Help understanding Vox AC15C1 schematic?

[artofharmony]

Hi all! I'm wanting to design a pedal based around the tremolo circuit in my Vox AC15C1. The circuit itself seems pretty simple, but I don't know it works. Anybody willing to help me understand? The schematic can be found at this forum post. And yes, that's my comment at the bottom lol.

[antonis]

MF1 acts as a straightforward phase shift oscillator..
(it uses C44, C49 & C46 to generate 180^o phase shift to create sinewave oscillation by means of negative feedback from Drain to Gate turned into positive feedback..)

SPEED pot alters LFO's oscillation frequency and DEPTH pot (depending on Q2 ON-OFF state) sets LFO's gain (output signal amplitude..) hence turning sinewave signal  to rectangular pulse..

Focus on Figures 2 & 3

[artofharmony]

Thanks antonis! Another question: where is the output? It looks like the input is at C, but I can't tell where the signal leaves the tremolo circuit.

[FiveseveN]

That's just the LFO and its output is at C. The actual tremolo effect is achieved by modulating the output tubes' bias with this "C" signal.

[artofharmony]

Thanks again! My last big question: what is the B+2 voltage powering the MOSFET?

[antonis]

[Ben N]

Here's an optical (Fender style) variant for your edification. http://www.geofex.com/Article_Folders/sstremolo/sstremolo.htm

[artofharmony]

Ah; I meant what is the voltage at that point? I don't know how to decipher the step-down transformers in the power supply.

[PRR]

Ah; I meant what is the voltage at that point? I don't know how to....

The amp uses EL84 power tubes. These are usually run near 300V-350V plate supply. The B+2 point is a second filter node so will be less but not a lot less. The key fact is that the EL84s do the actual tremolo and the LFO stage has to deliver a proportional voltage.... so not so much absolutes as relative voltage.

Are you trying to build a AC15C1? Prove it can work? Re-package it in a pedal??

[artofharmony]

Ah; I meant what is the voltage at that point? I don't know how to....

The amp uses EL84 power tubes. These are usually run near 300V-350V plate supply. The B+2 point is a second filter node so will be less but not a lot less. The key fact is that the EL84s do the actual tremolo and the LFO stage has to deliver a proportional voltage.... so not so much absolutes as relative voltage.

Are you trying to build a AC15C1? Prove it can work? Re-package it in a pedal??

I'm trying to repackage the tremolo circuit in a pedal. It's my favorite tremolo I've ever played, but I go use amp sims most of the time these days. Circuit seems pretty simple, so I figured I'd give it a try. Thanks to all of you for the help!

[PRR]

in a pedal. .... Circuit seems pretty simple

The actual tremolo is an 18 Watt tube amplifier. Not just the part you snipped.

An inspired designer might re-scale it smaller, at risk of losing the cherished tone, but I don't think it fits a pedal.

[artofharmony]

in a pedal. .... Circuit seems pretty simple

The actual tremolo is an 18 Watt tube amplifier. Not just the part you snipped.

An inspired designer might re-scale it smaller, at risk of losing the cherished tone, but I don't think it fits a pedal.

Yeah I'm starting to realize that the circuit is more complicated than I thought I might experiment with building the oscillator and using it to drive an optical tremolo.

[Ben N]

Not saying you can't or shouldn't use them, but the attraction of the LND150 here is that it works at high voltages. That's why you can sub it in fairly simply to do the same job as the tube LFO in a classic amp. If you're working with pedal voltages, small signal MOSFETS will do fine. Have a look at this thread, where the mosfets are used to simulate the power tube section - the part that gets wiggled by your LFO in the AC15C1, and where tone is actually being generated, as opposed to just a waveform. https://www.diystompboxes.com/smfforum/index.php?topic=121713.0

[artofharmony]

Not saying you can't or shouldn't use them, but the attraction of the LND150 here is that it works at high voltages. That's why you can sub it in fairly simply to do the same job as the tube LFO in a classic amp. If you're working with pedal voltages, small signal MOSFETS will do fine. Have a look at this thread, where the mosfets are used to simulate the power tube section - the part that gets wiggled by your LFO in the AC15C1, and where tone is actually being generated, as opposed to just a waveform. https://www.diystompboxes.com/smfforum/index.php?topic=121713.0

Thanks Ben! Your post actually inspired me to pick this back up. New question: What kind of phase splitter is used here? How does it work? It looks like it might be a long tail pair like the one described here, but I'm not sure what that jumble of resistors before the triodes is.


EDIT: found this post that describes the phase inverter more in detail, but I still don't get with those four resistors are for.

[PRR]

>

not sure what that jumble of resistors before the triodes is

This IS a LTP. It's drawn rotated, and the caps are 10X bigger than a guitar audio stage to handle slow wobble. But it even has all the same resistor values as a Fender LTP.

If you can't identify the function of each resistor, go back and re-study a single-triode design.

[Rob Strand]

Thanks Ben! Your post actually inspired me to pick this back up. New question: What kind of phase splitter is used here? How does it work? It looks like it might be a long tail pair like the one described here, but I'm not sure what that jumble of resistors before the triodes is

It's just a form of self bias.   The grids need to go to a DC bias voltage.

If you look at figures 5 and 6 you can see this version has ground referenced grids,
https://www.angelfire.com/electronic/funwithtubes/Amp-DB_Heart.html

In the case of a typical guitar power amp the grids are biased to a DC voltage relative to the cathode by connecting the grid resistors to a tap off the tail resistor.  Then the grids are AC coupled to the signal via caps.

You can do similar things with JFETs.  Compare figure 3 where the gate is bias to ground and figure 6 where the gate is bias by tapping off the source resistor.

https://www.nutsvolts.com/magazine/article/fet_principles_and_circuits_part_2

[artofharmony]

If you can't identify the function of each resistor, go back and re-study a single-triode design.

I love how you say "re-study" like I've ever studied tubes at ALL prior to this project

Thank y'all for the responses! Looks like I'm digging deep into how tubes work now.