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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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 Post subject: Re: Guitar amplifier ?
PostPosted: 16 May 2018, 10:39 

Joined: 12 Mar 2014, 22:28
Posts: 7
Location: Canada
I would argue that HiFi amps are based mostly on science, while guitar amps are based on art. Thankfully in recent years there have been a lot of excellent books etc released taking a scientific analysis on what makes a good guitar amp sound. This makes it MUCH easier for a beginner to get a head start with some simple "rules", after that it just takes time to tinker and listen to what distortion pleases you.

For example:

1) Classic guitar amps undersize input transformers and overload tube rectifiers to deliberately induce voltage sag under load. Modern guitar amps will have stiffer PS and SS rectification as they want more of that metal edge that comes from pre-amp distortion. PS sag is not usually desired in metal amps.

2) Pre-amp tubes are biased very hot or cold to create different distortion characteristic. Classic amps would have very few stages to keep the distortion minimal (well originally they were intended to amplify cleanly ;-), modern metal amps have many stages leading to almost square/triangle shape curves.

3) Phase inverters are deliberately unbalanced and over-driven to add their own flavour. Recently analysis on the Fender Bassman circuit claims this is a large part of the famous sound. Different inverter circuits will yield different tone when over-driven.

4) Lots of interesting analysis on AC/DC follower circuits (not the band on tour hahaha ;-) M. Blencowe (aka Valve Wizard) has some excellent books on all of these topics.

5) Power tubes are also intentionally over-driven, but not as much as guitar amp mythology likes to think. Very few players practice at full volume hence most modern bedroom amps are pre-amp shaped.

6) Output transformers also yield some character to the guitar amp's tone. A builder will definitely NOT source high quality components here. This is one area I have not had much time to experiment with, so I can't say much more.

7) Lastly the speakers play a huge role in the guitar amp tone. Speakers have been deliberatly ripped etc to add their own character. Again one needs to hear a lot of combos to know what works best together.

Guitar amp building is more art than science which is why I find it more fun and rewarding!

 Post subject: Re: Guitar amplifier ?
PostPosted: 17 May 2018, 15:26 

Joined: 26 Dec 2016, 03:46
Posts: 195
Location: Bayarea
I think guitar amps are 90% art and less than 10% science. The science part is very basic. The points you made are widely known for decades. It's all about try and true.

I experimented with difference output transformer. I first used the Classic Tone Bandmaster/Pro/Vibrolux Reverb OPT, I then change to the Weber Marshall Plexi 50 OPT, no noticeable difference. I can change the value of the components in the tone circuit and make a much bigger difference.

As I said, I used the cheapest of the cheap components and have great success.

One thing nobody mentioned.......the layout of the circuit. That is just as important as the design if not more. I use a Fender Bassman 100 chassis as an experimental and prototype platform, I experiment with the circuit until I am happy enough, then I build a brand new amp with clean layout.

I started with a layout on the Bassman, but then I start modifying and add circuit, the layout and signal flow got messy and signal crossing. I got my sound, but it always sound a little funny like it's under water. I built the new amp, with the layout optimized for the final circuit, the funny sound is GONE!!!

Key is to know that Tube circuit is high impedance, with high magnetic field running around. Layout can be sensitive with high gain circuit ( for the metal as you called it). Layout is science and knowledge. That I agree. But the circuit is art as it's very easy.

I think some people have bad layout, but somehow get the right sound!!! This is blind luck. Guitar amps has a lot.

I am more like an engineer or scientist. I like hifi amp better, it's more science. People don't give enough credit on the power amps. As an engineer with many years of designing experience with advanced circuitry, I am actually very surprised how deep is designing SS power amp for hifi. I used to think how hard can audio circuit be. But if you combine power, high current, ultra low THD, very high internal gain ( for low THD), things get very tricky. The ground current, lack of large signal model for simulations, and relative high speed........Yes, you don't design for 20KHz!!! I design to 300KHz, some amps design to 1MHz, all the get minimal phase shift, flat frequency response. Getting 100W with this specification can complicate the design very fast. It is a challenge. To me ( only to me), this make guitar amp a child's play. It's the high current that make it difficult also, You really need to know system grounding, signal integrity ( they actually have engineer call signal integrity engineer that specialize in grounding and crosstalk. I was contracted to KLA Tencor to layout their critical pcbs). It's nothing like low power small signal design.

I spent over 2 years on this and I still learning new things. In our industry, we come up with new ideas, implementing it, get a patent and into production in less than two years. People think hifi SS power amp is no big deal, never really dig into it.

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