Tube biasing question

[JoshT289]

Hello Everyone,

I bought a used Premier 11A which came with Svetlana 6550Cs. One failed, but the seller provided an extra, so all is well for now and the amp sounds amazing with these tubes. Still, I assume that the remaining power tubes are old in the tooth so I have decided to buy a quad of Gold Lion KT88 reissues from Jim McShane to replace them.

My question is, should I turn the bias pots to the left before putting in the new tubes? I assume that these 6550s might be biased higher given their age and wouldn't want to damage the new tubes when first inserted.

Thanks in advance for your advice.

Josh

IMG_2857.jpg (1.75 MiB) Viewed 7219 times

[admin]

No, you don't have to make any pre-tube replacement adjustments. Just follow the instructions on the biasing which is on page 9 of the manual.

[roberto]

If you want to skip the manual, turn off the amp. Install new power tubes. Turn the bias pot to the left (counterclockwise) and turn on the amp. Wait three minutes and bring the pot to where the bias led turns on, then go back when the led goes off and a little bit more. Have a listen and then the volume knob to off position…if you see a led on bring the pot counterclockwise. It is ok if a led flickers along with the music. All must be off when there is no music.

Happy listening!

[roberto]

At the amplifier's pic, I can see three on LEDs. You should adjust the bias pot screw counterclockwise until the led goes off, then a little bit more. This is all.

[Big Dog RJ]

Nice spotting Robbo mate!

Yes, a few of those LEDs have definitely come on. I reckon a bias adjust will sort it out or as advised by CJ a full output tube replacement, which Josh has already ordered. That will ensure the tubes are operating in top condition.

My only concern, he's currently enjoying the sound of the 6550C being those fantastic tubes from SED, which CJ used to highly recommend. However, now he's ordered the KT88's. Very different in nature and will not sound anything like a 6550C... then again Josh may like it, I wouldn't know. Only he can decide that!

Best, RJ

[roberto]

https://frank.pocnet.net/sheets/086/k/KT88.pdf
https://www.tubesandmore.com/sites/defa ... 50c-sv.pdf

RJ,
I don't have any doubt of your experience with tubes. You are an encyclopaedia of knowledge! (with much respect). I read the specs of both KT-88 and the 6550C, and they are very much alike. Both can be replaced vice-versa with no problem. I did use both, and I remember that the 6550C was my liking on an ARC model VT-100 MKIII. Also in the VTL-90 and the mono block 225. I did use the KT-90 supplied by VTL and came back with the 6550C.
I remember using both, KT-88 and the 6550Cs. Changing them, just a minor adjustment in the bias, of course this is right because if I did use the same brand new tubes, I must readjust the bias too.

The European KT-88 nomenclature, American made correspond to 6550C...as you said, his ears must decide which are the ones.

Happy listening!

[Big Dog RJ]

Yes, Robbo I'm aware of that. The KT88 can be used as a direct replacement from the 6550's.

However, that's not what I was referring to. I was thinking about the sound/ tonality variations you would experience between using a KT88 vs 6550's... there's quite a difference in the overall presentation, especially within the midband and LF factor.

I think with the 6550C tubes in his Prem11A, he was enjoying the fantastic tunes... now changing over to KT88's, this is going to change all that.

At the end of the day, as you correctly pointed out, only he can decide whether that type of sound is preferred or not.

Cheers mate, RJ

[JoshT289]

Hi Guys,

Thanks for your input. I feel I should answer a few comments you've made.

1. On biasing the tubes, thanks. I do have the manual and I had read it. I just wasn't sure if, as Roberto suggests, I should first turn the four biasing adjustment screws a wee bit counterclockwise to ensure the new tubes aren't being provided too much current at the get go. The manual doesn't say to do that so long as you go through the biasing procedure within about one minute after turning the amp on and again about half an hour later, so I'll just follow the manual.

2. As far as those lights being on, that photo was a few days old and I have further biased the tubes. For awhile, two or three lights would go on when I first turned the amp on but all would turn themselves off within a few minutes. Still, I backed them off a bit just in case. I think these tubes have been a bit finicky because of how old they are and expect new tubes to require fewer adjustments.

