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6N9S / 6SL7 and DC heaters

Posted 18th May 2012 at 03:03 PM by triode_al

I have been reworking my RIAA preamp the past weeks - main effort to install a SALAS shunt in the HV.
I had some problems looking like instability of the DC. I also noted that every on/off cycle, the problem got worse. And while developing, I would switch the preamp on an off often, to see the effect of the tweaks, such as setting the shunt cuirrent.

Later I found that one 6N9S tube was 'dying', really, it went black, but intermittent.
The heater wires probably had been stretched a bit and in cold state they made some contact (measured 3-4 ohm) but when heated up it lost contact. The effect cascaded into the Vb, and was seen on both outputs similarly.

I conclude The [Russian, Reflektor brand, big logo] tube is very sensitive to the heater.

I had the two tubes running in series from a medium impedance 12 V source (IRF250, gate connected to drain via 100k resistor and a 300 uF cap: relatively slow start of 2 secs. But too fast probably for these tubes in series.

So I went and made a new circuit that had a very slow start and minimised the ripple to zero. It has two stages.

First stage: a shunt based on TL431 and the TIP30C PNP, 1k1 anode resistor, reference with 1k/4k=12v5. The source resistor is 5 ohms. The total current is 550 mA; a tube takes some 300 mA (6N9S) - 400 mA (ECC35).

Second stage: a series pass device that makes a very slow start. It is based on a BUZ71 N-MOSFET, that has the gate voltage rising very slowly.
  1. Biasing: From the +12v5 I have a dropper with 68k/40k. Then a RC to the gate: 510k feeds 47 uF. This exhibits a very slow start cycle.
  2. But most of the time the device is off: and the transition to on would be too fast to improve anything over my IRF250 (that I use for the other buffer stages). The trick I have, is I by-pass the FET with a 68 ohm resistor. It allows 150 mA of current during 5 seconds before the FET turns on, in that time the tubes are luke warm, so there is no subsequent turn-on thump.
  3. The heater is connected between the collector of the TIP30C and the drain of the BUZ71A.

The residual noise is very far away, just a hint of hiss and some 50 Hz (transformers are not in a separate case).

I tried my preamp with a Philips brown base ECC35, and man, does that shine. The 6N9S has some distortion in the highs. Maybe it will improve as they burn-in.
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