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Step 2How did this project get started?
First, I like the sound of the 50L6 power tube in my vintage Kay amp. Although weaker than many common types (6V6, 6L6, etc.), nearly 80% of the output of a 6V6 can be achieved with a 50L6 (~4 watts for a 50L6 vs. 5 watts for a single-ended 6V6 tube.) And I have several spare 50L6's on hand... There's a long history of small practice amps with these tubes, yet they are generally ignored today.
Secondly, this build has always intrigued me: Super SE 6V6, a parallel single-ended (two output tubes together in parallel) 6V6 amp. Perhaps the same approach would work with two 50L6's, probably wringing 7+ watts from the pair--that would be a true test of their suitablilty.... A parallel SE design would be a true Class A amp, too, with all the of the richness and aural mystique of the class (and more punch than my Kay.)
Lucky Find
Thirdly, when gutting an old TV set I found a decent (and massive) power transformer appropriate for this build. A bit of explanation:
The 50L6 and it's variants (25L6, 12L6) are power pentodes with a maximum operating voltage of 200V. That's significantly lower voltage than most power tubes, which run at 300+ volts. Consequently, the majority of power transformers supply 250V or greater. A medium-voltage power tranny is actually tougher to find than the higher-voltage variety.
The TV transformer tested out with secondaries of ~140V and ~7V. AC voltage is RMS--essentially the average voltage for the wave form. Once it's rectified and filtered, it's higher. Depending on the rectifier, the DC output voltage will approach the "peak voltage" of the waveform. Immediately, I rejected the use of a tube rectifier--what I had on hand wouldn't be as efficient as a solid state rectifier. A 140V RMS transformer is nearly ideal. With luck, I could get very close to the 200V max using a SS bridge rectifier!
So finding the transformer first was the real motivating force behind the build...
Next, I chose the preamp tube, a 12AX7. That was easy--they're the most common preamp tubes, and the majority of guitar amps include one or more. 12AX7 tubes pack two triodes into a single tube--double the fun.
Enter the 6DG6GT as output tubes...
So I started planning how to supply the filament voltage for two 50V and one 12V (or 6V, the 12AX7 can be 6.3V filaments, also) tubes. A chance discussion with instructable member Ohm led to the 6DG6GT. While I was aware of the other variants, I hadn't heard of this one. Bravo, Ohm!
Sure enough, the 6DG6GT specs were identical to the 50L6, except for the 6.3V filaments. Now I could plan on three tubes that would run with 6V heaters, and the TV transformer included a 6.3V secondary...Well, I just HAD to build this! And I couldn't find any builds of this type (6DG6GT in parallel SE) on the web, at all. It couldn't be the first, but it looks pretty rare for guitar amps, anyway...
Secondly, this build has always intrigued me: Super SE 6V6, a parallel single-ended (two output tubes together in parallel) 6V6 amp. Perhaps the same approach would work with two 50L6's, probably wringing 7+ watts from the pair--that would be a true test of their suitablilty.... A parallel SE design would be a true Class A amp, too, with all the of the richness and aural mystique of the class (and more punch than my Kay.)
Lucky Find
Thirdly, when gutting an old TV set I found a decent (and massive) power transformer appropriate for this build. A bit of explanation:
The 50L6 and it's variants (25L6, 12L6) are power pentodes with a maximum operating voltage of 200V. That's significantly lower voltage than most power tubes, which run at 300+ volts. Consequently, the majority of power transformers supply 250V or greater. A medium-voltage power tranny is actually tougher to find than the higher-voltage variety.
The TV transformer tested out with secondaries of ~140V and ~7V. AC voltage is RMS--essentially the average voltage for the wave form. Once it's rectified and filtered, it's higher. Depending on the rectifier, the DC output voltage will approach the "peak voltage" of the waveform. Immediately, I rejected the use of a tube rectifier--what I had on hand wouldn't be as efficient as a solid state rectifier. A 140V RMS transformer is nearly ideal. With luck, I could get very close to the 200V max using a SS bridge rectifier!
So finding the transformer first was the real motivating force behind the build...
Next, I chose the preamp tube, a 12AX7. That was easy--they're the most common preamp tubes, and the majority of guitar amps include one or more. 12AX7 tubes pack two triodes into a single tube--double the fun.
Enter the 6DG6GT as output tubes...
