The Futterman is known for stability problems. It is a common mistake to assume that since this amplifier (Circlotron OTL) is an OTL, that it therefore is a Futterman. Its not.
The Futterman had a reputation of stability issues. By contrast this amplifier is unconditionally stable. I have shorted out the speaker terminals (no effect- when the short was removed the amp was playing as if nothing happened), pulled out tubes while the amp was playing music (almost no effect at all), swapped power tubes from one channel to the other while it is playing (again no effect), that sort of thing. It really is quite reliable. I've used the amp as a guitar amp, overdriving it very hard. It holds up to that easily, and has instantaneous overload recovery.
The Futterman had a reputation of stability issues. By contrast this amplifier is unconditionally stable. I have shorted out the speaker terminals (no effect- when the short was removed the amp was playing as if nothing happened), pulled out tubes while the amp was playing music (almost no effect at all), swapped power tubes from one channel to the other while it is playing (again no effect), that sort of thing. It really is quite reliable. I've used the amp as a guitar amp, overdriving it very hard. It holds up to that easily, and has instantaneous overload recovery.
It is always possible for a tube (or mosfet or BJT) short to dump B+ on one end of the speaker. A fast blow fuse in series with the speaker should save the speaker unless you are using a big amp with a low power speaker. You can always use an autotransformer like the Zero mentioned earlier.
I have experimented with the circlotron design. In fact the circuit wasn't too different. I used cascaded 6SN7 LTP's with CCS chips in the tails and mosfet followers to drive the output tubes. The driver circuit can be found in the 6L6GC in AB2 thread: http://www.diyaudio.com/forums/tubes-valves/133034-6l6gc-ab2-amp.html?highlight=6l6gc+ab2
Since I live in an extremely warm climate I chose to use just 2 output tubes per channel and use an OPT. As suggested here the 6AS7 type seems to be about the best choice. Triode wired sweep tubes work good too (6LW6). I am using 6AS7GA's without issue, but I am not asking for big current through them.
At first I was using a 600 ohm OPT made for SE amps, but got better results with some dumpster toroids from thermal printers. The power supply is also made from dumpster toroids making about 160 volts per supply. Power output is about 25WPC.
I have experimented with the circlotron design. In fact the circuit wasn't too different. I used cascaded 6SN7 LTP's with CCS chips in the tails and mosfet followers to drive the output tubes. The driver circuit can be found in the 6L6GC in AB2 thread: http://www.diyaudio.com/forums/tubes-valves/133034-6l6gc-ab2-amp.html?highlight=6l6gc+ab2
Since I live in an extremely warm climate I chose to use just 2 output tubes per channel and use an OPT. As suggested here the 6AS7 type seems to be about the best choice. Triode wired sweep tubes work good too (6LW6). I am using 6AS7GA's without issue, but I am not asking for big current through them.
At first I was using a 600 ohm OPT made for SE amps, but got better results with some dumpster toroids from thermal printers. The power supply is also made from dumpster toroids making about 160 volts per supply. Power output is about 25WPC.
OTL's for 31 years not a speaker blown. Many solid state amps and 2 speakers blown.
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I own 2 H3's (stereo blocks) a pair of H3aa's and several NYAL OTL1's & 3's never a problem. If you do not know how to operate them then you will have problems with them.
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I must say this reputation is without true merit. I own several and I will say this regarding all OTL's in general. No matter who builds, designs and sells them they all get pretty hot in comparison to every other tube amp I have ever seen. With that said they generally need their parts replaced after years (read 20 or so) of use, not unlike every other tube amp I have ever seen. If you get a futterman, set it up properly just like you would need to do for every other tube amp I have ever seen you will not have any other problems with it more then any other tube amp. I have NEVER lost a tube on a Futterman but I have on several Dynaco's, Bogen's, Scott's and Fisher's so again many of these Futts are 30+ years old and if you rebuild them and set them up correctly they will work perfectly for years (read 20 or so). I have NEVER lost a speaker on futterman's or any other tube amp for that matter but I have on 2 previous SS amps. I have collected, rebuilt and resold many futterman's and their associated brothers over the years to several tube amp lovers in my area and I have NEVER heard a complaint back and from time to time I still sit in those listening rooms and hear those amps and they still sound great. The Futterman & NYAL power transformers are some of the most durable PT's I have ever seen. If you have had a bad experience with a Futterman then I suspect you either set it up wrong, put the wrong tube in it, tried to use it without checking to see if it needed a rebuild, or finally you just had a bad part in it from jump just as could happen to any other tube amp rebilt or not that I have ever seen, although I have to say I have never personally experienced a part being DOA in my years of doing this. Tubes yes but parts never.
