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previous: Nils I've just built a Vibroverb clone, ... -- 8/3/2000 5:32 AM

Re: How to get more depth in a bias tremelo

8/3/2000 2:55 PM
Scott Swartz	Re: How to get more depth in a bias tremelo If you decrease the series 470K resistor and remove the .1 cap to ground on the oscillator CF cathode, you will have a larger amplitude waveform, but then you might get a “beating” sound. This can be eliminated by one of the two ideas below. The first one I posted here several weeks ago and another idea from “moocow” was posted as a response. The overall principle of increasing intensity is to be able to drive the grids more negative, which causes the output tubes to be cut off for a longer portion of the LFO waveform. Here’s the archive material: ________________________________________________ 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 a composite negative. 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. posted by 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.

8/3/2000 2:55 PM

Scott Swartz

Re: How to get more depth in a bias tremelo

If you decrease the series 470K resistor and remove the .1 cap to ground on the oscillator CF cathode, you will have a larger amplitude waveform, but then you might get a “beating” sound. This can be eliminated by one of the two ideas below. The first one I posted here several weeks ago and another idea from “moocow” was posted as a response. The overall principle of increasing intensity is to be able to drive the grids more negative, which causes the output tubes to be cut off for a longer portion of the LFO waveform.

Here’s the archive material:
________________________________________________

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 a composite negative. 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.

posted by 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.

Replies:

Nils Thanks Scott -- 8/4/2000 5:44 AM