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adding a emitter resistor, as little as 0.22 ohms, prevents current hogging. In fact, many ICs use emitter resistors, for example the LM317 uses 5 paralleled current mirror PNP transistors, each with its own emitter resistor; the LF351 uses both styles of current mirrors and the resistors used are 2100 ohms in value. Basically, the bigger the resistor value, the less we have to worry.
Subject: Heater Concerns (Jan 2000) & Screen driven tetrodes.
Nice article on ultra-linear mode. Any chance following on with an article on screen-driven tetrodes?
In the same issue a good solution to diagnosing a single faulty heater in a series string is raised. However, I think the solution should mention a resistor in series with each neon bulb, as the striking voltage of most of these bulbs is 80v (approx.) with 60-70 uA operating current. Without a current limiting resistor (and the heater string isn't likely to present the correct resistance, especially when cold), the neon will draw considerable current and may even be damaged, thus negating any diagnostic function. This is of course more of an issue here in Australia where the mains supply is 240 (and often 250) VAC.
Following on from this idea, I install a neon bulb with appropriate resistor on the highest B+ rail on amps I build or repair. It saves guessing when the caps have discharged (even with 220k across them it can still take a while), or laboriously checking every couple of minutes with a DMM.
Steven
Thank you for the tip about the current limiting resistors for the neons.
Had I actually built the amplifier, I would
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certainly have included the resistors. You see this brings up the issue of just how much detail to include in an article. Too little, and a reader will find that in practice the circuit will not work, for example it is a common in the literature to omit the cathode bias resistor from a circuit's drawing, so as to get to the essentials. Of course, this circuit will not work as outlined with the resistor. At the other extreme, too much detail will obscure what is essential.
I remember showing a friend a schematic of a preamp I had just built. He was pleased to find it so straightforward and he said to me, "This is great, this is so simple even I could easily build it." Later, when he saw the actual preamp he was disheartened. It looked magnitudes too difficult for him to build. What had been missing from the schematic was all the small supporting circuitry and embellishments, such as thermistors in series with the heater leads, a snubber network across the choke, protection diodes, many small bypass capacitors, lots of wire, switches, muting circuit and relays, and a very complex high voltage regulator.
Having said all of this, you are right, when there a chance that an unsafe condition or damage to a device might occur, it is best to provide all the details.
Subject: 600 ohm drive
Hello,
Wonderful site! Your really covering information not available elsewhere. Silicon/tube combined designs, OTL's, all sorts of great stuff.
Could you do an article about preamp output stages, especially for driving 600 ohm balanced lines? I'm working on a balanced preamp with this capability and could use some help.
-Dan
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