Bias vary trem 6G3 vs 6G11

[g-man]

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[sluckey]

The extra triode is a cathode follower. It's needed because the 6L6s need a stronger trem signal than the 6V6s. The trem should sound the same on both amps.

[g-man]

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[sluckey]

I'm a little confused, as I thought a CF always had gain of 1 or less.

That would be VOLTAGE gain. But a cathode follower has high CURRENT gain. The purpose of a cathode follower is to be able to provide more drive current for a low impedance device or any device that may draw current (therefore loading) from a source such as a hi z phase shift oscillator. IOW, the 6L6s will load the trem signal more than the 6V6s. This loading will drag the voltage amplitude of the trem signal down.

Put another way... The cathode follower acts as a buffer between the hi z oscillator and the 6L6s. So if/when the 6L6s draw current from the trem signal, that current will not cause the trem signal voltage level to drop. Without the CF, the loading effect on the trem oscillator may even be enough to cause the oscillator to stop oscillating.

You can bet that Leo would not have used that CF if it was not necessary for reliable operation. BTW, it wasn't long after that Leo abandoned this circuit in favor of the more reliable 'roach' trem.

[g-man]

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[SoundmasterG]

If you are playing around with the bias modulating trem circuit in an amp and you only have enough room for the oscillator stage, you can add a mosfet follower circuit that will do the same thing as the tube CF. It is a little more finicky to set up but it works just as well.


Greg

[Fresh_Start]

As Sluckey says, you need the cathode follower if you're diddling the bias on 6L6s. That's what I did with a modified Super Reverb anyway. Some guys have added it with MOSFET to Princeton Reverb circuit although stock without CF is fine for me with 6V6s.

I'm not sure the "roach" is more reliable than power tube bias vary tremolo.  More consistent in terms of not having to tweak the power tube bias to get tremolo dialed in, yes for sure. IOW the roach is far superior in production environment.

Cheers,
Chip

[tubeswell]

FWIW schematic of a Mosfet (IRF820) Source Follower after a LFO stage in a PR type circuit. (Use a high-voltage mosfet - IRF820 is rated for 500V) Everything is the same as if you had a CF stage instead of a SF stage, but you want between 100R-1k in series with the gate and a 12V zener between the gate and the source (banded end pointing at the gate) to protect from source-gate insulation breakdown. The advantages of using a mosfet follower are you don't have to worry about heater-cathode insulation (that can be problematic in cathode followers) and it doesn't add any load to the heater winding, and it doesn't change the sound of the amp or adversely affect the function of the tremolo.