Using 70v amp output to drive 500 ohm head

[mratx]

Does anyone use the 70v speaker output on an amp to drive their 500 ohm cutterhead? Currently I'm using an 8 ohm amp and a transformer to drive a presto 1D, but a friend of mine has a nice tube amp he's selling cheaply, 70v output but no 500 ohm. Not sure if would get enough output to drive the head, since that would only end up being around 10 watts. Don't want to swap out the amp transformer for one with a 500 ohm tap, I'm only interested if it would work as is.

Thanks,

Mark

[markrob]

Hi,

It depends on the power output rating of the amp. The output impedance is given by R=(V^2/P). Since V=70.7v, the output impedance = 5000/P. For a 10- watt amp, that would be exactly 500 ohms.

Mark

[Gus]

Does anyone use the 70v speaker output on an amp to drive their 500 ohm cutterhead? Currently I'm using an 8 ohm amp and a transformer to drive a presto 1D, but a friend of mine has a nice tube amp he's selling cheaply, 70v output but no 500 ohm. Not sure if would get enough output to drive the head, since that would only end up being around 10 watts. Don't want to swap out the amp transformer for one with a 500 ohm tap, I'm only interested if it would work as is.

Thanks,

Mark

As an owner of Fairchild 500 ohm usually you need 20-40 volt to drive the cutterhead at 70v starts to distort my opinion is go with tube amp will you get nice sound cuts.
Also a good idea is to connect parallel with your cutterhead an A.C. meter @ 100v.

[thelegr]

If you need the output impedance of your amp to be 500ohm so it maches the input impedance of your transducer, some 6AS7 in parallel push-pull will absolutely do the job! AND it will be OTL amp, so you will avoid output transformers, noise caused from impedance mismach etc

PS usualy, tube amps at this wattage have output impedance 2k-8k ohm. If something can reach lower output impedances it may be a lot of tubes in parallel, each configured to play at very low power OR (never seen that) 1-2 quite powerfull tubes (aka GU50) which drive the output from their cathode.

[opcode66]

I'm happy to recoil your head to 4 or 8 Ohms. Check out my site for mor information. http://cutterhead.repair

That makes a mono head much easier to use.