Today I heard a Futterman OTL powering a pair of Quad ESL57s. The Futterman has been recently serviced and is in nice shape. It sounded wonderful. I understand this is a rare beast.
I've been advised a set of tubes can last 10,000 hours. But these are not common tubes and they need to be closely matched, an their are 12 of them. If I were to buy this amp I'd immediately start worrying about putting together a backup set of spares, which could run into quite a bit of money.
So - Futterman owners - what say ye? Jump on the chance to own a legend? Or stick with my "set and forget" Quad 909 powering my ESL 2805s...
Roger Modjeski (Music Reference) held the Futterman OTL in high regard (the Quad ESL as well), and his RAM Tube Works has the tubes needed for them, computer tested, graded, and matched.
But Modjeski designed an amp using the Quad as the load, and is another great amp for the speaker: The Music Reference RM-10 (original, or Mk.2). Only a single pair of EL84's per channel for 35 watts per. Much cheaper to retube than a Futterman, and in the RM-10 the 84's last 5,000-10,000 hours.
If the price is reasonable, go for it! Futterman OTLs are the Holy Grail of tube amplifiers! I have three. A Harvard Music H3 using four 6HB5's per channel and a pair of H3aa's using six 6LF6's per mono amp. I have two pairs of Quad 57's and a pair of KLH Nines to use with them.
Awesome @roberjerman! State-Of-The-Art in the 60’s, and still great. One of Modjeski’s last designs was an ESL loudspeaker and a direct-drive OTL amp with which to drive it. No amplifier output transformer, no ESL input transformer, the tubes drove the ESL stators directly!
On his Audiocircle Music Reference Forum Roger discusses the design of the Futterman OTL, which he considered brilliant. Breaking current thinking on the subject, it employs very large amounts of negative feedback, one benefit being very low output impedance (in contrast to the high output impedance of all other OTL's).
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.
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.
Roger and I had some great discussions way back when about Futterman circuits. The genius was getting a pentode to operate at low plate voltage with high screen voltage, operated by a bootstrapped driver pentode. The oscillations partly came from paralleling the output tubes to get enough drive current. I’ve built a few over the years with PL519s and 6HB5 compaction tubes and can vouch for their transparency. The frequency compensation on the H3 I serviced was a bit of a compromise so I tweaked it to get better stability and make it more linear in the extreme top end. The gas filled regulator tubes sounded far better than zener diodes and solid state regulation in the screen driver circuit and so I decided the circuit did need to be as complex as the original. They look extremely cool too. Clio09, build it! If you need a hand to interpret Roger’s take on it let me know!
I have a New York Audio Labs Moscode 300 (which I may wrongly think is connected to the originator of the Futterman amps) and it was super nice. For some reason, it stopped working and it's been sitting at my brother's house for about a decade. Maybe it's time to look into reviving it!
@kacomess, New York Audio Labs was started by hi-fi provocateur Harvey Rosenberg. Harvey hired engineer George Kaye, and the two of them developed the company’s line of tube (input stage) / ss (output stage) hybrid Moscode power amps.
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. Harvey himself died at a fairly young age, but George Kaye continued to support the Moscode amps with service.
Eric, Thanks for the esoteric and informative news on NYAL. How do you know all this stuff? I’m wondering if Doug at Audio Specialties can fix it (provided I can get my brother to ship it to me from Chicago). Is it worth the cost?
@markusthenaimnut - you can email us at [email protected] and provide us with the relevant information. If your amp uses the 6FL6 we have plenty. We have some of the PL tubes as well.
Of the two H3s we bought, one was made by Harvard and the other by Tech Electronics. Both were much more reliable than the NYAL version. If you go back to some of Futterman’s published schematics in engineering journals you will note that they were of low power design. Futterman got caught up in the power game when the H3 was designed and it’s no coincidence that reliability issues started popping up with the increase in power. Roger’s design will be in the neighborhood of 40 watts pentode/25 watts triode.
@tomic601 - Yes Roger was UVA undergraduate, then a year at Stanford. While your offer is appreciated, Roger became dismayed at what they were teaching at UVA these days and also became disenchanted that the young engineers to be were not interested in audio circuit design. As such he decided not to donate to any academic institutions. He instead has made other plans for his journals, recorded interviews (including Saul Marantz and Futterman among others), rare and exotic tubes, antique electronics, and intellectual property.
@ndevamp - yes your help would be appreciated, I will email you to discuss.
Harvey worked with Mr. Futterman before the latters passing, and bought
the rights to the Futterman name and OTL design shortly before that came
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.
I had a Frank Van Alstine-modified Dynaco PAS pre-amp (quietest pre I’ve ever owned. Frank used 5751’s in place of the stock 12AX7’s), but decided to go with a passive pre, so got the then-new NYAL Super It phono stage, and sold the PAS.
Well, NYAL sent out the first samples of the Super It with the channel identifications reversed: left was right, right left. Shoulda been a red flag, right? It didn’t take me long to realize the Super It was a fairly substantial step backwards from the FVA PAS. Lesson learned.
But Harvey was a very interesting character, quite entertaining. He wrote a lot, including about the original Quad ESL and his other passion, the Decca cartridge. I wrote him (remember letters? ;-), and he looked up my number and called me in Burbank, California from New York! Over the phone he provided me with a lot of wisdom on Decca use and ownership, which has been invaluable. Miss ya, Harv.
Go for it!! When I spoke to Julius about my H3aa, he said: - Matched tubes weren't needed. The tube set can be "Balanced" via the balance pot and measuring pins in the 7pin socket to ground. Pins 5 and 7 comes to mind. But it has been a few years. No specific value. Just equal. But having a spares is a good idea. 6LF6s are getting harder to find. I notice a source for them in this thread. -Of course test the tubes. Especially for shorts. - If the balance voltages are way off, check the wire fuse in the cathode circuit. He had placed some replacements in a sleeve inside by the fan. -may have to swap an upper tube with one of the lower to dial in the balance. Do one pair at a time. He had always been able to balance them. As have I.
Other little info as memory serves The "Bias" adjustment pot is not output tube bias. They are "self biased". It adjusts the plate voltage of the, phase inverted tube? 90 volts? Measured on that 7 pin socket. Pin2 again comes to mind. Have to trace it out. Be very careful. Those photoflash caps can do harm. Even if the unit has been off for a few hours. The B +or- bus under those 6 capacitors run close to the edge of the top cover/chassis. Arced a good part of my 1/4" flat blade out. Feedback came up in the thread. The number 60db of feedback rings in my memory. I contacted George Kaye to source replacement tubes. None. But he has a modification to convert the 6LF6s to Triode. Eliminating the need for the Screen regulators. Another pair of 6LF6 would be installed their sockets. If a problem arises with the screen regulator/s, this might be a way to go. And finally LOL, Julius was my kind of guy. "Sailor mouthed". Some may take offense. Made me chuckle throughout the conversation.
I will look for them but recently I saw the hand typed instructions Julius provided for making adjustments to the H3. This was one of the areas where Roger simplified the design, using a 6 position switch and meter so one could easily take readings and adjust accordingly.