Idea: parallel input tridoes with different cathode bias/bypass. Good? Bad?

[mooreamps]

Quite on the contrary! In theory a diode will yield the exact same gain as a bypassed resistor. A diode will yield considerably more gain than an unbypassed resistor giving the same bias voltage.

for some reason, it does not. I've already measured it on one of my standard preamplier cards..

I tend to use diode biasing on the first stage to maximize signal to noise ratio at the input. Because there is effectively no feedback path I get wide frequency response, high gain and low noise. In this capacity there is no risk of overdriving them, but that's only because the signal in is very small. I dare say very few tube gain stages will be overdriven by a signal in the mV range. But hit it with a bigger signal, and it will be overdriven!

for some reason, it does not. I've already measured it on one of my standard preamplier cards..

The sensitivity of a tube gain stage (how large signal it takes to drive it to maximum unclipped output) is not determined by the bias method, but by the load line/tube characteristics. If a stage biased by a bypassed resistor will overdrive given a certain voltage, it will overdrive at the same input signal bias by the same voltage with diodes. Surely, you must agree that if we slam a diode biased stage with a 50 volts signal it will be overdriven?

It could. But since you are not looking at the schematic of my standard preamplifier, perhaps you are not knowing that you don't get those kinds of voltage swings until the control grids of the power tubes.

-g

[tmod]

for some reason, it does not. I've already measured it on one of my standard preamplier cards..

I'm more inclined to believe that you've made a wiring/measuring error than anything else.

I have to admit that I have never measured this myself by swapping back and forth between diodes and a resistor giving the exact same bias voltage. I have the equipment and know-how, but I have never seen the need. I have data sheets, I have SPICE, and the results I get in real life speak for themselves. When theory and practice line up there's little reason to question the theory...

Just for the hell of it I've just measured the amp on my bench atthe moment. First stage is 6N2P-EV (12AX7 equivalent) with fixed (diode) bias. I get a measured voltage gain of 48.

According to spec sheet, the mu of this tube is given as 100 +/- 15. Using a mu of 100, unbypassed gain is 26 at the operation point I'm at. Taking tolerance into account, Av should fall within the range 24.5-28.5. Datasheet value for fully bypassed is 55.6. 47-64 is within tolerance. So, does this seem like close to 26?

I know, this is just a single observation. Does this prove that the gain is the same as with a fully bypassed resistor? No, and I don't intend at this point to pick my circuit apart to swap in a resistor just in order to convince anyone. Based on this observation alone, is it perceivable that a bypassed resistor would yield an even higher gain? Yes, but I have not seen any theory to back that up. I have on the other hand seen theory indicating it would be pretty much exactly the same...

In order for the tube to give this voltage gain with an unbypassed resistor it would need a mu of more than a 1000. Either I have a tube with record-setting high mu, or diode bias yield a gain of close to double that of an equivalent unbypassed resistor. What seems more likely?

for some reason, it does not. I've already measured it on one of my standard preamplier cards..

I'm not set up for measuring frequency response at the moment, so if you (or somebody else) claim anything about different response with fixed (diode) bias vs. cathode bias I'm unable to test it.

It could.

Good. Then we agree on that one.

But since you are not looking at the schematic of my standard preamplifier, perhaps you are not knowing that you don't get those kinds of voltage swings until the control grids of the power tubes.

-g

I think we're getting at something here.

You're absolutely right. I am not looking at your schematic. I know nothing about the voltage swings in your circuit. I'm sure you're correct if you claim that none of your pre amp stages are set up for clipping. I simply don't know.

My guess is that neither did Merlin when he wrote on diode biasing in his book. I cannot speak for Merlin of course, but I'm furthermore guessing that he wrote the chapter with general guitar amplifier design in mind. As in "including but not limited to your amp". (Which I'm sure is a mighty fine amp btw.) Probably why he called his book "Designing Tube Preamps for Guitar and Bass" instead of something like "Reverse-engineering mooreamps's preamp, while not getting the bias right". Taking this even one step further I think that was why he wrote "[...]if the stage is likely to be overdriven" [...](page 35 in the book/website pdf, my emphasize).

