how does one calculate the power requirement for amplifier

[Hebert]

 I have a schematic for a one 6v6 amplifier but don't know how to make a calculation to determine what power transformer to get. for example these numbers: 550 center tap, 255-0-255. how are these numbers determined? is is based on the 6v6 tube's data? thanks

[PRR]

Easy cheat: one 6V6 is a Fender Champ. Get a Champ PT.

Yes, the other way is to use data. Do you have that? Do you want to chew it? Most builders do not.

[jjasilli]

Tube Chart:  http://electronbin.com/sheets/135/6/6V6GTA.pdf


Look @ Class A operation
Pick a plate voltage, say 250; Look down that column
Max Signal Plate Current 47 + Max Signal Screen Current 7.0 = 54mA. (Disregard current draw of preamp tubes as to small to bother with)


Rough estimate: PT should have output yielding 250VDC @ 54mA. 


With SS rectification, 178VAC PT secondary X 1.414 = 250VDC;  Tube rectification will have a lower multiplier, depending on which rectifier tube type. 


Anyway, that's the basic idea.

[tubenit]

Take a look at this "tube data cheat sheet".  LOTS of information there that may be useful. 

PRR also gave great advice in looking at what has already been made that is similar and seeing what PT & OT they use  ........... OR what replacement PT & OT is available for that specific amp and read the specification on it.

With respect, Tubenit

[d95err]

Easy cheat: one 6V6 is a Fender Champ. Get a Champ PT.

^THIS^

Data sheets are nice, but they are usually ancient, conservative and designed for audio/hifi/PA application. Guitar amp construction has different premises and manufacturers have abused tube limits ever since the dawn of guitar amps.  For example, if you look at data sheets for EL34 tubes, you'll find that their application examples look nothing like the actual voltages used in a Marshall amp.

The easiest way to find the right transformers is to pick an amp with similar characteristics to the one you want to build (e.g. a Fender Champ) and pick a set of transformer the vendor lists as a replacement for that amp. Theory be damned, you'll know that it will work!

[Hebert]

thank you everyone

[PRR]

> Guitar amp construction has different premises

Well, the PTs used in most 6V6 Fenders are the same spec (originally old stock) as good 6V6 radios. The change is that the last 6V6 radios had field coils shaving ~~100V, Fender had magnets and used that extra voltage for audio power. This is also why Champ uses 7k load while 6V6 radios are usually 5k load.

If you don't have to stand in a showroom and whup the other brand for sound/price, none of this is very critical.

[66Strat]

Power Transformer
https://www.mojotone.com/amp-parts/Power-Transformers/Tweed-Champ-Power-Transformer-120V-240V

Output Transformer
https://www.mojotone.com/amp-parts/Output-Transformers/Blackface-Champ-Output-Transformer

These should work well for the Valco circuit.

[pdf64]

...Data sheets are nice, but they are usually ancient, conservative and designed for audio/hifi/PA application. Guitar amp construction has different premises and manufacturers have abused tube limits ever since the dawn of guitar amps.  For example, if you look at data sheets for EL34 tubes, you'll find that their application examples look nothing like the actual voltages used in a Marshall amp...

The typical conditions suggested in the manufacturer info, unless otherwise indicated, don't include HT sag. At full load, many Marshalls conform pretty well to those suggested typical conditions, it's just that HT sag causes their idle conditions to appear somewhat different.

[jjasilli]

The question is how to calculate.  Plagiarism is great, but doesn't actually answer the question posed.  For that the tube charts are important. For ease of illustration, in my Reply above I pointed to the Typical Operation section of the Tube Chart. You can get deeper into the weeds.  Further down are graphs showing the tube curves.  Or, you can use your own test equipment & plot your own curves.  Draw your own load lines.  R&D various circuits & measure clean power output, etc.  This does not detract from the Typical Operation section as a useful guide, especially when compared to actual, well-regarded amps.

[HotBluePlates]

The question is how to calculate.  Plagiarism is great, but doesn't actually answer the question posed.  For that the tube charts are important. For ease of illustration, in my Reply above I pointed to the Typical Operation section of the Tube Chart. You can get deeper into the weeds.  ...  This does not detract from the Typical Operation section as a useful guide, especially when compared to actual, well-regarded amps.

"Deeper in the weeds" are "Book chapters on amplifier power stage design" (or perhaps whole books).

Lifting Typical Operating Conditions from the data sheet is "more elaborate plagiarism."

...  Further down are graphs showing the tube curves.  Or, you can use your own test equipment & plot your own curves.  Draw your own load lines.  R&D various circuits & measure clean power output, etc.  This does not detract from the Typical Operation section as a useful guide, especially when compared to actual, well-regarded amps.

