ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

How does tremolo work?

:

9/29/1999 6:41 PM

How does tremolo work?

Hopefully this won't seem like too stupid a question, but can anyone explain, or point me in the direction of an explanation for, how tremolo works in tube amps?

The point of the question is I'm restoring a Kalamazoo Model II that has tremolo with a freq control but no intensity control. I want to be able to adjust the intensity but need to figure out how to do that. I've been looking at other schematics but haven't seen a similar circuit yet.

Thanks for any help!

Steve Jones

10/1/1999 1:45 PM
Steve Jones	Was the question too stupid? Since no one jumped on I guess the question was too stupid? What may be obvious to some is a mystery to others! Anyway, if someone could recommend a book to read that covers the topic I'll try to figure it out. Thanks, Steve

10/1/1999 2:17 PM

Re: How does tremolo work?

Lemme try. No, the question isn't too stupid.

There are two ways tremolo works in most tube amps. One is by modulating the gain of a tube by varying its bias voltage, the other is by varying a circuit resistance, usually an LDR illuminated by a neon bulb. Fender amps have examples of both types.

In both cases, there is a Low Frequency Oscillator (LFO) that is almost always a phase shift oscillator running on one section of a 12AX7.

Very often, tremolo modulates the bias of the output tubes in smaller amplifiers without overall feedback around the power amp stages. The LFO output is just capacitively coupled to the grids of the output tubes.

In both cases, to change the depth of the tremolo, you change the size of the signal out of the LFO. I bet the Kalamazoo uses output tube bias modulation. Just find the place where the LFO is coupled into the bias and install a 1M or so pot there on the LFO side of the capacitor that couples it in.

10/1/1999 2:58 PM

Thank you R.G.!

quote:
"I bet the Kalamazoo uses output tube bias modulation. Just find the place where the LFO is coupled into the bias and install a 1M or so pot there on the LFO side of the capacitor that couples it in."

In the Kalamazoo circuit, there does not seem to be any coupling with the output tube bias. It is different from every other tremolo circuit I've looked at! The cathode of the 2nd preamp tube is coupled directly to the cathode of the tremolo tube. Then both tubes share a 4k7 resistor connected from the cathode to ground, bypassed by a 10mfd cap.

Other than the B+ connected to the plate of the tremolo tube and the grounding of the tremolo circuit, there is no other connection to any part of the amplifier circuitry. From the plate of the tremolo tube runs a series of caps. A .033 followed by a .02 followed by a .02, followed by a connection to the grid of the tremolo tube. At the junction of the .033 and the .02 cap, is a 68K resistor attached to the wiper of the freq control, a 500K pot. One side of the pot then goes to the on/off switch that turns the tremolo on/off, and the other side of the switch goes to ground. At the junction of the 1st .02 and the 2nd .02 is a 510K resistor to ground. At the junction of the 2nd .02 cap and the connection to the grid, is a 1.5M resistor to ground.

Hope the above description made sense. So is the circuit modulating the bias on the 2nd preamp tube?

Thanks for your help!

Steve

10/1/1999 4:01 PM

Yes. It's modulating the bias and therefore the normal audio signal gain of that preamp stage to get the amplitude modulation effect. I noticed this method is used on some of the simpler Valco (Supro) amps, for example.

To reduce the "depth", you have to reduce the tremelo oscillator output waveform voltage applied to the preamp cathode, so that it doesn't "swamp" the signal as much.

If you can find the schematic for the Supro model S6622 amp (it's in G. Weber's Hip Guitar amp book on p.639, but a recent discussion on Ampage had a link to another place where it was posted), you may be able to rework your Kalamazoooo (I wonder if they played kazoos in that town?)circuit in a similar fashion.

The Supro S6622 has a control for Speed, but not for Intensity. Looking at the Supro schematic, If you were to replace the fixed 2.2meg/220k voltage divider (where the 2.2k grid stopper taps off and goes into pin7 of V4) with a combination of resistors having a "volume"pot in the center, with the wiper being connected to the 2.2k junction point, you could vary the intensity of the modulation applied to the preamp's cathode bias.

Hope this gets you started.

10/1/1999 4:24 PM
Steve Jones	Thanks Doc! I have Pitman's book and there are some Valco Supro amps in there. I don't know if that particular model is or not, but I'll take a look. I did see one that was close, but there was only a speed control in it. I'll look this weekend. Steve

10/4/1999 4:46 PM
Doc	The amp I referred to has only a speed control. I tried to explain where/how I might add an intensity control to it. If your circuit is laid out similarly to the supro, then you could add an intensity control in the same location.

Page 1 of 4