How does this trem. work Gibson GA-40.

[Willabe]

Is the term in this amp modulating the ac signal at the plate? The LFO is just sharing the preamp input tubes cathode R/C?

Or is it shaking the preamp input tubes cathode and screen? If it's this then why the 2 series 510K's going from preamp plate to LFO plate with a .01 cap to ground tap from the 2 510R 's junction.

http://www.el34world.com/charts/Schematics/files/gibson/LES_PAUL_GA-40.pdf

And this Vox AC10 is almost the same thing.

http://www.el34world.com/charts/Schematics/files/Vox/ac10_2.pdf


                Brad    

[Willabe]

I just had a thought. Are these 2 trems biasing up the LFO cathode to the preamp tube cathode/screen dcv for a fast start up?


                  Brad      


Edit; This can't be right, I read the schematic wrong. I was looking at the screen voltage being on the cathode.

[HotBluePlates]

Is the trem in this amp modulating the ac signal at the plate? The LFO is just sharing the preamp input tubes cathode R/C?

No, it is injecting the trem by virtue of the shared cathode resistor.

There is a bypass cap, but trem frequency is down around 3-8Hz, so not much attenuation.

... why the 2 series 510K's going from preamp plate to LFO plate with a .01 cap to ground tap from the 2 510R 's junction. ...

Low-pass filter, mainly to get rid of any upper harmonics the LFO might generate while oscillating. The 1MΩ total resistance pretty well isolates the signal tube from the oscillator.

What is odd is apparently changing the Depth by varying the plate voltage of the trem oscillator. But since the oscillator causes trem by varying the current through the shared cathode resistor, maybe it makes sense.

Less oscillator plate voltage = less oscillator plate current = less trem current in the shared cathode resistor = less trem of the signal.

There is a multi-section high-pass filter after the signal stage; 0.005uF & 1M are -3dB at ~32Hz; multiple sections roll off more steeply and shift the -3dB point higher. If nothing else, the trem channel is brighter.

The point appears to be to kill pumping when you're not playing but the trem is on high Depth. the low oscillator signal is stripped out, but volume fluctuation of the signal for guitar frequencies is still heard.

[HotBluePlates]

And this Vox AC10 is almost the same thing.

And probably largely copied from Gibson circuits, like the Top Boost circuit was, apparently.

Key difference: Vox varies the screen voltage and bypassing of the screen to get changes of trem depth.

At one extreme, Amplitude wiper is grounded. The screen voltage is at its highest (most tube current) and the screen is partially bypassed by C5 (which would tend to reduce tube screen current/voltage variations, and therefore plate current variation). Oscillator current variation is small compared to total signal tube plate current, so trem is at a minimum here.

At the other extreme, C5 is shorted by the Amplitude wiper, and screen voltage is at a minimum due to the smaller lower end of the screen voltage divider. Tube plate current will be at a minimum, and the screen is not bypassed, which will make the oscillator current seem large compared to signal tube plate current, so trem is at a maximum.

[Willabe]

Thanks for the answers HBP.

I couldn't figure out why the LFO was tied to both the plate and cathode.


               Brad     

[kagliostro]

Many thanks for the explanation

K