Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: johngovan on September 18, 2022, 09:42:01 am
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I need some guidance in building this little amp. I wanted to build a 6L6 champ AA764 (without vibrato). Can somebody help me? I have some questions and plans in mind building this amp.
1. It's basically a vibro champ aa764 but without the vibrato section as i want the basic champ with volume treble and bass control.
2. I will use a Solid State Rectifier for this build.
3. I will use a 6L6wgb since this is the tube i have in my stash
The questions are:
1. Is 300v - 0 - 300v @ 100mA, 6.3v 3A PT can be use to produce 10w in 6L6wgb?
2. Is 15w OPT 5k : 4ohms correct for this 10w champ? Or should i use 3.5k : 4 ohms OPT?
3. Do i need a choke? And what value?
Thank you so much!
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here's a quick start, It's nice outside still
use the datasheets to find filament requirements
a choke is a personal choice, I use them in big amps, never in small ones, but I filter the PS real good
a SS rectifier will be 1.414 times your PS tranny so if the PT is putting out 300 X 1.141 = 424vac unloaded, maybe 380vdc filtered n loaded.
I have a PP 6v6 on the paper, wanting around 330-ish, SS rectifier so i'm looking at a PT of 0-200vac
the OT primary impedance is determined looking at the datasheet under single tube, the B+ the tube will operate at
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closed up the barn so I got a little more;
the red box is what I use to get an amp in the ballpark
so, match your PT to something 300vdc loaded
the OT suggests 4.5k
says 6.5W, don't fret that, a good speaker will make all your friends think it's a 15W :icon_biggrin:
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here's a quick start, It's nice outside still
use the datasheets to find filament requirements
a choke is a personal choice, I use them in big amps, never in small ones, but I filter the PS real good
a SS rectifier will be 1.414 times your PS tranny so if the PT is putting out 300 X 1.141 = 424vac unloaded, maybe 380vdc filtered n loaded.
I have a PP 6v6 on the paper, wanting around 330-ish, SS rectifier so i'm looking at a PT of 0-200vac
the OT primary impedance is determined looking at the datasheet under single tube, the B+ the tube will operate at
Thank you so much shooter. This is really helpful! I'm trying to find the data sheet of my tube. But tung-sol data sheet says that 6L6wgb is rated @350v plate voltage and a 4.2k plate load.
I really appreciate your help. I'll be posting my progress soon. I'm currently collecting the parts i needed.
I think 275-0-275 will produce 375vdc unloaded. If i got lucky enough i hope i can find a 600v tap and 550v tap @100mA PT.
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yes for single-ended you want to add a pi filter (CLC) before the output transformer. the bigger fender-style choke (4h 90ma) should work great for this (that's what i used on my 6l6 vibrochamp build)
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A single-ended 6P3s-e (wafer base) will go nicely at 400V B+ biased to 100% if you run the screens at 30-50V lower.
A 390R cathode resistor will get it running around 100%. You’ll end up with about 30V cathode voltage so the tube will hit 20.5w which is perfect for a 6P3s-e. With that amount of cathode voltage (or in fact with any ‘6L6’ type tube in a champ type amp), it will struggle to be overdriven with a typical champ pre-amp. So if your goal is more grunge, then think about adding another triode in the preamp. YMMV
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yes for single-ended you want to add a pi filter (CLC) before the output transformer. the bigger fender-style choke (4h 90ma) should work great for this (that's what i used on my 6l6 vibrochamp build)
Thanks for this. I'll add this one to my list of parts.
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A single-ended 6P3s-e (wafer base) will go nicely at 400V B+ biased to 100% if you run the screens at 30-50V lower.
A 390R cathode resistor will get it running around 100%. You’ll end up with about 30V cathode voltage so the tube will hit 20.5w which is perfect for a 6P3s-e. With that amount of cathode voltage (or in fact with any ‘6L6’ type tube in a champ type amp), it will struggle to be overdriven with a typical champ pre-amp. So if your goal is more grunge, then think about adding another triode in the preamp. YMMV
I see. I just wanted to hit the edge of breakup but not so much of a grunge.
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yes for single-ended you want to add a pi filter (CLC) before the output transformer. the bigger fender-style choke (4h 90ma) should work great for this (that's what i used on my 6l6 vibrochamp build)
By the way. Do you have schematic of your build? I'm thinking of what value of capacitor to use and also the value of dropping resistor.
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This is what i have done so far.
PT = 600v @100mA or 550v @100mA
OPT = 4k : 4ohms and 8ohms
Power Tube = 6L6wgb
Preamp Tube = 12ax7
No choke yet. Will be included in the layout soon
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I would spin those three cathode bypass caps 180°. :wink:
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I would spin those three cathode bypass caps 180°. :wink:
Oh no. I didn't notice that. 😁 Thank you sluckey. Your visio components are very useful!
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You're also missing a 15K resistor on the BASS pot. And the B+ dropping resistors should be 1K and 10K.
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You're also missing a 15K resistor on the BASS pot. And the B+ dropping resistors should be 1K and 10K.
I'll take note of that. Thank you so much. I'll update my layout tomorrow, i will also move the volume pot after the treble and bass to make the wirings more near.
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If you’ve never checked it out you might find the AX84 P1-eXtreme of interest: https://ax84.com/archive/ax84.com/p1x.html (https://ax84.com/archive/ax84.com/p1x.html)
It was designed to get the most out of a single 12AX7 plus big octal power tube (6L6, EL34, KT88 etc.). It’s a classic project with many successful builds and extensive documentation including schematics, layouts, and theory
The P1-eXtreme topology may be more tonally flexible than the AA764 by placing the tone stack after the two 12AX7 gain stages rather than in between.
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You're also missing a 15K resistor on the BASS pot. And the B+ dropping resistors should be 1K and 10K.
Here's my latest revision
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AA764 blackface champ layout
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I know, this is an old Thread, I reopen it as to as ask if you go further with your project
Thanks
Franco