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KOTriode wrote:

Without regulation, this amp would not work correctly due to its design: DC coupled and fixed bias. The high transconductance will make the bias current varied as much as 50-100ma due power line fluctuation without the regulators. It might work if the amp used cathode bias, but I chose fixed bias for various reason. The input stage (with or without the Cathode follower stage) bias the output tube from the plate voltage, fluctuation in unregulated B+ will cause the grid voltage to swing which in turn change the output tube bias current. Transformer with high current output, choke and capacitor would not help much with this design with line voltage fluctuation.Also since this is a setreo amp, good regulator definitely help crosstalk.All my systems are multi-amp and put a heavy load on the power line on the average of 1000W and more, so regulator in this amp is necessary, not an option. Why the adjustable voltage?  this tube to me is an experiment,  the regulator for me is very convenient to understand the bias of this tube with different plate and grid voltage in the amplifier, and how the sound will be affected with different operating point. I am not too crazy building a high voltage amp (the voltage is as high as 520V on this amp) with multiple power supply and wire hanging. This amp is never intended to work in A2, so does my other DHT amp. The amp can be played with or without the Cathode follower, my aim to get the driver as linear and wide bandwidth as possible, hence feedback will be unnecessary. I will try the amp with few different speakers as full range and later in tri-amp system, among them are mini monitor, Stacked Quad ESL, Horn system.... It would be very interesting to compare with the big DHT tube. Into the 10 hrs of break-in with a monitor, I am very happy with the sound so far.

KOTriode, I think you are missing what I am trying to say. It is very much not the debate of the usefulness of regulation but rather me stressing the point that neither yours nor my experience with regulation does not provide a universal answer if the 6C33C might be benefited by regulation.

It is not doubts that your design needs to be regulated to stabilize all your direct coupled operational parameters. However, your regulation does not address the needs of 6C33C but rather serves the environmental purposes. So, even in your design we do not know how 6C33C will sound in the same application with no regulation. BTW, I regulate my power AC supply – here is another way to stabilize operation without use of DC regulators.

If you look closely then you will see that in Melquiades I use regulation on the grid supply of the driver tube. I do have very substantial advantage on sound quality but here is an interesting question: do I have this advantage become of regulation of because some other reason?  The negative regulator in Melquiades drives the driver grid. The grid has own minute grid voltages that run back to the PS. Here on the PS side we have gas regulator – a very soft entry that instead of hard-shorting grid voltages or bouncing them back, the gas regulator very gently absorbs them. So, it is possible that in my case the regulation itself is not important but rather that I use the regulation as some sort of damper that sucks out the reflective voltages from driver grid. If cause if I go with voltage stabilization on negative side on the biasing grid resistor then I need to have one on positive side to lock the bias. BTW, try the same biasing approach on the 6C33C and it will not sound good at all.

What I am trying to say is that so far I do not see any evidence for or against regulation of 6C33C. Neither your use of mandatory for your circuit regulation not my “creatively-intentional” use of bias regulation does not say anything about the regulation and it benefits for sound of 6C33C.

About the A2. I do not say that you intended to be in A2 but driving the 86dB sensitive Quad in a room larger then a dog house your bass will be jumping for a fraction of seconds into A2. The  845/211, beside to having more power will handle this entering into A2 with some distortions but it will still be fine and the signal will be cared on. The 6C33C will not even go into A2 – it will flat cut the signal producing enormous amount of distortions across all bandwidth. All the I a saying is that being indirect heated the 6C33C is much deadlier with grid voltage approaching the bias voltage. If you have 65V on grid and 30-40 times gain in your driver stage then it is not difficult to be “right there” very fast. Sine you use an extra buffer stage, useless people do it to get current drive then I remind you that it would be fine with DHT 845/211  but it will not “help” to any indirect heated output stage.

About the 6C33C vs. 845/211. This might be interesting. Are your 845/211 also regulated and direct coupled with use of the same Tango magnetics?  Are they use the same Tango’s vintage? The Tango 20 years back was very much not the crap that Tango dose nowadays, if you know what I mean…

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