Yes it does- certainly the CS compliance increases! I didn't do any measurements from the lower impedance taps, but at 16 ohms, the source Z was over 100 ohms (I don't remember the number exactly and I'm too lazy to search at the moment, so let's call it 125 ohms). That means the 8 ohm tap will have a source impedance of 62 ohms, but also would most likely swing 29% less voltage. So in the interest of max compliance and tightest transformer coupling, I'd just stick with the 16 ohm tap.
Yeah, $1500 in car repairs shot the hell out of my budget.
As soon as the bank account is in the black....
Actually, I started cutting metal this weekend, so that's a positive sign. Mrs Y will be anxious to get those sharp, nasty bits of aluminum out of here, and the only way to do that is to assemble an amp. All's not lost, this gave me some time to go in and really tweak the heck out of the input and driver stages. I think I've got a good shot at having something going by New Year's.
As soon as the bank account is in the black.... Actually, I started cutting metal this weekend, so that's a positive sign. Mrs Y will be anxious to get those sharp, nasty bits of aluminum out of here, and the only way to do that is to assemble an amp. All's not lost, this gave me some time to go in and really tweak the heck out of the input and driver stages. I think I've got a good shot at having something going by New Year's.
Just to keep things in perspective, most power pentodes have a plate resistance lower than you might think; the output impedance after the stepdown of the output transformer will not really be amazingly high. A typical PP EL84 amp will have an output impedance somewhere between 50 and 100 ohms (at the 8 ohm tap) without feedback.
A quick look at the Mullard EL34 data sheet shows ra = 15k ohm at the 'typical' operating point of Va = Vg2 = 250V, Vg1 = -12.2V, Ia = 100mA. With a 2k:8 output transformer (again, from their typical operation) you'll get a 60 ohm output impedance.
Whether you want to call these current sources, voltage sources, or something in between depends on your perspective.
-- Dave
A quick look at the Mullard EL34 data sheet shows ra = 15k ohm at the 'typical' operating point of Va = Vg2 = 250V, Vg1 = -12.2V, Ia = 100mA. With a 2k:8 output transformer (again, from their typical operation) you'll get a 60 ohm output impedance.
Whether you want to call these current sources, voltage sources, or something in between depends on your perspective.
-- Dave
Dave,
My way of looking at it (perspective) is simplistic:
If the source impedance is less than the load impedance by say X3 then its a voltage source.
If the source impedance is greater than the load impedance by say X3 then its a current source.
In between its a POWER source.
some will argue the X3 figure should be X5 or even X10 BUT ....
Cheers,
Ian
My way of looking at it (perspective) is simplistic:
If the source impedance is less than the load impedance by say X3 then its a voltage source.
If the source impedance is greater than the load impedance by say X3 then its a current source.
In between its a POWER source.
some will argue the X3 figure should be X5 or even X10 BUT ....
Cheers,
Ian
Reviving a long dead thread ...
I have been playing with Gm amps for headphones lately, and in general, I like them a lot. I've done a SE one with a single 12HG7 (transformer coupled) which I like a lot, a PP with a quad of C3g's which is also pretty nice, and even a transformer coupled DN2540 based one which is a little less detailed (likely due to lower quality transformers, actually), but still pretty good.
Anyhow, I have a pair of DRD 300B amps which, while fine, have been sitting in their current form for a little too long. I have been contemplating either selling them off, or doing some sort of work on them. So, one thing that came to mind was to try to make a Gm amp with a pentode driver, but doing the whole thing as a cascode. That way, it still uses the 300B as the output tube, and hopefully does not require any real chassis work (assuming I can find a 9-pin driver).
Here's what I drew up. The two big potential issues I see, aside from whether it will work at all, are 1) finding a magical pentode that can take the current and that is reasonably linear (though maybe just an EL84?), and 2) is connecting the screen to the 300B's grid just asking for trouble. Better to build separate supplies? Any thoughts. Is it worth playing with this topology, or will it just end in disaster.
I have been playing with Gm amps for headphones lately, and in general, I like them a lot. I've done a SE one with a single 12HG7 (transformer coupled) which I like a lot, a PP with a quad of C3g's which is also pretty nice, and even a transformer coupled DN2540 based one which is a little less detailed (likely due to lower quality transformers, actually), but still pretty good.
Anyhow, I have a pair of DRD 300B amps which, while fine, have been sitting in their current form for a little too long. I have been contemplating either selling them off, or doing some sort of work on them. So, one thing that came to mind was to try to make a Gm amp with a pentode driver, but doing the whole thing as a cascode. That way, it still uses the 300B as the output tube, and hopefully does not require any real chassis work (assuming I can find a 9-pin driver).
Here's what I drew up. The two big potential issues I see, aside from whether it will work at all, are 1) finding a magical pentode that can take the current and that is reasonably linear (though maybe just an EL84?), and 2) is connecting the screen to the 300B's grid just asking for trouble. Better to build separate supplies? Any thoughts. Is it worth playing with this topology, or will it just end in disaster.
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