Tube biasing question

[Alanjett]

Thank you

[roberto]

G'day maties,

Just thought I'd share, yet something new again, although fairly old school...

So, today I went to my local supplier's place and we tested those Red Plating and sputtering tubes... works perfectly! Both on his testing rig and his CJ and Sonic Frontiers amps. So, what could possibly be the problem? I brought back three brand new tubes as replacements, plonked them in and biased accordingly. Out of the three, two are working fine but one is still Red Plating and this is a brand new tube. The Bias LED came on (although completely turned down) so I couldn't bias anything because there was no where to turn the trim pots... so the tube started Red Plating and went off with a very loud crack, like a whipping sound, then I immediately shut down the power amp. I replaced that tube (V4) with the previous old spare that was working fine just last night, and as I had guessed, that tube couldn't be biased either. Even turning the Bias pot all the way up to active the LED didn't help. Then I tried three different tubes, same thing, no Bias adjust was possible and no LED turns on... So this means that either the bias circuit of that tube has gone plonk or a resistor or something has popped. I contacted my service tech, and he said pretty much that's it!

After about the 10 year mark, apparently these auto-bias circuits need some servicing. Either a health check just to make sure the bias LEDs are functioning ok, that is tracking & setting idle current required for the type of output tube used, and checking if all other intricate surrounding parts are in working order. The amplifier will still work, with all four tubes nicely powering up but if one of these tubes bias cannot be set properly then that's a bias circuit failure. The good thing is, it's an easy fix!

So the monoblocks are off to service tech next week, he will do a full bias circuit check & fix, plus re-set the Class A bias again, check all tubes are healthy, and run-in the amps for 48hrs non-stop to make sure everything is tickety boo. So, as of now there's no music. Just silence, watching a movie as I'm typing this. Good movie by the way, called The Mule, by Clint Eastwood, outstanding!
I'll try to get hold of a loan amp over the weekend so that I could listen to some tunes but not all power amps are capable of driving CLX's effectively...

Cheers maties, RJ

Hola RJ, my dear wolf mate!

From a wolf to another wolf, check for an open 20 ohms/7 watts bias resistor. Change it and you are back to ride again!. Jeff is now recommending to use a little more wattage dissipation on that particular resistor. You can use 8 to 10 watts with no problem. You can manage to make it to fit right at the PCB.

Enjoy those fine tunes RJ.

Happy listening!

[Big Dog RJ]

Yes Robbo mate,

The Bias resistors have weakened over time... the tech will check all of these and replace them accordingly. Other than that the amps are good to go.
I'll be without tunes for a while, so I'm trying to see if I can borrow an amplifier to drive the CLX's reasonably well so that there are some tunes playing...

Hopefully, not too long a wait.
Best, RJ

[Big Dog RJ]

G'day maties,

Just got feedback from my tech, and yet again I've learned something new. So apparently, when biasing down your actually increasing current to the output tubes. Hence, if you turn the bias pots all the way down, you've actually provided the tube with maximum current, which in turn leads to Red plating. By turning up the bias (clockwise) you're actually reducing the current to the output tubes and setting the required idle current for the tube to operate. As volume increases, the amperage increases as well, and if monitored by a meter, you can actually see these ratings rise and fall as levels are adjusted.

Now, who would have known that?! I always thought bias adjustment was the other way round; less bias = less current and more bias = more current but not the case, in fact just the opposite!

So, the lesson learned: when replacing output tubes with new ones, do not turn the bias pots all the way down. Instead, turn down just one slight turn and leave it. When new output tubes are installed, they would normally sit at a higher required current simply because they're new and not too weak or anything like older tubes. Once the new tubes are powered up, they would usually trigger off the bias circuits, lighting up the LEDs. Then all that's required at this stage is adjusting the bias for each tube and setting their idle current as closely as possible. That's about it!

After so many years/ few decades in this audio field, what I had initially learned about tube bias was completely different to what the actual process is. Yet we cannot think we know it all... tube amplifiers and CJ in particular, are one of the most reputed and reliable brands money can buy. They're very well built and designed on such high standards, that if you don't know about tubes and not sure what you're doing, it's better to stick with SS. With tube amplifiers, it's always a learning curve, and more the experience, more the rewarding it is, especially when troubleshooting such areas that are common with tubes.

With careful adjustments, looking after the gear and no abuse, tube amplifiers will not only out-perform but will also out-last SS designs.
CJ is just one of those highend marquees that gets it right. Definitely worth the investment.

