circuitry question

[Tirol29]

hello 

i've found this tremolo output circuitry in many Univox amps :


question about the 12AU7 (yellow part) : what is that ? used for a kind of impedance matching ? 
thanks for answering

[pdf64]

I can’t make sense of the small section you’ve shown there; need to see what the 12au7 plates connect to. But also there’s no connection from the oscillator apparent, so as is, I’d assume an error.
This one uses the 12au7s plate impedance as a voltage controlled resistor, to progressively load then short out the signal, same as the ldr in Fenders roach.
https://el34world.com/charts/Schematics/files/Univox/Univox1000.pdf

[Tirol29]

...need to see what the 12au7 plates connect to...

that's the same as your schematic shows : each plate connected to the cathod of other triode, then one grounded, the other connected to output of first preamp stage.
only the cathod  graphical representation differs
thank you for your explanation

[sluckey]

Complete schematic...

     https://el34world.com/charts/Schematics/files/Univox/Univox1226.pdf

[2deaf]

It looks like somebody erased the connection to the cathode in order to fit the designation "10uF" in.

[Tirol29]

 
don't get it 
may be i'm too deaf 
i've tried to find the purpose of this circuitry, pdf64 answered but i'm still willing to know how "electronically" this works... but that will not stop me for poor playing

[sluckey]

Just think of the tube plate to cathode as a variable resistor. The LFO sine wave voltage on the grid controls this variable resistance. The variable resistance simply connects into the guitar signal path. When the resistance is high very little of the guitar signal is shunted to ground. When the resistance is low a lot of the guitar signal is shunted to ground. This is all happening smoothly at the rate of the LFO sine wave, typically 3Hz to 10Hz.

It's exactly the same effect as you turning the volume control up and down five times per second. Look at the complete schematic in the link I posted.

[Tirol29]

Just think of the tube plate to cathode as a variable resistor.
Look at the complete schematic in the link I posted.

thanks again,
i was a million miles away to think about this property of the triode, not found yet a word on that...
the rest, LFO, no pbm
many Univox schematics show this implantation

[edit]
found this
https://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1954/10/page27/index.html
too much for a friday night [/edit]

[PRR]

Yeah, the snippet has flaws. And this was not so widely used.

Plate resistance of a triode changes with grid voltage.

The variable plate resistance can be used to short-out a signal under LFO control. Tremolo.

The plate resistance only works one way(?). Here he used two triodes nose-to-tail to cover both sides of the signal.

Good 12AU7 datasheets have a plot of plate resistance under different conditions. These are always taken with large DC voltage and current. And also far away from the Contact Potential zone.

Accepting that reality of a "zero" voltage situation will differ from "real voltage", we can still see a trend. At Vg=0 we have rp=14k, -5V=25k, -10V=40k, -15V=75k... we can get a real variation of volume. The actual rp with plate voltage at zero will probably be much higher. We see that the designer used a 200k impedance for the 12AU7 to work against.

In my experience, this plan does NOT work with "all" 12AU7. They have to be selected across multiple factories and vintages to find one with the "soft" low-volt action. (A selected 12AX7 is about as likely to work, but needs far less LFO signal.)