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

All 16 matched 6LF6 tubes (that were running at about 65% of their original strength) have been replaced with a new matched set NOS 6LF6 tubes. The dozens of photoflood caps, which were about 1/2” x 1”, not as atmasphere described,

@unreceivedogma That's probably because they were replaced. Harvey told me on the 'phone what part he originally used (Rubycon 600uf 360V photoflash cap). Because at the time Japanese customers didn't like seeing Japanese parts inside their amps, Harvey told me he stripped the plastic covers off of the photoflash parts to disguise their identity. You can see banks of them in the original photos of the amps.

This amp didn't have the covers peeled, as they were not going to Japan.

He even told me where he bought them surplus: Bill Godbout Electronics. Because they were surplus and also photoflash, they likely didn't last long in that application, so if your amps were still working then those parts were likely replaced a long time ago, hence the newer (and likely new when installed) parts were a lot smaller!

It was a good move to use a different cap in the power supply. Photoflash parts don't have good ripple current specs.

Now if you are wondering why I might sound like I was trashing NYAL, the use of a surplus part that was never going to hold up in its application even if new is a good example of why. It might be because I had a bit more of the inside story since Harvey took an interest in my activities and so told me things he wouldn't have told anyone else. But if you go back and look at my two prior posts, you'll see I make some suggestions and comments, such as this one; do you think this is a dig?

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.

Harvey worked with Mr. Futterman before the latters passing, and bought the rights to the Futterman name and OTL design shortly before that came to pass.

One of the reasons the NYAL amps were unreliable was Harvey used surplus photoflash capacitors in his power supplies. They were rated 600uf @360V; about an inch in diameter and about 5" long. These parts were not suited for power supply duty as they didn't survive ripple current very well. So even if they were new when installed they would not have been reliable. If you see these parts in an NYAL amp they all need replacement.

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.