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low volume sustain

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7/11/2004 12:49 AM

low volume sustain

Hi all,

Does anyone have any tips for improving the sustain of a low power (1W), low volume amp? It has a couple of gain stages similar to a JCM800. Just not enough sustain for my liking. I was thinking of tinkering with a small positive feedback loop in the preamp ala m_e_s_a. Any opinions or alternatives?

Also, is the ubiquitous soldano/rectum/5150 lead channel still considered the best for massive distortion? What about bogner, VHT etc. I have not really seen schems of these.

thanks,

Craig

7/11/2004 2:46 AM

I built one of the 1 watt self split 12au7 amplifiers of which there are many variations. I immediately noticed that it had a tremendous amount of sustain, most likely due to the use of cathode biasing throughout the amp. I also think this is why a fender tweed deluxe sustains like it does even when played clean.

So how do you get sustain? Add cathode bias to everything. Cathode bias enhances sustain because the cathode resistor is in series with the current going through the tube. When you first hit your guitar strings, a high level signal is sent to the amp which induces high current through the cathode which in turn causes a large voltage drop across the cathode resistor. As the signal begins to die, less cathode current flows which in turn reduces the voltage drop across the cathode resistor. The net result is greater sustain.

The resistance through a tube recifier works the same way thereby also enhancing sustain. In general, the higher the value of cathode resistor or the higher the IR drop generated by the tube recitifier, the greater the sustain.

7/11/2004 4:23 PM
Bruce /Mission Amps	Hmmm.... I guess those cathode bypass caps are worthless then? Bruce

7/11/2004 8:44 PM
jbrew73	bruce i know what the bypass caps do but how exactly does this affect the sustain? another thing thats on my mind. does the note actually ring out longer or is it just the same volume for a longer time(kinda compressed).

7/11/2004 9:00 PM

Bruce /Mission Amps

As long as the cathode is higher voltag ethen the grid and lower voltage then the plate, the resistor's value is only for DC bias.
In this application, the bypass capacitor is what's allowing AC current to fluctuate through the power tube with no impedance by the biasing resistor.... if the cap is big enough in value, with the right amount of low Z reactance, it's supposed to completely bypass current around the resistor, so, the power tube's idle current level stays the same while the AC current "rotates" around it.
I think, as the power supply B+ voltage drops from more current delivered in and out of the OT, the amount of plate B+, screen B+ and the relative DC bias changes in the power tube, creating the uncompressed, compressed or tonal changes.
To really hear these things, try the same amp with no bypass cap and try to ignore the gain boosting from the cap in vs out.
In other words, pick a volume level that the non-bypassed version and bypassed version are the same power output and experiment with small value caps to big value caps.
Bruce

7/11/2004 9:12 PM

Bruce /Mission Amps

oops... forgot the other part... which is the same thing really.
The cap will "try" to charge up to the peak AC voltage, changing the apparent bias voltage, but the cathode biasing resistor's low DCR will drain it off fairly fast so the end result is that the bias voltage is slightly leading and dragging what you are actually playing.
The tube acts like it's trying to make slightly more or less power because of it's "bias level", relative to it's plate voltage... but the circuit as a whole can't quite do it, nor quite keep up because of the RC time constant on the cathode biasing resistor and cathode resistor bypass cap.
Since the B+ on the tube and it's screen is up and down and the cathode is held above ground and has a capacitor that is trying to charge up but drain off pretty fast through that "biasing resistor", it's a little out of synch with the B+ rail.
Try that setup with a DC voltmeter on the cathode and a steady single tone signal and you'll see.... try it with guitar signal and watch what happens.
Do the same test with an AC meter and look at the difference.
Bruce

7/12/2004 2:39 AM

You will get more sustain without a cathode bypass cap (at the expense of less overall output). Also, since current variation through the cathode resistor is highest in the later stages of the amp, the effect I described will be most pronounced in these later stages. For example, most self split 12au7 amps I've seen do not use cathode bypass caps in the output section which explain a lot of why they sound the way they do.

Also, the circuit for the legendary Trainwreck Express is an interesting take on this. It has three pre-amp stages before hitting the phase splitter. The first two stages are just the same old cathode biased stages we see over and over again with bypass resistors in the range of 1500-2200 ohms bypassed by a 22uf electrolytic.

It's the third pre-amp stage that is truely the most interesting. Suddenly the cathode bypass resistor jumps to 10,000 ohms and is unbypassed. It's clear this stage was set up this way to maximize sustain through cathode bypass compression. Without a bypass cap the compression effect is maximized. Furthermore the resistor value is extraordinarliy high to further enhance the IR drop across the resistor thereby further enhancing the compression. And finally the stage is late enough in the cascade chain to have lot's of current variation flowing through the cathode resistor.

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