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From:
Tyrannocaster
Date:
7/26/2001 8:45 PM
Subject:
Page 1 of 6
What do transformer specs mean?
270HX 176VA, sec. 275 -0-275, DC ma 200, Fil.#1(rct) 5.0v, 3a ct, Fil.#2(htr) 6.3v, 6.0a ct
That is for a Hammond power transformer. I understand the rectifier and filament winding
specs. What does the 176VA and DC ma 200 spec mean in practical terms?
Where do you look on a tube chart (RCA manual) to find the specs to add up to see if a
transformer will work with whatever tube combinations you want to use?
From:
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Bob I ([email protected])
7/26/2001 9:25 PM
Re: What do transformer specs mean?
I don't mean to be rude, but if I were you, I'd study some basic electricity before I get too far
into this. These are basic terms, VA is Volts per Amp, ma is miliamps, or .01 amp current. Both
numbers represent how much juice the transformer is cabable of handling.
You'll need to calculate the current draw from the power dissipation spec on the tubes.
Perhaps you could detail what you're planning to build and some of the more experienced
guys can help.
From:
Date:
Subject:
Tyrannocaster
7/26/2001 10:16 PM
Re: What do transformer specs mean?
No offense taken. I know it means Volts per Amp, and milliamps - that's not what I meant.
What I was getting at is what exactly is the process you use to figure out just what transformer
you would need. I don't have a specific project, but let's say you have some sort of a Vox AC30 thing, okay? First, you've got 4 EL84's, then you've got the 12AX7's (don't have an AC30
schem. in front of me so I'm making this up out of whole cloth)...let's say there are four
12AX7's - a PI, tremolo tube, and a couple of preamp tubes. Forgot the recto, let's call it solid
state. :-)
Okay, so you're saying that then you would look up the power dissipation spec on the tubes
and total them, right? Looking in my RCA Receiving Tube Manual all I see under 6BQ5 with
the word "dissipation" is Plate Dissipation, which is 12 watts. How does that relate?
From:
Date:
Subject:
Stan Cotey ([email protected])
7/26/2001 10:36 PM
Re: What do transformer specs mean?
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A very small correction to Bob's post:
mA = milliamp = 0.001 Amp of current.
From:
Date:
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Don Symes ([email protected] )
7/26/2001 10:04 PM
Re: What do transformer specs mean?
VA - VoltAmperes => Roughly, but not exactly, the same as power. The transformer is capable
of supporting a total load, including unavoidable core losses, of about 176 watts.
6.3V * 6A + 5V * 3A + 275V * .2A + 275 *.2A = 160VA
Add 16VA for core and rectification losses -> 176VA.
HTH!
From:
Date:
Subject:
MBSetzer ([email protected])
7/26/2001 11:27 PM
Re: What do transformer specs mean?
actually VA, as the name implies, is Volt-Amperes (voltage multiplied by amps) which is the
same thing as Watts when you are talking about DC power.
Volts per Amp would be voltage divided by amperes, that would not be the same thing.
If I'm not mistaken, the 176 VA is the rated maximum power that can be drawn through the
transformer from all the secondaries combined, even though it seems to me it is a rating
dependent on the robustness of the primary winding.
The 200ma DC is the maximum current that should be drawn from the high voltage winding
after it is rectified, you would add up the cathode current requirements of the power tubes,
preamp tubes, and any other currents whose power source comes from that winding to make
sure it is below 200ma, proportionately lower depending on what safety factor you want to use.
The operating point and service class of the amp's power section will give an idea how much
the current needs of the power tubes will increase from idle conditions to full power output.
This looks like a Hammond part number, the 200ma figure is derived from their standard way
of rating it, you will only use this figure as-is if you assemble the same topology of power
supply that they do when they rate them. For different types of power supplies, like capacitive
input, resistive input, or choke input, and different rectifiers, like half-wave, full-wave, and
bridge varieties, you would use a factor to convert from the 200ma by their rating method, to
your proposed power supply type, if it is different than theirs. Maybe the Hammond website
has this info.
To predict current requirements from tube manuals the old -fashioned way, it is best to have the
complete characteristic curves for the tubes, then you simulate your proposed audio design to
find what kind of transformer it needs. The other common approach is to design or adjust a
conventional audio circuit to perform within the limitations of a given power transformer. If you
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are really good you could use a slide rule just like the greatest tube engineers of all time. That
can be done by computer simulation these days, but most guitar amp builders instead develop
familiarity with classic vintage circuits then extrapolate from there depending on the confidence
they develop.
There is no substitute for experience, like Bob says, post more details of your proposed project
and there will be more than one Ampager that will *just know* what will likely work for you.
If you want to make a little progress toward using the tube curves, I have some basic info
posted, this is the second chapter from an Army manual that Randall Aiken has posted the first
chapter on his website. He has lots more good info also, check his link from the Suggested
Reading links in the right frame of this BBS.
http://www.coastalquality.com
the most advanced way to match transformers with tube circuits would probably be to follow
the guidelines in the RCA Radiotron Designer's Handbook, 4th Edition, affectionately known as
RDH4. Many people work up to it from less comprehensive documents, but if you already have
RDH4 you could just look at the sections you need, then if you don't fully understand it, locate
supporting publications or even ask more questions on Ampage.
Hope this helps,
Miie
From:
Date:
Subject:
benboom
7/27/2001 12:34 AM
Re: Maybe that's MY problem?
