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The idea presented here has not been tested, nor is it for the novice. It is an audio thought experiment. If you do build it, please send us your results.
Silk Purse Dynaco ST-70
The Dynaco Stereo 70 must be the most popular tube amplifier in the world. It is like the old venerable Volkswagen Bug: reliable, plentiful, and easily modified. At first I was going to add cheap to the list of virtues, but ST-70's have risen in price recently. In fact, I heard that completely stock units fetch much higher prices than modified pieces. So if you undertake this particular mod, or any for that matter, understand that you might lessen the resale value of your amplifier. Having said that, I think the there are better investments than audio gear, for example classic cars or even Beanie Babies, and the mod described here requires no alteration of the metalwork or PC board; in other words, a retreat is possible.
What is all the rage today? Para-feed single-ended amplifiers is the answer and the proposal for our trans-formation of the lowly ST-70. The conversion would be fairly easy and just might sound better than most of the high priced gear available today.
ST-70's have been converted to SE operation by re-stacking the transformer. This the process of pulling all the laminations apart and reconfiguring them so that all E's are stacked into a pile as are all the I's. Then the transformer is reassembled with a piece of wax paper separating E's and I's. The result is a transformer with an air-gap that will serve to prevent the transformer from saturating from the unbalanced current flowing through the primary. This is not for the faint hearted, as the laminations are glued together and are easily bent and will rust if contaminated by finger oil. Additionally, the air-gap requires some tweaking. Too little of an air gap and, while the inductance will be large, the maximum idle current will be very limited, i.e. low power output. Too much of an air gap and the maximum idle current will be comfortably high, but the inductance will be very limited, i.e poor low frequency response.
The advantage of the para-feed arrangement lies in the use of a conventional interleaved output transformer. In a para-feed SE amplifier the transformer is capacitively coupled to plate of the output tube; therefore, the transformer sees no unidirectional current flow and consequently it will not become saturated in use. A choke is used to give the output tube a current path to the power supply voltage. The advantage of the choke is that it buffers the output tube from power supply perturbations and it allows for voltage swings beyond the B+ voltage. The disadvantages to the choke are that it must be large and as high quality in design and
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