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Dr.dr. KT88
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([email protected])
1/9/2002 1:48 PM
Voodoo, Finesse, and lack of experience
Not to distract from the THD thread, all things Class A and single ended, After 1.5 years of push
pull DIYing and learning from whole lot of you tube dudes, I´ve come to the point of needing the
"Finesse Factor" of "That which only comes from experience."
To wit, I have several Q´s.
1. How´s does one determine PI distortion vs. Pre-amp Distortion without a scope?
2. How does one optimize PI for most headroom. High plate Voltage?
2. what is the best way to achieve the least amount of distortion in a Preamp design? High or low
plate voltage, high or low current, big plate resistors etc...
3. And the same for power tubes.
My goal is LOUD and clean but not sterile. I have empirically arrived at various scenarios , but I
keep missing the mark and the help from you tube dudes, are there any dudettes here?, Is most
appreciated.
Dr.dr. Kt88- exposing his ignorance, a rare event indeed
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kg
1/9/2002 2:49 PM
Re: Voodoo, Finesse, and lack of
1) pi dist vs. preamp dist. w/o scope.
that's a tough one. do you have a signal generator? how about a cd burner so that you can burn a
test cd? do you have a true rms voltmeter? this is one of those scenarios that is VERY hard to
overcome w/o a scope.
2) optimal PI design for headroom
that's easy... headroom comes from voltage. high supply voltage, and large value load resistors
will maximize voltage swing. you should be looking at schmitt/differentials, not concertinas/split
load splitters. you should be looking at a big hairy-balled driver tube, like 12at7, or 12bh7, or
6n1p, or 12bz7, or 6sn7, so that the load of the grid resistors on the output stage is less of an
issue.
now, as a side-issue, i STRONGLY suggest you consider using a follower of some sort to drive the
output grids. doing so increases headroom and output power in a very noticable way, and makes
the amp very quick to recover from transients. mosfet/tube, doesn't matter. this allows you to use
the highest value plate loads you can without voltage dividing the signal at the output grids.
you could also use a interstage driver transformer for the outputs, which is pretty much the same
thing, although the drivers if plate loaded should be low rp tubes. side benefit is inductive loading
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of driver allows it to swing symmetrically around the b+ rail, above and below, increasing output
voltage swing capability/headroom.
2b.) least amount of thd in preamp.
make all the grid resistors as high as possible. make sure all of your plate load resistors are about
10x lower than the grid resistors, and no smaller. make sure your tube's inherent plate impedance
is on par with the plate load resistor. make sure that the quiescent plate voltage sits at B+/2.
current is not too much of an issue, but do not starve any tubes (like running any tube at <1mA of
plate current) where they all become non-linear. the curves do become straighter at higher plate
currents, but as long as you're away from the low ip curves you'll be ok.
3) power tubes.
least distortion... make sure your plate supply is well regulated and does not sag too much. if you
run your outputs in tetrode/pentode, make sure your screen supply is very well filtered (if not
actively regulated, which is even better). make sure you have g1 and g2 stoppers installed, but
make them as low a value as you can get away with without giving up too much reliability...
certainly 1k or less. use a beam tetrode not a pentode. use tubes with hard vacuums instead of
soft. if you really want linearity consider an output triode (or a triode strapped multigrid tube) run
into a high load impedance. drive the output stage from a low impedance, be it a directly coupled
follower or an interstage traffo.
for best performance/headroom and lowest distortion, design your amp to pass a signal that is
much larger than it would typically see. a 500w amp run at 20w has PLENTY of headroom, dig? you
want to overdesign the specs so that the amp will loaf along with typical program material.
if you post more specifics i can offer more detailed hints... ie. cash, weight, tube types, etc.
hth
ken
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Dr.dr. KT88
([email protected])
1/9/2002 3:00 PM
Re: Voodoo, Finesse, and lack of
First digestion then heartburn!
Thanks! I´ll spew some questions back atchacome the new day.
Dr.dr. KT88
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KB
([email protected])
1/9/2002 3:23 PM
Re: Voodoo, Finesse, and lack of
that's a tough one. do you have a signal generator? how about a cd burner so that you
can burn a test cd? do you have a true rms voltmeter? this is one of those scenarios
that is VERY hard to overcome w/o a scope.
A Spectrum Analyzer can be of great help especially in your situation as THD% can be calculated to
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the .00001 even though your probably at greater then .1% it is much better than trying to see it
on a Scope which as Ken stated is a Vital Necessity. Analyzers will allow you to do Frequency
sweeps at almost any bandwidth imaginable and they offer precise level and Amplitude readings
and parameter programing for desired settings. If interested your not going to beat the Audio
Prescision Co. which is located at 5750 SW Artic Drive, Beaverton,Oregon 97005,Tel: 503-6270832 Fax:503-641-8906 US Toll Free 1-800-231-7350,email: [email protected] or
http://www.audioprecision.com
I would also recommend the Kevin O'Connor book Principle of Power as a bunch of the stuff that
Ken is saying like interstage Coupling from the plate side and Cathode side and much more that
not many besides Ken try. (Actually some do they just are not as vocal as Ken, he is really great
with info)His other books are good also. Good Luck and find your tone dude,
KB
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Jason
([email protected])
1/9/2002 3:32 PM
Single Ended Conversation
Pierre,
"My goal is LOUD and clean but not sterile"
First, for loud and clean, single ended is most certainly not what most of us think of. You may be
able to build a loud and clean single ended amp, relative to other single ended designs, but push
pull is much more suited to this. Sterility in an amp has mostly to do with linearity. We don't want
to play through a Hi-Fi.
I have built about 5 single ended designs, and rebuilt all of them looking for the "right" feel and
sound. None of them had the gusto, IMO. Basically, they were all derivatives of Champ variations,
some using 6550's, 6V6's, EL-34's, EL-84's, etc., parallel dual triodes for either input stage or
driving stage. I even tried Plexi type of preamps. BTW, all used the same Fender Iron. This was
probably the weak link, so I wont dismiss single ended amps for guitar. My designs were just not
as good as others I have heard and played.
For ultimate power and tone, I like large filter cap values. This will increase sterility [HiFi
response], but I like to compensate by using adjustable fixed bias, shelving response, Iron, and
tubes to keep the tone. In otherwords, I get solid power, bass response, feel, which borders on HiFi
push pull amps, but still kicks ass for guitar[at least IMO]. Push pull cannot be beat for versatility
and tone, IMO. However, for single ended amp design, I would probably go for larger filters, 2-3
stages of gain, good Iron, GZ-34 or SS rectification, a bit of feedback, fixed bias, shelving, and a
tone stack. This would probably give me what I look for in an amp. I would definitely try ultralinear
and pentode/tetrode operation. Since single ended amps run mostly in class A1 or A2, I would
keep power dissipation within limits. Lower design center plate/screen voltages. Using fixed bias,
tone can easily be altered and held. Large filters will give better feel, response, and feel IMO.
Steve Melkesethian's site is way cool [one of my favorites since 1994--I miss those printed
catalogues!] and gives lots of great amp ideas, but it is a very specialized field [single ended].
There are other topologies out there that may give you what you want, especially if you are new to
amp building.
THD's Univalve is one amplifier I would definitely like to own. I plan on buying one because it
sounds great, looks great, has some cool [and extremely useful] bells and whistles, and Andy has
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been building killer amps for a long time. I have never met him, but he seems like a top rate guy,
and he would definitely get my business. I really want a hotplate--my new house is located near a
few older folks--my amps are way way loud!
Jason C. Arthur
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