Cheers maties, enjoy those fine tunes!
RJ

[Joe Appierto]

Very useful information and it'd seem counter intuitive. I thought exactly the same as you did. Thanks for posting this, RJ.

[roberto]

With much respect, if you turn the screw to the right, you are increasing the current through the tube. If you keeping turning the screw, you might get a red plate, bringing the screw counterclockwise, you will decrease the current through the plate, and this will make not to have an unwanted red plate.

The truth is that this is in my Classic One Twenty SE, you could have a different circuit topology on different models...and turning to the right or clockwise, you might decrease the current. You must find which is in your case.

Happy listening¡

[AnotherJohnson]

I’m with Roberto on this one. But it does depend on how the circuit is set up.

An LED lights when the voltage and current come up to its characteristic specs. If it’s in series with a bias resistor, then it’s chosen so the when the bias current is at the right level, the drop across the diode should light the lamp. Turn the bias pot up until it lights, and then back off to put it at the threshold.

There are other, more complicated circuits that could be used, but CJ lives for simplicity.

[AnotherJohnson]

The reappearance of this thread reminded me to check the bias on the ARC REF75SE. There had been some drift, so I trimmed it up.

I would observe some things.

1. The CJ LEDs do give you a heads up by glowing continuously if a tube needs to be rebiased. You can check bias with a switch on the REF75SE, but you do have to consciously think about it.

2. The CJ bias pot screws are relatively forgiving compared to those on the REF75SE. Much less sensitive, hence quicker to a stable setting. With the ARC, the screws are pretty quick acting, and making a fine adjustment to put the needle right on the mark takes me longer.

3. The slot on a CJ bias pot is easy to get the official plastic driver into, and it also accommodates home made wooden chop stick drivers. The ARC slot is too tiny in comparison. It takes effort to get the official plastic driver into the slot. It is aggravating. I can do 16 KT120s on the CJ monoblocks faster than the 4 KT150s on the REF75SE.

[AnotherJohnson]

One of the beauties of the CJ biasing method is that you don’t have to understand it to do it.

Don’t bother with this “turn it down or turn it up” stuff.

Pull the old tubes, which were working because you did not wait for them to start failing … instead, you are wisely replacing them because they are getting close to 2000 hours. Plug the new tubes in, with no input signal, but with load attached (your speakers hooked up, your preamp off or at 0 volume). Wait 5 minutes and go through the light Kabuki dance. If the lights are glowing, you could just barely turn them off at the outset.

Play music for 30 minutes and go through the light Kabuki dance again.

You should be set for 6 months.

The oft repeated mantra of “turn bias down” is unnecessary unless some one who wasn’t knowledgeable has been fooling with it and gone overboard.

[admin]

I was not aware of the turn direction and current increase but it's good info to know. As AJ mentioned, CJ's recommendation on tube biasing makes it rather an easy affair.

[AnotherJohnson]

When you bias output tubes in a class AB amp, you’re basically changing the point where one tube hands off the job to the other in the pair. If you don’t get it right, you can hear the switching distortion and this is an audiophile no no.

You’re basically looking for the sweet spot of DC lift … and this is different for every tube type, and tube specimen.

If you really want to do it right, you’ll monitor the distortion on a scope. But virtually none do this, except at an authorized service center.

I’ve got a gizmo for guitar amps that plugs in between the tube and socket and allows you to display characteristics under playing loads. It works for 6550s, KT88s, and assorted other tubes you find in guitar amps. I’ve never tried it with KT120s or LT150s. You never see those tubes in guitar amps.

ARC writes stuff on their tubes. They charge two prices for their tubes, but if you tell them the cryptic numbers they wrote on your tube, they’ll match it. The numbers appear to be in code so that you can’t just pay Unkie Kevie a single price and get an equivalent.

The other comment is that ARC says that the KT 120s last about 2000 hours (as CJ also says), but ARC claims 3000 hours for the KT150s.

[Big Dog RJ]

Yes, it makes sense when we think that current is increased as bias pots are turned clockwise... & current decreases as bias pots are turned counter-clockwise... however it's quite the opposite. When the tech was doing this along with the multi-meter and scope, I could clearly see the rise & fall in current. So it was an eye opener for me, at least.

