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

[Joerg <joergsch@xxxxxxxxxxxxxxxxxxxxx>]

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 ;-)


> 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.
> 

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".

-- 
Regards, Joerg

http://www.analogconsultants.com/



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