Re: gEDA-user: [OFF] high current amplifier

[Gene Heskett <gene.heskett@xxxxxxxxxxx>]

On Friday 22 May 2009, Joerg wrote:
>John Doty wrote:
>> On May 21, 2009, at 6:00 PM, Joerg wrote:
>>> Chris Albertson wrote:
>>>
>>> [...]
>>>
>>>> I'm thinking about tube amps that had an output impedance of about 1M
>>>> ohm that used transformers to drive 8 ohm speakers.  About a 100,000
>>>> to 1 ratio.
>>>
>>> 1M? What kind of tube was that?
>>
>> Well, that's a typical plate resistance for a small signal pentode, but
>>
>> Power pentodes have lower plate resistance.
>
>Small signal, yes. But a 12AX7 won't be enough for a rock concert ;-)
>
And its not even a pentode, its a dual triode, designed for phono preamps and 
such.

>> For a large signal amplifier, the load generally isn't matched to the
>> output resistance. Instead, it's roughly (peak output voltage)/(peak
>> output current), which is different. If you match the output
>> resistance, in most cases you'll clip at an output power well below
>> the capacity of the amplifier.
>>
>> And finally, the real issue here is the current required. 100 amps
>> will melt the wire in any audio transformer I've ever seen.
>>
>> Everybody seems to think Mark's soldering gun suggestion is a joke,
>> but I don't know. I think I'd get one, pull the transformer, measure
>> its characteristics, see if it might work (maybe a couple of them in
>> series/parallel or something). They're light, cheap, and the closest
>> thing to the requirement here I can think of. I'm sure they won't run
>> at full power all day without overheating, but for a test set that
>> might be OK.

Full power? A minute thirty maybe then needs at least a 5 minute cooldown.

>It's not a joke, quite viable maybe. But the solder guns I have used
>can't quite get to 100 amps. Maybe the 100W Weller I got for a client
>does, it cost around $30 at a hardware store. Another option is to use a
>regular (fat) mains transformer that has a bit of clearance between the
>packet and core. Run a wide sheet of thick copper through there, only
>one turn and leave its usual secondary winding alone. Or if to be driven
>from a generator drive that other secondary and leave the primary alone
>(and don't touch it ...). This results in a huge current capability.
>Another option may be welding transformers. Even my cheap one can
>deliver 160 amps for quite some time. But those are huge.
>
>As usual, Levente needs to take every piece of metal off. Wedding band,
>wrist watch, etc. Best not to have credit cards close by either because
>their magnetic strip might later be stripped of its information. BTDT,
>quite embarrassing when you take the guys out for lunch and the waitress
>comes back with "Your credit card doesn't work".

:-)

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
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Life is a whim of several billion cells to be you for a while.



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