Is it possible to get "tube sound" with just a tube preamp and chipamp output? - Page 3 - diyAudio
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Old 8th May 2006, 07:33 PM   #21
teemuk is offline teemuk  Finland
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Seems like my post came here two times. Sorry.
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Old 8th May 2006, 07:33 PM   #22
teemuk is offline teemuk  Finland
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If you are using ordinary guitar speakers they will definitely color the tone (frequency wise), probably more than any power amp would ever do. Look at the frequency response of guitar speakers - they are anything but flat! The speaker system is an important part of the amplifier setup and can change the tone and your (stage) volume/headroom dramatically! Just think of it: How do a set of guitar 2 x 15" speakers with high SPL rating, roll-off frequency at 5kHz and free air resonance at 80 Hz compare with a single home stereo grade 6" speaker with a roll-off freq at 20 kHz, resonance at 120Hz and low SPL rating.

Then again, what really is "tube sound"? Is it the sound of the speaker system, the sound of a different transfer curve, the sound of having a transformer coupled circuit or just a nice soundingf tonestack circuit? Think about it.

I have heard so many guitar tones that i lost count. Many people are tricked to believe some amps have tubes just when you plug a compressor into the signal chain. Some people even believe my 12W SS amp is a tube amp when i have the right patch in my digital multieffect. You want overdrive tone and think you get it with just putting a tube to the signal chain? No way! You have to at least clip, pre EQ and post EQ - probably in few stages. Im quite sure you also have to compress before each stage too - at least that's what some hi gain amps seem to do. Blues or 60's tone? - Limiter and mildly overdriven discrete stage (that introduces a bigger amount of 2nd harmonics) - solid state or valve - shouldn't make a big difference. Just look inside some distortion pedals and think a moment what's inside of them...

All and all, you have to know what you need and make a circuit that provides it. Valve amps can be very linear and clip harshly if they are designed to do so - most of them are not. Why do you think opamp's clip harshly; because they are designed to be as linear as possible until the very bitter end. If you feed a very linear circuit with a full bandwidth signal it will not sound very nice - it doesn't make a difference whether you have a valve or a transistor as the active device. If you overdrive a single common emitter stage with a limited, hi-pass filtered signal and after that low-pass filter it you will get a vastly better tone. Valve amplifiers tend to do some of this automatically, however the resulting tone is not very "clean" and thats why people prefer separate distortion/clean channels. By the way, that common emitter circuit probably would have a dominant 2nd harmonic because it clips asymmetrically. Yes, transistor circuits clip asymmetrically and have a dominant 2nd harmonic as well as tube circuits - if you give a chance for that.

You really have to know what you need before yo know what questions to ask. Think about it. I hope i provided some help, not just confusion.

Teemu K
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Old 9th May 2006, 04:01 AM   #23
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Quote:
Originally posted by Shoog
I agree with the previous post, simply overdriving a preamp tube just isn't going to cut it. There are at least three sources of main distortion in a guitar amp and you are only addressing the simplest one. Simply raking up the 2nd order harmonics is likely to produce a horrible sound.

My suggestion of introducing an interstage transformer is simple and will offer at least two types of the distortion you are looking for. It also gets around the fact that the output of your tube stage is likely to be much to high to drive a chip amp with the result of horrible chip amp distortion. Clipping a chip amp is not a good thing so you need to keep the input well bellow clipping the chip amp. The other alternative would be to use the valve as a cathode follower - but this reduces distortion rather than increasing it.

Using a 10VA mains transformer in a parafeed setup, with a 240V to 6V step down ratio is likely to saturate fairly easily, cost next to nothing and give a useable input to the chip amp for a sensible tube preamp output.

Shoog

This sounds like an optimal solution. Using one or two tubes and a cheap transformer to make an okay-sounding preamp is ideal; I'm still kind of fuzzy on how tubes work, but I'd at least like to try making something with them.