No shot at you Atma just so many people say Futtman and the rest run for the hills. I to have pulled tube (One of the things you need to do when setting up the amp under certain circumstances) with no issues and left the amp running with no load on it and have had no issues. In general they are tanks and I am glad to see you support their durability!
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Cruel High End Audio Market conditions as the sharp blade divide really successful Audio aparatures creation & Manufacturer from average or less successful Audio aparatures creation & Manufacturer.
Thats cruel market condition `effect` is really obvious when we look for decades backwards.
Only the Best one Survives this . 😉
Best Regards for All
Thats cruel market condition `effect` is really obvious when we look for decades backwards.
Only the Best one Survives this . 😉
Best Regards for All
Best? Sadly many of the "best" are no longer around for various reasons, not just the ability to make a superior sounding and reliable piece of equipment.
Bose is.
They'll probably be the last one standing.
Best? Sadly many of the "best" are no longer around for various reasons, not just the ability to make a superior sounding and reliable piece of equipment.
Bose is.
They'll probably be the last one standing.
To be the one of the Best & successful (and to survive) Profesional Manufacturer on some specific part of the present cruel High End Audio market is not enough only to design & make good sounding and reliable audio gear(that is obligatory anyway),but include so many other serious factors like: Constantly : laboratory researches & aparatures improvements,quality control of each formed Hi -End aparature, improvements of aparatures aestetic & ergonometry,development & design of new aparatures ,Marketing researches,employments perfection & care......there is ten more serious relevant factors at least to count.One of very important factor is to be the Honest with customers,in past,today & future.So the HI End Manufacturer which fadeout from market scene made some mistake(s) somewhere for sure,they have to blame nobody but himself.
BTW : How many actual High End designers & Manufacturer participate and support DIY community ? - Only few (The Best ones).
Bose-is that some loudspeaker Manufacture ? Trust You that they are good one,wish i can have one pair of that Bose,but since I am poor (DIY) Man , that does not care. - Hey Man we goes so much offtopic now!-
Best Regards
I'm not a fan of Bose. My point was that many very high end companies have come and gone, but a mid-fi company with excellent marketing prevails. Sadly.
Yes- I imagine this is true.
The reputation is one thing, the actual is another. Certainly in the first 20 years of operations what we came to call 'the Futterman Legacy' of instability was our biggest marketing problem (its an OTL: therefore its a Futterman and thus unstable...). But like many myths in the world today, likely undeserved.
havent read the whole thread, but there was quite some discussion on the heater current and circuits for the output tubes...
It seems the popular way is to use a heavy 6.3V 20-30A supply and strap 'em all in parallel...
Personally I have good success wiring 6C33 in Series...
On average, there's only 0.2V variation between each tube, so around 0.1V per 6.3V section, Well within spec....
It works well with my 'Allan-Kimmel' severely modded Circlotron with 6C33....
Current breadboard is a stereo Inv. Futterman, the O/P tubes supplied by the one of the O/P stage +B 55V winding 'Underwound' down to 50V AC.
(I'm using a common and surplus 0-55, 0-55 V 500VA Toroid, with a couple of extra coils I added. One to reduce the 55V to 50 for heaters of 4 x 6C33C, and two coils to add a little extra +B for the O/P stage.)
It has good-old fashioned, Autobias--like you would find in any old style amp or radio, by cathode-resistor + de-coup cap and no extra negative supply rails, 100 ohm cathode-resistor gives me 180mA at a little under 100V per O/P stage rail.
The +B is only 95V as I need to add some windings to the toroid to allow higher volts, and more power than the 15 ish I have now, but currently, plays beautifully and has been utterly reliable, having done around 400 hours service since I built it before Christmas...
