Mystery semiconductor substitute

[Tom Phillips]

...It seems it might be a very subtle thing. It also seems you might have to hit the input hard to notice it. Maybe it's working and you just haven't hit it hard enough to hear the effect well?

I concur. Be very careful of trying to make something better than it ever was or trying to fix something that the customer didn't request.

[Randall]

Now that it is back together and I run a tone at just enough to light the gain LED, I can hear a difference with the tube knob, and I can see the lower half of the wave distort in an manner. There is no more data nosaj about the history, the owner doesn't know. I don't know if I fixed something, or if I chased rainbows all day, but the knob is doing something now and based on the input thus far, I am not proceeding further. I am increasingly believing this is a BS hyped "feature" that as some have said, doesn't really do much.

[Tom Phillips]

...I am increasingly believing this is a BS hyped "feature" that as some have said, doesn't really do much.

Yep. Even a single cathode follower stage is reported to have an effect but it is very subtle. And...there is some equipment hyped to be good just because there is a vacuum tube involved

[Randall]

"Look up the WT300 or 400 schematic, the tube stage is VERY similar to this, and is drawn in maybe a more readable format"

What Enzo, I thought you would have wanted to stick this drawing by your student on your refrigerator door?

[Enzo]

I did, it looks l;ike mine.

[J M Fahey]

Just quoting from memory: look at what Eden does on the regular heads: being high power SS amps, think 300W and more, they have symmetrical around +/-60V rails.
To be able to claim the "look Mom!!! a Tube Preamp" hype, they added an input cathode follower, with plates to +60V, cathode to -60V through a "magic" 56k or 100k resistor (hey!! Leo and Jim used those values, so they must have Mojo), and grid referred to ground, which nicely sits halfway between +/-60V.
So far so good.
Since itīs a dedicated rack preamp and probably "everything else" (Op Amps) runs from +/-15V, they had to build a dedicated supply for the tube (no "free" +/-60V available) they added a raw +124V one, regulated down to +117V to kill hum .
So grid is tied halfway to a voltage divider, I see a couple 1.5M resistors there which look the proper value.
Option 2 is that they simply self biased it with the floating 100 ohm cathode resistor (looks somewhat low to me, woud have expected 820 ohms or 1 k) and the 390k resistor is also floating , hopefully around halfway between +117V and ground.
Now if you measure grid voltage , multimeter input resistance pulls that down; and since input cap is ridiculously high 1uF, it takes a few seconds to go down.
Measuring cathode which *can* drive the multimeter well lets voltage go up again.

Cathode followers can add unsymmetrical distortion because they are unsymmetrical: when driving a load which is low enough and is driven hard enough to require some current, top half is driven by the triodes in parallel, they can supply/"push" some 6 mA easily; while the lower half is "pulled" only by the resistor.
If we re talking 56V and 56k that means about 1mA current capability.

So if you drive cathode follower *hard* , think 10 to 30V RMS , and it has to drive a load, say 10k, then upper half will be fine; lower half will clip because it is current starved.

That happens beautifully in a Tweed/Marshall/Valvestate tone stack driver, where signal can reach 70/90V RMS and it is loaded by the tone stack .
Now, on an *input* stage ... driven by, say, 1V RMS? (and that IF you use an active bass) ... and lightly loaded?
Just a gimmick.

Personal rant: tubes sound like solid state *unless* driven HARD.

Version II : Hartke amps also include a Tube Channel, which does sound different
How do they do it?
They cheat of course, what did you expect?
Preamp signal is amplified first, say, 10X, and split in two: half goes straight through to a "SS Volume" control; half to a 12AX7 stage with some 30X gain .
problem is that besides volume increase it still sounds the same as the other , so they added an internal, hidden, fixed EQ, think a classic Fender EQ, which returns volume to about normal (still somewhat louder than the other) and equalized, with more treble and bass and less mids.

So users "confirm" what they always believed: "tube sound is fatter and punchier than SS sound"
Funny thing is that I made the experiment of adding said EQ to the SS channel and left the tube one flat, only attenuated to give same volume ... and users now preferred the SS one

Smoke and mirrors

[Leo_Gnardo]

That essay / expose' earns a well deserved "like" Juan.

Something that tickles my funny bone: paperback-book size class D bass amps that include a preamp tube, y'know, for that "vintage tone." What a joke. I had one in for a fix, it would have sounded good tube or no. As it turned out its fault was due to corrosion on the tube pins. If it had no tube, it would not have failed.

[Randall]

Good stuff Juan. I wish I had known that a day ago, but as they say, timing is everything!