mino
New Member
Posts: 8
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Post by mino on Jul 3, 2019 7:37:19 GMT -7
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?
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Post by agsabq on Jul 6, 2019 8:36:58 GMT -7
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.
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mino
New Member
Posts: 8
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Post by mino on Jul 8, 2019 11:26:30 GMT -7
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.
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