Kamel
Well-known member
found this on the web
http://www.freewebs.co.uk/valvewizard/index.html
some good to start articles
http://www.freewebs.co.uk/valvewizard/index.html
some good to start articles
valve wizard
- Thread starter Kamel
- Start date
Help Support GroupDIY Audio Forum:
MasonAtom
Well-known member
analag said:
Did you check the link? Merlin's site offers quite a lot of information on tube design. His first book on guitar preamps is outstanding and he is currently working on a follow-up focused on guitar power amps. I've heard a few of his own designs in action and they are top-notch.
Mason
MasonAtom
Well-known member
MasonAtom said:
whoops... I just re-read your post and I think that you are implying that proper design requires more than just taking the manufacturer at their word in terms published grid curve vs. plate voltage plots. Sorry for the misunderstanding.
On first read I thought you were saying that his site was just a repository of tube data sheets.
Variance in tube performance is an interesting issue. I know that some manufacturers release anything within 10% or even 20% of specs. Didn't Marshall make a career out of buying cheap tubes that didn't pass mil spec? For my part, although I only have limited experience in designing tube gear, I've found that the measured voltages are usually within 5 to 10% of what I've plotted out in my load lines. I've never seen any horrific mismatches between published performance and actual performance, but again I only have limited experience. Also, it is one thing to calculate a load line to set your component values , and another thing entirely to understand how that translates in terms of sound. Some of my crappier sounding amps are testament to that.
Mason
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> usually within 5 to 10% of what I've plotted out in my load lines. I've never seen any horrific mismatches between published performance and actual performance
The full-complete spec sheets often claim +/-30% tolerance on key parameters.
The majority of tubes of one type will fall in a tighter window. But you can't count on it.
Real rejects are usually WAY off. Like 1/10th of what you expect. Or just dead. Half-right tubes are quite rare.
Much of the MIT RADAR "Vacuum Tube Amplifiers" book is about designing so that ANY non-reject tube WILL work. You can't hand-select tubes when the bombers are coming.
If you can't duplicate the "TYPICAL" conditions, +/-20%, after swapping a few OLD tubes, your test rig is wrong. (NEW production tubes are built different, and often adjusted in design to give "good" result in "popular" circuits, not to match the Show-Off "Typical" in the old data-sheet.)
The old graphs are correct (for old tubes) in the top and middle. At very low current (below 0.5mA on 12AX7), they were usually "drawn smooth" rather than based on real data. If you have a critical design working at the bottom of the curves, expect to have to trim the first prototype, and then verify with a box of assorted tubes to be sure they all land at tolerable conditons. But you can often do well by taking Known Good conditons, and scaling all resistors by the same factor.
The classic Fender-amp 12AX7 1.5K 100K stage is fairly tolerant. "Significant" cathode and plate resistors tend to fight any variation in the tube. It is common to find that most tubes bias-up with plate at 65%-75% of B+, very few as far out as 60% or 80%, gain generally within 10% of bogey spec. Good enough for guitar. Such technical variations have much less musical effect than the "flavor" some (possibly flawed) tubes add to the tone.
This is no different than building a house. How strong is wood? Even if you stick with "#2 SPF", one stud takes 4,500psi and another cracks at 2,700psi. You design so that the WORST POSSIBLE case only needs 3,000psi average (soft studs shed stress to adjacent studs).
Yes, Merlin knows his stuff. I wish more tube-writers were his level.
The full-complete spec sheets often claim +/-30% tolerance on key parameters.
The majority of tubes of one type will fall in a tighter window. But you can't count on it.
Real rejects are usually WAY off. Like 1/10th of what you expect. Or just dead. Half-right tubes are quite rare.
Much of the MIT RADAR "Vacuum Tube Amplifiers" book is about designing so that ANY non-reject tube WILL work. You can't hand-select tubes when the bombers are coming.
If you can't duplicate the "TYPICAL" conditions, +/-20%, after swapping a few OLD tubes, your test rig is wrong. (NEW production tubes are built different, and often adjusted in design to give "good" result in "popular" circuits, not to match the Show-Off "Typical" in the old data-sheet.)
The old graphs are correct (for old tubes) in the top and middle. At very low current (below 0.5mA on 12AX7), they were usually "drawn smooth" rather than based on real data. If you have a critical design working at the bottom of the curves, expect to have to trim the first prototype, and then verify with a box of assorted tubes to be sure they all land at tolerable conditons. But you can often do well by taking Known Good conditons, and scaling all resistors by the same factor.
The classic Fender-amp 12AX7 1.5K 100K stage is fairly tolerant. "Significant" cathode and plate resistors tend to fight any variation in the tube. It is common to find that most tubes bias-up with plate at 65%-75% of B+, very few as far out as 60% or 80%, gain generally within 10% of bogey spec. Good enough for guitar. Such technical variations have much less musical effect than the "flavor" some (possibly flawed) tubes add to the tone.
This is no different than building a house. How strong is wood? Even if you stick with "#2 SPF", one stud takes 4,500psi and another cracks at 2,700psi. You design so that the WORST POSSIBLE case only needs 3,000psi average (soft studs shed stress to adjacent studs).
Yes, Merlin knows his stuff. I wish more tube-writers were his level.
Also wish there were more writers who focused on voltage amplifiers in mic/line amp applications -i.e the nice vintage recording gear we all love. The majority are written for guitar amp and power amp applications. Lots of overlap but a different focus.
RDH is excellent as is Electronic Designers Handbook by Landee, Davis, and Albrecht -McGraw Hill Publishing.
A good bridge from these texts to the average DIYer level can be found here:
http://www.audioxpress.com/resource/audioclass/index.htm
As stated elsewhere on this forum by NY Dave, these are must read articles that cover it all from from power supplies to power stages and are written in a voice that's somewhat missing in the RDH and EDH - that of the in-class Instructor
RDH is excellent as is Electronic Designers Handbook by Landee, Davis, and Albrecht -McGraw Hill Publishing.
A good bridge from these texts to the average DIYer level can be found here:
http://www.audioxpress.com/resource/audioclass/index.htm
As stated elsewhere on this forum by NY Dave, these are must read articles that cover it all from from power supplies to power stages and are written in a voice that's somewhat missing in the RDH and EDH - that of the in-class Instructor
sahib
Well-known member
lassoharp said:
I think we should pay in advance for the purchase to PRR to encourage him to right that book. I mean it. He is the man who could and should fill that gap.
I was flying out to Istanbul for a week and I ordered Blencowe's book as my holiday reading, which I am enjoying very much. The language he uses and his writing style is straight to the point. But most importantly you can apply all that to design a mic amp or eq if you think a little broader. He does an excellent job on explaining frequency shaping. Take page 28 to 31 for example. He talks about cathode bypassing but the last graph you end up is very much like RIAA equalisation (reverse). You already killed two birds with one stone.
Blencowe's book sounds worth checking out. Funny you should mention cathode bypassing and reverse RIAA curve - just recently experienced that effect firsthand by mistake during a mic pre build.
Bruce Rozenblit has a couple of very good tube texts as well. A Begginer's Guide To Tube Audio Design and Audio Reality
Count me in for an advanced copy of a PRR book!
Bruce Rozenblit has a couple of very good tube texts as well. A Begginer's Guide To Tube Audio Design and Audio Reality
Count me in for an advanced copy of a PRR book!
