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b_force 20th April 2009 11:55 PM

The secrets of guitar amplifiers
 
To make a long story short, I'm planning to make a guitar amplifier by myself. I've (re)searched a lot over the internet, google, diyaudio and I lend some specific tube books from my father. (because nowadays those subjects aren't import any more on the study of physics :cannotbe: ). I also found this page, which is very useful: (TIP!)
http://www.pmillett.com/technical_books_online.htm

So I begin to understand how a lot of schematics are build that specific way. But there are still some questions left where I can't find the answer of on the internet or in books.
Now I know that guitar amplifiers are a very different kind of sport than most HIFI amplifiers. Reasons could be just production money, matter of taste or just some random choices because it was build that way.

One specific question to begin with is, why are most guitar amplifiers build PP?
Is that only because of the extra output power?

But why use two EL84 and not one SE EL34/KT77 ? That's almost the same price for the same power.
Besides, you need also an extra phase inverter for a PP amplifier.

For example, one idea was to make an KT77 SE amplifier. That must be enough for some 12-14W. Good enough to make the neighbours angry. :cool:

Wavebourn 21st April 2009 12:28 AM

Did you compare costs and weights of SE and PP transformers?

b_force 21st April 2009 12:31 AM

I don't know exactly, I know a few OT's for PP and Se that you can buy for almost the same price. I didn't looked at the weight.

HollowState 21st April 2009 12:46 AM

Re: The secrets of guitar amplifiers
 
Quote:

Originally posted by b_force

One specific question to begin with is, why are most guitar amplifiers build PP?
Is that only because of the extra output power?

While I'm not a guitar person (anymore) I can say the answer is mostly because of the efficiency of PP output over SE. In patricular the output transformer requirments are much different. A PP transformer can be much smaller and lighter then SE and thus less expensive. Even if you include the extra tube (or tubes) and handful of small parts, the overall cost can be less. The shipping weight will also be less.

Something may also be said for the overload and distortion characteristics of PP verses SE when a certian sound is desired. But I think it's mostly the above.

b_force 21st April 2009 01:13 AM

But I've read that SE amplifiers are have more second order harmonics (distortion). Can that not be a extra point to go for a SE design?

leadbelly 21st April 2009 02:29 AM

Quote:

Originally posted by b_force
But I've read that SE amplifiers are have more second order harmonics (distortion). Can that not be a extra point to go for a SE design?
What answer are you looking for? Commercial guitar amps went PP long ago for various reasons like cost, weight, and power. For yourself, there's nothing wrong with building a big SE guitar amp, and there are designs out there if you look for them. I'm building one right now with a super-6BG6GA tube.

flysig 21st April 2009 03:38 AM

The SE transformer has to carry a lot of current, and so it must have a heavier core and windings. That means that the SE transformer is a lot more expensive than the PP.

PP gets more efficiency out of each tube, so you can get more power with fewer tubes, or more power with a smaller power supply.

Two books that are very good for designing guitar amps are "Vacuum Tube Circuit Design: Guitar Amplifier Preamps" and "Vacuum Tube Circuit Design: Guitar Amplifier Power Amps" by Richard Kuehnel. These books will explain the details of how to design a guitar amp and what the benefits are of each type of design.

gingertube 21st April 2009 03:45 AM

A guitar amplifier (as distinct from a H iFi Amplifier) is a musical instrument in its own right.
Keep researching and you will find plenty of tips and hints as to how to get the particular sound you prefer - have you defined what you want?

Aside: I'm an EE in the day job. The most common mistake I see in design is not defining your requirements specification before you start and then trying to design something that is "all things to all men." This is even more critical for software design where the "Feaping Creatures" (Creeping Features) syndrome sets in. The requirements spec is the only way you know when it is finished.

Perhaps counter intuitively the modern high gain overdrive guitar sound is generated practically entirely in the preamp section and it requires a HiFi-ish fast and tight power amp and power supply section.

The old 60's and early 70's Vintage Blues sound requires a fairly clean preamp section with a power amp section (often using cathode biased output tubes and concertina phase splitter) which can be overdriven and a soft power supply. This sound is best achieved using a push pull stage with a common cathode bias resistor without a bypass capacitor BUT then requires the use of matched output tubes OR a single ended output stage.

The preference for push pull output stages was born out of the need for huge power output, before the days of big PA Systems.

