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Old 27th March 2008, 07:10 PM   #21
SY is offline SY  United States
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Are you sure about the operating conditions for the 6SC7 voltage amps? I have a hard time believing it will swing the required voltages. What's your assumed current and Vgk?

I'm also having trouble understanding why you'd use a cascode in the first stage for such a wussy voltage gain. I don't think that the bandwidth you're expecting is correct- I get more like 70kHz f3.

There's a bunch of more minor things, but I'll pick at the big issues first.
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Old 27th March 2008, 08:52 PM   #22
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Current on the 6CS7 (NOT 6SC7- I made that mistake myself... here's the one I'm using: http://www.nj7p.org/Tube4.php?tube=6cs7 ) on the LTP (using the smaller of the two triodes in the tube) is about 4.5ma. According to the sim, I'm showing like 17v negative bias between cathode and grid. That, multiplied by 8 gain, gives about 135-140v swing negative, before the tube goes to grid current. By my calculation, I need about 100-130v to go full-output (60w) on the outputs (400v swing at the output transformer from rest) depending on the actual gain of the output tube. Also, remember that only POSITIVE output will count... and I've got at least about 150v on the anode resistor at idle, IIRC... that should cover positive voltage swing...

SY, what input drive impedance are you assuming to get 70KHz? I'm assuming nothing but the 330K+47K input load (that's the HIGHEST impedance the input grid could EVER see, even with an "infinite impedance" driving it, which should be the absolute worst case for bandwidth)... and the program says that bandwidth (driving the input of the 6CS7, about a 550K load) is still "greater than 1MHz")...

As for the gain- that's about all I could get, with an un-bypassed cathode resistor (so I can inject feedback from the output). With a cathode bypass cap, the gain goes up to like 150+, easy...

I don't know why I'm having so much trouble with TubeCad apparently... but I get the SAME results on EITHER of the two computers I'm using (one is an old P2-266 and the other is a Celeron 700 machine) that I use... I'll get to the bottom of this, I swear!!

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Gordon.
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Old 27th March 2008, 08:57 PM   #23
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Default Dyslxeic? Me? mipossible!

OK, that changes things a bit. My bandwidth figure came from the source impedance of the cascode (basically, the plate load resistor) and the load impedance of the LTP (Miller capacitance). I'll recalculate for the proper tube.
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Old 27th March 2008, 09:09 PM   #24
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One more thing- why use such a low mu tube? Something like a 6GF7 strikes me as a better choice.
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Old 27th March 2008, 09:58 PM   #25
GordonW is offline GordonW  United States
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Quote:
Originally posted by SY
One more thing- why use such a low mu tube? Something like a 6GF7 strikes me as a better choice.
6CS7 can handle 500v on the plate, the 6GV7 only like 300 or 330. I looked at 6DR7s, 6GF7s and such, but the 6CS7 was the only one that had enough "B+ tolerance"...

While I'm writing- I was thinking, a few minutes ago, about something I read on TubeCad a little while back- using a pentode input tube in "ultra-linear" mode, by driving the screen of the pentode from a triode cathode follower, with the input for the cathode follower having its AC voltage voltage-divided from the output voltage of the pentode (IOW, using a cap and a couple of resistors, drive the grid of the cathode follower with full DC voltage from the pentode, but voltage-divide the AC voltage to like half). Anyone ever tried that in practice? I like the idea of keeping the output impedance lower, but having close to normal pentode gain... that could make a neat substitute for the cascode (and be able to swing more AC voltage, at a lower NET DC voltage fed to the next stage, compared to the cascode)...

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Old 28th March 2008, 01:17 AM   #26
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The 6GF7 (as an example) first section could be run with a CCS load at 2.5mA; with -4V between grid and cathode, the DC plate voltage is 350V. Dissipation is also well within margin. You have plenty of room to swing the 150V peak if your B+ is high enough. And that should get you a gain of nearly 30dB on each side of the LTP. Using a ruler on the curves, it appears that you'll have something like 2-3% 2nd harmonic (which cancels). Not bad at all. Get some decent gain out of the first stage (or cascade two LTPs) and now your talkin'.
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Old 28th March 2008, 12:43 PM   #27
GordonW is offline GordonW  United States
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OK.

Maybe I'm missing something fundamental here.

When the tube datasheets say "max plate voltage", I'm taking that, for most tubes, to mean the MAX voltage the tube should see for any part of its operational cycle. I know some output tubes are different... but if you look at them (say, the 6JB6), they have a MAX PEAK voltage figure listed as well... usually something in the few-KV range. But, with small-signal tubes, I take the plate voltage as the max... if that isn't right, then boy, I've been wasting a LOT of time looking for high-voltage tubes!!

SY, I plugged in that alignment for the 6GF7... and it looks like its IDLING at a plate voltage of about 318 volts. When that thing swings negative input bias, it looks like it's going to go WAY over the 330v max plate voltage of that tube, almost immediately... will this be a problem?

Also, there's another problem- the input DC grid voltage is only 3V. I need a MINIMUM DC input grid voltage of probably 60 volts, to make the Mulllard-style direct-coupled input (plate of the input tube coupled with the grid of the LTP without a coupling cap- getting rid of that cap was one of the FUNDAMENTAL things I wanted to do, to improve LF stability). Probably 100V DC grid voltage on the input of the LTP would be better. However, I've only got 550V rail voltage to play with. Hence, my alignment with the 6CS7... it gave me 135v swing. about 96v DC grid voltage, 4ma current... on that 550v rail. And the tube never sees over 500v on the plate (idle plate voltage is right at 300v)...

I know some of this might sound like stupid questions... but you gotta learn this stuff SOMEWHERE... and it's better to risk seeming stupid, and ask these questions now, rather than blow up some hard-to-find parts, and REALLY feel stupid then...

Thanks...

Regards,
Gordon.
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Old 28th March 2008, 01:25 PM   #28
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Note that on the datasheet for the 6GF7, the max voltage is listed as "Max DC voltage." That's true of nearly every audio tube. Otherwise, transformer coupled output stages would have to be run at half their voltage. The peak voltage thing is usually a spec for sweep tubes, since they're used in HV pulse operation.

Remember, when we talk about grid voltage, it's not grid-to-ground, it's grid to cathode. You don't need 60V grid to cathode! What you need is sufficient bias so that the tube can swing the required voltage at its plate without running into grid current. For a tube with a mu of 60 and a required swing of 150V peak, that means 2.5V bias minimum.
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Old 28th March 2008, 01:36 PM   #29
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Hi Gordon and SY

In the schematic the driver and output stages are designed to be driven from the same B+. I understand it from the practical point of view (one supply), besides that it is nice to have high voltage available to provide headroom for a lot of driver capacity...BUT, as the output stage works in class B how will the driver stages react to the sags in the PS? Or are you planning to use a zero output impedance PS?

A related question...if one uses a separate B+ for the output tubes, how well regulated should it be? SY, what are you using in your 6LF6 amplifiers (I know it's about 800VDC, but not how you get to it)?

Many thanks, Erik
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Old 28th March 2008, 01:54 PM   #30
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All good points. The regulation of the output stage becomes an interesting issue because of the high power and large current variations. That was probably the weak point of my big amp. The supplies were pretty simple, just big honkin' power transformers and CLC filters with a LOT of C.

For my input stages, I used separate supplies, and as a cheat, ran them bipolar. It's cheaper/easier to use +/-350V than 700V.
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