6SL7+6SN7 micro amp. Micro champ or micro 5E3?

Jerry garrcia

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Do an experiment. Set the master at the highest level you think sounds good with all levels of gain. Shut down amp and measure the resistance setting of the master volume. Then we adjust the 470K : 220K voltage divider to make the master useable all settings. Does that make sense?
01:39 o’clock here so not in my highest cognitive function 😀. Resistance setting where and to what? I think I know what is your purpose with the master volume but could you please elaborate a bit more regarding the voltage divider?
 

andrewRneumann

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01:39 o’clock here so not in my highest cognitive function 😀. Resistance setting where and to what? I think I know what is your purpose with the master volume but could you please elaborate a bit more regarding the voltage divider?

Set the MV as high as you think you would ever want it. Then measure this resistance (below image) and report back. We will use your result to adjust the 220K resistor in the following voltage divider so that your MV gives you more usable sweep. Did you use a 1Meg pot for the MV?

1660002753392.png
 

Jerry garrcia

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Set the MV as high as you think you would ever want it. Then measure this resistance (below image) and report back. We will use your result to adjust the 220K resistor in the following voltage divider so that your MV gives you more usable sweep. Did you use a 1Meg pot for the MV?

View attachment 1014901
Aha. Forgot to mark that. Yes it’s a 1 M. Will calculate the voltage dropping resistor tomorrow when the brain got a couple of hours of sleep..
 

Jerry garrcia

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Did @Jerry garrcia just whip out Ohm’s Law? You’ve come a long way my friend. :p:p:p:p However, I calculated only 0.9A of current for your two tubes. How did you get 2.2?
Ah. A mistake😬. Took the value from a previous build that I planned.
6.8-6.3= 0.5V
0.5/0.9= 0.56
My bad. 0.5-0.6 ohm per winding. Seems correct? What’s the acceptable difference from 6.3?
 
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Jerry garrcia

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Set the master at the highest level you think sounds good with all levels of gain. Shut down amp and measure the resistance setting of the master volume.
That was more tricky than I thought. Started with the planned 6” speaker but since rhat wasn’t attached to a baffle I did some change in speakers. The one that sounded the best was an alnico 12” p12n. When master was around 70% it started to break up in a nice output distortion. Since, as always, clean is the purpose I think 722K is the max value I fancy for the master volume pot. This amp is still loud even without a regular output tube. The small, light weight cab/amp will make it perfect for practicing ali Also,any tips on a small, light weight speaker that sounds better than the 6” Jensen MOD speaker?
An also. Was my filament drop
Resistor ok?
 
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andrewRneumann

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Ah. A mistake😬. Took the value from a previous build that I planned.
6.8-6.3= 0.5V
0.5/0.9= 0.56
My bad. 0.5-0.6 ohm per winding. Seems correct? What’s the acceptable difference from 6.3?

Is there a center tap for the heater winding? If not, I think you are ok with one 0.5 ohm in the circuit somewhere. The main reason for running tubes at the correct voltage:



1660081892772.png
 

Jerry garrcia

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Was that measured from coupling cap to wiper like I drew?
Just woke up since the dog is sick in the stomach. Gave her a whole cuw bone and she ate it all. Not good for that dog… third time she woke me this night. Like having small children.
It’s measured from one wiper to another on the master volume pot. Wrong?
Is there a center tap for the heater winding? If not, I think you are ok with one 0.5 ohm in the circuit somewhere.
no ct. artificial on the 6SN7’s socket. Two 100R from each heater to cathode. Thanks for the describing picture.
Where is the best spot to place the 2-5 W 0.5R? Filament lamp? Read that it could get pretty hot.
 
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andrewRneumann

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It’s measured from one wiper to another on the master volume pot. Wrong?

I’m just confirming you measured the resistance between the two arrows like I drew in my diagram. Yes/no?

Where is the best spot to place the 2-5 W 0.5R? Filament lamp?

Can you fit a 3 terminal strip on this nut? Run the resistor between the two ungrounded terminals before it goes to the lamp. Just an idea.

1660088193317.png
 

Jerry garrcia

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Ok, this may take a couple of tries. If you have a 47K, clip that in parallel with the 220K grid leak on power tubes. See if you like it. You can adjust up and down until you find what you like.

View attachment 1015205
Fourth times now. She is like a tube of mayonnaise but filled with fescues. Feel bad for her by giving that cow bone.

