You'd have less loop gain available, for a given closed-loop gain. I haven't done any calcs but my guess is you'd end up with a bit more distortion overall (because most distortion comes from the output stage), and definitiely higher output impedance. But if you're one of those people who likes the idea of 'low-feedback design' then go for it, it's a very forgiving topology.
I prefer the sound in triode with no feedback. For UL operation I suppose I would prefer low feedback.
So a 6SN7 would be OK? If it were triode operation, lower gain in the first stage would be an advantage. So the 6SN7 could work with either UL or triode.
So a 6SN7 would be OK? If it were triode operation, lower gain in the first stage would be an advantage. So the 6SN7 could work with either UL or triode.
It would be 'OK' in the sense that it will still make a functional amplifier. It's a simple topology, very easy to tweak. Simple circuits have simple shortcomings.
(At this point you will get a dozen people in this thread telling you to replace literally every resistor with a CCS because reasons. Don't let them overwhelm you)
(At this point you will get a dozen people in this thread telling you to replace literally every resistor with a CCS because reasons. Don't let them overwhelm you)
Don't worry, I don't use any active loads, just resistors. Resistors sound better for my tastes.
Thanks Bela for the simulation in triode. Do you have any views on UL operation?
Breadboard it.
When you're at it, you may want to try an ECC40 as well. Mu 32 and good sounding valve.
When you're at it, you may want to try an ECC40 as well. Mu 32 and good sounding valve.
I use the ECC40 a lot. That's another I'm considering. Best sounding valve around a mu of 30 for me. Useful as an I/V tube stage in DACs.
Hi @andyjevans , I gather you are a fan of the 6SN7.
I was wondering if you had tried any of the 6SN7-oids that have a similar vintage? I have acquired a few 6SR7G tubes, which is close to half a 6SN7, but there are also Loctal equivalents. Many of the alternatives are relatively cheap, but a Sylvania 6SR7G is going to be the same standard of workmanship as a Sylvania 6SN7 in my books, so potentially bargains?
6SR7:
6SN7:
I was wondering if you had tried any of the 6SN7-oids that have a similar vintage? I have acquired a few 6SR7G tubes, which is close to half a 6SN7, but there are also Loctal equivalents. Many of the alternatives are relatively cheap, but a Sylvania 6SR7G is going to be the same standard of workmanship as a Sylvania 6SN7 in my books, so potentially bargains?
6SR7:
6SN7:
Divide the first stage cathode complex into two parts.
The gNFB R and C values are hypothetical.
The gNFB R and C values are hypothetical.
Not much loop gain available, as already stated. With a concertina as the phase splitter the input stage has to do all the heavy lifting gain-wise. I've succesfully used a triode-pentode here, as have many others, including the Dynaco SR-35. I used an ECF82 (=6U8A) in my EL84 push-pull, and it works wonderfully well there. It has enough gain for maybe 12-14dB of feedback, while still leaving enough closed loop gain to function without an active preamplifier. I can dig out the schematic for you if you're interested.
Regarding feedback, UL designs really don't function without it. Or they do, but damping factor might be less than 1, so not really good for many loudspeakers. Through experience I've become partial to Bruno Putzeys' stance on feedback, that goes something like "if you're gonna use it, don't be a wuss and apply enough of it". Not his words, though, but that's what I understood between the lines of his whitepaper on the subject.