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The distortion thing!!! Ian's rave - hopefully there is something useful in it.
With a CCS in the tail then the total current thru' the 2 sides of the PI/driver are fixed. The CCS is set for 2 x the idle current of each triode or to think of it another way, to the idle current of the 2 triodes combined.
If you drive one side to cut off then the other side can go to 2 x it's idle current - no more. You get a clamping function.
When overdriven it is likely to clip really hard and that means lots of odd harmonics and higher order odd harmonics at that, the thing here is that the higher order odd harmonics are the ones which are musically "quint" (meaning no musical relationship to the fundamental note) and even small amounts of them are "finger nails down the blackboard" stuff. They just grate.
That means that it (PI/Driver) has to run in Class A, that is not overdriven.
That is why for a Class AB output stage where one side is regularly driven to cutoff then you need to use separate CCS for each of the push pull sides, and separate bypass caps.... BUT I abandoned this approach since it has the worst overload recovery of any output tube biasing methods and so is unsuitable for a Git Amp.
The overdriven PI/Driver sound will be somewhat different from a resistor "tail" of lower AC impedance and unlike a lowish restive tail PI which has a fairly pleasant overdrive tone, it is likely to be far less pleasant and maybe quite awful unless you set the amp up such that the output tubes go into overdrive long before the PI/Driver stage.
OFF THREAD ASIDE (FWIW):
AS for that HIFI schematic I linked, the balanced shunt feedback from the output tube anodes does something special, it enforces balance for a start.
Guitar guys who have listened to that amp with my corksniffer CD player and speakers have all commented on 2 things, its speed and the way it handles the attack of a guitar, and that it grunges pleasantly when driven hard. HiFi "dudes" talk about PRATT (Pace, Rhythm and Attack). This amp has great PRATT. Music like JJ Cale just is simply stunning with an easy relaxed loping pace.
It is NOT a low distortion design but appears to be a benign distortion design. I've had 210W per Channel 0.001% distortion SS Power amps , JA80 4 x KT88 Jadis mono blocks, 845 Single Ended Triode Mono blocks and a heap of other high end HiFi Tube and SS Power Amps in my system and I always come back the amp to that design, currently my HiFi Amp is exactly that design but with 6SL7 diffamp and old coke bottle shape 6V6G output tubes.
A glorious 12 Watts per Channel.
I have had a try the Baby Huey Power Amp for Guitar on my to do list for far too long.
For now I have to say for those interested try it at your own risk. What works quite delightfully for a HiFi Amp may not be ideal for a Git Amp.
The Balance shunt feedback trades output tube gm for reduced internal impedance (lower rp). That means that it better drives the lowish Primary Inductance of Git Amp Output Trannies primary and results in improved bass, but it also better drives the leakage inductance and inter-winding capacitance of the the Output Tranny for improved top end. In a Stereo HiFi setup that also means improved stereo imaging (mostly phase related information) and you tend to hear a 3 dimensional wall of sound rather than 2 separate channels.
So is this just some guy waving his primary sexual characteristic.
Well I hope not, and the 60 or 70 guys around the world who have built one or more of these seem to agree.
Is it all my own clever design - nah! like most things it is based on someone else's work, in this case Yves M's 6GW8/ECL86 design here:
Push Pull ECL86/6GW8
Now back to our regular programming.
Cheers,
Ian
With a CCS in the tail then the total current thru' the 2 sides of the PI/driver are fixed. The CCS is set for 2 x the idle current of each triode or to think of it another way, to the idle current of the 2 triodes combined.
If you drive one side to cut off then the other side can go to 2 x it's idle current - no more. You get a clamping function.
When overdriven it is likely to clip really hard and that means lots of odd harmonics and higher order odd harmonics at that, the thing here is that the higher order odd harmonics are the ones which are musically "quint" (meaning no musical relationship to the fundamental note) and even small amounts of them are "finger nails down the blackboard" stuff. They just grate.
That means that it (PI/Driver) has to run in Class A, that is not overdriven.
That is why for a Class AB output stage where one side is regularly driven to cutoff then you need to use separate CCS for each of the push pull sides, and separate bypass caps.... BUT I abandoned this approach since it has the worst overload recovery of any output tube biasing methods and so is unsuitable for a Git Amp.
The overdriven PI/Driver sound will be somewhat different from a resistor "tail" of lower AC impedance and unlike a lowish restive tail PI which has a fairly pleasant overdrive tone, it is likely to be far less pleasant and maybe quite awful unless you set the amp up such that the output tubes go into overdrive long before the PI/Driver stage.
OFF THREAD ASIDE (FWIW):
AS for that HIFI schematic I linked, the balanced shunt feedback from the output tube anodes does something special, it enforces balance for a start.
Guitar guys who have listened to that amp with my corksniffer CD player and speakers have all commented on 2 things, its speed and the way it handles the attack of a guitar, and that it grunges pleasantly when driven hard. HiFi "dudes" talk about PRATT (Pace, Rhythm and Attack). This amp has great PRATT. Music like JJ Cale just is simply stunning with an easy relaxed loping pace.
It is NOT a low distortion design but appears to be a benign distortion design. I've had 210W per Channel 0.001% distortion SS Power amps , JA80 4 x KT88 Jadis mono blocks, 845 Single Ended Triode Mono blocks and a heap of other high end HiFi Tube and SS Power Amps in my system and I always come back the amp to that design, currently my HiFi Amp is exactly that design but with 6SL7 diffamp and old coke bottle shape 6V6G output tubes.
A glorious 12 Watts per Channel.
I have had a try the Baby Huey Power Amp for Guitar on my to do list for far too long.
For now I have to say for those interested try it at your own risk. What works quite delightfully for a HiFi Amp may not be ideal for a Git Amp.
The Balance shunt feedback trades output tube gm for reduced internal impedance (lower rp). That means that it better drives the lowish Primary Inductance of Git Amp Output Trannies primary and results in improved bass, but it also better drives the leakage inductance and inter-winding capacitance of the the Output Tranny for improved top end. In a Stereo HiFi setup that also means improved stereo imaging (mostly phase related information) and you tend to hear a 3 dimensional wall of sound rather than 2 separate channels.
So is this just some guy waving his primary sexual characteristic.
Well I hope not, and the 60 or 70 guys around the world who have built one or more of these seem to agree.
Is it all my own clever design - nah! like most things it is based on someone else's work, in this case Yves M's 6GW8/ECL86 design here:
Push Pull ECL86/6GW8
Now back to our regular programming.
Cheers,
Ian
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