Hello,
I recently built a clone of the selmer little giant MKIII circuit, I posted about it here actually,
https://tubeamparchive.com/viewtopic.ph ... 55ea984180
I've finally got time to have a go at resolving some issues. The amp seems to be eating through power tubes, which will play fine for a while then the amp will start cutting out when played beyond a certain volume; I then noticed that when this occurred, the 5.6k resistor between the filter caps (I don't know the technical name) became red hot and burned out.
At this point I've only got 2 working EL84's remaining and they are being used in another amp (another selmer in fact), so I've been hesitant to continue searching for the problem. I did however, briefly put one of those tubes in so I could measure the plate voltage and the plate dissipation on the single EL84. Plate (Pin 7): 318v, Cathode (Pin 3): 9v. The cathode resistor is currently rated at 220r making the plate dissipation about 12.6w. Not terribly high I don't think…
I've got some more EL84's in the post.. any advice on how to proceed would be very much appreciated.
Thanks,
Ed
Eating through Power tubes, selmer little giant MKIII clone
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Eating through Power tubes, selmer little giant MKIII clone
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Stevem
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Re: Eating through Power tubes, selmer little giant MKIII clone
Well one issue is that the el84 is only a 12 watt tube, not a 12.6 watt tube,and in a single ended class A circuit like that the tube is under the greatest load at idle and not when playing though the amp, when you play thru the amp you are dropping some or all of that 12.6 watts across the load / speaker!
Also in a class A set up the tube should not have more than 300 volts on its plate.
These days we have the higher plate voltage due to our higher line voltage than back when the amp was made.
One thing to do to correct for this is to up that cathode resistor up to 250 to 270 ohms and a 5 watt one if it is not one now.
I would try to end up with a resistor that will place the tube at 11 watts of dissipation.
Another good mod to make that adds life to the tube is to add a screen resistor which all amps should have, but many do not as in this amp!
To do this place in series with the screen power supply wire to To the tube a 470 to 1000 ohm 2 watt resistor.
Another tonal mod to make is to audition different value cathode bypass caps, as that 25 uf stocker is a bit small.
I would say that a 150 uf would be pretty much what you might want and getting three 50 volt 50 uf caps that you can install step by step in parallel one by one will let you check them out.
These two minor mods should end your tube failures for ya!
Also in a class A set up the tube should not have more than 300 volts on its plate.
These days we have the higher plate voltage due to our higher line voltage than back when the amp was made.
One thing to do to correct for this is to up that cathode resistor up to 250 to 270 ohms and a 5 watt one if it is not one now.
I would try to end up with a resistor that will place the tube at 11 watts of dissipation.
Another good mod to make that adds life to the tube is to add a screen resistor which all amps should have, but many do not as in this amp!
To do this place in series with the screen power supply wire to To the tube a 470 to 1000 ohm 2 watt resistor.
Another tonal mod to make is to audition different value cathode bypass caps, as that 25 uf stocker is a bit small.
I would say that a 150 uf would be pretty much what you might want and getting three 50 volt 50 uf caps that you can install step by step in parallel one by one will let you check them out.
These two minor mods should end your tube failures for ya!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Eating through Power tubes, selmer little giant MKIII clone
Thats brilliant, thanks for the advice; when I eventually get my tubes I'll have a go at adding those in.
Its a british amp, and I think the line voltage over here (in the UK) has always been that high. The originals had Mullards in.. which perhaps could handle the punishment.
When it was functional, it did sound very good.. on the brink of collapse. I imagine that by lowering the plate voltage and the dissipation wattage, there'll be something of a trade-off?
What effect would different value screen resistors have? Would there be much difference between 470 and 1k ?
Its a british amp, and I think the line voltage over here (in the UK) has always been that high. The originals had Mullards in.. which perhaps could handle the punishment.
When it was functional, it did sound very good.. on the brink of collapse. I imagine that by lowering the plate voltage and the dissipation wattage, there'll be something of a trade-off?
What effect would different value screen resistors have? Would there be much difference between 470 and 1k ?
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Stevem
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- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Eating through Power tubes, selmer little giant MKIII clone
Get yourself two 470 ohm resistors and then try just one and take a good listen when cranked up, and then install the other in series to get up close to 1k and listen again, there will be a change in playing feel more than anything else.
One of the other main factors for that tubes life is to have the screen grid voltage when playing thru the amp drop below the plate voltage even only by 3 to 4 volts.
