Tube DIY Asylum: One stage is not enough. by Joelt Do It Yourself (DIY) paradise for tube and SET project builders.

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Posted by Joelt (A ) on November 17, 2003 at 19:01:02

In Reply to: ECC99 sounds great! posted by AudioGenesis on November 17, 2003 at 18:26:59:

> > I'm no longer a fan of high gain - this amp has plenty of gain for me, and easily drives my Reference 3A's to decent volumes. I say go without another gain stage if you don't need it.> >

Hi Robert,
I think you're confusing two distinct requirements: the output levels required by your listening environment, and the voltage levels required to bring the amplifier to its rated power. The ECC99 may be driving your amp enough to be loud - but it may not be doing a good job of driving your output stage to its potential, and within reasonable distortion limits. Have you measured the THD?
Swinging the 47+ volts needed to drive a 2A3 is not a job for one stage. It requires at least two IMO, and the last one should be choke, transformer, or CCS loaded.

There's no sense in using low distortion triodes in your output stage and then using a driver stage with 6% 2ndH.

You should never require any stage in an amp to swing its full bias. The lower the swing required, the more linear the operation. Hence why "high gain" is not a bad thing at all. Most amps don't have enough gain! Low mu tubes are great for driving transformers, lousy for swinging lots of volts linearly.

The single ended tube amps that have truly extraordinary measurements, without NFB, all use extraordinary means to do it. The "Axiom" 300-B is an example. Two stages of gain are used, and each stage swings through a very small region of its loadline, uses choke loading, and their PSU's are tightly regulated to boot.

regards,

Joel

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