Posted by Dave

August 22, 2011 - 08:01 am

Hi Dak -- Others have made mention of the input stage configuration as well. For this modification, I did not change the basic topology of the design (other than those presented) as the intent was primarily to focus on the changes that an improved match in the output stage, and improved HF stability would bring to the design. To your point however, I did the output stage/stability work presented here years ago, and at the time, also developed a triode input stage for it as well. However, that portion of the re-design was not presented due to the thoughts mentioned above. Still, for those interested, I am more than happy to make the information available. While a triode conversion for the input stage could use a single 12AX7 section as used in the HF-87/89 designs, maximum performance improvement comes from converting the input stage to a paralleled triode design, using a 6CG7 or equivalent tube. The only downside of this change is that the sensitivity of the unit then requires about 4 vac from the preamp to develop full power output. In reality however, this in itself is really a welcomed change, as the PAS, HF-85 and similar preamps all work well with this sensitivity level, with the added bonus that noise and cable issues are greatly reduced. If this additional modification is something you are interested in, let me know, and I'll look into posting that modification here as well. Thanks for your comments and interest! Dave

Posted by dak

August 20, 2011 - 10:52 pm

A very well thought out mod! It is a pleasure to read your thoughts on bringing this amp up to its full potential. I can't wait to start working on my units once i finish some of my other projects. I do have one question concerning the input tube. I am not very fond of the sound of the EF86 tube, and much prefer the sound of a triode such as the 12ax7 types or maybe the low mu like the 6cg7 or even the 6c4. Any thoughts about changing the input tube? best regards, Dak

Posted by Steve L.

August 07, 2011 - 02:44 pm

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