Posted by Keith

June 24, 2015 - 12:57 pm

There is something else you need to be concerned with... the low level stages of the HF-20 run at lower voltages than the driver and output stages, and receive extra filtering. These lower voltages are created by passing the main B+ voltage through an additional series resistor, and to an extra capacitor section (actually two stages of this). The voltage that you have at these points is determined by the values of the series resistors and the current drawn by the tubes connected at those points. If you disconnect the plates of those tubes, the current draw is gone, and the voltage will drift up to the value at the other end of the resistor. Since the different sections of the capacitors are NOT all rated for the same voltage, this may cause you to exceed the voltage ratings on the lower voltage sections. If this happens you will blow the capacitors. To hold that voltage to the intended levels, you NEED to provide some sort of load to ground at those points (to replace the load originally provided by the tube).

Posted by Steve L.

August 09, 2012 - 06:46 am

Hi Bktheking, Sorry that I don't really have much advice on tube brands. However, from our page on testing 7591 tubes, I can generally recommend any good brand of true NOS tubes. Of course, the problem there is being sure that they are new. If it must be a new-manufacture tube, the Electro-Harmonix brand generally did okay.

Posted by bktheking

August 08, 2012 - 11:56 pm

I've now got three units and have a matching pair of outputs so one unit is going to an ak'r. What is the best output tube to run in these?

Posted by Steve L.

July 29, 2012 - 07:20 am

Hi Brad, Actually, the temperature of the transformer will decrease, as the load is reduced. That is because the reduced current dissipates less power in the internal resistances of the transformer. Here is an application note, which discusses the subject: http://www.controlledpwr.com/whitepapers/uktempa1.pdf Thank you for your comments.

Posted by bktheking

July 28, 2012 - 11:33 pm

It was me that made the comment. My understanding is: If a transformer is designed to put out a certain amperage to run a load albeit heater voltage, removing a tube would increase temperature of the transformer due to the fact that the tube isn't present and the amperage that was "supposed" to be used it missing. A user answered the post buy saying a missing tube would decrease the temp of the transformer instead of increase. I've now got a second hf-20 on the way and will try removing the tubes V2 and running the tape out as an input once it's rebuilt. Cheers- Brad

Posted by Steve L.

July 05, 2012 - 02:58 pm

Hi again Wes, I just found the thread on Audio Karma. I see that the concern was for the lack of load on the power transformer, if you remove V2, instead of isolating the pin with heat shrink. Removing the tube, eliminates the 1.9W heater load, which the 12AX7 has. Since the total load on the 6.3V heater winding is normally about 19W, the voltage would not rise significantly, due to the removal of that one, low level tube. In fact it would help alleviate slightly, the fact that the HF-20 power transformer normally runs pretty hot. Thank you for giving me an opportunity to address the concern.