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Thread: Unknown SE EL84 amp schematic...

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    Unknown SE EL84 amp schematic...

    howdy folks,
    i'm curious if anyone out there has come across or has any experience with this schematic. i've been attempting to build a little mock version of this which doesn't seem all that hard, but this schematic is a little vague on how to wire up the heater elements to the 12AX7 and the EL84. i'm also wondering the purpose of the the connection between the 6.3v AC tap and the 3 pin on the EL84. should that not run to ground along with the 100uf bypass cap? experience tells me that it's definitely not advisable at it is drawn here.



    anyhow, thanks for the help.

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    Quote Originally Posted by abram View Post
    experience tells me that it's definitely not advisable at it is drawn here.
    Why not? What's your experience with this?
    It is a simple way of elevating the heaters to reduce hum (search "elevated heaters"). The connections are pins 4+5 / 9 on the 12AX7 and 4 / 5 on the EL84.
    The schematic doesn't look bad at all. For my taste the EL84 cathode resistor is too low, I'd go with a 270 or at least 220. Screen resistor is a bit high (safety?), most designs I know use 100 Ohms.

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    The heaters for the 12AX7 are at either 4 & 9 or 5 & 9 depending upon which half of the tube you use. The EL 84 heaters are at pins 4 & 5.

    The return of the center tap resistors to the cathode of the output tube will lift the heaters from ground by whatever the bias voltage is. This is done to reduce hum.

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    hah. by "my experience," i mean that the fuse would pop until i disconnected the heater taps from the circuit. this is the first amp i've attempted to build from scratch, so i wanted to get an outside opinion on the schematic. i'm sure i just messed up something in the wiring. the heater elements would heat up fine without running them "elevated" so to speak. i didn't have to time to test the signal through. it was late and i was trying to get to a stopping point. thanks for the recommendations. i'll definitely try out those values with the cathode resistor.

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    good to know. i'm correct in the understanding that connecting the 4/5 pin on the 12AX7 will result in 6.3v operation?

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    Quote Originally Posted by abram View Post
    good to know. i'm correct in the understanding that connecting the 4/5 pin on the 12AX7 will result in 6.3v operation?
    No.

    There are 2 complete triodes in the 12AX7. The heaters for the two tubes are connected from pin 4 to pin 9 and from pin 5 to pin 9. If you connect to pins 4 & 5 the two heater sections will be wired in series and will need 12.6 volts to work correctly. If you use the first tube section (pins 1, 2, 3) wire the heater to pins 4 & 9. If you use the second half of the tube (pins 6, 7, 8) then wire the heater to pins 5 & 9.

    You should look up the tube diagram online and this should all be very clear to you.

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    much obliged, good sir.

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    Umm... can someone tell me is this a guitar amp or something "bad HIFI" in between... And yes, why is EL84 grid resistor so high, 5.6 kilo ohms, wow what will damage that sweet distortion, i saw designs up to 2kOhms... but this is too high resistance...

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    Supporting Member eschertron's Avatar
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    I'd like to hear this amp when you're running with it!

    The 'extra' resistors in the screen supply for pentode may lower the voltage to the screens and help equalize the volume differences between triode and pentode mode?
    With only 1 gain stage in the preamp, I'd guess this is mostly 'clean', and so the EL84 won't be pushed to draw grid current. So I think the 5k6 stopper will be inaudible, except maybe to make triode mode a bit darker.

    Abram, this is great little amp to learn design chops on. Get a book (huh, showing my age!) and do some research on the intended functions and limitations of various components. For example, you can make the EL84's grid stopper about any value you like just to see what you like. AFTER the amp is running perfectly as built, of course Other components will also change the character of the amp, but as Albert mentions, some component values can be bad for the amp if the operating window is broken. That's the reason for the research. Have fun, and report back often!
    Last edited by eschertron; 04-23-2017 at 07:01 PM.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    Quote Originally Posted by abram View Post
    i mean that the fuse would pop until i disconnected the heater taps from the circuit.
    It shouldn't do that. Does the heater winding also have a grounded centre tap?

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    Senior Member Enzo's Avatar
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    Dave, abram's issue was 7 years ago.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    Dave, abram's issue was 7 years ago.
    Ha! I didn't notice that with the thread being exhumed today.

