[Whitespider]

As Pryso replied in Tube-O-Phile the SA-4 was designed by Roger Modjeski of Music Reference fame. Not all OTLs have reliability issues. But tube life is a fact of life. The 6LF6 tubes are no longer being made and stock is not plentiful. Nor you ever be looking at the tube being remade since it is not an audio tube but rather a video sweep tube. I own a Berning EA2100 amplifier which uses the same tubes.   This is not a OTL nor a ZOTL design amplifier but rather a output transformer amplifier using the tubes in a very conservative manner.  Ironically, the batch of tubes I purchased some decade+ ago came from a owner of a pair of SA-4 amps who had purchased a case of these tubes for future use.

David Berning is the only maker of custom ZOTL amplifiers. He has licensed the technology and some amp designs to Linear Tube Audio.  LTA is the only company that has successfully made ZOTL amplifiersbesides his own company.  I don't ever recall any DIY ZOTL amplifiers being made. This most definitely is not something a repair person would be able to convert your amplifier to ZOTL technolgy. Not plug and play.

You need to decide if OTL is the route you want to go. There are many, many tube amp manufacturers to choose from using different circuits, technology, and tubes.  If OTL provide the sound you want, go for it.  But do realize all amps using OTL circuits are going to produce lots of heat and may require a far amount of tube changing. It's the nature of the beast.

[Rusty Jefferson]

........ But do realize all amps using OTL circuits are going to produce lots of heat and may require a far amount of tube changing. It's the nature of the beast.

Agree with your post except this statement.  ZOTL amps don't run hot (even the class A models, though the tubes on his 845 amps are hot) and many owners never replace tubes in their ownership time which is commonly many years.

[Whitespider]

Sorry I wasn't clearer in my statement about the heat issue. I was specially referring to OTL amplifiers and not ZOTL which is an entirely different class of amplifier. I am very familiar with Berning's works having owned his preamps and amplifier going back decades. In general all of his stuff runs cool and and have very long lasting tube life.

But ZOTL is not technically an OTL in the classic sense. The lack of output transformer is about the only thing they share in common. And ZOTL is really one man and a few models from Linear Tube Amplifiers. On the other hand, there have been a number of OTL amplifier companies that have come and gone, as well as a number of DIY units out in the world. And of them are tube intensive and run warm i.e.Hot. Plus, they require a fair amount of care and feeding with tube biasing and tube replacing. Berning's ZOTL are self biasing, run cool, and have long lasting tubes. 

[thunderbrick]

You know the saying, expect the worst, hope for the best. Right now the expectation is that when I turn them on I will release the magic smoke, all 8 tubes in each amp will explode, and I will start a fire.

I understand AudioQuest is coming out with audiophile-grade fire extinguishers.  That should ease your fears.

[airhead]

I can answer this thread with a bit of experience.  I own a pair of StaxF83 speakers, extremely inefficient electrostatics with very high impedance through most of the midrange.  These speakers
are notoriously difficult to drive and are often referred to as "amp killers."  The very first amplifiers I tried with them were the Counterpoint SA4.  They went into oscillation; which the dealer was able
to fix but I took against it nonetheless.   I tried many other amplifiers, including some famous solid state amps and very famous transformer coupled tube amps.  Nothing really sounded good until I got a pair
of NYAL Futterman OTL3', which turn out to have a wonderful synergy with these speakers.  I have had these speakers and amps for over thirty years now.  It has not been entirely trouble free:  indeed one amplifier
did catch fire, and the speakers' had to rebuilt after twenty years.  But the amplifiers were repaired successfully, and have been re-tubed only once since then.  They do not run hot at all, the balance
and bias seem stable, and I still think they sound glorious.  Roger Modjeski looked them over recently; said the caps are still fine, and did the re-tubing of one the amps, saying the other did not need it.

Life is complicated.