Today, there is absolutely no reason why a single ended power amp section can not be used and in fact there are a number of amp manufacturers designing amplifiers with a "Body" control. This control basically allows one side of the push pull to be dialed anywhere between perfect balance and right off, effectively giving you a single ended sound. It is simply a pot between the phase splitter (one side only) and the output tube grid.

My own latest guitar amplifier (completed last weekend) has footswitchable Clean (2 stage) and Lead (4 stage) preamp using 6SL7 octal tubes driving a 6SL7 common cathode stage followed by a self biased concertina phase splitter driving Ultralinear connected Push Pull 6V6 Current Source biased output tubes. The Output stage is very HiFi-ish with a reasonably "tight" CLC power supply. That gives me all the sounds that I wanted from a guitar amp. Its a reasonably modern design but the use of 6SL7 gives it a vintage "bent". It won't do flat out full metal jacket shread and it won't do the early 60's flabby blues sound but it can do anything in between. In addition the use of a high mu tube (6SL7) for the concertina phase splitter emphasizes the concertina limitations and adds "warmth" as compared to the Stacked schmidt phase splitter which is used in 90+% of guitar amps.

Just one mans views/ideas.

Cheers,
Ian

chrish 21st April 2009 05:35 AM

G"Day Gingertube.

I am still living my tube building life vicariously at the moment. Moved house 6 months ago and still have not set up a workshop. Long story with some personal issues there so won't go in to it... Short story is I expect a mate of mine to purchase much of my woodworking machinery that is stored in an area that is too small for me to use them effectively. When they go I should have space enough to have a workbench and work area for handyman stuff and electronics projects...

Projects on my horizon are a pair of monoblock 7591 PP amps with Tamura output transformers (that is for another thread) and my son wants to learn electric guitar. My ex wife bought him a cheap guitar and (HORROR!!!) a cheap SS practice amp. I said we could research and build a tube amp for his guitar. From what I have gathered, he likes more of a classic rock style of music, but I am sure his tastes will change (he is 13). My hope is that we can build an amp with a reasonable power section that will last the distance for him. The amp you just describe sounds very interesting and something similar may well be suitable. Are you willing to share your design?

I have a few Russian 6V6 equivalent tubes and a whole bunch of Russian 6SL7 and 6SN7 equivalents, a power transformer 290-240-0-240-290 150mA, 5.0 3A, 6.3 2A * 2. Just really need an output transformer (Edcore 15 watt guitar transformers look like good value), a speaker and a design.

Cheers,

Chris

gingertube 21st April 2009 06:04 AM

Chris,
Look here:
http://www.powerscaling.com/communit...p?topic=1017.0

Some off topic rave on the pro's and cons of concertina splitters and the problems of reverse grid current in vintage high mu tubes (like the 6SL7) but all then schematics I used are there (starting at page 3).

Acknowledgment:
The Preamp circuit is a (slightly) modified version of Kevin O'Connor's London Power Standard Preamp (LPSP). The mods are some resistor value changes to suit the 6SL7 and to make sure (reverse) grid current with vintage 6SL7 is not a problem.

I would make a couple of changes if I were building it again:
1) I used J201 for the channel switching JFETS. They caused me a little grief as their cut off voltage is too low (as low as -0.3 volts). I would use J202 or J203 instead if I were to build it again.
2) The negative supply is a bit more complicated than needed - I include the bias set resistors etc. for possible future conversion of the output tubes to fixed bias - probably wont ever be used, the "Baby Huey-ish" CCS biasing of the 6V6s shown works really well and I like the idea of no idle current adjustment ever required. It is what I'm running. Note that due to the required 24 Volts or so of bias these CCS's dissipate about 2x the power of the ones in the Baby Huey and require heatsinks. I used tiny 39 degree Cper watt heatsinks and they get hot to touch (not burned finger hot but uncomfortable to touch hot). Bigger heatsinks would not be a bad idea.
3) The LED current set resistors in the switching circuit are shown as 15K. With the LEDs I used these had to be dropped to 8K2 for a bit more brightness.

Also a comment:
The Power Amp schematic shows Pentode Mode/Ultralinear Mode Switching. One mode gives better guitar amp tone and I leave the switch in that possition all the time. I need to check the wiring to see which mode that is. Will advise tomorrow.

Cheers,
Ian


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