00940
I wasnt aware that the manual mentioned such high voltage swings. They are pretty crazy headphones.
Regarding the Bijou & all that.
I personally wouldn't worry about it. 4vp-p isnt much compared to more, but it will drive most conventional high impedance headphones to formidable levels. I am curious how close my guess was to reality though. It may be as high as 4.4v, and could possibly be slightly less than 4v, but Id bet somewhere between 4 and 4.4vp-p.
The cathodyne is the second stage. Its the tube with outputs at the plate and cathode. It splits the signal into 2 out of phase parts (balanced!) to drive the output stage. Something of a problem in designing an amp with a cathodyne is that the balance of the cathodyne is entirely dependent on the load impedance. If you get the resistors wrong things go weird.
Now, running class A1, where the grid is never driven positive the input impedance of a triode is basically a lot. Several meg-ohms typically. In the bijou its bypassed by the 470Kohm resistors, but lets just say "a lot".
When we drive the grid positive things get weird. The input impedance of the tube DROPS extremely quickly. This is a big problem because while we are driving one tube positive the other one is being driven "more negative" the load impedances as seen by the top and bottom of the cathodyne are no longer the same and the balance goes away.
Unfortunately building an amp that will run class A2 is tricky. The crossover from A1 to A2 must be handled seamlessly, which requires a fairly low output impedance driver that does not react poorly to a changing load. Transformer coupling, cathode followers driving power tubes, and pentodes driving power tubes are good bet. Several well respected designers have commented that a lot of "the sound" of tube power amps is actually the driver stage struggling to drive the output stage properly. The other half says "I dont care what you say, I like the tone of my 12ax7 driving a 845" which is Ok, as long as you admit to building an expensive tone control.
Now the futterman design is a whole 'nother story.
The futterman design is actually a very neat combination of components. When I first saw it I was appalled by the use of a cathodyne (I am normally biased against cathodynes because they suck) but the way futterman integrated the various parts is toooooooo cool.
The key to the futterman design is a positive feedback loop nested inside of a negative feedback loop. The positive feedback loop is tricky. Positive feedback buys you silly-low output impedances, but if left uncontrolled WILL cause very serious oscillations. The neat trick is that the negative feedback loop keeps the positive feedback loop in check AND lowers the output impedance even further!
I think that there is some untapped coolness in the Biojou circuit, but have never tested. I dont think R5 and R6 are optomized to take advantage of the positive feedback effects of the Futterman. Alex has commented elsewhere (in the bijou thread) that the output impedance before the application of global feedback is (I dont remember what, I think 60ohms) - which is almost exactly the same as a simple white cathode follower without the benefit of positive feedback. Im pretty sure that with some tweaking of the values of these 2 resistors you can get output impedances in the single digit, if not fractions of an ohm range once global feedback is applied. I would say if you want to try this to lock global feedback down too - remove the feedback pot and replace it with a good old fashioned resistor once you settle on values.
I think its better that the stock schematics make no effort to exploit the benefits of the positive feedback loop: the popular mod of removing of the global feedback loop would turn the amp into an oscillator and result in many melted tubes and smoky amps.
Ooh, if anyone out there in TV land is crazy enough to actually try to get the positive feedback loop to work, please dont come crying to me if it dosnt work. I really hope it does, and think it will, but it might go up in smoke.
) you can only get about 4vp-p out of a bijou before it clips into an infinite impedance load.
