This tube tremolo is an old idea of mine , torturing me since few years (actually since I sold my tube wiggler from electro harmonix ) . The goals were to make it as simple as possible, with less components as possible , and obviously , great sound.
the first idea was to use only one 12ax7 to create the audio path and the oscillator . Then I realised that tubes (or valves) aren’t great for few things that you can find really easy to do in a solid-state circuit. With only one tube you get quickly ristricted in possibilies and choises. First of all, You need a transformer which can give high voltage, you need a secondary winding for the heaters and their big demand of current (300mA for one 12ax7) therefore you need big cap on the power supply and the audio path that can handle high voltage as well.
The idea started with one triode wired as a regular gain stage , but instead of connecting the grid leak resistor to ground it was connected to the wipper of a “depth” pot which fed to the grid a Sine wave . The Sine wave was created by the second triode, a phase shift oscillator . This is the classic and main desing of bias tremolo found on dozen of tube amps. The Sine wave modulate the bias point of the grid . (You can also change this by manipulating the cathode voltage)
But here come the troubles (for a stand alone tremolo).
Phase shif oscillators don’t have a lot of speed range , most go from 4hz to 8hz or so . And 3Hz is a quick speed (it corresponds to 3 cyles per second) . This wasn’t low enough for me. I tried few things to get a lower speed but with no success. If the frequency is too low , the amplitude of the sine wave is weaker and the oscillation stops. Its high output impedance can also make it works weakly because of the load connected to it.
The big voltage swing produced (the sine wave oscillation) has to be reduced to get a nice low sine wave to modulate the grid correctly (hence the limiting resistor before the depth pot on tube amp tremolo ). Feeding the grid with a big voltage will make some really weird sounds…. In this case this resistor has increased to few megahoms to reduce the signal to few volts (the required voltage to control the grid of a triode is way smaller than the voltage needed for a power tube). This oscillators was really limited (but in 50’s and 60’s that was a great thing ! ) .
The other thing is that tube gain stage has a really high output impedance , 40k or so (depending on the plate resistor) therefore you can’t really use it as a regular pedal . Actually you can but you have to be carrefull on your cable lenght . Lowering the output impedance would be a great thing to do .
A lot of things to take care of , too much for a simple tremolo project .
So here I’m now .
the next goals are : Using a solid state oscillator which will deliver a larger number of modulation sources if I want to (square, ramp, triangle, sine wave ect…) . A stronger signal less dependent on the load because of the low output impedance of solid state device (if not low enough, buffers will be made with transistors or op-amps). Therefore , the second triode could be used as a buffer for the audio path and keep this path all tube !
I’ve just to buy and install a 12V or 15V transformer for the solid-state parts. Not a big deal , this kind of transformers are cheap and small .
Another idea came in my mind, why not add one more tube and make a tremolo that can be used for other things ? the fact is the power transformer is rated for the heaters at 1A , and the high voltage at 10mA , 12ax7 gain stage needs only 0.5mA to 1,5mA to work (depending on the current asked) so there’s enough room to install another tube .
No need to say that this project will be housed in rack format , I don’t believe in tube pedals (tubes are too fragile by itself to be put at your feet) , with a relay switching for bypass and other remote switching.