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Did I blow my tube amp?

So here's the story: I recently received my tube amp and replaced the power tubes. So naturally, I wanted to bias them to make sure they work properly. I've never done any biasing before and checked some youtube videos on biasing. One of the youtube video had a Manley Stingray connected to power, no input signal, and NO speakers connected. I did the same. After the biasing is done, I connected the speakers and input signal, but for some reason so sound came out.

So the question is: did I blow my output transformer? Simply by not connecting the speakers while biasing or just having the amp on for a short while? From what I have read, it should be ok to have no speakers connected as long as there's no input signal.

I had another tube amp before this, and turned it on one time without speakers connected and it worked flawlessly after.

Could you guys shed some knowledge or personal experience on this topic?
vu001
jea48
4,451 posts
 
Though I have never powered up a tube amp without the speakers connected this thread has made me think, WHAT IF?.... What if a speaker cable spade come lose because I was changing ICs or what ever? How many here always check the speaker cable terminations before turning on the amp?

I read the owners manual for my ARC tube amp cover to cover twice.... No mention whats so ever.... No WARNING.... Nothing about not running the amp without the speakers connected. I know better.... But what if?

I am going to give ARC service a call and ask, WHAT IF!

Cary Audio.

How do you correctly bias a Cary Audio vacuum tube amplifier?

Disconnect the speaker cables and the input RCA cable or XLR cable from the input jack.
Start with the amplifier warmed up for 5-10 minutes and then turn it off to connect the bias cable.
Connect the provided Bias Cord into the bias connection jack. It has a 1/4 inch mono 'phone plug' on one end and a pair of alligator clips on the other end. Plug the phone plug into the jack on the chassis. The red alligator clip is for positive electrical connections, the black one is for negative electrical connections on the milli ampere (mA) meter.
Set your meter to the milli amperes (mA) reading range. NOT to the Milli Volts (mV) range!
Turn the bias adjustment screw on the chassis all the way counterclockwise so that the meter will read zero when you turn the power on. Now, turn the AC power on. Wait for 60 seconds to warm up the tubes.
SLOWLY turn the bias adjustment screw clockwise while you watch the meter for a mA reading.
Any version Rocket 88, for example, should be set to 175-200 milliamperes with 160-240 range mA setting range OK. Any version V12 should be 250-275 mA with 220-330 overall as the possible mA setting range. Let the amplifier run without an input signal for another 5-10 minutes and check it again. Readjust the bias setting again, if necessary. New output tubes should be checked a couple of times in the first two weeks as they 'burn in' to their normal operating mode. After that, check them every 3 months just to be sure all is well.

Turn off the amplifier, reconnect the speaker cables and the RCA or XLR input cable, turn on the amplifier and enjoy the music!

NOTE: Some other tube amplifier designs will oscillate or go into overload if they are used without a speaker or resistive load attached to the speaker output terminals .Cary Audio Design tube amplifiers are inherently STABLE DESIGNS and may be operated without a load on the output terminals for adjusting bias or while in burn in.
larryi
4,035 posts
 
I have, on several occasions, turned on my amps and noticed that one channel was not working because a spade has worked itself loose. Nothing has ever happened.

I never start a listening session with the preamp turned up very much, so I do not subject the amp to high level output into such a no load situation. I know it is not a good thing to do, but, I have never, nor has anyone I know, experienced damage from operating without a load attached.
dividebytube
50 posts
 
Things to check:

1 -Do the tubes still light up? If not, check the fuse.

2 - Check connections - both input and output. We all do silly stuff, like the time I plugged the speaker wires in to the 4-ohm and 8-ohm tap, hence no sound!

3) if you have the amp on - with the speakers plugged in - do you still get a bias reading? If so, that indicates that power is reaching the tubes.

4) I've been messing with tube amps for over 20 years and have never, ever blown an output transformer. There may be something else that went bad inside. If you don't have experience working with high voltage, then have a tech take a look for anything obvious. Luckily most tube amps are fairly easy to troubleshoot.
truemaineiac's avatar
truemaineiac
249 posts
 
It would be great to know what the outcome of this is.
vu001 OP
3 posts
 
So after taking the amp to an experienced tube technician, the problem was that 2 of the output tubes went bad, and took out a fuse and fried 4 resistors. On the outside, everything seemed normal since the amp turns on and all tubes light up as usual, but the 2 dying tubes were causing problems behind the scene.

I had a feeling those 2 tubes were going bad and bought replacements, but before I could bias the new tubes, the main fuse blew, cutting off all outputs, causing me to think my biasing procedure damaged the amp.

The amp is now fixed and working better than before. A few things I learned from this experience. If you buy a tube amp second hand, always re-bias the tubes right away even if the previous owner told you he already did. It IS actually safe to bias your tubes without speakers connected on the condition that no inputs is connected either.
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