At the end of the day, it's definitely worthwhile changing tubes whenever required, based on usage hours. Also as tubes weaken, they still sound OK, that is music still plays and tunes sound fairly decent. I guess the lesson learnt here is not to turn the bias pots either all the way up or down. Turning these pots full swing to either side can cause issues, as was the case with my tube change over experience.

Another chap had modded his Manley Labs Snapper monoblocks from EL34's to KT150's. My tech had done the mod for him, and had re-engineered the amplifiers to accommodate the KT150's. I must say, the results are quite stunning! Everything is completely different, from speed, power, heft, depth, scale, mids to HF extension control, and LF heft and control is outstanding! So, at this stage I was wondering weather to do the same mod with my LP125M's, and the onboard trannys can easily take on the KT150's without any problems. However, I'm not too keen on changing the overall tonality towards KT150's. If this was in favour then I would have gotten the ART150 or ART300's by now. Very pleased with the SE upgrades and Class A bias mods done on the amplifiers so far... modding further towards KT150's, I'm not sure something I need to think about.
The tech says it will alter the sound and presentation, without a doubt, but whether or not I would really prefer this is yet to be determined. Will obviously need to run a few home trials. Will see how it goes once the amps get back.

Till then, enjoy those fine tunes!
Cheers, RJ

[AnotherJohnson]

It could be set up either way, but the simplest would be in series, rather than as part of a bridge.

The only way to know without studying the schematic is to forget about the LEDs and use a meter. With older ARC amps, the meter is your only choice, although this is no longer the case.

I really think the operative principle is to NOT obsess over up or down, on or off. You know the target is for the light to be on the verge of turning on when no signal is coming from the preamp. Just plug the darn tube in, twist the pot to the light set point. Wait 5 minutes. Do it again. Half hour later, do it again. Then forget about it for 6 months. At that point, just do it after a half hour warmup. There should be no need for initial or 5 minute later tweaks. Just check/adjust it with no signal after half hour of warm up.

It really isn’t rocket science. Maybe Soviet rocket science.

We could have fun with jokes about CW and CCW being reversed on either side of the equator . Sort of like the reversal of the swirl when you flush.

[Big Dog RJ]

Yes, good advice AJ. That's pretty much the way to go.

BTW: Whiles the monoblocks are being attended to, I had the fortunate opportunity of visiting my ML dealer, same place I purchased the CLX's from. I'm picking up a pair of very high grade monoblocks from Dartzeel SS design and a Jeff Rowland preamp. I've always liked the Darts very much, and they have that typical sense of SS authority with full range Stats. In fact, I've used a Dartzeel stereo version before on the CLX's, and it was fantastic! On a lighter note, I've always wanted to try out Jeff Rowland power amps with the CLX's. He happens to have a stereo version as well as monoblocks but they are the only demo units at the store/showrooms, asking price 22grand and 45grand respectively. I'll try to bring these home tomorrow. At least will have some fun with SS amps. The one and only SS amp I wanted to keep but ended up giving away was the MF2550SE. Should have just kept it for days like this! Now, I'm regretting... Another one I should have kept was the Pass Labs XA30.8, another superb combination with stats but I guess that tube magic and its bug keeps biting me like a bad mosquito... so I just always end up with tubes.

My benchmark has always been Dan D's designs, the Relentless and Momentum amplifiers, however that price tag is over 100grand. So definitely not going there but just once in a while it's certainly nice to experience such gear on the same stats that you own, and boy is it a wonderful experience.
This time round, I won't be comparing the Darts with the CJ's, simply because both are completely different designs. What I will be comparing is the Darts & the Jeff R's with the Momentums & Relentless amps, that would seem a fair match up.

Obviously, overall I'll still prefer tubes, that's where my passion is, and getting round the CJ sound has been totally engaging. I've noticed between brands, such as ARC & VTL, these are more authoritative in the bass and LF weight in presentation, whereas with CJ it's more focused on the mids & highs, and mid bass. There's just something beautiful in that way those tones are rendered, with such great color, vividness and beauty, the natural sound is at its finesse. Similarly the LF is just right, well balanced and not too much of anything, hence if it was added heft/weight to the bass like ARC & VTL, then I think the CJ sound would become fatiguing... and this is probably where CJ rolls off that last bit of bass grunt. At least this is my analysis of it. However, now with the introduction of the ART150 & ART300's, that bass grunt is getting there, especially with KT150's, and this is where JF is offering this range to easily compete with higher end SS designs. Then JF offers the CJ Classic range, and this is where that typical CJ musicality lies, slightly between that golden glow and more natural tones. I believe this is where ARC comes into play, with great LF control, and added mid-bass slam. With SS designs, such as those mentioned above, along with Pass Labs gear, that LF heft and dynamic slam, seems limitless! Those transients just extend beyond anything I've heard, the last time I really experienced this type of freq extension was on Alsyvox Ribbons driven by CH Precision gear, most fabulous!

