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

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carlsbad2's avatar
carlsbad2
1,384 posts
 

To use a multimeter in the current mode it must be in series.  voltage probes are in parallel.  very few people have ever used a multimeter in current mode.

 

Bias currents are very seldom set in the current mode.  Generally there is a 1 ohm resistor in the circuit so if you read millivolts across the resistor, that equates to milliamps.  very convenient.  

Sounds like this one wants you to put the meter in the circuit.  Is there a jumper you remove and put the meter in it's place?

Ok so with all of that discussion, my point is that most likely you don't have the meter set up correctly.

You imply that you get sound out of the amp.  You would get no sound if bias was really at zero.  

So if you're getting sound, then the bias is not zero and you have a meter problem.

Jerry

 

Post removed 
inna
7,141 posts
 

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

newbee
4,751 posts
 

Not so much, but it is always nice. Same brand though, not mixed.

inna
7,141 posts
 

Thank you, that's what I thought.

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