Ok. I think you got me confused SY. The figure was illustrating the condition when the stage is delivering peak power to a load. At idle the dissipation is naturally much lower. I am sure the tube can handle the idle dissipation but can it handle those enormously high peak power dissipations? (Even when the music doesn’t contain them on frequent intervals). That illustration is for 12.5W average; at 25W average the device would be dissipating something like 400 watts at the peak power level. The only power rating quoted by the datasheets is the plate dissipation that is about 30 watts (IIRC). How much past that can one safely go?
If you run them with a continuous sine wave at full power, most OTL amps will melt. The (slightly) saving grace is that each tube is cut off for about half the waveform, so the dissipation requirement is proportionately lower.
In a 6C33 amp (or something similar using a high-perveance tube), the rails are generally run at a lower voltage than your example, too; for example, Kevin's high power OTL only used 130V. One reason that Futterman liked using pentodes was the ease of reducing plate voltage without requiring the driver stage to be able to take the outputs into grid current to get a reasonable swing.
In a 6C33 amp (or something similar using a high-perveance tube), the rails are generally run at a lower voltage than your example, too; for example, Kevin's high power OTL only used 130V. One reason that Futterman liked using pentodes was the ease of reducing plate voltage without requiring the driver stage to be able to take the outputs into grid current to get a reasonable swing.
SY said:
SY has hit the nail on the head, my amplifier ran on 140V rails, and I've seen designs using 6C33 that ran out 120V rails as well. Because one of the tubes is nearly cut off 50% of the time on you can effectively dissipate about twice the rated power for a half cycle because it is integrated over the full period.
Also most music has at least 10dB of dynamic range so the average power on most material is quite low.
You will melt most OTL outputs if you run them at maximum possible output power on a continuous basis as SY indicated.
Circlotrons are a bridge type topology and are somewhat easier to drive than a totem pole, is more symmetrical from an AC signal perspective, however the supplies are floating and you are driving both the power transformer leakage inductance and interwinding capacitance to the ac mains which can result in poor HF performance if care is not taken in the supply and transformer design. Hint: Use a large common mode choke in each floating supply to keep HF signal out of the transformer.
Good supply filtering is a must in a circlotron, (particularly if you do not use global feedback.) I'd use a full C-L-C pi filter in addition to the previously mentioned CM choke.
Hi,
OTL amps are inefficients monsters by their nature.
They are however the ultimate in transparency when/if given a copious and stiff PS and all input stages are designed properly.
The better the PS design, the better the OTL IMHO.
Take care of Zout, choose your input stage topology with your output tube's Cin in mind and all should be bliss.
Cheers,
OTL amps are inefficients monsters by their nature.
They are however the ultimate in transparency when/if given a copious and stiff PS and all input stages are designed properly.
The better the PS design, the better the OTL IMHO.
Take care of Zout, choose your input stage topology with your output tube's Cin in mind and all should be bliss.
Cheers,
fdegrove said:Hi,
OTL amps are inefficients monsters by their nature.
They are however the ultimate in transparency when/if given a copious and stiff PS and all input stages are designed properly.
The better the PS design, the better the OTL IMHO.
Take care of Zout, choose your input stage topology with your output tube's Cin in mind and all should be bliss.
Cheers,
IMHO Completely true on all counts.
SY said:
That's really true, indeed. But using such a cathode-follower power output stage in combination with a low-ratio output transformer, you can get amazing results - the usual transformer drawbacks reduce with the low winding ratios (for my 2x 6C33C Circlotron with about 150 Ohms output impedance I use a 160 VA toroidal OPT with a 4:1 ratio (primary winding : secondary). This results in an impedance ratio of 16:1, which adapts the cathode follower output impedance perfectly to that of standard loudspeakers.
And if you put this transformer in your boxes instead in your amp, you still can speak of an OTL amp... ;->
And the sound is - perfect. I never have heard such a powerful but well defined and clear bass with tube amplifiers. Voices come very natural with a perfect localization, and the highs are crystal clear.
And - I get nearly 60 Watts out of this pair of 6C33C (260 V plate, 100 mA quiescent current -> class AB)
Uli
Triodelington OTL ?
I think the triodelington can solve some "OTL tube problems".
Kind regards,
Darius
I think the triodelington can solve some "OTL tube problems".
Kind regards,
Darius
Defiantly a good idea. I've though of doing similar builds. The transistor in a sense, is just working as a transformer. The tube sees the load impedance x 10. The load sees the output impedance of the tube / 10.
