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Reduce Beating In Modulating Bias Tremolo

7/24/2000 9:30 PM
Scott Swartz	Reduce Beating In Modulating Bias Tremolo Over the weekend, I was working on a circuit that incorporates a modulating bias tremolo similar to the Fender brown 6G16 Vibroverb, and I noticed a very interesting phenomenon. When I initially switched on the tremolo, it worked but I got the dreaded beating at a relatively low intensity. I was powering this circuit from a Kepco power supply so I could vary the B+ and bias voltages at will, and I noticed a big change in the beating as I changed the bias voltage. As I made the bias voltage more negative, the depth of intensity increased and the beating at that intensity decreased. At a cold Class AB bias, the beating is almost completely eliminated. I had been wondering why this type of tremolo seem to work better in some amps than others and I think this is the answer – you need to have the bias set so that when then tremolo swings positive, the bias for the stage needs to remain at a reasonable composite negative (the instantaneous sum of the swinging positive plus static negative bias). Therefore, if the bias is set to class A or hot class AB, you will get beating, but if you back off to colder class AB, the beating is greatly reduced. The problem with setting a cold bias is that compromises the tone when not using tremolo. The obvious solution is to increase the negative bias when then tremolo oscillator is started. This can be accomplished by use of a relay with two poles to simultaneously unground the oscillator feedback path AND short or unshort across a resistor in the bias supply to increase the negative bias on the power tubes. This concept greatly reduces the beating sound and preserves the non-tremolo tone. I have already ordered parts to add this to one of my amps. I hereby place the concept of increasing the negative bias simultaneous with starting the tremolo oscillator as a means to decrease beating or LFO feedthrough in this type of tremolo circuit into the public domain. If you are using this type of trem, feel free to experiment with this concept.

7/24/2000 9:30 PM

Scott Swartz

Reduce Beating In Modulating Bias Tremolo

Over the weekend, I was working on a circuit that incorporates a modulating bias tremolo similar to the Fender brown 6G16 Vibroverb, and I noticed a very interesting phenomenon. When I initially switched on the tremolo, it worked but I got the dreaded beating at a relatively low intensity. I was powering this circuit from a Kepco power supply so I could vary the B+ and bias voltages at will, and I noticed a big change in the beating as I changed the bias voltage. As I made the bias voltage more negative, the depth of intensity increased and the beating at that intensity decreased. At a cold Class AB bias, the beating is almost completely eliminated. I had been wondering why this type of tremolo seem to work better in some amps than others and I think this is the answer – you need to have the bias set so that when then tremolo swings positive, the bias for the stage needs to remain at a reasonable composite negative (the instantaneous sum of the swinging positive plus static negative bias). Therefore, if the bias is set to class A or hot class AB, you will get beating, but if you back off to colder class AB, the beating is greatly reduced.

The problem with setting a cold bias is that compromises the tone when not using tremolo. The obvious solution is to increase the negative bias when then tremolo oscillator is started.

This can be accomplished by use of a relay with two poles to simultaneously unground the oscillator feedback path AND short or unshort across a resistor in the bias supply to increase the negative bias on the power tubes. This concept greatly reduces the beating sound and preserves the non-tremolo tone. I have already ordered parts to add this to one of my amps.

I hereby place the concept of increasing the negative bias simultaneous with starting the tremolo oscillator as a means to decrease beating or LFO feedthrough in this type of tremolo circuit into the public domain. If you are using this type of trem, feel free to experiment with this concept.

7/25/2000 4:43 PM
moocow	If the problem is with positive swings of the oscillator, you can use a diode to cut them offs. The diode goes across the intensity pot with the cathode (stripe) attached to the bias supply. I've posted this mod a couple of times here on AMPAGE, so it's already in the public domain.

7/26/2000 10:11 AM
Danny	i have this problem and it is exactly as you described it, but when you use two speakers it goes away easier, and the speaker is still moving

7/28/2000 12:32 AM
moocow	Traynor Tremolo After a bit of thought, I realized I've seen something like this before. The Traynor Studio Mate YGM-4 uses a tremolo intensity control that biases the amp colder as intensity in increased. However, is not switchable. You can find a schematic on Joe Piazza's website: http://home.rochester.rr.com/piazza/index.html

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