Tremolo oscillator use in triodes

[soundmasterg]

I have an amp that I was working on years ago that I want to get back to. It is basically 3 amps in one head, with a common power transformer, separate output sections, phase inverters, and output transformers in one chassis, and I am adding a preamp chassis where each of the 3 preamps will be separate from each other. Call it an ABC amp, one channel for each letter.

I have two tubes per preamp available. Each section (pre and power amp) has local star grounding on a bus that grounds to one point in the power amp chassis, except the first filter stage and safety ground are at different ground points respectively. For one of the preamp 12AX7 sections I was planning to use a single tube tremolo oscillator, and was intending to put it in one of the 12AX7 sections for the second tube in that preamp. Will that oscillator affect the stability of the other triode in the bottle (and hence that preamp channel) if the other triode is being used as a gain stage or a cathode follower?

Thanks!
​​​​​​​Greg

[R.G.]

Same reply as on MEF: you could sidestep the issue by using a MOSFET to make the trem oscillator.

See http://www.geofex.com/Article_Folder.../sstremolo.htm

The middle MOSFET in the link implements the Fender trem oscillator. The other two are a voltage regulator and lamp driver. You could use just the middle MOSFET.

This has the advantage of leaving you yet another half-tube to play with. I bet you could use it.

As a follow on observation, whether it's okay or not depends on the things you already know about: all triode sections are not equal, all power supplies are not equally well executed, all inter-electrode capacitances are not equal, etc.

[soundmasterg]

Thanks R.G.!

Yah I'll have to see how well it will work out in this amp. It might be a better approach to use the triode elsewhere and use a MOSFET as you suggest. This amp is quite complicated, with three separate amps in one head basically, all sharing the power transformer. I've done the best I can to isolate things, and when I had it working with the rev2 chassis for the preamp 10 years ago, two of the sections worked well, but the third (2204 circuit) was having issues. The layout on that rev of the preamp chassis was problematic, so rev 3 is going to be a LOT better, and will eliminate all the issues I had with layout on that previous one. I just need to bend the metal up and see.

I get a 404 error with that link you provided, but I was able to find it on your site anyway. Looks interesting!

Greg

[didit]

A 404 reply for me as well. This one works for me.

http://www.geofex.com/Article_Folders/s ... remolo.htm


..

[nuke]

I cobbled up something similar in this thread, using ordinary NPN transistors:

https://ampgarage.com/forum/viewtopic.php?f=6&t=37542

If you can find about 30-40 volts to power this circuit, it "drops in" to the Fender optical tremolo circuit very easily, using the same front panel pots and capacitor values. I have cobbled this together on a replacement bias-supply board for Fender amps. It uses the bias tap on the power transformer to make the usual negative bias supply, and it implements power and the oscillator using the darlington transistor pair and another transistor of suitable voltage to fire the neon lamp in the "roach" which could also be a red LED just as easily.

The point of it was to replace the Fender tremolo circuit entirely, freeing up a whole 12AX7's two triodes, without making holes or hunting down a rare reverse-audio pot to replace the stock Fender 3M reverse audio taper front panel pot.

If you can find power for an opamp or other solid state oscillator, there are many tremolo implementations you can do pretty easily.

[soundmasterg]

Thanks nuke!

I kind of want to use bias modulating trem on this amp, but we'll see. The power amp is a Bassman AA864, and there may not be enough swing to get the trem to sound nice enough. With R.G. Keen's circuit as he described I can use part of it only to get the bias modulating trem, and probably a lot better swing too. Once I find my schematics I'll have to sit down and look at the options on this amp and see what can be done with the intensive circuitry in there, what can be done with layout, and with the existing supplies in there. Space is a premium due to so much going on in the two chassis.

Greg

[nuke]

Thanks nuke!

I kind of want to use bias modulating trem on this amp, but we'll see. The power amp is a Bassman AA864, and there may not be enough swing to get the trem to sound nice enough. With R.G. Keen's circuit as he described I can use part of it only to get the bias modulating trem, and probably a lot better swing too. Once I find my schematics I'll have to sit down and look at the options on this amp and see what can be done with the intensive circuitry in there, what can be done with layout, and with the existing supplies in there. Space is a premium due to so much going on in the two chassis.

Greg

Yeah, once you have the oscillator running, the choice of interface into the audio circuit is usually just a matter of tweeking some last stage of hardware. Bias modulating tremolo is pretty simple to do as well as LED, or neon bulb optical. Just a matter of designing the right thingie to drive whatever pleases you.

My goal was really to just re-use the existing front panel pots and footswitch, and I came across Peavey's patent for the Transtube products. Then the light went on in my head when I read it and thought about how it worked for a little while and how it simulates the triode with a darlington pair of transistors. I cobbled one up and right off the bat, it worked perfectly with the same pot and phase shift network.

If one is working from scratch, an LFO is as easy as grabbing your cheapest OpAmp and about ten minutes looking up a suitable set of parts. To re-use the hardware already in say, a Deluxe Reverb, the "transtube" oscillator makes it a piece of cake.