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Post by afterbang on Apr 26, 2008 12:25:44 GMT -7
Is that normal? I'm getting 396v with my new mullard rect. with my GEC 6v6g's biased in their normal range (28mA). Tone is definitely tighter, thinner, cleaner and way way brighter than before. I even reinstalled the stock GT 6v6r's and got the same sound. Did I just get used to the dying old rectifier tube or have I thrown off something with the new tube? And if the higher voltage is normal, should I adjust to a different bias current? The amp almost sounds as though it is biased "cold" even when I have the tubes up toward 31mA. I should add that the stock recto had died and was causing the fuse to blow (could something else be going on?). Thanks.
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Post by afterbang on Apr 26, 2008 15:44:13 GMT -7
Gigged this z28 last night with the nos 6v6g's and 5ar4 and needed to dime my volume to get the amplitude and overdrive i normally get at 12:00-1:00. That really is my main concern. The tubes came from KCA and are tip top as far as I can tell. As previously stated, the amp feels very very "cold." Should I be biasing this thing up into class a range (35mA)? I just feel like I should ask before I start fooling too much.
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Post by Paul (TRANE) on Apr 26, 2008 17:57:14 GMT -7
Ask this one of myles in the ask the experts section. He is the tube & bias guru on here.
Paul G.
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Post by hdahs143 on Apr 26, 2008 20:16:25 GMT -7
28ma at 396 plate volts is a long way from cold. It's pretty flippin hot. The 35ma you were talking about in your second post, would be a very bad idea, and is way too hot for a 6V6 at this kind of voltage. If you look in the BIAS NOTES section of the site, there are instructions on biasing this amp, which instruct you to adjust the bias pot until you get a voltage reading of 375 VOLTS. I would double check yourself, and if you dont find the answer, contact Myles or the Doc, but DONT bias it any hotter. This link is great for bias values. www.webervst.com/tubes/calcbias.htmGood Luck!
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Post by afterbang on Apr 26, 2008 22:30:04 GMT -7
Thanks for the responses. Just got back from a gig having spent a good amount of time testing before. I definitely hear you with regard to the bias range and I spent plenty of time using the Weber calculator. However, dialing the amp with the new recto tube down to 375v got me a bias current up into the 40's (i found there's an inverse relationship with regard to setting bias and voltage on the pot). On top of that, using numbers equal to what the calculator indicated got me an amp that was nearly clean all the way to full clockwise (really thin and nasty). The Mullard 5ar4 is clearly looking to provide higher voltage and the tubes needed more bias current (if that's a correct way of putting it?). To make a long story short, the amp killed tonight as it has been for years when the amp was dialed into near class A range. The only difference was even more dynamics, more headroom, extended bandwidth, really tight lows, on and on (none of the biased-too-hot side-effects)... Time will tell if the tubes hold up (aren't these amps supposed to be pretty easy on tubes?), but I got it sorted and I appreciate the advice. peace.
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Post by myles on Apr 30, 2008 8:24:02 GMT -7
The Z-28 can range in voltages and your observed voltage is not out of line.
A Deluxe Reverb can run over 425 in some cases by the way.
I do not not like the method of setting bias by cranking the tubes up until the plate voltage hits 375. You can end up with some pretty hot running tubes and harsh tone along with very short tube life.
I your amp with a bias tool this is where you want to be in milliamps: ... 21-25mA
60% 70% 21 25
The Weber chart from the link in another reply here would say 21.3 mA is 70% but it looks as if he is figuring a 6V6 at 12 watts as an EL84. I find them to be 14 watt tubes. Some data sheets call them 12 watters and some 14 watters.
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