3. OK, now, on the type of tube for replacement, I am open to suggestions always, but have done a fair amount of research and of course there are many, many competing opinions on the inter webs, lol. I'm not inclined to go NOS given current prices, so I have looked at online options for new 6550s and KT88s. Eventually, I limited my choice to the following: (1) the Tung Sol 6550, (2) the Sovtek 6550, or (3) the Gold Lion KT88 (all Russian reissues). Having read many reviews, but also spoken with Jim McShane (a tube supplier in the Chicago area with a great reputation), I ruled out the Tung Sol 6550. Between the Sovtek 6550 and GL KT88, it simply came down to which tube purportedly sounds better, and the consensus seems to lean towards the Gold Lion. What I've heard is that the Sovteck 6550 is excellent in its own right, and in fact it's what ARC now recommends for their older 6550 amps, but supposedly it performs best in the mid bass region and the Gold Lion is a bit more balanced and stronger at the frequency extremes. Of course, I won't really know without trying one or the other or both in the CJ. One way or the other, though, I won't be using the Svetlanas anymore, so the sound will change some no matter what. But, please do advise if you think the Sovtek would be truer to the original sound and how the GL might change that sound.

As an aside, I do have a Rogue Cronus Magnum II, and I like the KT120s very much in that amp, so I could always go with new KT120s in the CJ over 6550s or KT88s. (the Rogue is in our winter home, though, so I can't just swap the tubes without buying a new set). I figured the KT88s in the CJ would be a closer fit than KT120s, but honestly I'd like to have all three sets available at some point if I like them all.

Thanks again for your input!

Josh

[admin]

1) You can certainly turn them to max as roberto suggested but I don't think is needed. It's not like a tube would suddenly blow because it's not ideally biased. Yes, it could cut down the lifespan after 100's of hours of use or lower the acoustic properties, but having it unbiased for a few minutes should not do any damage. As you mention, the manual makes no note of turning the bias to some "default" prior to a tube change. But I don't think you can go wrong either way and roberto is knowledgeable in these things so I trust his opinion as well.

2) This is fairly common. I did a "2nd adjustment" on some of the tubes in my ART amps after installing the tubes.

3) Can't comment but lots of our members have experience with tube recommendations and are far more knowledgeable then I so I will defer to their opinion. If buying from a reputable tube dealer, they can also often make suggestions. I've recently had good experiences with thetubestore.com.

[AnotherJohnson]

It is important to set the volume control to Zero if you want a good bias adjustments.

Depending on use, it is not a bad idea to revisit the bias adjustment semiannually. Nevertheless I often forget (if things sound good, it’s easy to forget), and check it annually instead.

I keep a record of tube replacement and bias adjustment dates on the back page of the owner’s manual.

[roberto]

I have a small notebook where I write all what I have done at it. It is a kind of a bitácora. Dated and bias adjustment is basically written. I had a bad tube KT-120, and wrote which it was. For the proper bias adjustment here is what I do:

1) with the amplifier powered off, and the preamplifier with the volume knob to "0" ( no possible signal to the amplifier ) bring the bias trimpot control to the counterclockwise maximum possible.

2) Turn the amplifier on, and wait al least two to three minutes before you do the first adjustment.

3) Now move the trimpot clockwise very slow to a point where the bias diode just lit.

4) Stop right there and then go back to a point where the Led goes OFF. Do a just a little bit more a counterclockwise movement.

5) Play some music at your desired volume level and after 20 to 30 minutes with no volume or no music playing, check for any possible LED is ON and if there is, readjust it as step 4.

5) All the power tubes must be adjusted with this procedure.

The normal operation of a tube is when it is hot and high temperature is on it. All the tube components are hot. Over bias might happens when the tube is not hot, so this is the reason you go back a little bit more with the adjustment.

Tubes are like a car. They do need service. An usually power tube failure, is when there is no way to adjust the bias, the trimpot is totally counterclockwise and the LED is on. That tube is a suspected for a possible failure.

Happy listening!

[JoshT289]

Thanks Roberto!

[roberto]

Josh,
My main language is Spanish, and I do have a barrier with my English. I hope you did understand what I was trying to explain. We are here to help. There are so many good advices here...I am always learning.

Happy listening!

[Wildcat]

As an aside, I do have a Rogue Cronus Magnum II, and I like the KT120s very much in that amp, so I could always go with new KT120s in the CJ over 6550s or KT88s. (the Rogue is in our winter home, though, so I can't just swap the tubes without buying a new set). I figured the KT88s in the CJ would be a closer fit than KT120s, but honestly I'd like to have all three sets available at some point if I like them all.

