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Old 12th March 2012, 04:31 PM   #1
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Default Frequency Response of Output Tubes

Is there any available data sheet that shows output tubes' frequency response? I know the typical "describing" words that people use (EL84s are chimey, 6L6s are glassy, etc) but is there any data out there that can actually show a particular output tube would be better suited for a particular application (bass clarity for example). Just curious, thanks.
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Old 12th March 2012, 04:44 PM   #2
SY is offline SY  United States
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No, not really. The performance of an output stage is dependent on the tubes, the transformer, and the stage driving it. Without knowing all of those things, you can't predict performance. The "glassy," "chimey," "cloudy," "chocolatey" stuff about tubes is audiophile nonsense, as much as ascribing engine performance to specific types of main bearings. An amp's performance is far more dependent on overall design and execution than whether you used current production Lo Hung Dong 6V6 or 1953-vintage Achtungsbieremesse NOS EL84.
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Old 12th March 2012, 05:12 PM   #3
DF96 is online now DF96  England
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A 6V6 is good from DC to somewhere around 10MHz. An EL84 may go a bit higher, as it is smaller.

If output valves have any effect on perception of frequency response, this will be caused by distortion masquerading as frequency imbalance. Conclusion? Don't fully trust your ears, especially when your eyes know what you have changed.
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Old 12th March 2012, 05:37 PM   #4
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As mentioned by both SY and DF96, tubes themselves have relatively high bandwidth. I'll just add that it is the output transformer that forms the bandpass filter and dominates the frequency response.

The 'chime' and 'glass' character you mention is typical of guitar amp reviews, and the way a guitar amp responds is much more determined by overall design, and not what tubes in use.
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Old 12th March 2012, 05:44 PM   #5
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Quote:
A 6V6 is good from DC to somewhere around 10MHz.
At least. Many typical audio tubes are good to 20 or 30 MHz with proper RF circuit design, assuming that you are actually making an RF amp. The old Metal 6L6 will work on the 20 meter ham band (14 MHz) and put out considerable power. Even the worst type of tube should cover up to a few MHz, so frequency response of the tube itself is not a factor in an audio amp.

Quote:
I know the typical "describing" words that people use
I have my own....from a guitar amp builders perspective.....6V6's were made to sing the blues but EL84's rock!
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Old 12th March 2012, 06:34 PM   #6
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Quote:
Originally Posted by tubelab.com View Post
I have my own....from a guitar amp builders perspective.....6V6's were made to sing the blues but EL84's rock!
Or vice versa ?

I don't think that output tubes, or any tube at all, will produce harmonic distortion in the audio frequency range. But, as told by the others before, non-harmonic distortion is different between the tube types. And this means different sonic perception by the listener or the musician.

Best regards!
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Old 12th March 2012, 07:41 PM   #7
DF96 is online now DF96  England
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Originally Posted by Kay Pirinha
I don't think that output tubes, or any tube at all, will produce harmonic distortion in the audio frequency range.
I suspect this is not what you intended to say.
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Old 12th March 2012, 07:47 PM   #8
tomchr is offline tomchr  United States
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Quote:
Originally Posted by howe0168 View Post
Is there any available data sheet that shows output tubes' frequency response? I know the typical "describing" words that people use (EL84s are chimey, 6L6s are glassy, etc) but is there any data out there that can actually show a particular output tube would be better suited for a particular application (bass clarity for example). Just curious, thanks.
The frequency response is dominated by that of the OPT. The LF pole is set by the magnetizing inductance and the DCR. The HP pole is dominated by the leakage inductance and parasitic cap as I recall. All of those depends on the winding style and materials chosen.

I actually put my OPT's on an HP 4194A impedance analyzer and had it calculate the parasitic components for me. Using those values in LTspice, the simulation predicted the LF and HF poles to within a few percent of the values I measured on the finished amp.

~Tom
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Old 12th March 2012, 08:10 PM   #9
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Quote:
Originally Posted by tubelab.com View Post
At least. Many typical audio tubes are good to 20 or 30 MHz with proper RF circuit design, assuming that you are actually making an RF amp. The old Metal 6L6 will work on the 20 meter ham band (14 MHz) and put out considerable power. Even the worst type of tube should cover up to a few MHz, so frequency response of the tube itself is not a factor in an audio amp.
No need to add proof to this but for exmple the gu48 has been used with success in audio amps and yet was designed for RF heating. 80mhz max if I recall correctly and no significant adavantage comes from this characteristic in audio applications over smaller tubes.

Last edited by Alexontherocks; 12th March 2012 at 08:14 PM.
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Old 12th March 2012, 08:13 PM   #10
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The frequency response is dominated by that of the OPT.
In some of my amps it is the only frequency dependent item in the signal chain. True there is one coupling cap, but it's pole is below 1Hz. There is no feedback otherwise the amp would be unstable.

I have been working on a monster sized breadboard that uses Plitron toroidal OPT's in P-P. The OPT's are rated for 400 watts at 20 Hz. The upper 3db point is 75KHz. Any tube that I have tried in it is capable of a frequency response of 5Hz (lower measurement limit) to 40KHz +/- 1db. It's just that some tubes are a little more powerful than others. It takes careful circuit design to make an amplifier thats not dependent on the tube type.

With smaller OPT's the tube's internal impedance will affect the frequency response at both ends of the curve. A given OPT will usually have a broader frequency response when driven by a lower impedance source. Every tube has a characteristic impedance that varies with the amount of current flowing through it. Negative feedback can be used to lower a tube's internal impedance.

Every tube will have an internal capacitance associated with it. This will affect how well it performs in a circuit. The tube or mosfet that is driving the output stage must have a low source impedance for it not to be affected by the output tubes capacitances.

A typical amplifier design with average transformers will exhibit different frequency response with different tubes. The differences will be dependent on the circuit design and OPT quality.
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