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Post by aarondavis on Apr 5, 2011 7:47:24 GMT -7
OK, i'm curious - When my Z28 is set with the volume at say 9 or 10 oclock (pretty low) and bass is at 10, treble 8-9 - in other words all controls fairly well attinuated; what is the slight bit of breakup always present if I dig in?
Is it the Preamp tube (NOS Tesla EF806)? Is it the phase inverter (NOS Mullard 12AT7) Is it the power tubes (JJ 6V6)'s Tube rectifier?
Etc?
I still would like more clean headroom; what can I do?
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Post by Phil (aka Phil) on Apr 5, 2011 8:34:08 GMT -7
It's not primarily the EF806 or the rectifier. It's mostly the phase inverter and/or the power tubes. Read this: www.guitaramplifierblueprinting.com/files/Phaseinverter.pdfThis is another great article by Myles. He's tweaked FAR more amps than I ever have or will...he's the man. He'll have some great insight here. You've already installed a relatively high current phase inverter tube to drive the tubes (12AT7). Make sure that the phase inverter tube you're using is balanced! The next two things I'd look at that are easily user-changable are the power tubes and bias setting. You may be able to go with later-distorting 6V6's than the ones you already have. Make sure that the output tubes you use are very closely matched. Make sure that the bias setting is high enough to avoid crossover distortion. If I'm using humbuckers I find that both my Ghia and Z-28 have a little hair when I dig in even at 9:00. I'm running a 5751 in my Z28 and Ghia's phase inverters. Running a 12BH7 in the Ghia's PI position makes it really clean but I haven't tried this in the Z28.
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Post by detuned on Apr 5, 2011 9:30:05 GMT -7
+1 on the 5751 thing... I'm using one as my phase inverter in my Remedy, & it really opens up the power section, & cleans the amp up nicely overall... A 12AT7 will get you there too, but I don't personally like them much, except as a tremolo or reverb driver. The 5751 is a much sweeter tube (IMO!).
Bonus: both the 12AT7 & 5751 are extremely reasonably priced in the NOS market...
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Post by aarondavis on Apr 5, 2011 11:37:08 GMT -7
Thanks Phil, that's a great article.
Maybe Myles can chime in on this thread.
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Post by Phil (aka Phil) on Apr 5, 2011 13:48:42 GMT -7
Also, I guess one thing I wanted to say was that I've never thought of the Z-28 as a *totally* clean amp, even with the 5751 PI setup. It's kind of like the Ghia in that it really doesn't come "on" until 8:00 on the volume knob and with the right guitar it does start getting traces of hair by 9:00. At least mine does.
This does also go back to what we've discussed on this forum before: differing perceptions of "clean". Everyone hears it differently, especially with tube amps.
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Post by aarondavis on Apr 5, 2011 17:27:28 GMT -7
Yeah, the Z28 is *fairly* clean. I can do decent clean volume up to about 10 or 11 o'clock; which with most of my gigs is plenty loud enough. But with my main guitars it always has some dirt waiting in the background. My main guitars are a couple of Kolls; one with Lollar Imperials and the other with Dimarzio Virtual P90's. So they're both pretty hot. It's not just the pickups though, with OD pedals you gotta go slightly above unity or when you kick one on live it sounds anemic.
If I bias the tubes a little hotter won't that give a little more headroom?
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Post by Phil (aka Phil) on Apr 6, 2011 13:41:11 GMT -7
Yeah, the Z28 is *fairly* clean. I can do decent clean volume up to about 10 or 11 o'clock; which with most of my gigs is plenty loud enough. But with my main guitars it always has some dirt waiting in the background. My main guitars are a couple of Kolls; one with Lollar Imperials and the other with Dimarzio Virtual P90's. So they're both pretty hot. It's not just the pickups though, with OD pedals you gotta go slightly above unity or when you kick one on live it sounds anemic. If I bias the tubes a little hotter won't that give a little more headroom? It's not as simple as just biasing it "hotter". I can't write a three page essay right now; I'm leaving for karate Basically you want to bias a guitar amp between the point when you get crossover notch distortion as you approach class "B" operation, and the point when the tubes glow red and are ruined That's not a huge range like "dirty Deluxe" to "clean Twin" all from the same amp. It's narrower than that. The way most folks bias their amps is to shoot for 60-70% of max plate dissipation when the amp's at idle (ID or idle dissipation). This is a "safe" ballpark for most guitar amps that takes into account what happens when the amp's not at idle anymore and your input signal is gyrating and you're going into a reactive speaker load. Without knowing a particular amp's plate voltage and the plate load impedance, and having an oscilloscope, that's about the best you can do, unless you're talking about going by ear. Somewhere above the cold biased crossover distortion point but below the over-dissipation point is in fact the maximum clean power before distortion point. However, I don't think that you'd see a huge difference in the kind of headroom you're looking for in optimizing the exact bias setting with a scope vs. going with 65% or so ID for a given tube. The tube's "distortion rating" (like going from a 5 to a 7 on the od GT scale) and rebiasing for that tube will give you a much more noticeable result than just rebiasing (optimizing the bias on your current tubes).
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Post by aarondavis on Apr 9, 2011 13:01:23 GMT -7
You know, I hadn't really turned the Z28 up since I had installed the Tesla EF806, Mullard 12AT7, a new "robust" Chinese 5AR4 recto and a new set of "late distorton" JJ 6V6's.
Last night I turned the amp up a bit, volume at noon, bass at noon, treble at 8 oclock. You know, it was quite clean... cleaner than with the older tubes. I dare say, I think it was plenty clean for gigging now. Sounded great too; what an amazing little amp.
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Post by Phil (aka Phil) on Apr 10, 2011 5:02:57 GMT -7
You know, I hadn't really turned the Z28 up since I had installed the Tesla EF806, Mullard 12AT7, a new "robust" Chinese 5AR4 recto and a new set of "late distorton" JJ 6V6's. Last night I turned the amp up a bit, volume at noon, bass at noon, treble at 8 oclock. You know, it was quite clean... cleaner than with the older tubes. I dare say, I think it was plenty clean for gigging now. Sounded great too; what an amazing little amp. Did you rebias the amp for the "later distortion" 6V6's?
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