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Checking tube bias

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Quote Originally Posted by microstrip View Post
Quote Originally Posted by Calle_jr View Post
I have the Jadis JA-80 and my friend has VTL MB-750. We don't have the same speakers, and I think that's what makes the biggest difference.
I think you should keep the MB-750. The Jadis are wonderful and I will keep mine forever for the wonderful dynamics they are capable of. The thing with Jadis is that they have extreme spec tolerances. You really have to match tubes every time. If not, caps and things start to melt in them.
A big advantage with the MB-750s is that they measure and calibrate each tube and you even get a warning to replace a tube if it's not proper.
With Jadis, a fuse blows the first time it happens. This fuse is connected to a faint red led light on the back. Unless you see that faint light when it happens, you wont see it again.
I really love the sonics of Jadis power amps, but the practical handling and care can be very annoying. If it wasn't for the sonics, I wouldn't keep them.
I think you are confusing the VTL MB-750 with the VTL Siegfried's. The MB750 have individual bias but we must check tube current individually with a meter. Although it does not need matched tubes I found that it sounded (and measured...) significantly better with well matched tubes.

Practical care of tube amplifiers can be very simple if we have an open mind and like modern measuring gadgets .Every tubophile should own a Bosch PTD-1 to measure his tubes from distance. Point it at the tube anode plates and you immediately know if a tube is defective in two seconds. I also use it to check bias in solid state amplifiers - you point it at the zone of power transistors and can get an idea of any bias unbalance.

Since long my preferred audiophile tools are from Bosch - a distance laser meter with an accuracy of 1.5mm and the contactless thermometer - also very useful to check for heat leaks in the house!
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  1. microstrip's Avatar
    The procedure only detects strong imbalances or dead tubes - it is not intended to check bias accurately!
    My third tool of from Fluke - a Fluke 77 VOM.
  2. c1ferrari's Avatar
    Relevant to individual tube evaluation -- Amplitrex AT1000 and Tektronix 576 Curve Tracer are recommended.
    In situ measurement -- a true-rms reading Fluke is recommended.