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    Supporting Member eschertron's Avatar
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    Quote Originally Posted by Dave H View Post
    Ha! I didn't notice that with the thread being exhumed today.
    !!! Slipped by me, too I may never hear those sound clips now.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    Old Timer J M Fahey's Avatar
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    And as far as grid stoppers, I have seen as high as 220k in *good* amps. (think Soldano/Mesa Boogie and most high gain ones).
    Juan Manuel Fahey

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    sure you will! hah. but yes, your guess was right. the single stage was mostly clean. unfortunately, the power transformer i wound up getting at the time was horribly under powered (cutting teeth), but it sounded pretty good as drawn here even with low plate voltage. i agree with your statement about the grid stopper resistor though. since we're dealing with audio frequencies in this amp, there is practically no current flow into the grid so the voltage there should be the same on the grid regardless of the resistor value. i imagine that the value of the resistor would affect the frequency response of the amp. i doubt 2k vs 5.6k would be much of a noticeable difference except maybe with the volume knob cranked, but i haven't tried it myself. i may have to crack it back open and swap 'em out just to see what the effects are.

    anyhow, here's a link with the very first sound clip. i wound up putting a mojotone oil and paper cap for the 22 nF DC blocking cap between the preamp and power stage. i'd definitely recommend it at a first build for sure.

    No. 5 speaks. | notes to self
    Justin Thomas likes this.

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    Supporting Member eschertron's Avatar
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    Excellent! Thanks immensely for the follow-up. Sounds nice, low noise... can't really ask for more than that for a first build.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    I made that amp, but i modified the schematic a little bit, removed a power transformer (that dual 220V transformer), i know its not safe but, what can I do... because i live in europe i used the direct 220V faze from plug with full-wave rectification 1N4007... Made a preamp section a little bit "deeper" and added alittle bit more distortion, great build... i would recommend it to anyone... Reminds me of 1956 Knight EL86 amp... thus its great even for veteran builders or rookies... lol it does not have the stomp or "stop" as fender Showman TFL's have but its exeptionally nice... HAHAH and one thing... i didnt have the money for the Output trafo so i used a big 220 to 6v transformer... aded some resistors to not blow the speakers out of the cabinet... yup i damaged the ohmage... but it works just fineee... Sweet sounding

    And for the ones who asked seeing the rig in action, i added 400v 47uF cap on the beginning to raise the plate voltage and have more distortion, but all others are 350v 47uF... Plugged my SG to it... Jesus that thing rocks...imag1041.jpg

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    Keep in mind that i am 13yr old and I am not still quite sure what am i doing lol...

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    Quote Originally Posted by Dr. Tube View Post
    i used the direct 220V faze from plug with full-wave rectification 1N4007...
    Direct AC from the wall into a rectifier without a transformer is strictly forbidden.
    None of us would be pleased to find out about a 13yr. old casualty due to ignoring safety precautions.
    The Dude and eschertron like this.
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

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    When you are working on that circuit, there is absolutely NOTHING between you and the full current of the mains coming out of your wall outlet.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Dr. Tube View Post
    I made that amp, but i modified the schematic a little bit, removed a power transformer (that dual 220V transformer), i know its not safe but, what can I do... because i live in europe i used the direct 220V faze from plug with full-wave rectification 1N4007... Made a preamp section a little bit "deeper" and added alittle bit more distortion, great build... i would recommend it to anyone... Reminds me of 1956 Knight EL86 amp... thus its great even for veteran builders or rookies... lol it does not have the stomp or "stop" as fender Showman TFL's have but its exeptionally nice... HAHAH and one thing... i didnt have the money for the Output trafo so i used a big 220 to 6v transformer... aded some resistors to not blow the speakers out of the cabinet... yup i damaged the ohmage... but it works just fineee... Sweet sounding

    And for the ones who asked seeing the rig in action, i added 400v 47uF cap on the beginning to raise the plate voltage and have more distortion, but all others are 350v 47uF... Plugged my SG to it... Jesus that thing rocks...Click image for larger version. 

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    This has to be a troll. Nobody could be that stupid.

