Futterman. Jump in? Or, not so fast...

Roberjerman - Thanks for the comment. I had sent you a message a day or two ago and it appears you might not have noticed. Can you tell me how many hours you get out of a set of tubes on the H3aa's? I've read somewhere that they will go 10,000 hours, but I'm not really sure how many hours the current set has on them. The  amp comes with test results for all 12 tubes. But I can't imagine not buying a back-up set of tubes, which will cost quite a bit. How often are you having to re-tube your amps?

Yes we have some of the tubes for the Futterman amps. We need to know the specific tubes and amp so we can check the inventory. Prior to his passing Roger designed a new OTL circuit based on Futterman's work. There was a point in the prototype development where Roger was a bit stumped as to why he was getting some oscillation from the amp. So he purchased a Harvard H3 in hopes of figuring out the issue. Turns out that in the H3 schematic Roger was using as a guide, Futterman "conveniently" left out a critical design component which was easily seen when you opened up the amp - the plethora of ferrite beads in the circuit. That solved the issue.

Once manufactured we intend to publish Roger's design philosophy for this new amplifier. It uses a significantly less number of parts, not nearly as many ferrite beads (Roger used a neat trick to solve for that), has improvements in the circuit, and is simpler to set up than the Futterman. Roger's appreciation for Futterman's work includes his recorded interviews with the man that contain a wealth of knowledge, as well as multiple files on various Futterman designs, some that were given to him by Julius himself.

Damn @clio09, now you've gone and reminded us of why Roger's passing is such a loss to us all. ;-(

The Futterman amplifier proved to be the main impediment to our running a business. This was simply because the circuit (especially under the Rosenberg/NYAL name) did more to convince audiophiles that OTLs blow up more than any other topic (analog vs digital and so on).


For many years convincing the public than an OTL could be reliable seemed a Sisyphean task. But eventually the public forgot.

The reason they had the reputation of reliability issues is oscillation. Futterman, as was hinted above, was better able to keep his amps running due to a simple trade secret (one which he never imparted to Harvey Rosenberg) mentioned above. But even so, one must be careful- the amplifier has a lot of feedback and is at the limit of its phase margins. Those schooled in amplifier design will realize that this combination can lead to oscillation at a very high frequency; hence the RF beads. In a nutshell what this all means is that as frequency goes up, there is (as is the case in all amplifiers) phase shift such that at some point the feedback becomes positive rather than negative. In OTLs this is at a much higher frequency than it is in transformer-coupled amplifiers. The Futterman is usually set up to run at a point just shy of where this is so. So if the amplifier is driven into clipping, is on an adverse load or if a tube arcs, any of these events are known to be things that can set the amp into oscillation.  

Many of the Futtermans used the 6LF6 power tube as this was one of the more robust pentodes you could use for this sort of thing. These days they have gotten a bit harder to find! Alternates nowadays (which may require modification of the circuit in order to use them) are the EL509, EL519, PL509 and PL519 (the latter not being exactly the same as the former). Like the 6LF6 these tubes can support a lot of plate current (usually in excess of 1 amp) if operated at lower voltages (this kind of tube is known as a 'sweep' tube or 'horizontal output' tube as they were used for the horizontal sweep circuitry in televisions- and therefore ran at very high voltages and had to support a fair amount of plate dissipation; its a bit of fortune that they can also be run a the low output voltages typically seen in the output section of OTLs). The 6LF6 was slightly more robust so it one were to rewire their amplifier to use the available alternates the output power is likely to be slightly less. However no-one except Futterman owners are looking for 6LF6s so they are not that expensive on ebay, however one must be careful with such purchases (with a casual look on ebay this morning one of the tubes I saw for sale was obviously gassy) and matching power tubes really helps out the performance of this amplifier! 

One thing you'll find with OTLs is that they rule the roost when it comes to transparency, speed and bandwidth. Its not subtle- its the sort of thing that you hear immediately. Keep in mind that the speaker choice is important- but if you have the the right speaker the combination can be good music quality that few audiophiles experience.