I have no doubt that you have a nice pre amp using fixed bias (so have I!), and sure you may have tried out different bias arrangements, and now settled on one that gives a lower gain than some of the other arrangements you tried. But if you claim on a general basis that using fixed bias yield lower gain than resistor bias, contrary to what tube theory would lead us to believe, I feel you'll have to dig up some pretty convincing documentation. "for some reason, it does not" just won't cut it in my opinion...

By the way, I would love to see schematics for your amps, I'm sure they sound great.

[R.G.]

...
I think we're getting at something here.
...
But if you claim on a general basis that using fixed bias yield lower gain than resistor bias, contrary to what tube theory would lead us to believe, I feel you'll have to dig up some pretty convincing documentation. "for some reason, it does not" just won't cut it in my opinion...
...

You're having a "discussion" with a guy who carried on an argument about the action of fuses, taking a position which actively ignored the manufacturers' published design data.

I'm guessing that you're not going to get any convincing documentation.

Just a guess.

[mooreamps]

You're having a "discussion" with a guy who carried on an argument about the action of fuses, taking a position which actively ignored the manufacturers' published design data.

I'm guessing that you're not going to get any convincing documentation.

Just a guess.

I was going to post some schematics of how this works. But since you people do nothing else but patronize me, then you all can just go figure it out for yourselfs.

[R.G.]

Oh my! What a surprise!!

ROFLMAO

[octal]

Well, it looks like you did it guys. Now the world will be denied the secrets behind connecting two diodes in series from the cathode to ground and/or some magic regarding fuses.

[tmod]

You're having a "discussion" with a guy who carried on an argument about the action of fuses, taking a position which actively ignored the manufacturers' published design data.

I'm guessing that you're not going to get any convincing documentation.

Just a guess.

Now that you mention it I remember that thread.

I'm guessing you're right...

[tmod]

I was going to post some schematics of how this works. But since you people do nothing else but patronize me, then you all can just go figure it out for yourselfs.

You know, after re-reading my post I was thinking that maybe I came across as overly harsh. It was not my intention to be patronizing.

But I don't think you have much reason to complain. If you make extraordinary claims you should be preparedto back it up with facts. When you accuse a named person (although you seem not to aknowledge his real name) for stealing your unpublished work that's something you should expect will get reactions. Especially when you accuse said person of ignorance/lack of understanding. Particularly when said person has demonstrated his knowledge, insight and helpfulness time and again over the years.

You also seemed to make generalized claims that run contrary to accepted tube theory. You also came across as patronizing when you took for granted that I hadn't used fixed (diode) bias in reallife. When I showed you my findings and spent time demonstrating why you must be wrong, I don't think that is patronizing. Particularly not under the circumstances. If you want more info/schematic/data/measurements from the amp I measured, by all means let met know.

If you can demonstrate why I'm wrong I'll listen respectfully. I have only been meddling with tubes the last six years or so, so I still learn something new every day. I'm positive that you, like most others around here, have a lot of insights that I would find useful. But claiming that my measurements somehow cannot be correct despite being in tune with all the available information is not very useful without any further explanation/proof.

And I'm sorry if you feel put off publishing your schematic. I'm curious as to what it's like. I think we all should be humble enough to accept that even should you be wrong about the theory, you may still have created a masterpiece by trial and error or accident. The poorly designed cathode follower in the Fender Bassman is a case in point (see Merlin's site, DC-coupled CF, or better still, buy the book). I have serendipiously made decent amplifiers myself, so there's nothing to be ashamed of...

There is of course also the possibility that you have made a real and important new discovery. I'm still pretty open minded about this, but so far I haven't seen any indication that there's anything new under the sun. That doesn't mean there isn't, but as long as you don't post them, your seemingly wild claims are all we have to go by. Hope you change your mind, and hope you buy Merlin's book. Even if there should be omissions relating to your (possible) invention I'm sure there are other tricks for you to learn.

[diagrammatiks]

You invented the diode constant current sink?

Nobody tell Morgan Jones.

No wonder I couldn't understand any of the schematics in his book. You hadn't invented them yet.