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Post by bigben55 on Jan 3, 2019 14:28:44 GMT -7
I've been using a Shuguang 5AR4 in my Z28; it's what Dr Z recommends. Lately, I've noticed tube rattling. I use JJ 6V6S power tubes(also Doc recommended) and have had rattle issues with them in the past and have 2 tube dampers on each tube. I ordered the "double mica" ones, for less rattle in combos. The set has seen 8 months of hard use, figured it was time to replace them so I did(bought a matched quad). Still have audible rattling, kinda bad rattling! So i put on a glove, played some offending notes and grabbed the tubes to narrow it down. It totally was the 5AR4. So i pulled it and replaced it with a used known good one, a ri Genelex Gold Lion GZ34. NO rattle.
Questions: I know different recto tubes can have different voltages. Is doing this, just plugging in a different recto tube, "bad?" If so, how? Next question(hypothetical), if I "just stuck" another pair of 6V6s in there without rebiasing, would the potential "sounding bad" be the only issue? Could doing so damage my amp?
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Post by nicholas on Jan 3, 2019 18:11:56 GMT -7
Check this out.
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Post by bigben55 on Jan 3, 2019 19:59:39 GMT -7
I've seen this. But it doesnt answer my question. It's a great piece, describing what the different tribes do/dont do to the circuit and what can be subbed in what for what.
In my experience, recto tubes should be replaced with power tubes and a rebias. I'm asking if it's ok to pull out a 5AR4 and stick another one in. And the 6V6 question.
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Post by headshrinker (Marc) on Jan 3, 2019 21:10:24 GMT -7
A new rectifier likely won't change the current too much so it would hurt the amp or tubes, but that's not a guarantee. I personally wouldn't do it, but if I did, I'd play and listen to the amp while watching the power tubes to make sure they didn't red plate. As for the power tubes, these days there is such a wide range in them, that you are taking a risk that the bias could end up way off. Not a big problem if the tubes end up running cold, other than bad sound, running hot is a problem. Doing the rectifier and the power tubes together, in my opinion, is asking for trouble. Always best to check the bias on a fixed bias amp when you change the power tubes and/or rectifier. Now if you are only doing it briefly to find the tube that is rattling, I would do it, but there is still a risk.
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Post by nicholas on Jan 4, 2019 8:04:58 GMT -7
Your post is missing some info that would be helpful. From what I gather you bought a matched quad of 6v6s and have been using 2 of them for the last 8 months. You didn't mention if you had it properly biased for them. If you did, and you are just going to use the other two from the matched quad I'd say your ok.
Just changing a rectifier to the same type shouldn't effect the bias very much.
I'm reading between the lines here, but I'm assuming you are asking this because you don't have the means to bias it yourself. Personally I would check the bias when changing either, even in your situation. It's not difficult and the equipment needed isn't very expensive.
As far as the rattling, I've had issues with JJ 6v6's. Even the double mica ones. Electroharmonix 6v6gt can handle similar plate voltages and don't seem prone to rattles (so far from my actual experience). They sound good enough to me. I've been using them instead.
Anytime you replace the power tubes you need to have it biased. The exemption would be if you bought a pair from a respectable tube vendor that assigns the tubes a rating number, then have it biased for that rating and you continue to purchase tubes from the same retailer with the same rating number. In this case the tubes should be close enough to not cause damage or failure.
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Post by bigben55 on Jan 4, 2019 11:04:22 GMT -7
You are correct. I always buy matched quads and pay a tech to bias the amp, to Z specs, 28mA. I pulled 2 used ones, put in 2 new ones, discovered the rattle was definitely the Shuguang 5AR4(which I had the tech install with the new tubes/bias), so I pulled it and installed another recto tube. My ears can't tell any difference so I guess we're good.
I've flirted with buying a bias probe snd the stuff needed but for as infrequently as I do it, I'll just pay my guy $50 every 2 years. I'm not scared to bias myself and one day will, but for now dont.
My other question, regarding power tubes is, what "bad stuff" could happen if I just put 2 6V6s in it WITHOUT biasing? I dont plan to, but am curious. I understand it might not SOUND good, but what damage if any COULD occur.
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Post by daddyelmis (Greg) on Jan 4, 2019 11:50:14 GMT -7
My understanding is that possible damage from improper bias would be burning out the tubes with too hot a bias, and I suppose that could also impact related items (screen resistors).
I don’t know that a too cold bias would do anything other than affect tone.
FWIW, I got a 4 tube bias rig on eBay for $40 (plugs into a multimeter) and it works great and very simple - biased my Z28 in about 10 minutes when I put in new power and rec tubes.
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Post by bigben55 on Jan 4, 2019 15:33:10 GMT -7
Got a pic part # and the instructions you use for biasing? I have a cheap Centech multimeter
[quote
FWIW, I got a 4 tube bias rig on eBay for $40 (plugs into a multimeter) and it works great and very simple - biased my Z28 in about 10 minutes when I put in new power and rec tubes. [/quote]
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Post by daddyelmis (Greg) on Jan 4, 2019 16:06:53 GMT -7
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