I put KT120s in my Premier 11 and have enjoyed the sound. The set of 6550s in the amp were a tired pair of 6550s, GE manufacture I believe. I think C-J even recommends KT120s for replacement in the 11 now.

My 11 is down at Music Technology right now--it started sounding bad, like a mistracking record (sibilance), and I realized that I couldn't adjust the bias to where it should be (I either couldn't get the LEDs to go on, or to go off--can't remember which). Probably a minor component failure in the power supply or bias circuitry, I'm thinking. Once Bill can get to it (I'm thinking another month or two yet--they usually have a 10-12 week backlog), he'll get it sorted out for me.

[Big Dog RJ]

G'day maties, thought I'd share a few points on output tube biasing and tube replacement, since my monoblocks required a full output tube replacement done just now.

Also a quick note to Wildcat: trust you received the Prem 11A back in full working order, and you're enjoying those fine tunes!

Now, since I had these monoblocks, I had done a full tube replacement on both just before and after the full SE upgrades were done. This was carried out over a period of two years, plus a slight mod was done on the Class A bias operation, which sounds fabulous but at the same time, really heats things up... so this arvo I picked up a pre-ordered full matched set of Quads X 2sets.

This evening I turned down each bias on each output tube before installing the new KT120's. All eight were fine, except for three naughty tubes...

1. Left monoblock V4: was having a hissy fit upon power up, causing the LED to heavily flicker with a loud sputtering noise, also causing the CLX to activate. Definitely no good, pulled it out and replaced promptly. All good

2. Right monoblock V5 & V6: were Red plating!!! V5 was Red plating whenever bias was adjusted up to.. causing it to look as if it was about to blow like the 4th of July's Canon fire...

3. V6 was heavily Red Plating even before bias was attempted. That is, bias was all the way down but still LED was on and adjusting bias on V6 didn't make any difference. Again looked like it was about to contribute to Canon fire...

Also *Note* both tubes were dam hot to the touch, even after allowing to cool down for 10mins... very hot to touch!

That Red plating was pretty serious, the last time I had this sort of phenomenon was on one KT90 output in one of my Manley monoblocks. But that was just one tube, not two having the same issue at the same time. Anyway both tubes were promptly replaced and now all is good

So overall, not a too shabby experience, and it's a good thing the supplier is Melb local, so replacements were done promptly. Another good thing is just 8 output tubes to deal with, compared to 16...! Been there, done that and no thanks!

I also spoke with my main tech who did the Class A bias mod, said that the initial draw modded for Class A operation was probably out of spec on those three particular tubes, although they would have been matched and tested at factory prior to shipment. So, again by the time these tubes ship out, travel across several continents and reach our doorstep, some of them do fall sick. Afterall, these are very delicate electronic instruments. Must be handled with care, and that is one thing the shipping & handling crew just don't do. They can't be bothered.

Even though these have big "Fragile- handle with care" stickers on their cartons, that's the very reason for them to play a little Footy once in a while... I've also seen them do this with other Fragile packages. No wonder on top of air travel, these tubes go bad.

Anyway, these three phenomenon I haven't quite faced in this exact manner before, hence thought I'd share this so that anyone installing new output tubes, should look out for these issues.

BTW, the amplifiers are sounding absolutely marvellous! As they ought to! Cheers to all, and do enjoy those fine tunes!
RJ

[Wildcat]

No Premier 11 here yet, but when I emailed Bill a couple of weeks ago, my amp is near the front of the queue and he'll be working on it shortly. I don't know if I mentioned it, but he had me include the tubes I intend to use in the amp so he can make sure it's all operating to spec. (The way the amp is built, when the tubes are out, I was able to make a nicely padded pair of boxes to enshrine the tubes in for shipping.)

[Big Dog RJ]

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

[admin]

Sorry to hear it wasn't just a bad tube but glad to hear the fix will be relatively easy. Let us know how it goes.

[Alanjett]

Just purchased a vintage PV10 with phono. Can someone tell me do the tubes in the amp have to be biased? or as they are only small tubes it doesnt apply thank you

[admin]

No biasing for the PV10.