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  


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. 


  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. 


Just speaking generally on push-pull tube amps: 0 mA is perfectly safe, unless the bias reading mechanism is severed (very unlikely) and it’s not actually sitting at 0 mA. In any case, you would see the KT88 plates start to glow orange (usually a large oval patch) and then cherry red as your final warning to quickly SHUT DOWN the amp.

The bias comes up slowly, and you should start to see some reading after a minute or 2. It will increase gradually from there before hitting stability (after ~ 10 minutes as per the manual). But looking for something at 15 - 20 seconds is way way too early.

I’d give it a few minutes, then start adjusting the screw to increase bias until you see a non-zero reading. If you go all the way to the spec’d mA bias early on, it will get too hot later on. So start low and gradually increase. Just keep checking and adjusting (iterate) so it doesn’t run away. You’re a human servo lol. If you’re in trouble you will see the plates glow as your warning. After 10 - 15 minutes do a final adjustment and enjoy.

If it’s using a trimpot you don’t want to overuse that mechanism, so don’t make biasing a daily or even weekly occurrence.

I scanned through the manual and it looks like there's just 1 bias adjustment / reading for all 4 KT88 tubes (cumulative). That probably means you need to stick to tightly matched quads. 

I agree with mulveing, Check your manual and you will probably find that it has  automatic muting on turn on to protect it tubes. With my amps I let them warm up for a couple of minutes, then set the bias, let them run for about 20 minutes and check/reset the bias as necessary. Then I check the bias the following day after running the amp an hour or so to make sure the tubes hold their bias. If they do I check the bias about every three months.

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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.



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

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

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. 

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

@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 ?

I was somewhat surprised that VAC Avatar's manual says nothing about tube matching, even output tubes matching. 

@inna  VAC does recommend sourcing replacement tubes from themselves - which would include the necessary matching and low noise screening for your amp. 

mulveling, this continuous auto biasing, is it done only for sound quality or also for tube longevity ?

VAC's stance is a 3-way benefit: sound quality, tube life, and reliability / fail safes. I can only say that as a user of iQ amps for 4 years, they've been 100% reliable, with no incidents, and never any worry nor need to recheck bias.

If a tube looks like it needs replacement soon, a front panel LED corresponding to that tube is supposed to light up. If it starts to go REALLY bad, a different LED color lights up and the amp shuts off. I haven't had any LEDs light up on me yet. 

I've compared the 200iQ directly to its predecessor Phi 200 (without auto bias) and yeah, the iQ absolutely sounds better. Since this year, I've been running Master 300iQ.

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.

STATUS: 11-10-23
I have left the amp on for over 3 minutes and still 00.00 on the Milliamp setting. All tubes light up, all seem about the normal brightness, so now I am now going to try a different multimeter. 


Thank you all so much for your detailed responses. The response was far more than I ever expected. Content and detail is exceptional. 
Thank you all! 

I will continue to follow and conclude this thread. 

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. 

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.