6072 tube preamplifier.

[Padel]

Hello all,

This is my DIY tube preamplifier, it's working and sounds great for years.

The first stage is classic grid drive, the output of this gain stage goes from it's anode through an 1Kohm resistor (direct, no capacitor) to the grid of the cathode follower (buffer).

I want to add two more buffers (three per channel total) to drive a six channel power amplifier so, each channel have it's own buffer.
Here is the schematic.
Will this work?

Kind regards
Padel

[SY]

What's the plate voltage of the first stage?

When direct coupling, it's a good idea to add a diode from grid to cathode of the CFs, reverse biased. This protects the tube during turn-on, but has no significant effect once things are warmed-up and operating.

[Padel]

Originally posted by SY
What's the plate voltage of the first stage?

When direct coupling, it's a good idea to add a diode from grid to cathode of the CFs, reverse biased. This protects the tube during turn-on, but has no significant effect once things are warmed-up and operating.

Thanks for the tip. I'll try it, allthough I have no ''blow up''problems with the current schematic.

Any comments about the schematic I posted?
Is this okey or I'm in ''dangerous''?

Regards
Padel

[Padel]

You mean like this?

[NickC]

The location is fine. But reverse the diode orientation.

When the tube is first turn on, the diode allows the voltage from the first tube to follow into the 33k resistor. Once tube 2 turns on the path changes and diode does not play any role


CHeers
NIcholas

[SY]

What's the plate voltage of the first stage? This is important to know to determine if the second stage (the CF) will operate properly.

[Padel]

Originally posted by NickC
The location is fine. But reverse the diode orientation.

When the tube is first turn on, the diode allows the voltage from the first tube to follow into the 33k resistor. Once tube 2 turns on the path changes and diode does not play any role


CHeers
NIcholas

Yes, you are right, my mistake.

So, is this going to work or should I find a good helmet when power on for first time?


Padel

[Padel]

.....and the correct is...

[cogsncogs]

@Sy

I've simulated 188V on the first anode, 187V on the cathodes using Ritek's model. I haven't checked the accuracy of this model and right now I'm too tired to whip out the plate curves and a pencil.

Cheers
Wayne :B

Edit:
P.S. And that exceeds the cathode/heater rating of 90V.

[SY]

Yeah, the CFs will have to be run off a separate heater supply biased up to 140-150V or so.

If your sim is accurate, the CFs at least have a decent (120V) potential across them. 6mA per section current draw, which is much too high for that tube- you'll be in the grid current region. Best to double the cathode resistors to drop the curent to 3 mA, maybe 69K.

Source impedance and drive capability will not be great since this is a low transconductance low current tube.