• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Aikido and SE KT88

kacernator said:
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.

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.

kacernator said:
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.

kacernator said:
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?

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


:scratch: ??

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


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.
Apparently I was writing the previous reply while you responded.

Let me add that you would want to use the output safety divider in cases where you don't want a large signal swing into the next stage - a transistor following stage for example. The KT88 (most tubes, in fact) can easily tolerate a large momentary swing. But, as I said before, adding the safety resistors there won't harm anything.



2004-10-14 10:13 pm
I Built my Aikido KT88 SE as orginal 12ax7/12au7 circuit and used UL 400v over tube and -39 on grid . Sounds very clean ,only problem is terrible high driver gain . I changed input tube to 12at7 with 1k on cathodes . Helped littlebit bus didnt sound nice ? Maybe 12au7 is suitable in input ? Wondered if the original circuit is only theoretical ? That gain would be enough to drive 845/GM70 SE ?