2. amps-preamps
  3. 187018-186107

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:

  1. Should I have some reading on my meter after 15-20 seconds?   
  2. How long before I should have some Milliamps showing?
  3. How long before I should have music coming out? 
  4. Could a bad tube cause this situation?


Thanks for any thoughts. 
Graham

128x128regenav

Showing 4 responses by inna

inna
7,170 posts
 

Thank you, that's what I thought.

inna
7,170 posts
 

Is matching output tubes necessary if I can adjust bias for each of four tubes ?

inna
7,170 posts
 

It is tempting to experiment with this, to buy a few brands of totally unmatched tubes, mix them randomly and see what happens, see if I can hear the difference with perfectly matched same brand quad. Judging by what you all say, I should be able to hear the difference.

In my conversation with Kevin about tubes he said absolutely nothing that could be interpreted that he recommended getting replacement output tubes from him. He kind of suggested getting 12AX7 tubes from him but not 12AU7 or EL34. It was an excellent conversation, by the way, I liked Kevin.

inna
7,170 posts
 

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 ?

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