5. S950 Tremolo

 The tremolo circuit of the S950 is unique.

The oscillator is a standard design, using half of a 12AX7:

In the schematic above, the speed control is at lower right, the intensity control upper left, and the oscillator signal leaves this picture at the upper right.  What happens next is a little weird:


The schematic above shows the oscillator output entering the schematic at the lower left. The instrument signal from the first stage transistor enters through the 100n capacitor at the far left goes through V7A, the second gain stage (as described at 4. Preamp).  The amplified signal then feeds V7B which is a concertina splitter.  There is no gain at this stage, here the signal is split into two 180 degree out-of-phase signals which are sent to V5A and V5B.  This is a push-pull pair that pushes and pulls through transformer T4.  This looks like a standard cathode-biased output stage, except that V5 is a 12AX7  It does not drive a speaker.  This is all happening within the reverb/tremolo channel preamp.

The sine wave (or something close to a sine wave) produced by the oscillator acts upon the grids of V5 to vary the bias of V5 up and down, producing  the tremolo effect. 

This is similar to the way tremolo was engineered into the famous “bias tremolo” circuits of low-wattage amps from the 1950′s, and early 1960′s.  The brownface Fender Vibroverb is an example of this approach.  Many people claim this is the coolest sounding tremolo in existence.  And many knowledgeable people say it’s a bad idea because it works by varying the bias on the output tubes, and that output-tube bias should not oscillate because it can create stability/reliability issues.  Maybe the Lectrolab solution makes everybody happy by doing things the same way but different!  That is, this circuit varies the bias of a low-power 12AX7 within the preamp circuit, which will not threaten the output tubes or the output transformer.

After being treated with the “mini-output tremolo section”, the signal goes through a “mini-output” transformer. It’s about the size of a reverb transformer.  From there it is treated to volume and tone control.  Here’s the whole picture so far:

Click to enlarge

This is not an economical circuit.  The tremolo function requires two entire 12AX7’s.  That’s one-and-a-half more tubes than a blackface Fender or a tweed Vibrolux. Many more-economical designs were readily available to copy at the time the Lectrolab S950 was engineered, around the mid-sixties.  So why did Lectrolab opt for this complexity?  It seems doubtful to me that they did this for sonic benefit.

And how does it sound?  I haven’t a clue.  If I ever clear off my bench and get the honked-up power section of this amp reassembled we’ll find out!

I have not seen this type of circuit in any other amp, and I’ve looked at the circuits of a lot of guitar amps.  If you have seen this approach before, or have any ideas about why anyone would use it, please let me know!  Thanks.

4 Responses to 5. S950 Tremolo

  1. I have 2 functioning examples of this amp. One is unmolested and the other (that I use), has a Fender type of inputs, no germanium transistors, no boards there. I use my own cabinet for one used, the twin twelve cabinet for the other. HAD jensen c12r’s in it.

    • alexage1 says:

      Hi John – thanks for informing us. Not just one, but TWO other S950’s are out there! And being played. If you have pics or sound files, send them in, it would be great to see/hear.

      Thanks again,

      George

  2. The tremolo here isn’t exactly the same as the brown Vibroverb, which uses power tube bias modulation. There are a few other amps that use a similar tremolo scheme to the Vibroverb including a bunch of M-series Magnatones, but I’ve not seen a setup like this before.

    Looking at the schematic, we have a concertina phase splitter feeding a pair of small-signal tubes whose bias is modulated by the oscillator. The Vibro-Champ used a very similar setup, but without the bridge-tied load (BTL) in the output. The Vibro-Champ gets a bit of crunchiness and grit from its tremolo at the limits of bias excursion, so this might be an attempt to keep the signal cleaner? A BTL will produce very large voltage swings in its output, although the interstage transformer may cut this back down depending on its turns ratio.

    I’m very interested to see the phase inverter – what makes it so unique?

  3. Rocco Egizio says:

    Great work! I just picked up a 400C, almost done servicing it. Once you get this amp done, if you haven’t already, you may want to try a Marshall 18 Watt style output transformer. Triode has a very good one, with 4,8, an16 ohm taps, good price too. It will improve the overall tone , and volume probably, of the amp. I built one of their 18 watt kits and it sounds great. Too bad you don’t have the original OT, they used good transformers in these amps and odds are they were built in Chicago, maybe even by the same company that makes Triodes transformers. Very cool amp you got there.

    Rocco

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