The terminal strip can fit the bolt of the OT as you wrote. It should be ok to just put on resistor I’m one of the windings as I understand. Thankful. Quite a lot of amperage that passes that resistor so maybe a bolted off one is an option?
I’m just confirming you measured the resistance between the two arrows like I drew in my diagram. Yes/no?
yes.
Yes it it measured there. After the coupling cap off V2B (one wiper of the 1 M pot. Next spot the other wiper of the pot that goes to the V3.
I’ll try that added resistor.

It’s a loud well and sounding amp. Since the chassis isn’t to heavy I built the cab out of 12 mm birch plywood. Can’t decide on the front yet. Looking forward to tha Class 2A build and trying to learn how it functions.
 
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Jerry garrcia

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I'm not sure what you mean here. If you buy a 2W or 3W resistor, I'm sure you can use a regular axial leaded resistor.
Just forget what I wrote. Was searching the web regarding this matter. Didn’t use my brain. Found a lot of thread that covered this but all was about bigger amps with a wast number of tubes. This one just has two small ones so it doesn’t apply here. Will parallel two 1R 2w for one of the filament windings. Sorry. Need to think it through before I ask.
 

Jerry garrcia

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Just a short update as the sun is starting to rise here in the Stockholm archipelagos. Gave the dog a real big ox bone and wish I hadn’t. The intestines are now producing enormous amounts of stuff. Need to go back to sleep but started to read about class 2A amps.
Regarding this one I’m starting to finish the cab. Tomorror (today since it’s 5 am) time to do the speaker cut out. Just a jigsaw and a dremmel so might have to use grill cloth to cover the baffles in this one.
Placed two 1R 2w in parallel on one of the windings of the heater windings and the filament voltage dropped to 6.33VAC which one has to be pleased with. Also changed the secondaries to 10K, 8 ohms. Will wait to manipulate the attenuator resistor since I haven’t played it with the planned speaker. I guess it will sound a bit different than through a well made pine cabinet with a P12N than with the planned cab out of 12 mm plywood, 9 mm baffle and a 6” Jensen Mood. Might use a Jensen C8R instead or a fender/eminence 8” depending on weight and sound compromise.
As a bonus a elk neighbour picture.
66B2CFD2-47BC-4802-942A-1133EA9D3DEA.jpeg
6E1EB4D0-0CDD-46EF-8035-4BA597D839B2.jpeg
 

Jerry garrcia

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The amp is now complete. @bebopbrain went for the micro champish circuit due to wanted more of a simple circuit and the boy doesn’t really care fore a tone control when playing the archtop. V1A plate 99V, V1B 130V, V2 plate 336V. Due to the 0.5 Ohm resistor on one of the filament windings the filament voltages are now 6.13VAC. It is crazy well sounding and the tone is amazing even though it is in a 12 mm birch plywood cabinet and a “cheap” 6” Jensen MOD speaker mounted on a 9 mm baffle. Have only tried it with a Epiphone Wilshire and a home built Tele with Biltoft pickups.
The weight of the amp in the cab is almost nothing so @NTC don’t think the boy need to do that well needed workout 😀. He’s like BMI 17 and has luckily not inherited my short, fat fingers without any reach. He’s more like Bucketheads reach, which is good for those crazy jazz cords that Pasqual is utilising. @andrewRneumann 🙏🙏🙏 didn’t need to modulate the attenuator resistor with this speaker. I’m totally blown away by the sound. Have started with cheap tubes until I know everything works fine. JJ 6SL7 and a Soviet military version 6SN7. If Gain is below 7 out of 13 (always 13 as max volume) then the Master can be cranked up to 8-9 before it starts to break up. In previous builds I haven’t really cared so much for the NFB but in this amp together with the boost switch it totally change the tone in a good way. The grid leak/cathode bias switch is also adding a lot. Grittier sound with the switch in grid leak mode. Thanks @printer2 will now add it in all the previous 6SJ7 builds.
Also thanks to @2L man for the input on the OT and grounding issues. Works like a charm and you can’t really hear that it is on if not being played.
Also @robrob for designing this circuit that I modified a tiny bit to suite them loved octals.
Will post some pictures and sound samples in a day or two. The amp is still missing grill cloth and my cabinet is not all square but love the not perfect look.
Now the wife is coming to the summerhouse and need to clean up the empty bottles, components and wood pieces all over the place.
Have a week or so to get my sh*t together and start to manage my hospital before Class 2A hopefully starts😀.
 
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