We were discussing this on the 300b thread currently...
https://www.mennovanderveen.nl/cms/...keling/publicaties/transadventures/5-Ajay.pdf
Should sound really nice.
https://www.mennovanderveen.nl/cms/...keling/publicaties/transadventures/5-Ajay.pdf
Should sound really nice.
The 6SL7/6V6 setup employs Bandersnatch's "exolinear" short loop NFB and looks pretty good. FWIW, I'm not comfortable having a single tap on the O/P trafo serve both the "exolinear" connection and a UL connection. I'd go full pentode mode, with gas discharge regulated g2 B+, and leave the tap solely for short loop NFB. If you want a 100% Octal look, use a 0D3 and a 0C3 in the gas regulator stack. Read GE's excellent documentation here.
BTW, full pentode mode will be easier for a high RP/low gm type, like the 6SL7, to drive than UL mode.
A full pentode SE 6V6 yields 5 W. Your O/P "iron" can process 15 W. Plenty of magnetic headroom to support a few dB. of global NFB is present, should distortion and/or damping factor considerations make it necessary.
Eli - you're very familiar with pentodes. I was looking for a schematic of an input pentode with gas discharge regulated g2 and couldn't find anything on the net, strangely. Do you have a diagram of how to implement this on a typical small pentode input stage? Is it as simple as feeding G2 with something like a 150v VR or are there any other components involved?
@andyjevans
Some small signal pentodes don't need much in the way of a + voltage on g2. Most of the time, a dropping resistor and cap. to ground are what you see. If the datasheet shows a g2 B+ voltage in the range of gas discharge regulation, there's every reason to believe things will work out well.
The 6AC7 is (IMO) interesting. It will definitely work with 0D3/0A2 g2 B+ regulation and the datasheet has a clear warning about a dropping resistor off anode B+ causing an undesirable remote cutoff characteristic. With DC heating, I can see the 6AC7 as the 1st gain device in a phono preamp. Yes, multi-grid partition noise is always an issue, but the high gm is compensation. I'm guessing the 6AC7 is at least as good as the 12AX7, in the noise dept., and you gain freedom from adverse Miller capacitance interactions.
BTW, don't forget a high quality 0.068 μF. cap. in parallel with the VR tube. The cap. kills the noise gas discharge generates. However, excessive parallel capacitance will create an unwanted relaxation oscillator.
Some small signal pentodes don't need much in the way of a + voltage on g2. Most of the time, a dropping resistor and cap. to ground are what you see. If the datasheet shows a g2 B+ voltage in the range of gas discharge regulation, there's every reason to believe things will work out well.
The 6AC7 is (IMO) interesting. It will definitely work with 0D3/0A2 g2 B+ regulation and the datasheet has a clear warning about a dropping resistor off anode B+ causing an undesirable remote cutoff characteristic. With DC heating, I can see the 6AC7 as the 1st gain device in a phono preamp. Yes, multi-grid partition noise is always an issue, but the high gm is compensation. I'm guessing the 6AC7 is at least as good as the 12AX7, in the noise dept., and you gain freedom from adverse Miller capacitance interactions.
BTW, don't forget a high quality 0.068 μF. cap. in parallel with the VR tube. The cap. kills the noise gas discharge generates. However, excessive parallel capacitance will create an unwanted relaxation oscillator.
