Topic: tube biasing

[Tim]

I was wondering how you guys bias the tubes on your amps.

The ASL is my first tube amp so this is all I know. It has a 5 position knob on it with a meter below it. The knob has positions for "off" and then one for each of the 4 el34 tubes. The meter is just like an analog vu meter and is clearly marked (green on the left, red on the right, and a little blank spot in the middle). I just turn the biasing screws until the meter is centered.

is this meter setup significantly different than most tubes amps?

[Lil Kim Jong-Il]

The conrad johnson gear is about the easiest ever to bias.  There is an indication LED and a bias pot for each tube.  All I have to do to bias the tube is turn the pot untill the LED is just below the illumination threshhold. 

[cheshirecat]

Dynaco gear is a little different... you need a VM and need to measure the Bias voltage at a particular pin on an octal socket on the front of the amp.  On the ST-70 there is one pot for each pair of output tubes (so closely matched pairs are essential), you measure the voltage from a particular pin of the octal socket to ground, when you hit 1.56V you are all set (this is based on the voltage that a new D cell battery puts out... right).

[ducati]

That sounds right for your ASL.

For my ARC amp, and every other tube amp I've had in the past, I connect a multimeter to a set of taps (one for each tube) and dial the bias pot (one for each tube) so the reader shows the correct bias (for my ARC that is 65 mV DC).

I prefer the multimeter setup or the built-in analog meter to a bias LED as I feel it's more precise, and I like a quantitative number as to how much bias has drifted since the last time I maintained the amp.

FWIW, it should be mentioned that you should always let your amp warm up for 20-30 minutes with NO SIGNAL to the inputs before biasing.

[Lil Kim Jong-Il]

The bias LEDs are great for convenience sake/ease of use.  But every time I adjust the bias, I wonder if the bias intended for use with the OEM recommended 6550s is really appropriate for the KT90s that I'm running in there right now.

When installing new tubes, shouldn't they be biased initially and then again after warm up?  Is it possible that when replacing tubes the old tubes have drifted enough that new tubes would be operating at an unsafe bias or is this not really an issue?

[ducati]

Ah, you speak of tube rolling...  A hobby in itself  

I have never bothered with setting bias on a brand-new tube until it's warmed up...  Never had any problems. 

Since you have tried this--how much does bias drift from the initial turn on of a brand-new tube to the warmed-up state immediately thereafter?

[Lil Kim Jong-Il]

Since you have tried this--how much does bias drift from the initial turn on of a brand-new tube to the warmed-up state immediately thereafter?

When I installed the 6550s or KT90s initially and set the bias, then again 30min later, there was a very slight adjustment.  Not much but enough change on warm up that the bias LED illuminated.  That was the recommended procedure by CJ.  Both 6550 and KT90 quad sets were from the previous owner so this doesn't really apply to brand new tubes.  Also the amp was already at the correct bias for the 6550s when I recieved it.

I can't recall clearly but I believe that there was not a signifficant difference between old and brand new tubes when I replaced the 6550s in an MV50 that I used to own.  But the old tubes were from the previous owner and I really don't know that they needed to be replaced - I just wanted to try a new set.  I remember that the bias setting procedure was the same and the results were the same: there was a slight adjustment after warmup with the brand new set.

My comment about drift between old and new tubes was a question/speculation.  There is a decent range on the bias pots and I'm sure they designed the range to be safe.  But in general I do not know what happens to tubes if operated at a bias out of that range and how does the bias setting need to change as the tube ages.  I assumed it would drift until it can no longer be biased within the safe range, bit I don't know.  I never had a tube fail so maybe they don't age like that.  I was hoping you guys could tell me.

[cheshirecat]

When I start with a fresh amp (new rebuild or whatever) or different set of tubes is set the pots to the middle then fire the things up wait 30 seconds or so then I measure the bias and adjust accordingly.  Then every 30 minutes or so I recheck and re-adjust for say 4 hours or so.  I usually find over the first 2 or 3 adjustments there is the most drift... On my ST-70 I have had things start at 1V and go to 1.4V (40% increase there) and also down to .70 or so.

If something is clearly wrong with the bias voltage you will get the ever popular 'glowing plates' where the plates of your tubes are glowing red.  When a tube goes bad this is what will happen, you will not be able to bias it properly, the plates will glow red and eventually you will blow either the filament in the tube due to heat and the tube will shut off (like a blown light bulb) or you will blow the fuse on your amp.

