Not so much, but it is always nice. Same brand though, not mixed.
Tube amplifier - tube bias and time to warm up
Power Amplifier
I have a Cary Rocket 88R tube power amp, it is not an integrated, power amp only, but my questions are pretty basic.
The specs for biasing my amp are 195-220 Milliamps.
Currently it has:
- Quad set of KT88's
- Pair of EL84's for the voltage
- Pair of 12AX7's for the preamp drivers
Status
All tubes light up.
No tubes are glowing hot.
All fuses (2) are working.
When I power up my amp to do the tube biasing, I'm not getting any milliamp reading on my Multi Meter. Am reluctant to leave amp on after 30 seconds with no milliamps showing. Worried I'll blow something.
I get a rise up to about 40 Milliamps when I turn the amp off. But nothing within 30 seconds when it is on. The Bias LED's are lit, which indicates that there should be bias current available.
When I remove the Bias headphone jack, I do not get any sound out of the amp.
All tubes light up - but no sound. But again, I'm shutting the amp off after 30 seconds for worry that I don't have the Bias set properly and I could damage it.
I've had the amp in the shop months ago and I don't recall how long it takes to warm up to get a bias read, and long enough to put out sound.
Questions:
- Should I have some reading on my meter after 15-20 seconds?
- How long before I should have some Milliamps showing?
- How long before I should have music coming out?
- Could a bad tube cause this situation?
Thanks for any thoughts.
Graham
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- 17 posts total
Matching is not nearly as critical in that case, but still good to have. A bias reading is measured at just one point (idle), and you want the push / pull tubes to behave symmetrically throughout their operation range. Of course your VAC does it the "right" way - having individually biased tubes. VAC's continuous individual auto-bias since their iQ series is absolutely wonderful - it accounts for tube drift over its lifetime and even during operation. |
@inna Yes! Its one thing to set the bias point, which defines the operating point of the tube. But the tube has a dynamic operating curve; ideally each of the tubes have matching curves to minimize distortion at all power levels; IOW they are working perfectly in tandem all the time. This is true even if the power tubes are biased class A1. To do this properly, the tubes are matched on something called a curve tracer. They can be created using a computer platform, like this one, which is a kit that allows you to create a very accurate test platform. There are a number of kits and construction articles on how to build one for yourself. One that is already built is likely going to be a couple of grand$. |
Ralph, I see. I think, Brent from Audio Tubes does this kind of matching for extra charge. I was somewhat surprised that VAC Avatar's manual says nothing about tube matching, even output tubes matching. mulveling, this continuous auto biasing, is it done only for sound quality or also for tube longevity ? |
- 17 posts total