Dynaco ST-70 Tube Amplifier

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Dynaco ST-70 stereo tube amp At the moment, I cannot accept Paypal, but am required to list it as an electronic payment option This unit is one of the original Philadelphia based kits. The chassis is original and has the early version 3 screws per side to secure the base. Original power transformer meet all specs New output transformers New Triode driver board - built to original specs, with 1% tolerance resistors, 2uF coupling caps, NOS 7199 Curcio capacitor board upgrade - using diode rectification 4 new JJ EL34 tubes Unit sound is excellent Quad cap included for appearance, is not secured and can be used or not Van Alstine improvements per 1982 Audio Basics Article is what was done - Frank Van Alstine is one of the few people around that know this amp as well or better than the originatorAnother common problem with vacuum tube amplifiers is the value chosen for the interstage coupling capacitors. In the case of the Dynaco St-70 for example (see attached schematic) coupling capacitors C10 and C11 are 0.1 uF. This introduces another large low frequency roll-off within the feedback loop. Since the amplifier actually amplifies the difference between input and feedback, and since the feedback is the difference between input and feedback, and since the feedback is taken off the output of the amplifier, at low frequencies the difference signal becomes very large partially due to the roll-off caused by the 0.1 uF coupling capacitors. Now remember that we have shown that any circuit becomes less linear with increasing amplitude and at the frequency extremes. The roll-off caused by the small value interstage coupler makes the front end work very hard to generate a large low frequency correction signal. This causes the front end to run in a very nonlinear mode at low frequencies. You hear it as "muddy bass." The "cure" is quite simple, make the interstage capacitor large enough in value so that the loop roll-off is minimized, thus reducing the correction required, and letting the front end run in a more linear mode. The low frequency correction signal is easy to see on an oscilloscope. Using a low frequency square wave as a source (20 Hz is fine) look at the signal on the output side of the interstage coupler. Note that it looks much like the input signal. Now look at the signal on the input side of the coupler. You will find the circuit is generating a signal with a large bass boost! (This is true in most tube preamps too!) What is happening is that the "flat" input signal is rolled off by the interstage coupling capacitor. Then the rolled off signal is fed back to the input and a correction signal is generated with a large bass boost to make up for the roll-off. The boosted signal is then rolled off again by the coupling capacitor and its output looks just fine. But the "monkey motion" has ruined the voltage amplifier's linearity at low frequencies.Now let's look at what is wrong with the original Dyna St-70 in detail. Refer to the audio channel schematic again, keeping in mind that the "dashed" section is our addition, the original has the input connected to V2 directly with a piece of wire.What we have is a typical vacuum tube amplifier with unlimited bandwidth input acceptance (DC coupled) but with limited bandwidth output transformers and small interstage coupling capacitors. The power supply is also limited bandwidth, being pretty feeble at both low and high frequencies.A very low frequency signal is rolled off by the interstage coupling capacitors, turned into a "lump" as the output transformer core saturates, and is further distorted as the power supply runs out of steam. The feedback signal, being taken off after all the disasters occur is very different from the input signal. This generates an enormous "difference" signal which drives the front end into 100% distortion trying to make an impossible correction. Inasmuch as the circuits are underdamped too, the "blob" makes the amp ring for a few cycles attempting to digest... read more

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