Now the OT that was right for the EL84's may not be right anymore, causing dynamic range compression in that way as well. It also occurred to me that when you remove a cathode bypass cap, you cause the plate Z of the tube to go up significantly. I totally agree about the current hogging with mis-matched tubes (same issue exists in differential or long-tail PI circuits, especially when they use a CCS in the tail - which is why I don't use that topology for a PI). I read that if both P-P tubes share a common cathode R that is not bypassed, it amounts to positive feedback between the tubes, and allegedly caused spurious oscillation in one case. I forgot to mention that I used separate cathode bias resistors on my EL84's mentioned above. Its just a bigger power tube reliabilty thing. Using separate cathode bias resistors allows each tube to find its own idle point independent of what its pair is doing.īypass Caps are not required for Class A but are required for lower distortion when using Class AB Operation. It is alleviated somewhat by biasing at 70% of maximum dissipation where the max Rg1 value can be X2 the datsheet value. It is worsened by the "usual" abuse of the max Rg1 value. This is less of a problem with smaller power tubes. The higher idle current in that tube means higher grid current and more disturbance (reduction of the bias) of the bias point which worsens the problem to the extent that one tube can eventually take over most or all of the idle current to the point where it will red plate an destroy itself. If tubes are not tightly matched one tube can hog the idle current. With a single shared cathode bias resistor you are entirely dependent upon the tubes being matched for them to current share properly. I believe that you are all overthinking this. I'd love to have some realtime compression for the sustain it gives me, but not if I've also cranked up other distortions. But what about when both output tubes are operating simultaneously (because they are each biased close to class A)? Is there still compression at the low or medium signal levels? Is there expansion? Is there excessive harmonic or IM distortion? The amp sounds real good (driving G10 Greenbacks), but I guess I need to go borrow my buddies spectrum analyzer and do more testing. So I would expect dynamic range compression. When the EL84 is pushed hard to near clipping, since it's got unbypassed cathode bias, the bias voltage on the El84 will increase as the waveshape voltage increases (in real time). But since I'm operating the EL84's closer to class A, per tube, there are questions. My belief, although not actually measured yet, is that if the output pair is operated class B (which it's not), the lack of bypass cap would suggest real time dynamic range compression, which creates more sustain, which most guitarists like. An article in Tube Cad Journal from 1955 that was highly recommended suggests that there's not likely a lot of difference in distortion. Gain went down obviously but I had too much gain anyway. So now I'm trying to find out if I've increased distortion in any bad ways by removing this cap. The screen R's are 1K (which goes to B+) and the cathode bias R is 330R. OT is from a Fender Deluxe Reverb amp with 6.6kohm primary Z. So there's all this talk about blocking distortion being about the coupling caps on the grids of the EL84's, but it was the removal of the cathode bypass caps on the EL84's that actually got rid of ANY blocking and crossover distortion.ī+ is 332VDC, bias is 11.8VDC, 36mA at Q per EL84, 11.8W at Q per EL84. I later remembered that I had increased the cathode bypass cap from about 25uF to 200uF, which may be why the Paul Ruby method no longer worked. Eventually I tried removing the cathode bypass C, and boom, no crossover distortion at all, even well into clipping. Then later after changing the cathode bias R and screen grid R it appeared not to work any more. Pure crossover distortion (from under biased output tubes run push-pull) will give you a relatively smooth transition across zero volts, as opposed to blocking, which creates a very abrupt sharp cornered glitch that spews much high frequency spectra that's not particularly musically related to the signal.Īt first it looked like the Paul Ruby zener diode thing worked. I tried all the methods on the Aiken site but blocking was still real bad. I was determined to get rid of any crossover distortion that was related to blocking distortion. In my latest guitar amp project, I've got a 12AX7 paraphase PI driving EL84's push-pull with cathode bias.
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