Now this has got me curious. I have been having all kinds of trouble with an amp I made which
I thought had an appropriate power transformer, but I wonder. I made one with two EL84's,
which according to my tube data sheet (GE data) draw 65 milliamps each. (I didn't have this
data when I got the transformer.) My transformer is a Hammond that's rated at 83VA, 90ma isn't this way too low for a pair of EL84's, then? Could this cause the hum that I'm getting and
can't get rid of no matter WHAT THE !@#$ I do with the amp? It's also got to power the pair of
12AX7's, too.
From:
Date:
Subject:
Mark Lavelle ([email protected] )
7/27/2001 6:23 AM
Re: Maybe that's MY problem?
I think that drawing too much current would be mostly a matter of overheating & eventually
killing the tranny, but you're definitely stressing that unit in a big way...
From:
Date:
Subject:
jean
7/27/2001 6:34 AM
Re: Maybe that's MY problem?
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thats means its better to have a more powefull tranies????
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Don Symes ([email protected] )
7/27/2001 11:58 PM
Re: Maybe that's MY problem?
More current capacity is better.
Of course current is the thing that drives cost - larger wire sizes and (I think) more/larger
laminations to transfer the energy.
Yes, higher voltage for a given current is more power, but only if your circuit can use it.
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MKB ([email protected])
7/27/2001 4:56 PM
Re: Maybe that's MY problem?
Just looked at the EL84 data sheet; two EL84's in AB PP draws 2X46mA plate plus 2X11mA
screen, this is 114mA total at 300V. That is about a 25% overload for a 90mA TX; this isn't too
bad IMO. I would try it myself but I'd check the temp of the tranny closely to see if it overheats.
Idle current, AB PP is 2X36mA plate\2X4mA screen, for 80 mA total. It won't be stressed at all
there. BTW, you may be able to reduce the stress by leaving the 5V winding free or lightly
loading the filament winding.
Another thing to check is B+ sag from idle to overdrive. If you are stressing the tranny, you will
get a large voltage drop in overdrive.
Of course, this all assumes your amp is run in AB. What is your B+ voltage? 65mA @ 300V is
19.5W, the absolute max for a EL84 is 12W according to the Amperex data sheet.
From:
Date:
Subject:
Mark Lavelle ([email protected] )
7/27/2001 5:27 PM
Re: Maybe that's MY problem?
Aren't you forgetting a few 12AX7s?
From:
Date:
Subject:
MKB ([email protected])
7/28/2001 3:29 PM
Re: Maybe that's MY problem?
Okay, the preamp load is pretty small compared to the power amp needs. Around 1mA per
12AX7 triode stage maybe. But that does have to be considered in overall B+ loading.
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benboom
7/27/2001 7:22 PM
Re: The hell with it!
I think the new Hammond PT I got from Angela is defective. I am getting 295 volts out of it
AFTER the 40 volts of zener drop I had to put in in the first place because all the voltages in
the amp were coming out too high. (Remember , the PT is rated 275 at 115 volts AC, and my
AC is usually about 122, but still...) Plus, the Voxy Lady runs its EL84 plates really hot - 358,
Bruce said. This morning when I checked I was getting over 360. I don't think that with that
much zener drop the PT should be giving me all of this voltage.
I've had it with this thing. The amp has bitched and moaned every !@#$% step of the way, and
it's never sounded good enough to be worth all this trouble. It's now history, along with its
damn power transformer. I'd use it for target practice if it wouldn't break my arrows.
I guess this one was the devil's price I paid for making my other amp, which is an angel. :D
From:
Date:
Subject:
Stephen Conner ([email protected])
7/27/2001 10:08 AM
power stage design
To start with, you find out how much DC power you can get. Your transformer will give about
360-370V once rectified (multiply 275 by 1.41 and knock off 10-20V) and the spec is 200mA.
So you have (360*0.2)=72 watts. Now, power tubes in the usual class-AB setup are about 50%
efficient, sometimes more. So you can say that your transformer will probably be good for a
(72*0.5)=36 watt amp; either a red-hot 6V6 setup or a laid-back 6L6/EL34.
The next step is to get hold of a proper data sheet for the tube type you're interested in. An
entry in a tube manual often won't tell you enough. You are interested in data for two tubes in
push-pull class-AB. Here's the relevant part of the 6L6GC data sheet which runs to 9 pages,
you can download it from the web.
In theory, you would now find the hookup that ran off 360V B+ and drew about 200mA. Then
you would choose the screen voltage and output transformer (OT) accordingly. In practice,
most people use whatever OT they can find, and adjust the screen voltage by adding resistors
to get the right amount of power.
Steve C.
From:
jean
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7/27/2001 11:52 AM
Re: power stage design
yes thats very interesting
do you know about preamP tube???
how calculate them
i think its important because those tube too will draw some power so maybe need a litlle more
current ???
i mean in your exemple is i want to add 3 12ax7 in front of the power amp
Thank
Jean
From:
Date:
Subject:
Stephen Conner ([email protected])
7/27/2001 4:02 PM
Re: power stage design
do you know about preamP tube???
ya... each triode section in a 12AX7 will draw about 1.5mA from B+. So you can figure 3mA
per tube. 12AT7s and 12AU7s are capable of drawing more, up to around 20mA per tube, but
most guitar amp circuits don't bias them this hot. Don't forget each tube draws 300mA heater
current at 6.3V.
Steve C.
From:
Date:
Subject:
Don Symes ([email protected] )
7/28/2001 12:02 AM
Re: power stage design
Don't forget each tube draws 300mA heater current at 6.3V.
Each preamp tube. (Sorry to be pedantic -striving for clarity).
The power stage tube heater currents are higher.
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