Aikido and SE KT88

[kacernator]

Has all ECC88 Aikido preamp enough gain to drive SE KT88 fixed biased in UL/Triode mode? B+ 460V.

How much gain do I need to drive it?

[kacernator]

How can I know how much gain is needed to drive the desired output tube?

[Sheldon]

Originally posted by kacernator
How can I know how much gain is needed to drive the desired output tube?

First, divide the desired quiescent dissipation in watts by the plate voltage to get the quiescent current. Look up on the plate curves, the grid voltage at that point. You will need to swing from zero to about twice that voltage. Your input swing is 2.8*RMS input voltage. Divide that number into the swing needed at the output tube for your required input/driver gain.

Sheldon

[kacernator]

As I look at these scheme: http://www.tubecad.com/2006/03/aikid...mplifier_2.png

There are left the 1M safet resistors on the output's stage tubes from Aikido, why?

I tried disconnecting the 1M in my Aikido build and I like the sound more WITH them.

[Sheldon]

The safety resistors are there to keep the Aikido output stage grid at about half the rail voltage if the input tube fails or in the event of a poor connection at the socket. Otherwise, the grid will be driven to the full rail voltage. They will have virtually no effect under normal operating conditions.

Sheldon

[kacernator]

Maybe they should have no effect on sound, but there is a little one. Try it, you will hear it.

So is it OK to put them back in the posted schematics above?

[Sheldon]

Originally posted by kacernator
Maybe they should have no effect on sound, but there is a little one. Try it, you will hear it.

It's possible that the output impedance of the 12AX7 front end is high enough so that the safety resistors are audible, though I remain skeptical. But it's your ears that matter in this case.

Originally posted by kacernator
So is it OK to put them back in the posted schematics above?

Let's summarize: It sounds better to you with the safety resistors in, they protect the output stage in event of an input tube fault, the circuit designer believes that they are important, and he's by all accounts quite knowledgeable in audio circuit design.

What additional information to you need to answer your own question?

Sheldon

[kacernator]

I am just wondering why are those 1M not at that schematics, thats all.

http://www.tubecad.com/2006/03/aikid...mplifier_2.png

[Sheldon]

Originally posted by kacernator
I am just wondering why are those 1M not at that schematics, thats all.

http://www.tubecad.com/2006/03/aikid...mplifier_2.png

??

They ARE on the schematic. What am I missing??

Sheldon

Edit: I think I see the problem. In some examples on the TubeCAD site, he uses a 100k resistor divider from B+ to ground on the output of the Aikido (different from the 1M divider on the grid of the Aikido output tube). That one is not so important, because the Aikido is cap coupled, not DC coupled to the output stage here. It would prevent a loud thump in event of an output tube failure on the Aikido. You can use one there, and it won't hurt anything. But the resistor value should be about 100k or so, not 1M.

[kacernator]

They are missing on the output tubes of preamp.

Here are labeled R7 and R12:
http://www.tubecad.com/2006/04/17/output_buffer.png