For transformer coupled, I tend to only do all-tube designs. But for direct coupled the hybrid route defiantly seems like a good way to go.
If, and only if, the temperature (and beta) of the BJT is held constant.
It strikes me that it may be easier to oven stabilze a BJT to constant
repeatable temp and optimal beta, than attempt to run consistantly
cool in the presence of other hot devices nearby.
There are Thermaltrak Bipolars with a temperature sensing diode on
the same die. Could be abused to regulate a nearby heater, perhaps
on the flip side of the same sink...
I am not suggesting anyone overheat a BJT into thermal runaway.
-------------------------------------------------
If the Triodlington in actual circuit were to be stabilized by an emitter
resistor, then thermal runaway of Beta is less of an issue. But some
enjoy circuits without feedback in that part of the circuit, temperature
has to be watched by other means.
It strikes me that it may be easier to oven stabilze a BJT to constant
repeatable temp and optimal beta, than attempt to run consistantly
cool in the presence of other hot devices nearby.
There are Thermaltrak Bipolars with a temperature sensing diode on
the same die. Could be abused to regulate a nearby heater, perhaps
on the flip side of the same sink...
I am not suggesting anyone overheat a BJT into thermal runaway.
-------------------------------------------------
If the Triodlington in actual circuit were to be stabilized by an emitter
resistor, then thermal runaway of Beta is less of an issue. But some
enjoy circuits without feedback in that part of the circuit, temperature
has to be watched by other means.
Just what we needed, triode with negative thermal coefficient.
And positive intrinsic thermal feedback. Great, you do that...
Beta ain't like Mu, you can't trust it with even one eye closed.
A pure sand guy might simply use an emitter resistor to fix it,
poison to several triode circuits of potential interest.
And positive intrinsic thermal feedback. Great, you do that...
Beta ain't like Mu, you can't trust it with even one eye closed.
A pure sand guy might simply use an emitter resistor to fix it,
poison to several triode circuits of potential interest.
I do not require anyone to believe.
http://www.nxp.com/acrobat/datasheets/BU4525AX_3.pdf
This is the same part as used? Correct datasheet? True?
For "middle of the graph" Triodlington current of 75mA:
Current gain drifts aproximately from 6.5 to 26.5
over a temperature range of 25C to 85C.
But I only tell prokies, whatever the heck that is...
And so does Phillips.
----------------------------------------------------------------------
I merely suggest that holding the temperature constant
at 85C might be easier to regulate than holding at 25C.
Its a fine circuit so long as Beta is not permitted to drift.
http://www.nxp.com/acrobat/datasheets/BU4525AX_3.pdf
This is the same part as used? Correct datasheet? True?
For "middle of the graph" Triodlington current of 75mA:
Current gain drifts aproximately from 6.5 to 26.5
over a temperature range of 25C to 85C.
But I only tell prokies, whatever the heck that is...
And so does Phillips.
----------------------------------------------------------------------
I merely suggest that holding the temperature constant
at 85C might be easier to regulate than holding at 25C.
Its a fine circuit so long as Beta is not permitted to drift.
Attachments
@SY
Just log in there:
http://www.vintage-radio.net/forum/showthread.php?t=13624
You'll find photos, technical data, schematics ...
I am going to build up a triodelington OTL soon.
At the moment I am busy with the line pre
you know the EF86 volume pot.
Darius
Just log in there:
http://www.vintage-radio.net/forum/showthread.php?t=13624
You'll find photos, technical data, schematics ...
I am going to build up a triodelington OTL soon.
At the moment I am busy with the line pre
you know the EF86 volume pot.
Darius
Thanks for the link.
To see if it actually does work "perfectly," it might be interesting to compare distortion spectra of the bare tube and the tube augmented by the bipolar and see if they scale. The ECC82 has quite high distortion, so the harmonics and their ratios are quite easily seen.
I have to admit that I'm surprised that you would do such a thing after your strong objections to using LED biasing, solid state CCS, and source followers for screen drive.
To see if it actually does work "perfectly," it might be interesting to compare distortion spectra of the bare tube and the tube augmented by the bipolar and see if they scale. The ECC82 has quite high distortion, so the harmonics and their ratios are quite easily seen.
I have to admit that I'm surprised that you would do such a thing after your strong objections to using LED biasing, solid state CCS, and source followers for screen drive.
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