Please pass this info along if you see someone post about a related issue, i think this issue is more common than we think and is being overlooked. Thanks MI Ever have a problem Biasing your Power Tubes? Maybe the pot is maxed out and your bias is too low or you set the bias at an average setting only to come back later and see that the bias is now running hot? You might get on this forum and people are quick to tell you that you need to change a resistor etc. But a recent experience of mine has revealed a new source of this problem. Line voltage fluctuations outside what is considered normal. In other words your outlets in your home are fluctuating voltage more than is deemed "normal". I have a brand new 100w Marshall DSL100HR that kept going through the V5 power tube. The tubes would sometimes bias high enough and other times wouldnt. I have the bias set between average an hot at about 78mv a side. Considering what my plate voltage had been reading, 78mv was closer to average than hot. And at times i had the bias even lower and still had tubes go bad. So it turns out my line voltage in the am is about 118v but in the daytime it has dropped as low as 100v. It never made sense that i needed to change a resistor cause this is a new amp using the tube type (EL34's) it was designed for. So this is what was happening. Id set the bias during the day when my line voltage was likely low say 105v and id have a hard time getting the bias up to my preferred 78mv so id settle on the highest i could get them while still being matched. Sometimes id have to roll the tubes to get them to bias higher. So then my bias pots would be maxed out. Then the next morning before work id go power up my amp for my morning practice routine and at this time of day my voltage is closer to the 120v i should be seeing. Because of this my bias has now skyrocketed and im marching my Power tubes to their early demise. CHECK YOUR LINE LEVEL AT SEVERAL TIMES THROUGHOUT THE DAY. This problem went over my head cause the few times i checked the outlets in my home they were "close enough" its only when you zoom out and look at the bigger picture that what was happening becomes clear. In my case i believe it could be a few things causing above average voltage fluctuations but after checking the breaker box voltage i believe my next move is to call the power company and have them come out an see if this is an issue on their end (EDIT: It was an issue on their end, they said they'd fix it but my voltage at the box is still too low. I found a solution though and I posted about it. And I won't cost you $1000, more like $35. It's the opti ups 2000 regulator.). If not ill have to track down where this power is getting lost. In this case i believe its a power company problem not a wiring in my home problem. Anyways this info should be suggested alongside the "you should lower the bias resistor" advice. If the amp is new and the tubes being used are what the amp was designed to handle you shouldnt need to change a resistor. That advice is likely just so common its being suggested without any consideration to the fact that a new amp not biasing high enough is likely not due to a resistor. If your amp is new and your using good tubes that are the type the amp was designed for you should never have to change a resistor or a bias pot(infact doing so when voltage fluctuations are your problem would just make things much worse as your bias would then be jumping even higher when your line voltage goes back to normal). Anyways, please pass this info along so others in this situation might avoid burning through power tubes as i did or worse! -M. Inglis Edit: This has fixed my voltage problem and keeps my voltage much much closer to 120v. It boosts your voltage if the line voltage goes below 116v (on my unit, this may vary by +/- 1v). So when my line voltage is at 116v or below the regulator boosts it by about 10v. And if your voltage ever gets too high it will do the same thing. This unit is more precise than the more expensive units but it won't work if your line voltage drops dramiticaly(under 100v for example). More expensive units like a Tripp Lite can handle greater fluctuations but won't regulate until your line voltage is below 102v so it requires greater fluctuations to be useful. Anyways I'll post the specs for this unit now since I've already left a comment describing the pros an cons in detail in another thread of mine. Last thing I'll say is this unit is exactly what I was looking for an so far it works great, many not as good of quality as others but there's nothing else in this price range that actually works.