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OTL amps for guitar duty
OTL amps for guitar duty
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Old 9th February 2017, 02:26 PM   #21
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by AquaTarkus View Post
Deliberate irony ;-)
Ah. My apologies for being as thick as a whale omlet.

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Old 9th February 2017, 03:13 PM   #22
goddlediddles is offline goddlediddles  Ireland
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OTL amps for guitar duty
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Originally Posted by Gnobuddy View Post

There are only a few components that can potentially create that much THD in a guitar amp, and those include the actual valves, the speaker itself, and the output transformer.
-Gnobuddy
Not forgetting of course that sag created by the tube rectifier.
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Old 9th February 2017, 03:26 PM   #23
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by goddlediddles View Post
Not forgetting of course that sag created by the tube rectifier.
I agree totally that "sag" is very much a part of guitar amps designed for at least some genres of guitar music.

But I think "sag" is more like building an accidental compressor into the circuit. Not quite the same as creating THD because of a nonlinearity in the transfer function.

Put another way, THD-generating mechanisms involve a gain that changes instantaneously, varying rapidly during each individual half-cycle of (guitar) audio. Meanwhile, sag is a much slower gain variation, with gain changing rather slowly over a period of many cycles of audio, so it is the audio envelope that is "squished", rather than individual half-cycles of the waveform.

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Old 9th February 2017, 04:10 PM   #24
atmasphere is offline atmasphere  United States
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Originally Posted by Gnobuddy View Post
I have heard this many times, and I do believe it's quite plausible. The logic being, "tone" actually is a misnomer for "timbre" in this context, and anything that creates roughly 10% or more harmonic distortion has the potential to change the timbre.

There are only a few components that can potentially create that much THD in a guitar amp, and those include the actual valves, the speaker itself, and the output transformer.

So it's certainly plausible that the output transformer contributes to guitar amp "tone". But it bothers me that I've never heard of anyone actually doing an experiment to prove it.

Oh, we have plenty of people saying that their guitar amp sounded better after they put in a $200 Mercury Magnetics transformer or something like that. Trouble is, there is no AB comparison possible, and human psychology dictates that you usually will think something is better if you just spent $200 on it. So these sorts of claims more or less fall into the "that could possibly be true, but it could also just be the placebo effect" category.

In principle, it doesn't seem too hard to come up with an experiment to test if guitar amp output transformers actually contribute to the amps timbre. Say, drive a guitar speaker with increasing amounts of sine-wave 500 Hz power, first from a low-distortion solid-state amp direct-coupled to the speaker, and then from the same amp via a 1:1 ratio output transformer made with the same-size iron as the actual valve amp OT. Or even two back-to-back OTs, one stepping the voltage up, the other stepping it back down, for an overall 1:1 ratio (less losses).

If you hear significantly different timbre via the transformers, the hypothesis is proved. If you don't, well, it turns out to be yet another amp-guru myth. Either way, we all learn something useful!

-Gnobuddy
An output transformer can have quite an effect, as depending on the design, might be limited on the bottom and or top. Phase shift accompanies the cutoff frequency and is as a rule of thumb going to be present to about 10X or 1/10th the cutoff, depending on which pole we're talking about. Guitar OPTs cut off fairly high (20-40Hz), so its plausible that phase shift can exist to 200-400Hz or higher. The ear translates broad spectrum phase shift like this into tonality, so its easy to surmise that's part of the 'sound'.

The thing is, in an OTL, you just impose similar bandwidth limits, like smaller coupling caps...


