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Topic: Preamp tube questions |
Michael Pierce
From:
Madison, CT
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Posted 3 Jul 2009 4:28 am |
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1. Would someone explain the difference between a "regular" preamp tube (e.g., a 12AX7) and a "balanced triode tube for phase inverter" use (e.g., an ECC83S, a type of 12AX7, I believe). Can the balanced triode version of the 12AX7 be used in place of a regular 12AX7, or is its usefulness limited to the phase inventer role?
2. I've read here on the Forum that substituting a 12AT7 for a 12AX7 in the preamp position (V1, for example) can result in a "thinner" sounding amp. I've also read that it can result in less gain, but more note clarity and definition. Are those inconsistent views? I have a couple of brand new JJ 12AT7's that I'm wondering if I should hang on to for use in my 1970 SF Princeton, which currently has 12AX7's. Mostly play at bedroom volume...
as always, thanks, mjp |
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Jon Light
From:
Saugerties, NY
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Posted 3 Jul 2009 5:55 am
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It is my understanding that 'balanced triode' means that they test out as balanced as opposed to that they are made any differently----in other words 'they happen to be balanced'. I could be wrong. But in any case there is no harm substituting any 12A*7 tubes and the worst thing that would happen if an unbalanced tube were used in the inverter slot is that it wouldn't sound great. And there is nothing bad that could happen with a balanced tube used as V1 or V2. The inverter is the only place where the relationship between one side of the tube and the other matters.
The only thing thin about using a lower gain tube might be that there is less distortion from not pushing the stage as hard. If one is accustomed to higher gain then I guess the lack thereof might sound 'thin'. Simply, try all options in V1 and use what you like. 12AT7 is a standard check-list item if you are trying to increase headroom.
btw--I don't know the Princeton but all the Fender ampsthat I've known use a 12AT7 (ECC 81) for the inverter. |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 3 Jul 2009 6:36 am
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I think the "balanced triode" designation is mostly sales hype. A 12AX7 tube is a dual triode, essentially two simple triode tubes in the same envelope. The two sections are designed to be as close as possible in performance. They do this by making them mechanically and dimensionally identical. While a tube that had been subjected to improper use might exhibit differences in the performance of each section, a new tube likely would not. Any differences would probably be too small to notice in our applications (audio amplification). A balanced tube would also be fully interchangeable with any other tube of the same type and manufacturer.
Substituting tubes with different characteristics is an old idea, and it works for some amps. In others, what the manufacturer chose proves to be the optimum. |
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Brad Sarno
From:
St. Louis, MO USA
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Posted 3 Jul 2009 7:33 am
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If I recall correctly, the 12At7 has a more stringent spec for the two sections to be balanced (equal). It's also a lower voltage gain tube, and does a better job at driving current. That makes the 12At7 popular as a phase splitter, where matching is more critical. A 12At7 is a lower gain tube than the 12Ax7, so it will sound different, and some people do say it's "thinner" sounding.
Matching the two sections of a 12Ax7 exactly just isn't critical in most applications. Maybe if it's in an audiophile stereo amp and each half drives a left and right channel and people want those exactly matched, or in an amp using a 12Ax7 in a long-tailed phase splitter. But I don't recall seeing much regarding 12Ax7's being perfectly balanced. But surely a tube dealer could test a bunch of them and hand select the ones that happen to be matched.
An ECC83 is the European name for a 12AX7. And I believe that an ECC83s is one with "spiral" filaments to reduce hum in amps with AC heaters. No indication of being exactly balanced though.
Brad |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 3 Jul 2009 8:15 am
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On 1: http://www.dougstubes.com/faq.html#g1
On a "balanced" triode, the current draw is approximately the same for each triode section when subjected to the same pin voltages. Tubes like 12AX7, AT7, and so on are designed to be balanced, but the issue is how close they get, and that varies from tube to tube for a variety of reasons. Whether or not "closely balanced" is a good thing depends on the circuit and one's preferences.
On 2: The nominal gain and gain ratio between various tubes are in this table:
| Code: |
Gain Ratio Relative to
Tube Gain 12AU7 12AV7 12AY7 12AT7 5751 12AX7
12AU7 19 1.00 0.46 0.42 0.32 0.27 0.19
12AV7 41 2.15 1.00 0.91 0.68 0.58 0.41
12AY7 45 2.37 1.10 1.00 0.75 0.64 0.45
12AT7 60 3.16 1.46 1.33 1.00 0.85 0.60
5751 70 3.68 1.71 1.56 1.17 1.00 0.70
12AX7 100 5.26 2.44 2.22 1.67 1.43 1.00 |
There's nothing inherently "bad" about 12AT7 tubes in an early preamp position, but my experience with many examples tells me that most I've tried sound a bit thin in the early preamp sections. I don't know why - perhaps they were more intended as utility tubes for applications like phase inverters and so on, as opposed to being an integral part of the tone stack. If I want to lower the gain a bit, I have had better luck with the 5751 or 12AY7 - that's what I usually use in the first tone-generating section of BF and SF Fender amps for pedal steel application. I sometimes find that a hotly wound pedal steel pickup overdrives that first gain section.
But overall - as long as preamp tubes meet basic specs and aren't too noisy or microphonic for a particular amp application - I judge these tubes strictly based on how they sound in that particular amp with the guitar(s) I want to use. It's nice to have rules of thumb, but I am sometimes surprised at the results, good or bad. The good side of this, however, is that it's possible to change the nominal sound of the amp by changing tubes.
My take, anyway. |
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Jim Sliff
From:
Lawndale California, USA
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Posted 6 Jul 2009 3:05 pm
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The 12AT7 was not originally (and still isn't ) made as an audio-signal tube. It excels at other functions - tremolo oscillators and phase inverters - but is not usually a good choice in a preamp slot.
Dave's gain chart is a good guideline - but you'll find most preamp tubes of that type are 12ax7/7025's, 5751's and 12AY7's in descending gain order.
But you can mix and match, and sometimes good accidents happen. I've found superb 12AT7's for preamp use (but VERY rarely) and I've found many 12AX7's to be horribly microphonic.
I would suggest, if you don't know much about tube types, that you stick with the stock types and just try different brands. Once you get a feel for the sound of different tubes you can try different gain values - but stay within reasonably normal limits.
None of this applies to output tubes, where replacement is a more complicated project, especially in a fixed-bias amp (like the blackface and silverface models) where proper test equipment and ability to make adjustments are required.
Oft repeated, but still an issue - tube amps are NOT for everyone. IMO they are tonally superior to SS amps for almost every application - but they are not "plug-'n-play", and even come from the factories set to very conservative levels to save tube life but at the expense of optimum tone.
_________________
No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional |
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