Just a quick update on my tube tremolo project . I replaced the DC coupled LFO with a LRD/LED combo . I’ve 2 VTL5C3 or similar . As expected , no noisse or bleedthrough! Perfect matching! here’s the schematic :
obviously it’s a known and used circuit , but hey , not millions ways of doing attenuation! and it works well so… Note that a compressor would be quite similar. The trick here is to isolate the DC voltage needed to set the proper bias of the gain stage, to focus on the (periodic) attenuation of the AC signal. Caps are perfect for blocking DC but letting pass AC.
the depth control works a wonder ! the feel is quite different though . A beefier tremolo which remind the great Bo Diddley . Instead of controlling the gain of the amplifier with the original idea (you move the bias of the grid or the cathode with the LFO signal) the LRD/LED put the AC signal at ground, creating a periodic attenuation.The DC bias point , for the tube, is kept fixed.
The drawback is high price of the LED/LRD combo , almost 5 euros. But I think last time I saw it the price was 20euros (?!) on german sellers. Obviously you can make your own combo but LEDs and LRD can have a large difference in specs. You have to try and find the good ones , or/and tweak your circuit to have them working properly . I had an old mutron type circuit that I’ve really used because of the saturation of the input so I took the Vactrol form there.
Anyway, still a lot of work to do with it ! Now I’ve to think about the incorporation of the “harmonic tremolo” , the switching system and the output buffer. Will the signal go in the buffer all the time , a buffered ouptut ? Will the tremolo have a on/off switch to use the circuit as a regular preamp ? Will the circuit be tuned and designed to be used in front of an amp (less gain) or as a preamp , both ? Will I use subminature tubes ? Will I put serveral features in the LFO ? a square/triangle mode ? two speed controls ? lot of interesting questions ! Some have answers already though…
The fact is I would like this circuit to be as functional as possible , offering serveral possible ways of using it. Such design can be really expensive to build for the price of multiple use . Beside, I want this circuit to be used with a lot of instrument, organ , wurlitzer keyboard , voice . I don’t want this effect to be limited .
Just to clearify , It will certainly live in rack format , espcially with all the electronic junk I’ll put in it….
Update 07/15 = the “harmonic tremolo” implentation is now done and works good. Unfortunately some circuit arragement has to be made to avoid volume loss . The mixing of the two signal and the volume pot. form a voltage divider (half of the signal is out). Two choises , a gain boost or a different mixing circuit. The frequency of the Low and High have to be re-calculated to take the miller effect in the equation (the miller depends of the gain of the triode). The low and high must have the same cut off point. I decided to put mine around 630hz . As the impedance is high little cap are needed. Even if precision isn’t really required I’ve to select the proper frequency cut off .
THe LFO will be a LFO that I used for serveral project now,. It works really good and can give the feature I want to add in this project.
I need to buy a case now and I don’t know what model …. a traditional rack (expensive) or a simple aluminium box (cheaper) ? Also need to find a two buttons footswitch , I’ll certainly used a used marshall or other brand footswitch.
to be continued…
thanks for reading