Don't want class B! I know cathoded bias sound warmer than fixed bias, but I already know how that power amp sounds (fixed bias, placed in a Fender Deluxe reverb from 1971). BTW, this amp from 1971 still sound good and I listened with original tubes, really worn, I think, and sounded awesome!
I know 18W or 22W (cranked or not) sound at the same volume, practically, so, if I can do tubes last longer, better for them! Although I can't understand something, or mabe I'm missing something: if cathoded bias run about 100%, this could be similar to class A, right? But, how can tubes last longer if they're biased hotter in cathoded bias?
First off I gotta say just b/c I prefer cathode bias, there are countless great amps with fixed, and if you want the Fender sound then perhaps you should let'em run on the edge.
I think Printer was thinking of class-A amps, and it is probably more common with cathode biased amps that run in 'pure' class-A b/c max power out isn't the issue.
My cathode biased amp I've mentioned runs each 6V6 at about 25mA and they have about 375volts on the plates. Here's a sound clip while it was under construction, at this point I only got 13watts:
Lucky13 tubeamp - YouTube
This reminds me about one thing regarding cathode biasing. If you use a simple resistor and a bypass cap, you will only get 13-14watts with a 400V supply. This is b/c as the tubes reach the point where the current increases beyond idle, the bypass caps will start to chrage up and the voltage on the cathode will increase, reducing the available plate voltage and limiting power. This gives an increase in compression and can be very cool. To fix this I have a voltage referance at a value slightly above the idle cathode voltage, say 30volts, and when the cathodes reach that voltage b/c the caps are charging up, the voltage stopd at 30volts and keeps the cathodes steady for an increase in power, in my case close to 20watts.
I'll try and draw up a schem, easier than all this typing
First off I gotta say just b/c I prefer cathode bias, there are countless great amps with fixed, and if you want the Fender sound then perhaps you should let'em run on the edge.
I think Printer was thinking of class-A amps, and it is probably more common with cathode biased amps that run in 'pure' class-A b/c max power out isn't the issue.
My cathode biased amp I've mentioned runs each 6V6 at about 25mA and they have about 375volts on the plates. Here's a sound clip while it was under construction, at this point I only got 13watts:
Lucky13 tubeamp - YouTube
This reminds me about one thing regarding cathode biasing. If you use a simple resistor and a bypass cap, you will only get 13-14watts with a 400V supply. This is b/c as the tubes reach the point where the current increases beyond idle, the bypass caps will start to chrage up and the voltage on the cathode will increase, reducing the available plate voltage and limiting power. This gives an increase in compression and can be very cool. To fix this I have a voltage referance at a value slightly above the idle cathode voltage, say 30volts, and when the cathodes reach that voltage b/c the caps are charging up, the voltage stopd at 30volts and keeps the cathodes steady for an increase in power, in my case close to 20watts.
I'll try and draw up a schem, easier than all this typing
It is not the available plate voltage that is the cause of the change, the amount of voltage change is not significant as compared to the plate voltage, but the change on the voltage of the cathode capacitor is significant as compared to the bias voltage on the tube's grid. The shift causes the tube to be biased colder. This helps some people get away with biasing their output tubes up to 100%. At lower volumes the amp is in Class A (which it is for a great period of time anyway), it is there a bit longer if biased hotter. Some amp outputs are said to sound fizzy when really pushed (due to crossover distortion because the bias has shifted as compared to what it was when it was set with a lower level signal) but sound better with an increase in bias. Anyway this is from what I have read, I am just getting my feet wet with this stuff.
That could be avoided with negative feedback, right? Fizziness, I mean. Don't want really that heavy crossover distortion.
I disconcur, as far as the tube is concerned the bias is not getting colder when the cathode voltage increases due to 'dynamic demands'. As long as the signal is below the tube's quiescent plate current the signal on the cathode is zero (assuming cathode cap shunting all frequencies). This is because the net delta-current is zero, it goes just as much up as down. As soon as the tube gets into class-B, actually a bit before that point, the signal starts to go more up than down on the cathode, and the cathode cap starts to charge and we see the rising cathode voltage. Appearantly this is the same as a colder bias, however as far as the tube is concerned the grid-cathode voltage is not getting colder, it is still trying to turn more 'on' but the grid starting to conduct simply means the cathode is following the grid, it's bias is not getting colder. The result is a higher cathode voltage that gives less plate-cathode and screen-cathode voltage which lowers the available max power. This is what gives cathode biased amps a different response and compression compared to fixed biased amps.
