daz
Junior Member
Posts: 54
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Post by daz on May 31, 2009 16:00:12 GMT -7
I have a spare set of JJ 6v6's I'm thinking of popping in as the original GT 6v6's are at least 2 years old.I bought this Z28 slightly used and for the past year I've been gigging this amp 6 nights a week on cruise ships. I just have a couple of queries 1st before I go ahead.
I've read the GT 6v6S and JJ are the same tube.I can see they are of the same internal construction, just larger bottles with the JJ's. So why have I seen posts suggesting biasing up to 28ma when the Z site suggests 24-26 for the GT's if they're the same tube ?
I have on old Mullard GZ34 plus the stock Sovtek and a Weber Copper Cap.All 3 give me different results when I check the bias : with the Sovtek reading highest and the Weber lowest across a spread of about 6ma.I assume this is normal but am surprised the Weber read lowest.I'd have thought the opposite.
I have an SRS dual bias kit and meter and have some experience.I'm just assuming it's about time to change tubes and would appreciate any advice. Cheers Daz
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Post by fishman on May 31, 2009 17:59:12 GMT -7
I am not an amp tech by any stretch of the imagination however I don;t think 1 or 2ma is going to make a monster difference. I am going to assume, thats a dangerous thing, that the bias will by nature drift within certain parameters depending on various factors concerning electricity of which about all I know is if you touch something wrong you will have an experience not unlike ejaculation minus all the good stuff that goes with that, and you will be pretty much numb and stupid for a variable amount of time...it is possible to pick up VOA and ships at sea after these types of experiences. anyway I swap out the JJ's with no rebias, if I am within a ma or two and the sound is right I leave it alone. We used to do it by tubeglow in the old fenders because we did not know what a bias meter was or would we have known how to use one, if the tubes were glowing just right we knew we were close enough, too cold and it sounded crap, to hot, really sounded good but you could smell it .... you could also cook a thanksgiving dinner right on the amp...this is the for runner to the TV dinner...all the old time music guys used to cook on their amps, hence the phrase...man you were cookin' last nite..... Better quit now before I get myself into trouble, apologies for the rant, but I coulden;t help myself... DISCLAIMER: If I were in the least bit shaky I would pay a tech, like I did, to show me how to do this correctly, quickly, and stress-less...well worth the 50.00 he charged.
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Post by bustertheboy on May 31, 2009 23:01:29 GMT -7
I have a spare set of JJ 6v6's I'm thinking of popping in as the original GT 6v6's are at least 2 years old.I bought this Z28 slightly used and for the past year I've been gigging this amp 6 nights a week on cruise ships. I just have a couple of queries 1st before I go ahead. I've read the GT 6v6S and JJ are the same tube.I can see they are of the same internal construction, just larger bottles with the JJ's. So why have I seen posts suggesting biasing up to 28ma when the Z site suggests 24-26 for the GT's if they're the same tube ? I have on old Mullard GZ34 plus the stock Sovtek and a Weber Copper Cap.All 3 give me different results when I check the bias : with the Sovtek reading highest and the Weber lowest across a spread of about 6ma.I assume this is normal but am surprised the Weber read lowest.I'd have thought the opposite. I have an SRS dual bias kit and meter and have some experience.I'm just assuming it's about time to change tubes and would appreciate any advice. Cheers Daz Different rectifiers give different voltages, so yes , you'll get different readings. Generally 28mA is what the Doc suggests but a couple either way (especially lower) won't hurt. See what sounds best- I'd guess the Mullard and 28mA. Let us know Brett
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Post by Curt on Jun 1, 2009 7:00:39 GMT -7
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daz
Junior Member
Posts: 54
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Post by daz on Jun 1, 2009 7:53:24 GMT -7
Thanks lads for the replies.Being on the cruise ship my internet access is via satellite, therefore sometimes painfully slow and always costly, so I'm not always able to do topic searches, tho I had seen that last post before.It's the one that made me ask my initial question about biasing at 24mA (as per the Z site) or as one poster suggested 28mA.
There was another point about how bias is affected by the input voltage.I had set it at home 2 months ago to around 24-26mA and having checked it on the ship I saw it was at 18-20mA.In the process of adjusting I tried 2 different outlets and got slightly different results. That's to be expected normally, but in my case I use a Furman AR1215 voltage regulator after far too much frustration with inconsistent ship voltage.One outlet showed 120V on the Furman and the other 125V.The 125V outlet gave bias readings of about 4-5mA less than the 120V. Is it normal that a higher voltage outlet will produce lower bias readings ? On that same topic I'm surprised I see any difference at all seeing as the Furman is supposed to provide the same outlet voltage regardless of it's input voltage. Hmm, anyone else have similar findings with Furman regulators ?
Thanks again for your time and thoughts.Amongst cruise ship musicians and electricians etc I'm the leading (read only) authority on tubes ........ a scary thought.So any advice is most welcome. Cheers Daz
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Post by bustertheboy on Jun 1, 2009 14:08:03 GMT -7
Hi Daz being in Australia I have to use a stepdown transformer to come down from 240v to 120. The Doc says his amps were set for what most US outlets give- about 125v. Because of this, I could never get the same voltage to the output tubes as given on the site- I was struggling to get 160v at an acceptable current rating (somewhere around 25-28mA). So what you're finding is completely normal- you may just get a slightly browner tone and slightly lower on set of output tube clipping. The way I found around it when I need more headroom is use the famous Z28 tone knobs to your advantage- wind them back a bit. A great jazz or tele clean tone is to be had at vol 4 o'clock and treble and bass 11. Brett
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daz
Junior Member
Posts: 54
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Post by daz on Jun 24, 2009 12:03:23 GMT -7
Just an update : I put in a set of JJ's , biased at 28mA (from suggestions I've read) and so far I find the amp sounds barky in the mids and brash in the top end. I replaced the stock GT 6V6S which are tired.I'm aware they're the same as the JJ , tho the JJ's are a larger bottle, and the amp was much warmer sounding. I don't have a lot of opportunity to work on it as I'm on a cruise ship and there's not much free time in our lounge except when we're in port. Just to round out my info I have a Mullard EF86 and the stock 5751 , plus the stock GZ34 or a Mullard. I'm playing an R8 Les Paul with WCR Green/Moore pu's. Any thoughts on biasing etc ? I'm simply using a dual bias tool from SRS.
Thanks for any thoughts. Daz
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