So I started planning how to supply the filament voltage for two 50V and one 12V (or 6V, the 12AX7 can be 6.3V filaments, also) tubes. A chance discussion with instructable member Ohm led to the 6DG6GT. While I was aware of the other variants, I hadn't heard of this one. Bravo, Ohm!
Sure enough, the 6DG6GT specs were identical to the 50L6, except for the 6.3V filaments. Now I could plan on three tubes that would run with 6V heaters, and the TV transformer included a 6.3V secondary...Well, I just HAD to build this! And I couldn't find any builds of this type (6DG6GT in parallel SE) on the web, at all. It couldn't be the first, but it looks pretty rare for guitar amps, anyway...
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It comes to mind would a 6W6GT also be suited for this project. I think they also have a higher plate voltage and maximum power dissipation. I'm certain they are as available as a 6DG6GT as well.
cheers
(I also kind built this around the transformer--that had something to do with the power tube choice.)
Literally, the best sounding instrument I ever heard was an experimental Hammond Model that used BATTERIES to purify the electricity flowing into the instrument. This guy had taken apart a (I think it was a B3) Hammond Organ that had a huge speaker box and Tubes for amplification. I tried to find out why, exactly, but all I could get out of him was something about the electricity is cooler and the tubes lasted much longer. This chap was nearly 70 and could he ROCK that Organ! He would sit and listen to on record after another and play right along with then all! Uh! Record, the kind that you drop a needle on to listen to them. OH! And the turn table sat on a cement piling that passed through the floor into the ground and must have weighed close to 10 tons! He said that was the only way to make sure the needle never skipped! Oh! And this Organ ran 12 volts D/C exactly.
I like the nature of this website. But it shames me to notice there is not more tube enthusiasm !
I am working on a revision of your design with 6W6GTs and all Edcor transformers looks like its gonna be about $100 in transformers alone. And i respect the nature of your project completely. I am hoping to do a follow up instruction set on how add active bass and treble to your design as well as modify it for 6W6GTs if you have no objections ill start work on that soon.
I suck at making cabinets. So i am grateful for you added that.
Use or abuse the info in the project as you please. I stole ideas from other circuits, so have at it.
I look forward to seeing your amp...
I looked closely at the 6W6GT datasheet, and you'll never get 20 watts from a pair in parallel (or even push-pull), but rock on!
Ill note i have seen plate dissapation form 10 watts GE to around 14 for these tubes.. depending on manufacturer.
But your right i grossly over estimated the power output.
You should definitely learn about voltage and current flow before you attempt anything with tubes.
As an electrical engineer i would suggest trying to reverse engineer simple populated PCBs and create schematics to them try to understand how they work. Avoid surface mounted devices and more than 2 layer boards for the beginning.
If you want to use a very useful piece of software that has no tubes, but can get you started understanding schematics and laying out PCBs.
http://www.cadsoftusa.com/
its cross platform. OSX Linux and Windows.
PS if you don't understand what I'm talking about Google can answer faster than I can. Also a very powerful resource for electronics questions is stack exchanges electrical engineering forum.
Try some simple stuff first--like a guitar effect. There are lots of designs and discussions over at diystompboxes.org.
Is it tube, solid-state, or just controls? A preamp module? An amp itself?
Why?
-- There's a power rectifying (4 diodes on the left) and filtering circuit (two large caps on the left.) Looks a little over-kill for just a preamp.
-- And there are two power transistors attached to a large metal heat-sink. It could be two channels, or it could be a single-channel push-pull amp.
A preamp wouldn't need the big transistors and the heat sink--that's something you only see in power amps. Or the two "transistors" may be discrete power amplifiers in a TO-220 package.
There may be other transistors (or ICs) I can't see behind the POTs, and they'd serve as the preamp. Somewhere off camera is a transformer or a wall power supply.
So this could work on it's own as an amp...
Anyway, sorry for my stupid questions, but I'm a beginner in electronics, and I just don't know anything (but I want to learn it, it's interesting). :)
Higher efficiency speakers are real--look at speaker specs, and there's usually a chart for SPL (Sound Pressure Level), measured in decibels. Look at the sensitivity column on this page. A 3dB increase in SPL is the equivalent of doubling the amps wattage.
Double the wattage doesn't really equal twice as loud (that's log function.) But it is noticeably louder.
Rather than spending lots of cash on new guitar speakers, you should try it with other speakers you have on hand. There should be an ideal "load" for the amp (speaker ohms rating), and normally it's loudest at that point. So try to match the load, if possible.
50L6, 12AX7, 6DG6GT Tubes