| Posted by Dak | April 08, 2015 - 12:39 am |
| Dave, thank you for replying. I guess I should mention that the amp in question is the "RH84 v2" which uses a 22v zener to set the G2 voltage of the el84 tube. I personally don't like using SS devices if at all possible and I already fried one as I was checking voltages when I accidently grounded the meter probe at the G2 tap. If I don't install the zener I would need a 2 watt 2K resistor to set the G2 voltage. Would that resistor be adequate to prevent arcing? cheers, Dak |
| Posted by Dave | April 07, 2015 - 05:37 pm |
| Hi Dak -- Thanks for the good wishes. If your thought is to replace the Screen Stability resistors with a Zener, then the protection will be lost. The idea of the resistors is to actually introduce some impedance into the screen circuit, where as the low impedance of any diode simply cannot achieve that. If you're worried about the negative effects that the added resistance has on screen regulation, then consider a worst case scenrio: True pentode output tubes. Such tubes do not enjoy the grid alignment that beam power tubes employ, and therefore invariably draw more screen current than their beam powered cousins do, all else being equal. Therefore, in a Dyanco Mk II for example (that employs EL34s), the screen current can approach 50 ma when power output reaches a full boil. At 25 ma per screen grid then, a 100 ohm Screen Stability resistor would produce a drop of just 2.5 volts. Even accounting for the AC influence due to the UL connection of this amplifier, this still represents an incremental regulation of <1% which against the regulation of the B+ supply itself, can all but be discounted. At typical use power levels then, the regulation effects of the resistors are nil. In the literally hundreds of folks I have corresponded with regarding these resistors, no one has ever claimed any sonic coloration due to their presence. I hope this helps, and best wishes to you as well! Dave |
| Posted by Dak | April 07, 2015 - 03:06 pm |
| Hi Dave, I hope all is well with you and yours. My question is , if one uses a zener diode to set the G2 voltage, will that zener also prevent or decrease the arcing event? thank you for sharing your tube knowledge with us, best regards, Dak |
| Posted by Dave | December 19, 2014 - 01:43 pm |
| Hi Pete -- Thanks for providing your input! Your experience has mirrored those from nearly all four corners of the globe now, as well as my own experience. Screen Stability resistors are not required in every application, but are cheap insurance, and hardly cause any performance problems when installed where not required. Thanks again for your input, and good luck with all your audio projects! Dave |
| Posted by Pete M. | December 18, 2014 - 04:47 pm |
| For years my ST-70 had occaisionally allowed a tube to 'cherry'. So, I had to replace one after another. Then I read this article and added 470 Ohm, CC resistors to the screen grids. It's been over two years now and asbout 500 hours of running time and I haven't had a tube cherry yet. I now expect the EL34s to simply wear out, not short out. In fact, I have done this to all my tube amps. and all that I repair. Thanks Dave. |
| Posted by Dave | September 15, 2012 - 10:08 am |
| Joseph -- First, I apologize for a way, way late response to your question. I would address it in two parts: 1. A thermistor in the AC line has very little to do with the output tube arcing issue. Oh, it's slower ramp up could conceivably minimize a possible transient trigger, but it ramps up much more quickly than the tube heats up, so its presence in the circuit -- while always a good idea for minimizing peak turn on currents -- will do little for the arcing problem, because it is not addressing the root cause. 2. The idea of adding the relay and timer components is only significant if the delayed application of B+ is important to you. I mention it in this fashion because the idea of cathode stripping in receiving tubes due to the instant application of B+ before a tube has warmed up has all but been shown to be insignificant. It is a valid concern in transmitting applications which involve literally thousands of volts on the electrodes, but not in home entertainment type equipment. Still, I included the discussion of it in my article because there are other reasons folks may wish to delay the application of B+ -- be it from not exceeding the voltage rating of various capacitors in the unit before current draw from the tubes stabilize the operating voltages at their normal values, to simply having a personal preference to delay its application. Apart from the issue of need however, is the fact of the wide spread use of the concept. Therefore, I was primarily concerned to show the effects that delayed switching of B+ can have on tube life if it is not implemented correctly -- and offer a simple solution to show how it can be safely implemented when needed. Ultimately then, the absolute need to delay B+ is rarely required for the health of your tubes. But the inclusion of screen stability resistors is mandatory in many stock vintage circuits that did not originally include them, but do include the operating conditions I outline in the article. The need for their installation is even more imperative then if modern power supply upgrades are installed. Thanks for your interest! Dave |