Tube DIY Asylum

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All pentode el84se no GFB

Posted by Paul Scearce on March 22, 2011 at 20:34:35:

I've built a number of different amps, including some zero feedback SETs, and some push-pull amps. I thought I'd try my hand at single ended pentodes. I'm aiming for an output impedance similar to zero feedback SET designs, without using any global feedback around the transformer. My speakers are pretty efficient--I'm currently getting plenty of sound from a 2.5 watt EL34 screenmode amp. I have several el84 tubes in my junk box, so I decided to try designing around them.After mulling over the data sheets, I think I've come up with a design I like.

The input tube is an EF184. The two 50kOhm resistors connected to its plate form the load and a local feedback network around the output tube. If I calculated right, this should work out to about 20dB of negative feedback. This brings distortion from the output tube to below 1%, and output impedance to a bit below 5kOhms. The 100kOhm resistor from the plate of the el84 to the cathode of the ef184 provides a bit of additional feedback. I think it works out to around 6dB. Since this loop includes the ef184, I'm hoping it will bring distortion from that tube down to around 1% as well as raising the damping factor of the amp to around 4.

The amp is still in the design stage, so I'm open to suggestions or comments.