Again, it all boils down to personal preferences, stick with what you love and what brings you the most enjoyment from recorded music. After all, it's all about the music.
Cheers to tubes!
RJ

[AnotherJohnson]

I share your regret about the 2550SE.

I wish I’d kept mine as a spare.

This haunts me as I wait for the Levinson 5805 to come back.
I reconcile myself to the idea that eventually I’ll pare down to one system, so the ultimate spares may be the components in the system that isn’t chosen.

[AnotherJohnson]

Back to the bias circuit.

It is not simple. The MV 75 schematics I’ve looked at are difficult to see on my phone, so I will have to see if they’re clearer on my desktop. But it looks to me like there’s an op amp that’s being tweaked for adjustable fixed cathode bias. So much for the idea of simplicity. I picked the MV75 because I think that was the first to incorporate the LEDs.

Regarding all the impressions of what have to be described as spectral imbalances among the different manufacturers, I believe that they are all trying to deliver neutrality, and that they have all always had this goal. The imbalances attributed to each manufacturer would manifest as distortion. In my opinion the tonal imbalances reflect how well the transformers couple to the real world speaker loads. Better transformers and better power supplies track music better, but frequency impacts this since speaker loads are complex impedances that have a frequency component.

As a practical matter, we all know what we think we hear and we all know what we think we like. I have been amazed at how beautifully the ARC, CJ, and Levinson amps have handled my F228Bes. They all sound like music. And, once again, I challenge anyone who was not present at the performance and in the mixing to make assertions about which is more accurate.

[Big Dog RJ]

G'day maties,
Just a quick update: the monoblocks are back! Alive and well! Sounding marvellous!

So the bias circuits were adjusted, resistors were changed over where necessary and the Class A bias operation was further tweaked to get back within proper spec. That Class A bias was actually the culprit, which put things out of whack once brand new tubes were installed... hence had to be adjusted on a tech bench.

In the meantime, those SS amplifiers were given a short spin, and pretty much a very short spin! The Dartzeel is still a very highend design and an absolute gem to own but still has that typical SS presentation to it. Sounds very powerful, dynamic and realistic but after a while listener fatigue starts to set in... and same with the Jeff Rowland. Although the JR is Class D operation, it was fairly decent driving CLX's but nothing close to the way CJ tube amps drive them. The Class D switching was very fast, LF's well controlled and transient information was plenty & colourful. However, it lacked the freq extension of the mids & highs that Class A/ AB and tubes offer. The JR didn't quite have the extension control that I'm used to with the CJ tube monoblocks, and more so with the MF2550SE that does supremely well. Damn! I should have never let that MF2550SE go... it surpassed both the Dartzeel and JR amps, and it's less than half the price!

Anyway, that was a learning curve once again, and we're back with the monoblocks in full swing! There's one other amplifier that I still would like to have on standby, just for fun and something that does drive the CLX's to their full potential in SS design, the Pass Labs XA30.8. Only issue is my dealer mate wants 10grand for it. Need to think about that one just for a standby amp...

Cheers maties, and do enjoy those fine tunes!
Woof! RJ

[admin]

Glad to hear they are back and up and running. $10k is a pretty steep price for a backup set of amps.

[roberto]

Listening to the news of what is happening in Ukraine, you have good news. Enjoy those fine tunes RJ. Welcome back to the wonderful sound of the tubes by CJ.

[Big Dog RJ]

Here they are maties, in full glory and sounding even better!

I'm not sure what it is but thus is another 15-20% improvement in performance. Whether it's the resistor change but that's only on the bias circuits not signal path..., the careful adjustment done on the Class A bias, and these amplifier stands by Solid Steel, certainly contributes to a wonderful change.

They sound magnificent! Everything seems to be tighter, more focus, definition and clarity. Not sure if this is all psychological but there's definitely something special going on here. Couldn't thank my tech enough for his skill and dedication towards CJ tube gear, this guy is a legend!

Cheers, and enjoy those very fine tunes!
Woofty woof'n!!! RJ