Amp biasing question

I'm not sure the preamp should be on... unless you have a DC coupled preamp and amp, but I should leave this to the tube heads.

My previous tube amps did not require the preamp to be connected, but if it was then it should have no signal going to the amp.  What is critical for most tube amps is that the speakers are connected as they are a critical part of the output circuit.  This was true any time my tube amps were powered up.  

I think what you are asking is really whether signal should be playing when you are measuring and adjusting bias (" biasing should be done with no input from the preamp").  I only check bias after the amps have warmed up for at least 45 minutes with signal playing to get the amps to normal operating parameters. That means the preamp/sources are connected, but when I check bias, I mute the preamp and no signal is passing into the amps when the meter is applied. Does that make sense?

I believe what you meant to say is that amp tube biasing should be done with the speakers connected.  It doesn’t matter is the preamp is on or off as long as it is not passing a signal. 

@whart  and @testpilot 

I understand what you are saying.  My question was due to reading separate threads that: if I was understanding them correctly, were stating that prior to turning an amp on, not only should it be connected to speakers, but the preamp should be turned on first (and shutting the system down should be done in the same order--amp off first and then preamp)

AND

that (again, if I was understanding correctly) that biasing should be done with the preamp turned off.  Perhaps that was what I misunderstood and what was meant was that no signal should be being sent from pre when biasing. 

Yeah, the turn on/off sequence is mainly to avoid a nasty "pop" noise from preamp turn on or off going through the system. You got it, @immatthewj 

Separate issue from biasing in my book. 

@whart  , yes, I agree that is separate from biasing.  And I may have misunderstood the postings I have been referring to.  I just was getting the impression that before turning the amp on, the pre should always be on.  If I had that right, one would have to have his or her preamp on while adjusting the bias.

The point of sequencing is in case your preamp causes a thump when turned on.
This way I f the amp is off, the thump does not go through to the speakers. If your equipment doesn’t do it, then not something to worry about.

My preamp or up stream equipment does not produce any noise, so it doesn’t mater. I don’t think I owned a piece of electronics that thumped for decades.

All of my equipment is tube and has a delayed soft on. So, no racket regardless of the sequence. 

@ghdprentice  , do you bias your ARC with the preamp turned off or on?

ghdprentice

The point of sequencing is in case your preamp causes a thump when turned on.
This way I f the amp is off, the thump does not go through to the speakers.

Yes, agreed.

If your equipment doesn’t do it, then not something to worry about.

Here I disagree, because preamps can fail on startup, and especially tube preamps can send nasty noises in such an event that will reach your speakers if the amplifier is powered first.

In an unusual sloppy mood last year, I failed this best practice that I'd followed for years. That's when tubes in my ARC REF5SE blew and it shook the house. (Fortunately, no collateral damage.)

@immatthewj

Depends on what your intentions are.

Are you planing on only checking the tube bias and then shutting the amp off and not listening to music, Then there is not any need to turn on the preamp.

If you want to check the bias and then listen to music then turn on the preamp and then turn on the power amp. Preamp not playing music. I leave my preamp on mute. Allow the manufacturer’s recommended time for the power amp to stabilize before checking the bias for each channel. My ARC amp says allow at least 15 minutes of uninterrupted stabilization time. I have have found to allow 30 minutes or so of uninterrupted stabilization time works best.

If you want to listen to music first, and then check the bias, first stop playing music then mute the preamp. Wait at least the recommended manufacture’s time for the amplifier to stabilize before checking the bias. In my case I allow the amp to stabilize for at least 30 minutes.

Also,... YMMV But but my two ARC amps are getting old. I have found I need to reset the bias on each channel, when I reset the bias on one channel and then reset the bias on the other channel. I usually have to fine tune the bias on the first channel again. I then recheck the bias on the second channel and sometimes I have to fine tune bias again on the second channel. Older ARC amps are a pita to bias the power tubes.

Also worth noting when it’s time to buy new power tubes buy pre burned in power tubes. It saves a lot of time checking the bias of new tubes without any hours on them at all... At least 48 to 72 hours pre burn in times on the tubes works well...

Also before installing new power tubes adjust the bias pots down a little first. Especially if the old tubes were biased regularly and were near their end of life... You can damage new power tubes if you don’t.

.

My Audio Research amps are self biasing. 

@cleeds 

Ok. I guess that could happen. Fortunately never to me.
 

My ARC Ref amps have a 3.5 minute warm up before coming on-line and the preamp (Ref 6SE) has a 1.5 minute warm up before coming on-line. So unless I go get a cup of coffee between turning on the preamp and amp that will not happen. I guess ARC thought of everything. Cool.

Thanks, @jea48  , I just remember reading here that an ampifier should not be turned on unless it was hooked to a speaker load and had a preamp  connected to it and turned on.  And I am pretty sure I have read that an amp should be biased with the preamp turned off.  The two seemed to me to contradict one another so I was just wondering how folks were doing their biasing.

With all that above being typed, it is possible that I misunderstood something along the way.

Thanks again for the input. 

If an ARC amp does not have auto bias, then the amp should be on for 20 minutes before adjusting bias.

ARC amps can be on with no input cables, and without speakers cables connected. They are perfectly stable. This is not true for some other tube amps so it's best to check with them first. 

jea48's recommendations above are spot-on. Turn the bias pots down before putting in a new set of tubes!

@arcguy  , I once owned a pair of ARC VTM120s, and for all the good things I can say about them, I would hesitate to use the term "perfectly stable."