The problem is any B+ from 100v to 400v is probably acceptable, requires different current & OT primary impedance.  One can make the design process as elaborate as desired, beginning with "how many dB SPL, and what's my speaker's sensitivity?"  And then work through the whole design process with some arbitrarily-selected PT, only to find out the desired/ideal OT impedance is not a readily-available part.

So there's a wide range of "right answers" to be found by designing, relatively few "parts on the shelf" available to buy.  Most of the available parts are for standard amp circuits, widely known & copied.

I have a schematic for a one 6v6 amplifier but don't know how to make a calculation to determine what power transformer to get. for example these numbers: 550 center tap, 255-0-255. how are these numbers determined? ...

Transformer makers produce (standard model) transformers that match typical circuits.  Jjasilli is right that it might come from tube data sheet examples (Application Notes published way back when a type is introduced show the manufacturer trying & documenting "optimum operating conditions").

Personally, if I wasn't copying a known plan, I'd start by assuming zero voltage drop from the peak voltage of a PT's high voltage winding.  If the PT says "225-0-225v" it implies a full-wave rectifier, and 225vac * 1.414 = ~318vdc.  The current rating is important, and if you want to idle the 6V6 at 100% dissipation you need 12w/~300v = 40mA (or a bit more), then add 5-10mA for the screen and another 5mA or so for the (simple) preamp.

You will find higher figures on (for example) Classictone's Champ PT but they're using a higher voltage tap, assuming a tube rectifier dropping some of that higher voltage, and they're true to the blackface Champ amps that used the same PT that was also used in the Princeton & Princeton Reverb amps (which need more current for 2x 6V6s and higher power output).

IMO, there's no way to get only "a little in the weeds" on this stuff without omitting enough related considerations that would lead to false assumptions & misunderstanding.  That book on tube amp power sections is starting to look really good right about now...

[jjasilli]

Long time no see! 


Anyway, yeah - so Typical Operation is a good place to indicate how the designs we now plagiarize were originated.

[shooter]

Long time no see! 

+1

[pdf64]

Long time no see! 


Anyway, yeah - so Typical Operation is a good place to indicate how the designs we now plagiarize were originated.

Unfortunately 6L6 and 6V6 info doesn’t seem to include anything close to how Fender operated them in push pull arrangements, after the ‘B’ series tweeds.
That I’ve been able to find, anyway 

[shooter]

how Fender operated them in push pull arrangements

 

That's like figuring out Gibson's why they.....

[jjasilli]

Long time no see! 


Anyway, yeah - so Typical Operation is a good place to indicate how the designs we now plagiarize were originated.

Unfortunately 6L6 and 6V6 info doesn’t seem to include anything close to how Fender operated them in push pull arrangements, after the ‘B’ series tweeds.
That I’ve been able to find, anyway 

Do you think you're being of help to Herbert, who asked a perfectly good question.  How's this to sum up: sorry Herbert, the answer to your question is an impenetrable mystery with no right answers.  There's no way to get merely a general idea about calculating PT criteria without first plowing through a dense textbook.  Whatever you do, don't even glance at Typical Operation stated for your benefit by the Manufacturer, or you'll be turned into a pillar of salt.

[PRR]

> Unfortunately 6L6 and 6V6 info doesn’t seem to include anything close to how Fender ......

No. RCA/GE wanted to sell more hi-price tubes and didn't want a lot of tube failures. They kept suggestions modest even after they improved materials, suggesting you move to the hi-price 6L6 above 14 Watts. Even reprinting all the odd old 6L6 suggestions on the uprated 6L6GC datasheet, with just a few suggestions to use the higher rating.

There is also no maker data how to make my 148 horsepower Honda deliver 400 horsepower. But they do this every weekend at the dragstrip. Gosh, how is this possible??

Leo probably *experimented*. Same as the Honda nuts. Crank it up and see what blows. He may also have had informal advice from the makers; GE published a high-THD short-life suggestion for 6L6GC in amateur radio use.

However I don't think Hebert was asking about hot-rod application. And there is not a lot you can do with a SE amp. Your B+ supply should be a "good voltage" (though rarely what the tube maker suggested) with enough current to meet the tube's dissipation plus cathode resistor loss. So for 6V6: 300V 45mA to 400V 35mA.

In fact nearly all the later 6V6 Champs used the PT from the 2-6V6 amp, because the difference in cost was small, and there was economy in stocking just one part.

[pdf64]

...Do you think you're being of help to Herbert, who asked a perfectly good question...

With specific regard to the SE amp query of post #1, no.
But I remember reading various threads querying the discrepancy between the operating conditions of Fenders and those suggested in tube info typical conditions.
The thread title wasn't SE specific, and so searches regarding p-p configurations may turn this up.
And Herbert may get the taste for this and move on to a p-p build later.
So yes, I think my post, and especially the counter from PRR, is helpful.