Quote:
Originally Posted by goddlediddles View Post
Not forgetting of course that sag created by the tube rectifier.
Rectifier sag does not have to translate to sag in the power supply. Much depends on the design of the supply; you can engineer sag using semiconductors as well (I prefer HEXFREDs as they tend to be lower noise similar to a tube rectifier).
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Old 9th February 2017, 04:38 PM   #25
pamaz is offline pamaz  Italy
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I have been a professional guitar amp manufacturer for some years ( and still I'm building them for friends or myself).
Hi fi and guitar amps share just the fact that they use the same kind of electronic components, and nothing else.
In a guitar amp, generally , you are searching for some distortion ( also on clean sounds) in order to enrich the harmonic content of the sound that, otherwise is quite sterile and poor. Think that the guitar loudspeakers, that are another fundamental part in the soundchain of any guitarist, have their boxes made to resonate, and if you make the dead and stiff, the sound will be " dead and stiff". So really, there a not great contact point between hifi and guitar amps. A different perspective could be the amplification for Bass guitars or for acoustic guitars and basse, where there is some request of "hifi" treatment of the signal.

regarding otl as a guitar amp: I know there has been a commercial attempt with a 6c33 amp ( it was presented at a namm show a few years back) but as far as I can see it's been not succesful. Not even mentioned as a curiosity among the guitar guys, neither in the Hi End of guitar amps builder....

Last edited by pamaz; 9th February 2017 at 04:43 PM.
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Old 9th February 2017, 04:46 PM   #26
atmasphere is offline atmasphere  United States
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I don't think the 6C33 is a practical OTL guitar amp tube as it runs way too hot and eats sockets. That is why I use the 6AS7G, although we've used PL519s as well.
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Old 9th February 2017, 05:09 PM   #27
pamaz is offline pamaz  Italy
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Not that an otl cannot sound decently for a guitar, but there are some basic facts that are to be considered whe you are designing a guitar amp in my opinion.
1) guitarists are sticky to some sounds ( and circuit) topologies (fender, marshall etc)
2) the approach in making a very good guitar amp starts not from the sound, but from the feeling that the entire amplification chain is giving you. Starting from the feeling you have under your strings related to the sound you hear. That's where a good amp builder is different form a normal one. What you need to do is "translating" what a musician wants to feel in a physical amplifier. Technically, this is where you play a lot with power supply, sag and feedback, that eventually have an impact in the timbre. Balancing all these elements is what makes for a great guitar amp.
3) replacement of tubes anywhere you are. Sourcing an el34 is much easier than any otl tube. This means that if you step in a music shop anywhere in the world , you'll find always an el34 or a 6l6 , or some el84.

These are some basic points you might want to consider when professionally designing an amp.
OTOS, If you are making an amp for your personal use that will sit in your practice room, forever, you can use any kind of topology ,style, or inspiration, that once the sound will be pleasing at your ears, it will always be a perfect amp.

edit I forgot to point out a fact: a guitar amp, needs to satisfy the player first, and only in a second moment the listener....

Last edited by pamaz; 9th February 2017 at 05:14 PM.
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Old 9th February 2017, 05:55 PM   #28
JMFahey is offline JMFahey  Argentina
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You are coupling the bad traits of SS amps with the cost and complication of tube amp ... which to boot will not sound like a TUBE amp anyway.
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Old 9th February 2017, 07:42 PM   #29
atmasphere is offline atmasphere  United States
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Quote:
Originally Posted by pamaz View Post
3) replacement of tubes anywhere you are. Sourcing an el34 is much easier than any otl tube. This means that if you step in a music shop anywhere in the world , you'll find always an el34 or a 6l6 , or some el84.
This is actually the big deal. The other points are all doable.

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Originally Posted by JMFahey View Post
You are coupling the bad traits of SS amps with the cost and complication of tube amp ... which to boot will not sound like a TUBE amp anyway.
This statement is outright false. OTLs most definitely sound like tube amps!

As far as bad traits of solid state??

Hard to say what that might be- soft clipping? -that's a tube thing.

Greater bandwidth? - that's a design thing and has nothing to do with tube or transistor.

Parts availability?- the tubes we use are 12AX7s, 6SN7s and the output tubes which are 6AS7Gs. They are available cheaply, so it would not be much expense to include a spare set if actually selling the amp.
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Old 9th February 2017, 10:37 PM   #30
Yugo is offline Yugo
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Originally Posted by SemperFi View Post
SemperFi....,would you mind posting the schematic for the guitar OTL with 6AS7...?

Thank you,
Yugovitz
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