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yesmynameisking

Can you use a Fisher 400 as a Preamplifier only?

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Without modifying the unit, is there a safe method to use a Fisher 400 just as a preamp without damaging the output transformer? I see it has a Speaker on/off switch. Does turning it to the "Speakers Off' off mode solve this issue?

Please no guesses, only reply if you're 100% certain. Hope I don't sound rude, just cannot risk damaging the unit.
 
 

 

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I just read through the manual, and the Speakers Off switch is only used when utilizing headphones, that's it, no other functionality.

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When the speaker switch is "off" there is a 25 ohm resistor that is in parallel with the load; the load is the headphones or in your case a preamp in series with a 330 ohm resistor. A typical power amp with a 100k in input when in parallel with the 25 ohm resistor pretty much still a 25 ohm load. Technically 1 / ( ( 1 / 100000 ) + ( 1 / 25 ) ) = 24.99  

 

 

Since there is a 330 in series with the headphones the minimum value can only ever be 23.2 ohms. So there is not much difference if using headphones or a preamp.

 

A load mismatch of 100% is safe with tube gear. BUT you should switch the speaker tap to 16 ohms though, the 24.99 ohm load is within the 100% mismatch (32ohm).

 

In short, yes it's fine just switch the speaker tap to 16 ohms :)

Edited by xxJPMxx

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I probably wasn't very clear now that I reread what I wrote. So step by step this is how I would do it.

 

Switch the speaker on off switch to the off position.

 

Change the speaker tap to 16 ohms. This isn't necessary but I think it will yield the best results.

 

Use the headphone jack as the output.

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I think you should email or call Craig of NOS Valves, and ask if he would know.  Better safe than sorry.

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The schematic is very clear, when the speaker switch is "off" there is a 25 ohm resistor loading the transformer's secondary. No harm can be done to the amplifier or it's transformers.

 

See page 7, R114 and R118.

 

No harm can be done to the power amplifier you are trying to use it with because it's input is of high impedance and even if it was low impedance like headphones R119 and R120 limit the amount of power.

 

http://vintagevacuumaudio.com/schematics-manuals/fisher/fisher-400-service-manual.pdf

Edited by xxJPMxx

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