HNY2025 to All ! 😀
@fabio1983 : The speakers you have from your Father are - I guess - similar to the one I have built (at left below) :
Their sensivity is circa 93 to 95dB, so yours shouldn't be very far...
T
I just completed the distortion measurements on the smaller OTL Futterman that I mentioned earlier. (4 6AS7 per ch version) It is delivering 17w per ch into 8 ohms with 3% distortion. The plans say that it is good for 25w and it might do that. After 17w the IMD increases very quickly.
I still have to get the spec for the larger Futterman (6 tubes per).
The M-60 clone was doing fine at 20 wpc with just a little more distortion than the futterman. It will do a lot more than that but my load was beginning to get hot. I have found a 150w @ 8ohm R and will retest later.
I got distracted by a 120hz hum at .0025V (loaded at 8R) on the output of the 4-toobe. Not enough to notice unless you stick your ear next to the woofer, but the other two amps are completely quiet. So I am hunting ground loops.
I still have to get the spec for the larger Futterman (6 tubes per).
The M-60 clone was doing fine at 20 wpc with just a little more distortion than the futterman. It will do a lot more than that but my load was beginning to get hot. I have found a 150w @ 8ohm R and will retest later.
I got distracted by a 120hz hum at .0025V (loaded at 8R) on the output of the 4-toobe. Not enough to notice unless you stick your ear next to the woofer, but the other two amps are completely quiet. So I am hunting ground loops.
Last edited:
Well I found the 120hz coming from the bias supply. It is a voltage doubler. On the first Futterman I built I used the bias supply as it was on the original schematic. At the time I had to modify to get the -260V I needed even though I used all of the speced parts and even with the higher line voltage. SO, I used regulated supplies from then on. I am either going to have to build a bridge or full wave to get away from that and it will take another transformer (replacement for original with a 115V secondary) need about 165 - 170V. Probably just upgrade to the regulated supply. It looks like they were trying to save $1.50 ;-)
Yes. You understand why I use 13200µF (6x2200µF/200VDC) and 2 reverse-coupled double-coil low DCR (1R per coil) inductances, in order to make the final stage silent on my U-OTL, for only 3x6080 ! 😳
T
Yes, I do. The B+ (and B-) supplies are quiet. But the voltage doubler on the bias supply just won't do.
FWIW Dept.: Just a few corrections...
OTLs tend to be excellent at square wave reproduction as they don't ring and have wide bandwidth. IOW, better than most PP tube power amps using OPTs and as good as solid state.
OTLs are usually not as heavy for a given amount of tube power as they have no output transformer. A pair of our M-60 monoblocks weigh less than a single H/K Citation 2 power amp for example.
The heat they make is a function of the class of operation. It seems to have little to do with the filaments. For example if any of our 6AS7 based OTLs are in standby (no B+ on the power tubes) you can grab the power tubes and hang on without being burned even if they've been in standby all day. But within 30 seconds of B+ applied the tubes are too hot to touch. But we run them pretty hard. You can run OTLs in class AB2 no problems- at that point they don't make any more heat than other tube amps of the same power.
You do need suitable speakers. The smaller the amp the more sensitive they are to the speaker for best success. But we've yet to run into a 16 Ohm speaker our S-30 (30 Watt/Channel) amp didn't play nice with. There are a lot of suitable speakers out there for the larger OTLs. If you run feedback in the amp the number of suitable speakers is increased. SETs have a similar 'suitable speaker' problem FWIW...
The Klipsch have chokes inside, not transformers (at least the ones I've had). The Quads and a number of other ESLs do have transformers. That is so you can run them with conventional amps. So is it better with two transformers in series or is one enough 🙂
The power tubes for OTLs tend to be the 6AS7G (6h13C or 6N13PJ), 6C33, PL509, PL519; these are the ones you can still find. But yes, if you think about it an OTL has to produce current and Voltages that drive real world loudspeakers so most high Voltage tubes like an EL84 need not apply.
Ha ! Coïncidence ? I fronted the exact same issue on my U-OTL bias supply ! I ended to increase the filtering as below :
T
atmasphere,
Your points are well taken.
I however try and look at things from a systems viewpoint: Starting with the sheet paper music and music stand, artist playing, instrument, microphone, . . . . All the way to the loudspeaker, room, and the listener's ear(s).
As to transformers, I will not throw out all my vintage music recording sessions on LP, 7-1/2 inch tape, and from master tapes to re-issue CD recordings.
