Is It Possible to Incorporate NFB onto the Tail of a Constant Current LTPI?

[capehead]

I think I would choose the first circuit with the zener diode. The second circuit has the advantages Teemuk mentioned but its temperature stability isn’t very good. I calculate a 10V drift in plate voltage for a 30C temp rise. The circuit with a 6V zener should be a tenth of that (if the diode has zero temp coeff.) or better still pick a zener with a temperature coefficient of -2mV/deg C to compensate for the transistor Vbe drift.

I’ve just run a simulation on the circuit you posted (with 380V on the PI and 100k grid resistors on the 6550s) and the PI clips at about 50V peak which is very close to your bias voltage of -48V. It is possible that the PI is clipping first. You’d get a lot more voltage swing out of it by changing its 820 ohm bias resistor to 470.

Ok so I built the first version schem and all I can say is wow! Such a drastic improvement, such to the point that it is almost too clean. The high end response is greatly improved and the clarity/dynamics of the preamp stays intact practically until all the up on the master, because of this it does sound louder, much louder. The only complaint is that the .022 grid blocker capacitors or output caps may be too small, it could use a touch more low end. The type of preamp circuit that seems to work the best with this PI is one that is very saturated so the second channel in my amp, which is a Jose style, may have to be changed. I made a couple changes to the circuit which were a 3k trim pot between the emitter and ground and a 10k resistor instead of a 5k pot on the second grid. So again Dave, thank you very much for your help and everyone else as well with this. I will continue to tweak this circuit and report on any changes that yield cool results.

[jazbo8]

If you can post some before and after soundclips, that will be great

[Dave H]

What zener and emitter resistor values are you using is it 6.2V and 2k2? That gives about 60V peak drive into a 100k grid load. Changing the emitter resistor to 1k8 makes it 75V peak (assuming I’ve worked it out correctly). That would make the PI squeaky with the 6550s biased at 48V.

[capehead]

If you can post some before and after soundclips, that will be great

I unfortunately don't have a decent means of making an accurate recording. The only recording device I have is a Boss digital 4 track, which works for what I typically use it for (writing demo songs, remembering cool riffs, ect), but it doesn't do a very good job at recording what comes into it via microphone with any degree of clarity.

What zener and emitter resistor values are you using is it 6.2V and 2k2? That gives about 60V peak drive into a 100k grid load. Changing the emitter resistor to 1k8 makes it 75V peak (assuming I’ve worked it out correctly). That would make the PI squeaky with the 6550s biased at 48V.

Yeah, I used the 6.2V zener, which actually biased the base voltage to 5.8V. And for the emitter resistor, I used a 3k trim pot with a 100R minimum resistance. When I was playing with it yesterday, I simply messed with that pot until I thought it sounded the best. By my guess, I think that resistance value was around 1k5, ie half way up on the pot. At some point today I will be messing with the input and output capacitors and feedback networks in order to bring the frequency response closer to that of a standard LTPI

[Zozobra]

First my apologies for the very deep necrobump but this thread has caught my attention as I've been musing some of the more off-piste topologies as I'd rather build something odd than something I can just buy. My current ongoing build is a 2xKT88 into 4k primary with 500V on the plates and the screens regulated down to 400V which I'll be using to power the PI. The first schem posted by Dave H looks worth a try so I'm going to give that a go later this week when I get a few hours to myself and I should hopefully have time to sim it beforehand. Considering adding a pot in series with the emitter resistor to ape the scale control used in Mojave amps which increases the LTPI bias resistance to reduce output swing, although I guess this topology wont get more distorted like the usual LTPI would.