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Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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Old 17th February 2013, 08:37 PM   #91
DF96 is offline DF96  England
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3k is fine if that is the best that can be achieved: high anode resistor from a lowish supply rail. Given an active load we can do better by using a bit more current, so smaller Rk. Ideally approaching 1mA as that is where the mu curve becomes flat. The fact that the wrong value for the upper grid resistor gives less distortion (in a simulation?) suggests that the valves are running non-linear, which means the wrong bias point is being used.
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Old 18th February 2013, 07:36 PM   #92
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Hello from Salem MA.

There are at least one thing wrong with using a vacuum tube as a load in an amplifier.
My number one objection is that unlike transistors, indirectly heated vacuum tubes normally have a heater and a cathode. I know that the manufacturers spec is that there can be a significant voltage differential between the heater-coil and the cathode tube.

The way the cathode is constructed is: A small cylinder of nickel has a folded thin wire of usually tungsten inside. This tungsten wire has an insulating coating so that there effectively is an insulating barrier between the heater and the cathode.

This heater / cathode barrier represented a significant technological challenge in production. I learned from my Dad, who was an expert at these things that for a reliable design you do not apply more than a few volts between the cathode and the heater.

There is no realistic way of preventing a noisy current to flow between the actual cathode and the heater wire. If this voltage is much more than about 20V there will be a current flowing between the heater and the cathode terminal.

I know that in indirectly heated rectifiers the heater to cathode voltage is permitted to be as large as 450V DC. that is OK for a rectifier tube, because the power-supply is normally bypassed well.

In an application where we can expect relatively low signal voltages the unintended current between cathode and filament may become a major source of noise and signal.

I therefore firmly believe that using a tube as a load in a high quality preamp is counter-indicated as my patent lawyer would say.

Specifically, if the heater DC voltage is negative with respect to the cathode, the heater wire will emit electrons that lands on the cathode, this is going to be a noisy current because the heater is not designed to be an emitter, it just is.

Conversely, if the heater is more positive than the cathode, the heater will become the anode and again a noisy current flows.

This heater / cathode current-flow is not something new, but may resurface again because of lost knowledge. In other words: keep the differential; voltage between the heater and cathode as low a possible, and worry about the noise later.

Resistor load: no problem, tube load will become noisy with time.

Hans J Weedon.
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Old 18th February 2013, 09:25 PM   #93
DF96 is offline DF96  England
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All the people who have looked into this seem to agree that lowest noise and longest valve life are given by making the heater positive, not zero as you suggest. The optimum appears to be about +40V, but it is a broad curve so the exact value is not too important. It does not act as an anode with respect to the cathode because the active cathode surface does not face the heater.

Active loads and cathode followers have been used without problems for a long time. At the output of an active load the signal has received significant amplification so noise will not be a huge issue anyway.

One proviso: it is said (by some) that NOS valves may have better heater-cathode insulation than modern valves.
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Old 20th February 2013, 07:03 PM   #94
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Well, with this in mind and max. amplitude: No problem... ;-)
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Old 20th February 2013, 09:53 PM   #95
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Hi there.

Thinking back many years, I may very well be wrong in claiming the there should be no DC voltage between the Cathode and the Filament in a vacuum tube amplifier. Sorry about that. I remember back to the 4+1 Audio amplifier published by Phillips and Mullard in the 1950s. It used an EF86 pentode input amplifier and an ECC83 as phase inverter, driving a pair of EL86s in push pull.

Since the phase inverter was a long tailed pair the cathodes were biased up some 35V and the recommended DC bias for the heater centertap was indeed 40V. Under those bias conditions the filament in the indirectly heated cathode was always biased positive with respect to the nickel tube used as a cathode. My guess is that since the heater was hotter than the nickel cathode, no noisy emission current could flow from the heater to the cathode, and the cooler nickel cathode did not emit enough current to worry about.

I stand corrected. Thank you very much. When I am wrong, I am wrong.

Hans J Weedon

Salem MA.
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