After many years of playing hard my Z28 I decided to consider changing it's rectifier & o/p tubes. But before doing that I thought of check it bias and found un-expected results with the plate voltage using a Weber Bias Rite - as follows: A = 408 volts B = 407 volts However, the current looks good at 27.6 & 26.7 mA. I recall from somewhere it should be around the 370 to 380 area. So what is the expected plate voltage and would these figures prove that my amp is healthy?
Your plate dissipation is ~11 W (408V x .027 A), which is well within the 14W limit of the JJ 6V6S tube that Dr. Z typically uses on the Z-28, and your tubes are well matched. The last time I checked my Z-28 I had a plate voltage of 381V and plate current of ~25 mA on both tubes. This gives a plate dissipation of 9.5 W, which is about 70% of the rated wattage. Your amp is at 79% of rated the rated wattage, and it's generally not recommended to exceed this value. Dr. Z has a very useful video at in which he measures bias voltage and current starting at the 9:30 mark. Unfortunately, he doesn't show the final plate voltage after adjusting the bias. Hope this helps.
Thanks for your reply. It looks like my amp is biased a little on the hot side. I may consider tweaking it around 24mA for for a little more headroom, but wondering if it would make an audible difference - i.e. playing a mildly over driven riff then boosts into a louder over driven riff - for example at point 3:04 on Times Like These by Foo Fighters (but I put this down to its production). It would be interesting if it could achieve that effect when kicking in a boost pedal or using the guitar volume nob. Its re-assuring watching the QA checks - good vid but it'll be good to know typical plate voltage variations.