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    Okay! Thanks for reminding me, really, I was stupid to build it whitout Isolation transformer... I knew before it wasnt safe but, I thaught if old radios and old Gretsch and Knight amps had no Power tr. That i could build it... Look up on youtube: Alamo Dart; restored from death" A video made by uncle doug... That amp has no Power tr. Anyways thanks, i made myisolation by connecting two toroidal transformers... They re not autotransformers, but they isolate... Primary:220v sec:12v to secondary of transformer no2... And there it is... Much safer, i watched a yt video how to make isolation circut... And that popped up, luckily i had 2 toroidals, same voltage and same amperage...
    Does somebody know where to buy 220v isolation transformers... I have a little bit of a problem with money and I cannot afford a fancy 325V transformers for big amps... Thanks for reply

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    Okay... I was really stupid, now I added a Power Transformer, i hope i am safe now, I dont know a whole theoretics of tube amps but i know I can do something... That was really stupid of me, i thaught i will save the money that way... Yup... Thanks again for pointing me to right direction, and amp even sounds beter isolated. But there were amps whitout PT like the Alamo Dart from 1965, look it up on youtube and google

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    Quote Originally Posted by Dr. Tube View Post
    Okay... I was really stupid, now I added a Power Transformer, i hope i am safe now, I dont know a whole theoretics of tube amps but i know I can do something... That was really stupid of me, i thaught i will save the money that way... Yup... Thanks again for pointing me to right direction, and amp even sounds beter isolated. But there were amps whitout PT like the Alamo Dart from 1965, look it up on youtube and google
    I recently did some refurb to the bathroom in our house. Basically just replacing one part of a system requires the whole system to meet code. Vacuum breakers, larger size drain pipes, all that had to be added in order for the county to allow me to do the work. Same with electronics. As safety regulations evolve, so do our techniques to meet those regs. A professional tech would not (could not, by law) release a repaired piece that doesn't meet today's standards.
    It's not just a good idea, it's a human life.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    Quote Originally Posted by Dr. Tube View Post
    But there were amps whitout PT like the Alamo Dart from 1965, look it up on youtube and google
    Yes, we know. We warn people about their "widowmaker" finds all the time, not a week goes by without one.

    Does somebody know where to buy 220v isolation transformers... I have a little bit of a problem with money and I cannot afford a fancy 325V transformers for big amps.
    Transformers are cheaper than coffins. You're in Europe? Mouser and Element 14 have huge parts selections, parts of allsorts, including power and iso transformers to suit your needs. There's a couple of smaller companies that focus on guitar amp repair & builders too, TAD for instance. I'm sure other MEFsters will ring in with their suggestions.

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    In the schematic that abram posted, 12AX7 is biased with 1.5 kOhm resistor and a 25v 25uF cap... That cap was not in the original schematic imag1045.jpg

    ... and can someone tell me the value of EL84 bias cap, it says 100uF... but how many volts? I had a wild guess and soldered a 100uF 100v cap...
    Attached Thumbnails Attached Thumbnails 5w-tube-amplifier-circuit-el84.gif  

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    Quote Originally Posted by Dr. Tube View Post
    ... and can someone tell me the value of EL84 bias cap, it says 100uF... but how many volts? I had a wild guess and soldered a 100uF 100v cap...
    100u 100V will be fine. The cathode voltage is only 10V or so.

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    And if somebody could tell me is it ok to use 12v PT as an output transformer(and could it be the cause of hum in my amp) I know that current saturates inside EI iron... But if its grounded...? So i saw that people in history used cca 6-24v toroidals in Hi-Fi amps, but is it okay for this SE amp?

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    Quote Originally Posted by Dr. Tube View Post
    So i saw that people in history used cca 6-24v toroidals in Hi-Fi amps, but is it okay for this SE amp?
    I wouldn't try to use a toroidal PT as an output transformer in an SE amp. Toroids don't have an air gap and will saturate with the DC current of a single ended output stage.

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    i concur with Dave here. also, the purpose of the output transformer is to match the impedance of the output tube (which is usually a few k-ohms) and match it to that of a speaker (4-16 ohms typically). voltage transformation is inversely proportional to the turns ratio, but the impedance ratio between the primary and secondary taps is the square of the turns ratio. in a 120V-12V or 220V-12V PT, that's a pretty poor match which makes for wasted power. it could also put undue stress on the output tube and maybe even result in oscillations. probably best to just look for an output transformer.

    this is the one i wound up using in my build.
    https://www.tedweber.com/w022905m

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