Third step: Design details

Looking at the plate curves for the ECC88, we see that with 90V on the plate, a cathode-to-grid voltage of 1.7V will give us the desired 10mA minimum current. The significance of this choice will be clear later. Nonetheless, this is a common spot to run these tubes and for good reason: it’s a low distortion point which doesn’t drive the plate dissipation too high. The tradeoff in choosing a lowish plate voltage is the limitation on swing, but that’s not an issue for our requirements here.

The volume control is arbitrarily set to 100Kohm because that’s an easy value to source and I happen to have some Alps Black Beauties in that value. I will not sneer if you use 50K or 250K instead. A stepped attenuator would be lovely here. Whatever you choose, you will need to parallel it with whatever resistance is necessary to load the transformer secondary with 10K. If you’ve got a high quality 10k attenuator in hand (15k if you’re using the Cinemag), you’re set!

Note that the grid is at DC ground.

The bipolar current source may be done several different ways: I’ve used single bipolars, ring-of-two, and cascodes. Single bipolars have mediocre performance. Ring-of-two is a good choice, but I’ve had one oscillate on me. So in a fit of pique, they have been sent down to the minors. And I sent them down because the cascode is a perfectly good alternative and has behaved well for me. This particular cascode will be powered by a +/-12 volt supply, a complication which benefits us slightly here and a great deal in the next couple of implementations. The -12V supply allows the tube to easily swing the required 6-ish volts peak-to-peak while allowing for sufficient voltage to keep the current source in compliance.

The easy bits are the choice of grid stopper and cathode stopper resistors. They tend to be non-critical (sometimes even omitted entirely, as they are in the simplified schematic above) and usually picked by resorting to tradition and experience. I will not omit them because I want this preamp to be rock-stable. The grid stopper can typically be between 1K and 10K. Since I have a box full of tiny 1K resistors and the input capacitance is low, that’s the value I use. The output stopper is slightly more problematic since it adds to the preamp’s source impedance, but I’ve never had a problem with 200 ohms in that spot. Half that would no doubt be fine if you’ve got some real drive issues to worry about. The main thing about stoppers is getting them as physically close as possible to the tube pins.

The output coupling cap is chosen to give a low frequency cutoff at least a decade below the lowest frequency of interest with the lowest anticipated load. Using the 10Kohm figure previously bruited about, for a 2 Hz cutoff the output capacitor needs to be 8uF. I have a coffee can full of 4.7uF polycarbonate caps, so a pair of those in parallel will more than suffice.

Let’s look at a few of the remaining values. The cascode current source uses a red LED for a 1.7V reference; these parts have relatively low noise, low impedance, and provide a nice visual indication that all is well in their portion of the circuit. The resistors feeding it are chosen to provide about 5mA of current. The transistors can be any general purpose high beta bipolars. Double checking, the current source will be set to 10mA, so with betas on the order of 100, we will only lose 0.2mA to the transistor base. This will not appreciably disturb the LED string. With a reference voltage of 1.7V, a quick Ohm’s Law check indicates that the emitter resistor will need to be about 100 ohms. A trim-pot of 250-500 ohms will allow plenty of range. If you’re the fussy type, use a 100 ohm trim-pot in series with a 47 ohm fixed resistor. If you’re not and you just want a box you can put in your system and use for the next 20 years, skip the trimmer completely and just use a fixed resistor of 100 ohms and an ECC88 for the tube.

And if you are building this version as a final unit and will not be pursuing the later enhancements, you can get rid of the +12V supply and run the resistor feeding the constant current source to the cathode follower’s anode supply. In this case, to get 5mA flowing, the resistor’s value must be increased to 20K:

The 1M resistor from the volume control wiper to ground is intended to keep things stable in the event the control’s wiper goes bad, a common occurrence. The 15K resistor shunting the control serves to load the transformer with its optimum 10K load.

The circuitry involving the MOSFET and relay is a dethumper- when the preamp is powered up, the output can have some dramatic bangs as voltages come up and the tube heats. This portion shunts all of that to ground until things have had a chance to settle themselves. The parts choices were mostly dictated by stock-on-hand; the 33uF cap is a 35V unit (a 47u will give a bit longer delay), the transistor can be any small MOSFET that will handle the relay current. In my first go, the relay was a 12V unit, so I just used the 12V rail and ground as the endpoints- that’s what’s indicated in this schematic. In another version, I used an IRF510, just because it was under $2 at Radio Shack. In this version, the circuit was connected between the +12 and -12 rails and a 24V relay was used. Either way is equivalent, your choice will depend on what relay you have in your own personal coffee can.

Alternatively, one could use a 555 timer or even a CMOS inverter to fire the relay. This circuitry is optional if you can guarantee that the preamp will not be powered up into a live power amp. This entire circuit is quite crude, so if you want to use something a little more elegant and robust to things like power line bounces, I will not be offended if you jettison my handiwork.

Power supply requirements are fairly nominal- the cathode follower runs a constant current, a lowish one at that, and sports fine power supply rejection. DC ought to be used on the heater, and the +/-12V source can take care of that duty. The series resistors are calculated on the assumption of a 300mA heater; if the tubes you use are 360mA, they will need to be changed to a pair of 100 ohm units.

I’m a crude, simple guy so I used a crude, simple cabinet- a $6.50 black plastic box with an aluminum lid, sourced from Radio Shack. You may wish to do something nicer.

A note on inputs and grounding: the input transformer allows you to drive the preamp via balanced or unbalanced sources. For balanced sources, the two ends of the primary are used as the inverting and noninverting input. For unbalanced sources, the terminal marked “ground” is connected to sources’ ground- the source ground is isolated from the rest of the preamp grounds. That’s the beauty of input transformers. If switch contacts don’t bother you, you can easily use a DPDT switch on the primary side to do polarity inversion. That will give you even more fun things to play with while listening to music.

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