(Used at least 10 hours per day)
Is there any fundamental objection for series wiring of the heaters? It reduces the heavy currents and mag fields with a lower volts/high current, I would have thought, and there is no apparent 'hogging' of power/difference in heating effect between bottles ...
It seems the popular way is to use a heavy 6.3V 20-30A supply and strap 'em all in parallel...
Personally I have good success wiring 6C33 in Series...
On average, there's only 0.2V variation between each tube, so around 0.1V per 6.3V section, Well within spec....
It works well with my 'Allan-Kimmel' severely modded Circlotron with 6C33....
Current breadboard is a stereo Inv. Futterman, the O/P tubes supplied by the one of the O/P stage +B 55V winding 'Underwound' down to 50V AC.
(I'm using a common and surplus 0-55, 0-55 V 500VA Toroid, with a couple of extra coils I added. One to reduce the 55V to 50 for heaters of 4 x 6C33C, and two coils to add a little extra +B for the O/P stage.)
It has good-old fashioned, Autobias--like you would find in any old style amp or radio, by cathode-resistor + de-coup cap and no extra negative supply rails, 100 ohm cathode-resistor gives me 180mA at a little under 100V per O/P stage rail.
The +B is only 95V as I need to add some windings to the toroid to allow higher volts, and more power than the 15 ish I have now, but currently, plays beautifully and has been utterly reliable, having done around 400 hours service since I built it before Christmas...
(Used at least 10 hours per day)
Is there any fundamental objection for series wiring of the heaters? It reduces the heavy currents and mag fields with a lower volts/high current, I would have thought, and there is no apparent 'hogging' of power/difference in heating effect between bottles ...
I think the main issue would be if a tube or socket (more likely the socket) had a problem, it would be a little harder to find. I've done a few prototypes with series filaments and they work fine.
The main issue with series wiring is that a bad contact will both cause the other tubes not to work, but worse, series wiring is much more likely to burn up the tube with the bad contact than parallel wiring.
I can't explain it better than this:
http://www.jacmusic.com/nos/images/6C33/6C33-HEATER-ERROR.jpg
I never really thought about it before I read the article above.
Now, if someone for some reason did want to build an exclusively Class-AB version, it looks to me like it would only need ~50V secondary power transformers. Am I reading the schematic correctly?
I'm referring to the power transformers for the floating supplies for the output tubes, as shown in the original ps schematic you posted. (Oops, maybe I have it reversed.) It looks like when switched to Class-AB mode that half the transformer primary is taken out which would result in the secondary voltage being doubled (not halved). So, I think, amend my previous question to be: "For an exclusively Class-AB version, should the secondaries of the power transformers for the floating supplies be ~200V?"
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Circlotron Stereo Amplifier
Atmasphere,
I have taken your suggestions on developing speakers that will work with OTLs.
(in stead of feedback) I have found and are building some Altec Equipment.
My needs for Power have diminished,I was wondering how a Circlotron
stereo amp as the Atmasphere S-30 thirty watt amplifier is designed.
Do all phases need their own B+/B- and filaments supplies? Does this call for a special power transformer?
Thanks
Darrell
Atmasphere,
I have taken your suggestions on developing speakers that will work with OTLs.
(in stead of feedback) I have found and are building some Altec Equipment.
My needs for Power have diminished,I was wondering how a Circlotron
stereo amp as the Atmasphere S-30 thirty watt amplifier is designed.
Do all phases need their own B+/B- and filaments supplies? Does this call for a special power transformer?
Thanks
Darrell
Funny, I just noticed in the new PS schematic v1.1AB kindly provided by Mr. Mullardel34 that the ClassA/AB switch operates on the secondary side of the floating power supply transformers (rather than the primary as is the case in the original Atmasphere PS schematic). And, the new one seems to operate as follows: in Class A the secondary is 100 Vac, and in Class AB the secondary is 50 Vac. Surely I'm missing something obvious. 😕
I have built amps with 200V supplies. In practice, they run a little lower than that- maybe 190V. You sure can't run much plate current with them...
I did this so the amp could make some significant power into 32 ohms- the result was that I get about 50 watts from 4 tubes.
The S-30 uses 4 B+ windings for the output tubes but the filament supply is common to both channels. Certainly a custom transformer is used 🙂
Sounds to me like there is an error in the drawing. This is a big thread- do you have a link to it?