Also, if I can get an appropriate tube, I think I'll try a stab at the single-tube solution mentioned above. With a spare guitar amp speaker (something cheap, probbably) it might make an interesting, and extremely simple, first amp.
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Old 9th May 2006, 07:34 AM   #24
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Buy an Epiphone Valve Junior and hook up one of these:

http://www.scopeboy.com/ssim.html

Or really - just go try some of the tube pedals out there. They are popular for good reason.
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Old 9th May 2006, 08:29 AM   #25
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The last post by Raintalk looks as if it gives a bit of useful information about optimising this arrangement.
I suggests that you go ahead with a simple single valve preamp stage, a step down interstage and finally a simple LM3875 implementation. This will be cheap and very simple to realise and it will surprise you with the quality of its sound.
You have nothing to loose and you might just create something that others will want to follow.
If you haven't built a chip amp before, try to get hold of a couple of chips in case of accidents. You may have serious DC offset issues if you don't include a simple time delay circuit, this will allow the valve to warm up before it starts driving the chip amp- which will fry your speaker !!!

Shoog
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Old 9th May 2006, 04:12 PM   #26
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Part deux

Look at the first chart at: http://milbert.com/articles/TvsT/tvtiega.html

Note the frequency response of the tube amp.
This is due to several things, but one factor is the reactive load of the speaker. You can't just hook up a resistor in place of a speaker and expect it to emulate a speaker. You could hook up a resistor and use EQ to mimic the same curve. Or you could use a more reactive dummy load:
http://www.duncanamps.com/technical/dummyload.html

Personally - I'd just use EQ.
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Old 9th May 2006, 05:32 PM   #27
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Raintalk can you express an opinion in plain words as to whether you believe a simple overdriven interstage transformer will produce a satisfactory distortion spectrum. The power stage will never be clipped so all the distortion will come from the preamp stage and the transformer.

Shoog
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Old 9th May 2006, 11:24 PM   #28
teemuk is offline teemuk  Finland
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Quote:
Originally posted by teemuk
what really is "tube sound"? Is it the sound of the speaker system, the sound of a different transfer curve, the sound of having a transformer coupled circuit or just a nice sounding tonestack circuit?
I think this thread is still lacking the definition of the term "tube sound" that the thread starter has been looking for. Yeah, surely the transformer coupled reactive load will make some "EQing" to the signal as well as the tube will have some non-linearities but the question is are they enough. I still have no clue whether the thread starter is after for a "tube" clean tone, "tube" overdriven tone, or perhaps "tube" hi-gain tone. And on top of that, what makes them "tube" for the person who started this thread?

What i'm saying is that anyone will definitely get a "tube sound" just by implementing a tube as the active device... pretty logical, right; it has to be a tube sound since it's coming from a tube. But is it the tone that the thread starter is really after?

You can implement transformers, tubes - all kinds of stuff - and make a circuit that distorts like hell - or stays clean. When you say "tube sound" what do you mean spasticteapot?

Teemuk
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Old 9th May 2006, 11:54 PM   #29
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Quote:
Originally posted by teemuk
When you say "tube sound" what do you mean spasticteapot?
Look, stop pestering the guy. You showed off how much you know about the subject with your lengthy post. Stop the contest and try to offer help to someone at a more basic level.

I think this thread has gotten too philosophical for what it started off as, a beginner (relatively) asking if this was a good path to pursue. Yes, you can get good results with just a tube preamp stage and a chip power stage. There are commercial rackmount tube distortion boxes that have no iron and no output tubes and no Celestion in them AFAIK and can produce a wide range of sounds, as well they should with their 4 figure price tag. Unfortunately, I haven't come across a good published design for one of these yet. The other suggestions in the thread are a best bet.
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Old 10th May 2006, 01:46 AM   #30
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I quite like Teemuk's answer and question.

I could think that teapot, not being a guitar player, could build something with a tube, take it to his guitar player friends and have them plug in and comment: "Where's the tube sound?" or maybe "That's over the top" or unlikely: "That's perfect"

Maybe the philosophy would trim down if the question were clarified with "Sound like X... Brand amp" Or "Sound like on this cut of music"
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