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Old 24th February 2015, 09:02 PM   #1
diaz028 is offline diaz028  Canada
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Default 6L6 / EL34 / EL84 / 6V6 frequency response curves?

Looking for a comparison of the frequency response cyres for these tubes in clean operation.. thanks!
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Old 24th February 2015, 09:07 PM   #2
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Not sure what you're asking here.

VTs aren't like BJTs: there is nothing comparable to the BJT's ft. All these types will work up to ~50 -- 60MHz. As for BW in audio applications, that's dependent on the circuit design and OPT quality.
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Old 24th February 2015, 09:11 PM   #3
Merlinb is offline Merlinb  United Kingdom
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You can calculate it from:
1/(2 * pi * Cout * ra)

In most cases Cout is around 10pF, and ra varies from say 15k for an EL34 to 50k for a 6V6, so you'd be looking at flat responses up to 1MHz and 318kHz respectively. Multiply by gmra and the gain-bandwidth products would be about 160MHz and 63MHz respectively. Like Miles said, what exactly are you asking?

Last edited by Merlinb; 24th February 2015 at 09:20 PM.
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Old 24th February 2015, 10:01 PM   #4
diaz028 is offline diaz028  Canada
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Apologies for the open ended question, I meant for audio / guitar amp applications.

People often describe each tube in terms of what they hear. I would simply like to see how they reproduce the audio frequency spectrum; much like a speaker 20hz-20khz SPL chart.

Of course these properties get thrown out the window once the output tubes are pushed into overdrive, but my uses are mostly in the clean / non-clipping region.
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Old 24th February 2015, 11:07 PM   #5
Keit is offline Keit  Australia
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True, some people describe different tube types in terms of what they think they hear.

Some people even claim that different tube brands sound different. Or even diffrent examples of the same type in the same brand.

Whole textbooks and many papers in learned journals have been written by electronics engineers and psychologists about why people think there are differences in things where there is in fact no difference at all. People imagining non-existent or reversed differences is a major problem in the drug industry and seriously frustrates development and optimisation of medical treatment.

There is a well known difference, easily demonstrated by instruments and known about by electronic engineeers since tetrodes were invented in the 1930's between triodes and tetrodes and pentodes. Pentodes have slightly higher distortion compared to beam tetrodes intended for audio, but the difference is so slight that in various circuits the difference is often reversed.

For tetrodes and pentodes, the sound doesn't inherently depend on the tube. it depends on teh circuit and the quality and type of output transformer.

All the types you nominated are tetrodes (6V6, 6L6) or pentodes (EL34, EL84). However these particular pentodes have higher gain than these particular tetrodes.

There is no intrinsic diffrence in the sound of an EL34/6CA7 compared to the 6L6 for example, but the EL34 has much higher control grid gain (11 mA/V vs 6 mA/V) and different screen characteristics. So, if you start with an amplifier designed for 6L6, and drop in an EL34, the amplifier may become unstable. You hear the instability as distortion. But an amplifier intended to take a EL34 won't be unstable and will sound the same - other aspects (eg transformer design) being equal.

If the amplifier is an ultralinear type there is a measurable (with instruments) difference with 6L6 and EL34 in the same amplifier due to the different screen characteristics. It is quite unlikely that anyone's ears can tell the difference. And the slight difference that is detectable on instruments can be easily reduced or eliminated by adjustment of the transformer screen tapping point.

The human ear cannot detect small changes in volume (1 dB or less) as a change in volume. But we DO hear the change - as a change in bass and clarity. Quite a number of journal articles have appeared that describe how double blind auditioning has identified a diffrence in clarity, bass, or treble, investigation by engineers shown that there was a diffrence in volume. When this was corrected, the auditioners could not detect any difference - even when differences in distortion beween amplifiers were easily detected on instruments.

Due to the peaky nature of music, all but the highest power amplifiers in audio service are routinely overdriven momentarily in any musical selection, unless reproducing vinyl or radio braodcasts, which are compressed. With CD's, even when played at moderate volume for personal listening, momentary overdroves are routine. Even with vinyl, overdrives occaisonally occur except for listening a quite levels.

The ear cannot easily detect occaisonal momentary overloads. Some people cannot detect it at all. (We can all, of course, easily detect overdrive once it occurs continuously.) However, less than perfect amplifier circuit design can result in an amplifier than does not instantly recover from an overdrive, and remains parallised for a short time after. This parallysis is easilty detectable and sounds horrible. A type of overdrive response called re-entrant clipping, once not uncommon in solid state amplifiers, also sounds bad.

Make no mistake. The behavior of audio amplifiers (both tube and solid state) under overdrive is VERY important and is a major reason why some sound a lot better than others, and why some sound different than others. Overdrive behaviour is determined by circuit design, not the tube type. Unless of course the tube is worn out or faulty.

Last edited by Keit; 24th February 2015 at 11:22 PM.
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Old 25th February 2015, 12:18 AM   #6
diaz028 is offline diaz028  Canada
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Thank you very much for this answer. That is something I had always assumed was part of the equation, but never read it anywhere.

Basically, a circuit designed for a 6l6 will sound like it has more treble or presence with el34 because the circuit has now changed? But if two amps were each optimized for each tube it would be very difficult to tell the difference?
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Old 25th February 2015, 12:25 AM   #7
Keit is offline Keit  Australia
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Quote:
Originally Posted by diaz028 View Post
But if two amps were each optimized for each tube [type] it would be very difficult to tell the difference?
Correct. Very difficult or impossible.
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Old 25th February 2015, 12:31 AM   #8
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Audio and guitar amplification can be quite different.
Audio is usually hi fi type amplification.
Guitar amplification can be clean or have lots of harmonics for a richer sound.

Marshall made a fortune supplying rich sounding valve amplifiers.
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Old 25th February 2015, 12:41 AM   #9
cerrem is offline cerrem  United States
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The Frequency response is dependent on a number of parameters...
Not entirely the tube, but each of the tubes you mention can have the same frequency response if the circuit is optimized for a given set of requirements...
Guitar amps are a bit more non-linear.... since you work primarily in square wave most of the time.... and square wave in the output stage spend 98% of their time in the "triode" region of the plate curves, left of the knee...This drops the effective plate resistance down to 400 to 800 ohms range.... thus changing the frequency response in a dynamic way....
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Old 25th February 2015, 02:15 AM   #10
Keit is offline Keit  Australia
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Both posts #8 (Nigel) and #9 (cerrem) are correct (through cerrem's 98% figure is an exaggeration). Note however that none of what they said implies that one tube type will react differently to another.

In any case the drop in plate resistance causes an increase in bandwidth, which is not much affected by the tube type. Frequency response is determined by the output transformer, the grid coupling capacitors, and any cathode bypass capacitors.
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