6N9S / 6SL7 and DC heaters - diyAudio
Go Back   Home > Forums > Blogs > triode_al
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Rate this Entry

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.
Posted in Uncategorized
Views 1840 Comments 0
Total Comments 0


Hide this!Advertise here!

New To Site? Need Help?

All times are GMT. The time now is 02:37 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2