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In Reply to: Advice needed re firing up new power tubes posted by desert dweller on August 30, 2006 at 10:13:09:
but then seeing an tube go ballistic will do that to anyone.
Running a tube at the lower limits actually creates a lot of distortion. You may not hear it at low levels, but the distortion may not be good for your speakers or your ears .. 8^)
Plus the tubes may not be running balanced (more paranoia).Leaving in the tubes for a five minutes is plenty of time for the tubes to settle down if they are new stock. If you see no anomalies, that is if all the heaters light up, etc. and the tubes look OK visually, then turn up the bias of the tubes one by one. After you set bias, doublecheck, as your amps shares one bias supply with all four tubes and the bias will change as you bring up the the other tubes. It's regulated, but the current draw will drop the voltage slightly at the tubes.
As per your readings, bias is a floating point actually, and there is actually a range through which the tube will operate quite well. For your MV 52, especially when running, say NOS American, I run the bias slightly lower: I adjust per CJ directions, and then turn the bias pot lower by a 1/4 turn. The lower bias runs the tube closer to class B but rolls off the top end slightly, which is OK as the US tubes seem to have a lot more top end than the Russian or Chinese tubes. While it is supposed to create more distortion, it is most apparent at the lower volume levels and is not very noticeable at normal levels. If you are running Russian made tubes, they are generally very tough, and I would run it very close to CJ's recommended point.
Stu
Follow Ups:
Thanks, Stu. Yes, I am so paranoid that I even turned off my noisy swamp cooler so I can hear anything the amp does. It's only about 95 out but it's getting a little warm in here, time to turn it back on.2 things that may or may not be normal:
1. The right side biased fine about 90% of the way up. The left side however, wouldn't bias. I played around with changing the right and left side screws to see how they affect each other. The best I can get is if I back off the right screws each about an extra 1/8 of a turn, then I can bias the left side with the screws turned just short of all the way up.
2. I can see a bright white spot in one of the right side tubes, it looks like it's inside just below the top of the plates. The plates are not red but I don't see this in the other tubes. It gets brighter as I turn the bias up from zero to the final setting. It doesn't look scary bright like when that tube blew, but it's noticeably different than the others.
Does the amp just need some more time to settle down or is it time to call the tech? It sounds ok but I don't have a normal operating baseline to compare it to.
Thanks for your help.
Sue
Let's do one thing first: swap the left side tubes into the right side and vice versa(be sure to turn the bias down before making the swap) If the right side still biases and the left side doesn't, then we can eliminate the tubes. The tubes may have too high a transconductance for the bias to be set properly. BTW, what kind of tubes did you get?Or possibly the amp needs further service. However snce you say that the tubes will bias if you turn the right side down a bit, it may be the tubes.
The white spot you see is not on the plate structure itself, correct? It then is probably one of the screen elements glowing and located inside the plate wall struture. It may be a sign that the tubes may have gotten jostled about a bit more and the screen grids have physically been displaced. You may want to check with your vendor about the problem and ask him. I would think that he would replace it since you say the glow varies with the bias: not a good sign.
Sorry, I wish I could be of more help, but I can't quite visualize what you are seeing.
Stu
OK, I'll try swapping the tubes and see what happens. BTW, they're EI 6CA7s. When I first got the amp a few years ago, another inmate raved about these tubes with this amp. That's what started me on this whole tube rolling quest.I have to look down in between the plates to see the bright spot but I can't see exactly where it is. Maybe I'll have a better view after I swap them.
Thanks again!
Sue
I don't have much experience with the EI's, maybe some others can pipe in their experiences.
Stu
They have screen alignment issues on some tubes too. Stu, I'm sure your diagnosis of a glowing screen wire(s) is what's happening to Sue's tubes.Ei quality just wasn't the same after the NATO campaign.
You may not find this particularly encouraging.I used Ei 6CA7's in my GTA SE-100 amp about 4 or 5 years ago. Sonically, I thought they were a winner and my favorite, preferring them over Sovtek and SED. However, they had a tendency to fail rather dramatically and after the second failure (each time bringing down a bias resistor), I simply threw them away.
I didn't have any issues biasing them -- each would bias properly. It was just general reliability.
Joe
Sue:
I would invest in some new tubes: screen misalignment is catastrophe just waiting to happen. If the screen wires are glowing that hot they will either start to sag and then short out, or simply break and short out also.STOP using that tube with the bright glow! and swap the tubes for something you already have. The newer Sovtek/EH/Mullard reissue series seem to have become much more reliable. By all means get another set of tubes: if not for your peace of mind, at least for ours!
Stu
What do you mean by newer? I thought the Sovteks and EHs have been around for years. Should I avoid certain vintages? Is there a way to tell when a tube was made?Thanks,
Sue
Hi Sue:Yes, the companies have been around for a long time but they have been modifying their tube production techniques and materials. The Sovtek/New Sensor conglomerate has been steadily working to increase the quality of their production and have been really successful. Of course you have to remember that I'm an old fart, and I can remember those days when Genelex was still selling new tubes from England, as well as Tungsol, and many others. Too bad I didn't know then what I know now!
About five years ago, the New Sensor people started to slowly revise their designs and their tube quality has increased: So have their prices, but it has been worth it. At one point I wouldn't be caught with a Sovtek tube in my possession, but now they actually last and sound decent. I still prefer my NOS stock, but then I am prejudiced in that regard. Even the latest Chinese tubes are better: they still fail but now they seem to go out quietly, and NOT create fearful pyrotechnics when they do.
A decade ago the quality of much of the Eastern Block tube production gave tubes a very bad name.
Just my opinion, your experiences may vary.
Stu
Stu, thanks for another very informative post. Any way to ensure I'm getting a recent production tube rather than say a NIB but few years old EH or Sovtek?ps I envy you guys who got into it when NOS tubes were still affordable. My buddy here in Tucson is set for life and doesn't have to deal with current production stuff. AFAIK, he has not had a tube go down since I've known him, about 5 years.
Hi Sue;The newer EH boxes (the black and white with the ram's head) are all new, as are the Mullard reissues. I believe the old Sovteks had those cheapy yellow and blue boxes, but I haven't been keeping close tabs on them, so the experience of others would be helpful.
I believe most current dealers have the new stock tubes.
BTW, Jim McShane, until recently, had a some nice NOS tubes, and probably still has a few lying around. I know I bought a lot of GE 6550A's from him.
Stu
I still have about 65-70 GE 6550As left - but if you want to try NOS look at the small tubes Sue. The financial strain is much less!
nt
Stu, Jim and Joe,thanks for your input. I shut the amp down and will switch the tubes when it cools off. I guess I'll have to call Kevin and see what he says.
I have some SEDs coming from Jim which will hopefully be here today or tomorrow. I'll be out of town for 2 weeks starting Friday so I don't know if I'll have this figured out by then.
Thanks again. You guys are awesome.
Sue
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