Output transformer for a 6EM7 headphone amp

[firestorm]

I've decided to build my self a spud headamp. I'm thinking on the lines of Squeezebox --> spud --> AKG701.

I've got a few 6EM7 that should do the trick. The only SE transformers I have are 5k->8R scavenged from a reel to reel tape machine, I think the original amp had EL84 as output tubes.

I'm quite new the tube side of life, so can any one tell me if a 5k - 8T SE transformer will work ok ?

6EM7 --> 5k..8R --> 65R (AKG701)

If not I have in mind a multitap transformer, made by Oxford Electrical Products, part number N35A002F, RS part number 210-6475 (rswww.com), not sure if this link will work http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2106475

The windings with ratios can be found here:

http://img-europe.electrocomponents.com/images/L217567-01.gif

If I can avoid spending more money that would be ideal, otherwise, I believe I can go for the OEP transformer.

Comments much appreciated

FS

I'm based in the UK, most US items end up costing lots of money with S&H charges applied

 

[Tom Bavis]

With triodes, maximum output is achieved with a load impedance of 2-3 times the tube's plate resistance. Higher impedance will reduce output somewhat, with lower distortion - a good trade-off with headphones.

5K will work fine, and you can even deliver close to a watt to an 8 Ohm speaker if you want. Depending on the sensitivity of the headphones, you may want to attenuate the output with a series resistor or voltage divider. You should terminate the 8 Ohm output with 8-10 Ohms.

 

[firestorm]

excellent !

I've got all the ironware, I've just need to sort the caps and Rs and I'm ready to go.

I'll report on my progress as appropriate.

Thanks for your help Tom

 

[dsavitsk]

You may be fine just using the phones as the load. Try it, and if you lack bass, put a resistor in parallel to the load to see if that helps. Start with about 30R, and move lower if you think it helps.

 

[firestorm]

Hi dsavitsk
won't the 30R just lower the static resistance, but not affect the load impedance?

 

[dsavitsk]

no. This is the same advice Tom gave, I just said to start with 30R while he said to start with 8 to 10R.

 

[firestorm]

Sorry Dsavitsk,
my mistake - but my Q still stands, adding a resistor will let the secondary see the right DC load, but the AC impedance will not be changed.