltages are very close per tube to get the current on the money. Just wondering if serarating the grid resistors in the modified set up could create an issue that I'm not aware of. My imagination could be the main issue here....but the bass might not sound as tight as it did before the mod. Being the new guy on the block....hope to hear something on this.
Follow Ups:
I don't think it would alter the sonic character of the output tubes. You could get into some problem if you didn't double the values of the pot and individual resistors that you replaced simply by drawing more power from the bias tap supply, but I don't think that's a problem. Also, now that you can use output tubes that are not perfectly matched you might be adventurous enough to try grossly mismatched output tubes. Big mistake. I think you'll find that worn out tubes, even pricy vintage ones, don't sound that good when they're huffing and puffing in combination with fresher tubes. Certainly, that's going to alter the sonic character of the music.
Thanks....I agree with everything you said 100%. I still use and will use very closely matched pairs. The JJ's have been awesome for me and stay on the money...super sonically and super reliability. I think just hearing back from you gives me some peace of mind that all is A-OK with my amp and that she sounds just fine. That tweaker mentality can still play havoc on trusting ones ears from time to time....thanks again.
If you are concerned about the solidity of the bass through that section of the bias circuitry, you could also add another 50-100uF of filter capacitance at the wiper of each of the four pots. I don't think you'll benefit much, but you can do it. Modifying equipment is kind of a black hole. The goal, ostensibly, is to produce a better piece of equipment. However, when confronted with the question wondering which is the best plan, prepare to be confounded with a thousand answers, all of them reasonable and compelling in their own way. For instance, you could use electrolytics to bypass the wipers of your amp. Others might say that that plan if foolish, what you really need is just a small, less than 1 uf of capacitance, filter to stabilize the DC and provide a grounding path for the AC through the grid resistor, and others will demand that if you aren't using paper in oil caps you are tempting providence. I don't know. I'm pretty sure they're all good plans including just kicking back, listening and thinking about it for a while. I could be wrong.
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