Most of them had a transformer in the microphone, and a transformer in the preamp, and a transformer in the sound board, and a transformer in the recorder.
If transformers are all bad, lets go back to Edison Cylinders, 78s and 80s recorded on one side only, not both sides.
A little levity never hurt anyone.
$0.03
Adjusted for inflation.
Your points are well taken.
I however try and look at things from a systems viewpoint: Starting with the sheet paper music and music stand, artist playing, instrument, microphone, . . . . All the way to the loudspeaker, room, and the listener's ear(s).
As to transformers, I will not throw out all my vintage music recording sessions on LP, 7-1/2 inch tape, and from master tapes to re-issue CD recordings.
Most of them had a transformer in the microphone, and a transformer in the preamp, and a transformer in the sound board, and a transformer in the recorder.
If transformers are all bad, lets go back to Edison Cylinders, 78s and 80s recorded on one side only, not both sides.
A little levity never hurt anyone.
$0.03
Adjusted for inflation.
The Klipsch speakers I know of use Autotransformers. One winding of wire, with multiple taps.
That is a transformer. There is transformation.
Example, the full winding to a tap with less windings.
All of that has incuctance, and all of that has leakage inductance, and distributed capacitance.
Just like every transformer that has a separate primary and secondary.
The transforming of the mind, comes to mind. Obscure reference, I know.
That is a transformer. There is transformation.
Example, the full winding to a tap with less windings.
All of that has incuctance, and all of that has leakage inductance, and distributed capacitance.
Just like every transformer that has a separate primary and secondary.
The transforming of the mind, comes to mind. Obscure reference, I know.
Yeah, that is the one. I think that it is left over from the early designs that used NO transformers including power. They were probably getting the "115V" off of the line. Back in those days, a little hum was expected. When Glass Audio added the other power trans they should have added a winding for this.
The particular problem I have is that I run the B+ an B- off of big isolation trans, so 120V out (or more) which gives me about 160V B+ and B-. So I need about -261V out of that circuit to keep my cats in line. (like herding 6AS7s - cats). The bias on the upper tubes (totem) is around -90V but the lower group is near -260V. That is one of the issues that people have building that amp. They are used to biasing their 6L6GCs at ~ -40V.
I have decided to go with a regulated supply and a new Antek 50VA. it has 200V @100ma windings (2) and I can use one of the 2 6.3V windings to buck it down to around 193V I was shooting for 191V. I need the other 6.3V winding to keep the 6EM7 hot.
Russ
Yes, that's True ! 🙂
For an OTL amp, it's very difficult to find a SINGLE power transformer which features ALL the required windings PLUS the suitable voltages AND currents... So I / We had to use "expedient" solutions.
- For biasing the upper 6080, I use a voltage divider taken on the lower 6080 plate supply.
- For biasing the lower 6080, I use a negative voltage doubler, as mentioned above.
Note that my 6080 runs in Class AB1, with only +115VDC/0/-115VDC supply, and idle current of 100mA per triode section. I have no regulated supply nowhere. No Class AB2 nor B operation here - with their permanent large voltage variations - that would indeed require another kind of supply, in power and size !
In short, it would de facto no longer be that simple and compact U-OTL design !
T
I'm not saying all transformers are bad! I've run a recording studio for the last 40 years; 'nuf said.
But all transformers make distortion; you might find this article interesting; output transformers make the most.
Yes, true. That's one of the reason why some premium models of output transformers are very costy, because they are primarily very difficult to build in order to minimize distortions at best !
At right : my U-KT120, with its cheap but surprisingly very good quality Chinese SE output transformer, and 6dB NFB loop.
At right : my U-OTL, which is a direct coupling output stage to speakers, so no output transformer and no NFB loop, 16R load and 10.00V RMS out.
T
My pick for OTL tubes would be 7236 on the output ,they have mu of 3.5 an impedance of 350ohms,I have 6 of these for sale, if interested please PM me.
We tried that tube. Works well until it shorts. So you have to be a little more careful about fuses. You can probably reduce the shorting issue by proper preconditioning if the tubes are NOS. One thing to note is the grid heatsink. If you are employing a direct coupled driver as seen in our M-60 amplifier, it can easily drive the tube into grid current. That is why its nice to have a large grid heatsink like you see in the 6AS7G (and not in the GA variant, which seems to be why its so much less reliable).