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Old 31st March 2005, 02:52 PM   #11
crtubes is offline crtubes  United States
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Sharing LED is good idea. Besides lower linearities, it also reduces temperature effects, and improves balance.
If LED you're using can't handle 10mA (lot's of LED can handle 10mA), why not use some power diodes?

Wow, all film capacitors for power filters? This is an expensive design.

BTW, what's your bias point for KT88? Since your cathode resistor is only 1ohm, I assume the bias point is very close to Class B. have you tried to move the point to class A as much as possible? if yes, is there any sonic improvement? (I am curious to know)
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Old 31st March 2005, 03:28 PM   #12
Giaime is offline Giaime  Italy
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That's a fixed bias output stage: there is negative grid bias from that 220k resistors (btw the guy missed the bias supply in the schematics). I would think that the design is very towards class A... just because KT88s have so high Pa that running it in class A would result in an acceptable output power...

those 1ohm resistors are there just to check total cathode current: basic ohm's law... And no, with cathode bias and 1ohm cathode resistor that's not class B (tubes at cutoff or the like): it's just two saturated tubes that would have big THD... see that the grid will have the same DC potential of the cathode. That is saturation!

Hope this helps
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Old 31st March 2005, 04:06 PM   #13
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Dunno what's the primary inductance of the transformer but driving it from a 50k pot may be a bit optimistic. Unless you're not so much into deep bass at low levels
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Old 31st March 2005, 04:42 PM   #14
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Hello,
Back from work now so I can play around with ideas for this again

Percy - cheers for them, the first one is a solution I haven't seen used too much, I'll have to check it out

crtubes - finding LED's that handle 10mA isn't a problem, but I would like to find one that has low 'resistance' in the 5-15mA range if I can (i.e. not much change in voltage drop accross the LED for a given change in current). As far as caps, even the 75uF one is about 14 so it shouldn't break the bank too much... I'm it would be possible to spend more on 'lytics. Bias point will be around 350V at 100mA per tubeish, I'll have to see how hot things get when it's running, but yes it's a long way into class A. As the amp stands now I can tell the difference sonically between 40mA and 75mA

Giaime - Yep, Ill still keep the adjusting pots easily accessible though in case I need a few more beans once in a while. Any thoughts on how well a regulated bias supply would work, specially as far as stability wrt mains level and things goes. Maybe the output impedance of the supply effects things?

Analog_sa - Do you think maybe a 25 or 10k would be more suitable then? Or am I thinking in the wrong direction? I only have 2mV offset from the cd player I'm using, so at the moment it has no coupling caps so the RC filter between cd coupling caps and potentiometer shouldn't be too much of an issue. Hopefully the input transformer will be able to take 2mV accross its primary?

Cheers all again!!
Steve
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Old 31st March 2005, 07:33 PM   #15
crtubes is offline crtubes  United States
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Quote:
Originally posted by Giaime
those 1ohm resistors are there just to check total cathode current: basic ohm's law... And no, with cathode bias and 1ohm cathode resistor that's not class B (tubes at cutoff or the like): it's just two saturated tubes that would have big THD... see that the grid will have the same DC potential of the cathode. That is saturation!

Hope this helps
I know this is grid biasd, but with 1ohm cathode R, the power supply overhead for class A operation is very small (you can draw a static load line and a dynamic load line on the Ia-Va curve to see what I mean) . If you set the bias current too large, you get very large distortion.
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Old 31st March 2005, 07:36 PM   #16
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Hi,
I dont see what you mean....those resistors needn't be there at all really, they're just a means of measuring current through the tube (by measuring the voltage drop accross them). They have no active role in the circuit
Steve
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Old 31st March 2005, 08:14 PM   #17
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or have I missed something? I'm pretty tired
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Old 31st March 2005, 08:24 PM   #18
crtubes is offline crtubes  United States
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The confusion may arise from my mis-understanding of class A in a p-p design.

I understanding is that only when both tubes (in a p-p design) are operating in pure class A (like those in single end design), we can call it class A P-P.

But seems that people define class A P-P as 'as long as the tubes are not in cutoff region'.
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Old 31st March 2005, 08:34 PM   #19
SY is offline SY  United States
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Default No cut-off

That is the definition of Class A. "Pure" class A is a marketing term, not a technical one.
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Old 31st March 2005, 09:04 PM   #20
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Hi,
Here's my bias supply. Should work ok I think... I find the 20k pots a little oversensitive at the moment, so I've put some resistors in above and below to narrow the adjustment range, to between -30V and -45V. Hopefully I've got my sums right! Decided against regulating at the moment, i think....maybe....
Cheers,
Steve
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