EDIT: (now I start to talk about fixed biased amps) Note that the fixed grid bias also shifts downwards as the tubes start to near saturation and draws grid current, the grid current must come from somewhere, (the supposedly 'fixed' voltage can shift downwards 20-40volts). But in a fixed biased amp you have a stiffer screen-cathode voltage, so the compression isn't quite the same.
In a 'pure' class-A amp we should always have a net delta-current of zero on the cathode, and thus none of this shifting of voltage on the cathodes.
Slideman...don't worry too much about X-over dist. And dont try to correct it too much with NFB if not necessary. As long as the 6V6 is biased comfortably they are linear enough to cross over quite well even without feedback. The amp on my vid has zero NFB and bias is 25mA, and everyone who tries it loves it's clean tone. But then again, if the amp you want to clone uses NFB, well then you know what to do...
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Good point and this is why Fender introduced the standby switch (Not to protect the tubes, but to protect the caps from high voltage. But I wonder if he realized he started the myth that all audio/receiving tubes must have the warmup sequence that he created?...uhoh...not gonna start that discussion again...)
(Not to protect the tubes, but to protect the caps from high voltage. But I wonder if he realized he started the myth that all audio/receiving tubes must have the warmup sequence that he created?...uhoh...not gonna start that discussion again...)That opinion was something I was looking for (even in other post of mine, about EL34's). Don't want to clone an amp, really, but I only know that Fender DR, sounds great, and has fixed bias. That compression you're talking about sure makes that warm sound in cathode biased amps, and that really interests me. Probably I'll try both bias methods, then I'll choose the one I like most.
The project will be a ~20W 6V6 amp with 6SL7 as LTP, and 3 stage preamp, (sort of hi gain preamp) with high SPL speakers.
But a lot of NFB makes an amp that sounds like a transistor amp. When it clips you get more gain and a sharper square wave.
Not really sure where you got 'dynamic demands' from, I was talking about when the output clips.I disconcur, as far as the tube is concerned the bias is not getting colder when the cathode voltage increases due to 'dynamic demands'. As long as the signal is below the tube's quiescent plate current the signal on the cathode is zero (assuming cathode cap shunting all frequencies). This is because the net delta-current is zero, it goes just as much up as down. As soon as the tube gets into class-B, actually a bit before that point, the signal starts to go more up than down on the cathode, and the cathode cap starts to charge and we see the rising cathode voltage. Appearantly this is the same as a colder bias, however as far as the tube is concerned the grid-cathode voltage is not getting colder, it is still trying to turn more 'on' but the grid starting to conduct simply means the cathode is following the grid, it's bias is not getting colder. The result is a higher cathode voltage that gives less plate-cathode and screen-cathode voltage which lowers the available max power. This is what gives cathode biased amps a different response and compression compared to fixed biased amps.
EDIT: (now I start to talk about fixed biased amps) Note that the fixed grid bias also shifts downwards as the tubes start to near saturation and draws grid current, the grid current must come from somewhere, (the supposedly 'fixed' voltage can shift downwards 20-40volts). But in a fixed biased amp you have a stiffer screen-cathode voltage, so the compression isn't quite the same.
In a 'pure' class-A amp we should always have a net delta-current of zero on the cathode, and thus none of this shifting of voltage on the cathodes.
Slideman...don't worry too much about X-over dist. And dont try to correct it too much with NFB if not necessary. As long as the 6V6 is biased comfortably they are linear enough to cross over quite well even without feedback. The amp on my vid has zero NFB and bias is 25mA, and everyone who tries it loves it's clean tone. But then again, if the amp you want to clone uses NFB, well then you know what to do...
I think this is what I was trying to say, if you did not get that, my apologies.
Great tip! I was tunning a friend's chinese amp, used to be EL84 cathoded biased but had tremolo, so I took it off, and bias ran directly to class A, so had to make it fixed bias, I haven't finished yet (had to tweak it quite much). It doesn't have NFB, and sounds great! I think it sounds better with fixed bias than with cathode bias.
I base most designs in what I have listened, or tried, but I have a lot of thing to try yet. That's why I ask this sort of things, don't have much experience yet, but enough to mess with some stuff.
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