[ducati]

Definitely always keep an eye out for glowing plates.

In my experience, being off on the bias (even radically so) won't cause tubes to "lock" to that bias as you're afraid, El Barto.  They are operating outside their normal range, tho, and this is hard on the tubes for sure.

I think you're quite right that a manufacturer wouldn't have bias pots on there that have such a range of adjustment that massive problems (like causing the tube to run away) could occur...

BUT, some food for thought: I was reading this morning about the new-ish BAT VK-55 and BAT included auto-biasing.  They claim this is good to (obviously) offset sonic changes with tube wear, but ALSO to lessen wear during line voltage changes and (directly in reference to this discussion) to lessen tube wear upon start-up.

So there--I think you have a better procedure than I do

[BobW]

Definitely always keep an eye out for glowing plates.

Fact !
You should clearly hear distortion at that level as well.
The tube's useful life is going sharply downhill every second the plate glows.

If you need a manual, try posting a request on Audiogon.com or in AudioAsylum.com.
It should have the correct biasing procedure.

[Lil Kim Jong-Il]

I do have all the manuals and follow the manufacturers recommended biasing procedures.  I've had a couple of tube amps, a tube DAC and a couple of tube pre amps over the course of 8 years and honestly I have never had a tube fail.  I've replaced tubes just because I had tubes with unknown hours and wanted to see if new tubes made a difference.  So I really don't have any experience with tube failure modes.

I'll keep an eye out for glowing plates.  Is that in power tubes or small signal tubes too?  I have the power amp cage off but the preamp is enclosed.  So I can't really watch it.

Thanks for all the info guys.  This forum has been really good these last couple of weeks with all the tube gear discussion.

[jk labs]

I was wondering how you guys bias the tubes on your amps.

The ASL is my first tube amp so this is all I know. It has a 5 position knob on it with a meter below it. The knob has positions for "off" and then one for each of the 4 el34 tubes. The meter is just like an analog vu meter and is clearly marked (green on the left, red on the right, and a little blank spot in the middle). I just turn the biasing screws until the meter is centered.

is this meter setup significantly different than most tubes amps?

Yours sounds like a good method. The LED method mentioned was new to me: I would think the sharp LED trip point along with the sensitivity of the human eye would result in very accurate adjustments.

In many older amplifiers the adjustment is done by dial metering but the user must use an external meter (analog dial or digital readout) and clip the test lead sequentially into each tube manually in order to do the measurement.

Lastly some amplifers don't require adjustments - there is circuitry there forcing a preset current through each tube.   

After the amplifer has warmed up, careful adjustment of idle currents will set the operating point for the tubes. With pairwise equal currents there will be no net magnetizing current flowing in the output transformer.  But what when the amp starts passing a signal? To maintain pairwise balance in push-pull amplifiers the tubes should be matched. The first level of matching would be to measure all tubes in the same socket (or with the same bias voltage at least) and see if the DC currents match enough to form pairs of tubes. 

The next level is AC matching.

[jk labs]

My comment about drift between old and new tubes was a question/speculation.  There is a decent range on the bias pots and I'm sure they designed the range to be safe.  But in general I do not know what happens to tubes if operated at a bias out of that range and how does the bias setting need to change as the tube ages.  I assumed it would drift until it can no longer be biased within the safe range, bit I don't know.  I never had a tube fail so maybe they don't age like that.  I was hoping you guys could tell me.

The tube depends on the emission of electrons from the heated cathode.  Through use the cathode simply wears out and becomes less efficient at emitting electrons. On the meter the bias current drops.

Another mechanism for wear is that the glass/metal seals lets air through. The flow of tube current is hampered. 

I think in this same manner possibly every single mechanism behind tube aging lead to a drop in the current.  At least I can't think of one that would increase the current.

Mechanical damage or meltdown on the orher hand ...   

[macdaddy]

Definitely always keep an eye out for glowing plates.

what do glowing plates look like..?

[ducati]

Bright red color on the plates.  Normally you will see a nice even glowing orange color...  If it's bright and red, that's BAD.  Bias is either way off or the tube is going bad fast. 

I have only had this happen once--it was an amp I bought used, and one day I noticed the bright glow in the third output tube.  I am not *positive* how long it was glowing, but as soon as I noticed it I shut off the amp and replaced the tube. 

You have to be careful because if the tube is glowing, it's being run very hard...  And if it fails, depending upon your amp you may be in for a repair.

I have never had this happen to a small signal tube in a preamp, only that one power tube.