If you like it, and it works for you, fine.
But your 'tests' are incomplete.
1st off, as has been said already by others, all you have to do is add a 25K L pot between the 15K R, coming off the -bias balance pot, to ground and you'd have both -bias balance and adjustable -bias. Fender should have done that but the bean counters most likely stopped them.
Your leaving out some important things that PRR and HBP wrote and some other things.
If you read HBP's response, you will see his test results show that "Using cathode bias causes the individual tubes to "pull toward an average" and exhibit less variation in measured characteristics."
Yes, I did read it.
But he wrote that was with
full K bias - not partial K bias, you would have to test to see if
partial K bias does pull the 2 tubes closer together as much as full K bias, it very well may not or very little. We don't know.
And your leaving this part out;
But the mixed-bias didn't hang around long... Perhaps Fender figured out the bias-balance pot had enough range of adjustment to be effective without the normalization from partial-cathode-bias.
And if that circuit is best, mixed -bias, why did Fender drop the mixed part? We know why Fender went to -bias balance, tubes weren't as good as they used to be.
Early 1960s, RCA/GE could make a crate of 6L6 like peas in a pod (and not like that misbegotten mixed-up ear of corn we just got at Carrols), All Alike. By the late 1960s all the best workers and managers had been moved over to the Semiconductor division, the tube racket was dying, take it or leave it, nobody cared.
This circuit does not trim average or total bias current but allows an offset bias so a mismatch push-pull pair does not idle-hum so much.
Allows current draw to be balanced "perfectly" between both tubes, eliminating the need for matched sets of tubes.
Yes, that's what a -bias balance circuit does.
But it doesn't automatically eliminate the need for matched tubes for at least a couple of reasons.
As tubes age, they drift apart. Your supposed to keep re-setting that -bias balance as you play the amp. Sooner or later, depending on how far apart the current draw was on the tubes to start with and how fast, how much they drift, you run out of adjustment.
So even with a -bias balance control you still better off starting with a matched set of tubes because, they'll last longer.
And it's not just about keeping the tubes "well within spec for the tube."
You were lucky that none of the tubes you tested red plated. The current draw from a different set of tubes can be pretty different.
And it's not just looking for red plating when you pop in a new set of tubes.
You have to watch them while playing at full volume to see if they red plate. If you didn't do that, they very well may be red plating when playing turned up.
It happens here often enough that guys have to re-build the -bias circuit on their amp so they can stop the tubes from red plating and to get that -bias centered on the -bias range the circuit has. So they can use different tubes.
Guys also like to set their power tubes hotter or cooler for the difference in sound. You leave that out completely. It does make a difference.
Gerald Weber/Kendric amps/author, wrote in his tube amp books, and Vintage Guitar Magazine, you can bias your tubes by ear to where they sound best to you, but watch them to see if their starting to red plate.
The only thing the circuit does not allow for, as you mentioned, is to set an exact plate dissipation percentage.
Which is important, which I've laid out why above.
In my opinion, this is the least useful thing to have, simply because you have to start with a well matched set of tubes to be able to do this.
And it is just that, your opinion. What's the big deal about buying a matched set? Everybody sells them now a days.
Is that what's this all about? You don't want to pay a little extra for a matched set?
Topic: Trying to understand the Fender "mixed bias" circuit, as found in the AC568 (Read 6632 times)