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?
It is my understanding that you never under any circumstances
power up a tube amp without connecting to your loud speakers.
Yes there is a very good chance you have damaged your amp.
One of the youtube video had a Manley Stingray connected to power, no input signal, and NO speakers connected.
Not sure why the guy did that......
Manual does not say to do it that way.

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?
I don't think so.... Not from just biasing.

Reading from the manual,

Connecting your Stingray.
(1) "You MUST have speakers or (load box) properly connected to the speaker terminals before power is turned on ........."

Reading the manual it says to make sure when connecting the speaker wires to the amp to make sure the ends of the wires do not come in contact with the case of the output transformers.... (Dumb way to build something)
So make sure the speaker wires are not contacting the output transformers.

Double check the input jacks. Make sure interconnects are seated properly.

Post back your results.

You didn't post the model year.....


I am no expert in the matter but my Cary amps are biased with all connections removed as recommended by the manufacturer. Best would be to check with manuf. Or trouble shoot with the user manual.
Did you check the B+ fuse?

When I changed the power tubes in my older VTL/Manley designed tube amps, I had the whole system connected and I changed one tube at a time, turning the amp off each time I replaced a tube and turning the amp on to bias the new tube. I also made sure after changing each tube that the other tubes were biased correctly, as changing one affected the bias on the others.
Hi all ! The VTL amp I had said to connect the speakers for biasing. My Cary amp says doesnt matter . Call Manley and ask .
Jea48, Regarding your comment "(Dumb way to build something)". The output transformer case may be grounded. Most people use terminated speaker cables eliminating this problem.

Also, powering up a tube amplifier with no load is very hard on output tubes and stresses the amplifier. Youtube is good for some things, but apparently not for biasing tubes.
Thanks guys for the inputs so far. I must add that the amp I have is not a Manley, I'm just following the steps in the youtube video, which had a Manley Stingray in it (I think it was EveAnna Manley from Manley Labs that did the biasing).

I know that some manufacturers recommend having speakers connected at all times before turning on an amp, but I also know that for Cary amps it don't matter. In my case though, the owner before me had it on once without speakers connected, and it was fine after he passed it on to me.

I checked all connections several times, even swapped in a solid state amp and music played normally at the speakers. I even had experience turning on a Yaqin tube amp before without speakers connected and it was fine after. In my case, there was no input signal connected at the time of the biasing, so there should be no load and the output transformer should be at idle.

The amp still turns on and all tubes lit normally, just no output at the speakers. The tubes usually get really hot when playing, but they seem to be just idling without getting hot. Would a dead fuse cause this?
Normally, when I bias a tube amp, unless the manufacturer says to have a load on the output, I don't. I do however, have shorting plugs for the single ended inputs. That way I know there is no signal going through the amp while I am biasing it and therefore not driving a signal through an unloaded output transformer.
Purposely withholding the name and model of the amp will not get you any meaningful info.
I would be very sure that the amp is connected properly to the pre-amp and the amp is configured properly for that input. As well, make sure that the speakers are properly connected as well.

It is interesting that you have never had the amp working properly so it is not so straightforward to blame the biasing exercise on this. I would contact the manufacturer for assistance.
yes: check the fuse(s)
it's a bad idea to turn on a tube amp without a speaker load since tube amps always have a power output transformer where the primary winding is connected to the output tubes & the secondary winding is connected to the speaker binding posts & expecting a speaker to be connected. If not connected to a speaker, there is a high probability that the tuned circuit connected to the secondary of the output transformer can start oscillating & destroy itself.

Additionally, this cut & Pasted from David Berning's website:
Audio-output transformer-coupled amplifiers should not be operated without a load connected because the inductance of the transformer's primary can cause the plate voltage to reach a sufficiently high voltage to break down insulating materials, resulting in amplifier failure and even fire. Some amplifiers are even unstable without a load.
I used to own a Jolida & AES Six Pac tube amps & even tho' Cary said it did not matter whether or not there was a speaker connected I always made sure that a speaker load was present during biasing.
Some info for you to read:

It now needs to be repaired. Looks like its time to either send it to the manufacturer or to a repair shop for diagnosis.
Jea48, Regarding your comment "(Dumb way to build something)". The output transformer case may be grounded. Most people use terminated speaker cables eliminating this problem.
04-07-12: Rrog


I was only repeating the info from the user manual of the Stingray of that particular production model.

Here is the full text.

This can short out the amplifier and possibly cause damage to the Stingray, besides losing signal."

See page 7 of link below. Diagram is also shown.


I notice the design flaw was changed on later models.
Your in a world of hurt!!
It is not that likely that having no speakers connected accounts for your problem. That could overload the primary side of the output transformer, but, that usually happens when there is an input and the amp is supposedly putting out a lot of power, except, there is no load attached. If you had no signal going to the input, I don't think you caused the problem.

Perhaps an output tube was bad (shorted) and that caused the main fuse to blow. Check all fuses first; if you are lucky it is just the fuse.
I ran across an interesting white paper by
Hartley Peavey.
I can't say from the facts stated whether or not the output transformers are blown, or if something else could be causing the problem. However, I would make two points:

1)Whether or not damage will result from powering up a tube amp (that has output transformers) and subsequently powering it down without having anything connected to the inputs and outputs depends on many aspects of the design of the amp. Experience with a different model amp has no relevance.

2)Regardless of whether or not damage might result with a particular amp design, IMO doing that is ALWAYS bad practice, especially if shorting plugs are not used on the inputs. In addition to the possibilities that have been stated, turn-on or turn-off transients could in effect put a signal through the amp. That is particularly likely to occur when the amp is being turned off, if shorting plugs are not connected to the inputs. Transients generated at the instant the power switch opens could couple into the inputs of the amp (to a greater extent than they normally would, because of the high impedance that is present at that point when the inputs are unconnected), then be amplified through the amp's signal path, resulting in huge voltage transients being caused by inductive kickback in the unloaded output transformers (as described in the paper Jim provided).

How likely is that to occur and cause damage with any given amp design? Who knows? But IMO it is a risk that there no point in taking.

-- Al
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.
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.
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.
It would be great to know what the outcome of this is.
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.
Also the thing to remember is, no one ever sells an amp with new output tubes in it. I always tell my customers to expect to need a partial or full re-tube when they bring in a used amp to have it checked out.