The para-feed confuses many people. The output tube is in parallel with the load in both a conventional single-ended amplifier and in a para-feed amplifier. In the first case, the idle and signal current flows through the output transformer; and in the second case, only the signal current flows through the output transformer. In both cases the output transformer sees the same impedance: the rp of the output tube.
The choke load can be replaced with an active current source, but it will require doubling the B+ voltage. Still, the active current source has a few advantages: it presents a much more frequency independent impedance to the output tube and it allows for a safer application of feedback across the amplifier. In May 1999 issue of this journal you will find an article on converting a Dynaco ST-70 to para-feed.
Subject: free transformers
I just thought I would let you know that the "free heater supply" circuit (July 2000) on page 4 doesn't work. When first powered up the tubes are cold, so there is no cathode current to light the heaters. A jump start is needed, then the circuit is self sufficient.
You can (as you mentioned) power the preamp tubes from the cathodes of the output tubes, but the output tubes should be operated in class A or the preamp tubes will get brighter as the volume is turned up.
Using the reactance of a capacitor for filament dropping works, but since the capacitor's impedance drops with increasing frequency, nasty high frequency transients are passed directly to the tubes. I don't know if this is a real problem with tube heaters, but I used to make EPROM erasers with a UV sterilizer bulb from a clothes dryer powered with a series 4 microfarad Mylar cap off of the AC line. Most of the dryer bulbs lived a short unhappy life. The harmonics from an electric drill on the same circuit would make the bulb glow brighter.
What happened to August? Vacation?
Keep up the good work
George
Ouch! You are absolutely right. The heater must be hot before it can conduct. No conduction, no heat; no heat, no conduction. Oh what a tangled web we weave… A jump start circuit is shown below. It is a transistor based current source that only conducts until the capacitor fully charges up; thereafter, it ceases conduction. The zener provides a current limiting point and a discharge path for the electrolytic capacitor at turnoff.
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A better approach to making some use of all that current consumed and heat generated might be to place the heater string across the B+ power supply of the amplifier. Yes, the B+ power supply will have be much more robust, but as the need for a separate heater winding is eliminated, it should not increase the weight of the power transformer. What advantage have we gained by this new arrangement? Choke based power supplies increase in effectiveness when terminated by a low resistance, as the low resistance gives the inductor something to bite on. The 50 ohm load presented by 20 6AS7's heaters strung in series is a very low impedance termination. An added benefit is that cold heater elements have only 15% of their hot resistance. This means that at startup, the B+ voltage will climb more slowly, protecting both the heaters and cathodes. |
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