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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
whart's avatar
whart
3,037 posts
 

Maybe somebody with real technical chops can correct me, but I don't even bother to check bias when I first turn my main system amps on-- Lamm ML2s. I run them for 45 minutes to warm them up and may check bias at that point, using a Fluke meter, but often, let them play in for a while longer--usually three LP sides after the 45 minute warm up. One amp is always rock solid; the other varies by a few increments. It used to bug me out, but as long as it isn't red plating, I'm good.

One thing I found- I also have 12ax7s on the front of the amps- and the "pulls" typically varied. Getting actual never used old stock made the amps far more stable. Go figure. 

newbee
4,767 posts
 

Whart, FWIW that works when you are starting up an amp wherein the tubes have already been biased, but for putting in new tubes you start with 0 bias and work upward 'til you reach the appropriate bias needed.

jond's avatar
jond
5,807 posts
 

Graham does the amp work? Have you tried playing music through it?

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