Does that output transformer only have two wires coming out on the side of it that feeds the speaker?
If you have the room on the chassis to add a octal tube socket you can wire to use / swap in a 6V6 output tube which has a bit of a different nature / play feel to it due to it being what's called a beam type output tube which the 84 us not.
One of the other main factors for that tubes life is to have the screen grid voltage when playing thru the amp drop below the plate voltage even only by 3 to 4 volts.
Does that output transformer only have two wires coming out on the side of it that feeds the speaker?
If you have the room on the chassis to add a octal tube socket you can wire to use / swap in a 6V6 output tube which has a bit of a different nature / play feel to it due to it being what's called a beam type output tube which the 84 us not.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Eating through Power tubes, selmer little giant MKIII clone
That is right, there are just the two wires that go to the speaker output jack
http://www.ampmaker.com/store/5W-single ... ormer.html
http://www.ampmaker.com/store/5W-single ... ormer.html
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Eating through Power tubes, selmer little giant MKIII clone
The amp will sound better if at some point you get a 16 ohm speaker, and or wire two 8 ohm drivers in series and then use the OTs 16 ohm output tap.
Some other little tweaks you might want to try out would be to place a .068 uf to 22 uf cap across that 2.2K resistor on V1Bs cathode, just make sure that the cap has a 25 volt or better voltage rating.
The bigger that caps uf value the more Bass you have on the low E string and the other way the more upper mids you will have, but the added signal drive level with a big cap may make for farty too loose Bass from the output tube being pushed too hard.
This is a matter of just trying it out and or if you have Humbucker pickups or not.
Also adding a .047uf to .22uf cap across each of the two power supply node filters will do some good to the tone, just have the caps voltage rating be 50 volts or better than what the full B+ is in the amp!
Some other little tweaks you might want to try out would be to place a .068 uf to 22 uf cap across that 2.2K resistor on V1Bs cathode, just make sure that the cap has a 25 volt or better voltage rating.
The bigger that caps uf value the more Bass you have on the low E string and the other way the more upper mids you will have, but the added signal drive level with a big cap may make for farty too loose Bass from the output tube being pushed too hard.
This is a matter of just trying it out and or if you have Humbucker pickups or not.
Also adding a .047uf to .22uf cap across each of the two power supply node filters will do some good to the tone, just have the caps voltage rating be 50 volts or better than what the full B+ is in the amp!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Eating through Power tubes, selmer little giant MKIII clone
Hi everybody. I m building a little giant clone too it seems that i ve solve mist of the problems by myself but this time i need help. I ve a ground loop causing 50hz ( eu frequency) presence in my el84 circuit.
I tried higher values for first reservoir cap just after the ez80 , it just atenuate a little but the hum is still very present. I try to isomate the more that i could this "reservoir" cap ground but no result. I even tried a resistor between the ground of the two 32uF but nothing happen.
Could you help me please?
I tried higher values for first reservoir cap just after the ez80 , it just atenuate a little but the hum is still very present. I try to isomate the more that i could this "reservoir" cap ground but no result. I even tried a resistor between the ground of the two 32uF but nothing happen.
Could you help me please?
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Eating through Power tubes, selmer little giant MKIII clone
If you pull the preamp tube out is the hum still there?
Does the level of hum change with the volume control setting?
Are your output and power transformer cores at 90 degree angles to each other like they should be, or are they inline?
If this is wrong many times even with no tubes in the amp it will hum.
Sometimes the magnetic 50hz flux of the PT will couple right into the speaker with no tubes in the amp.
Are your filament wires twisted tight, are any of the preamp tube grid wires (pins 2 and 7) laying on, Or too close to the filament wires?
Are the B+ wires feeding the recto twisted as they should be?
Try a known good non humming preamp tube as this could well be your issue!
With your meter set for low ac volts how much ripple do you read at the power supply side of the preamp tube plate load resistors ?
Does the center tap of the power transformers filament winding have a solid ground ?
I list these in no set order, just a bunch of things to check out!
Does the level of hum change with the volume control setting?
Are your output and power transformer cores at 90 degree angles to each other like they should be, or are they inline?
If this is wrong many times even with no tubes in the amp it will hum.
Sometimes the magnetic 50hz flux of the PT will couple right into the speaker with no tubes in the amp.
Are your filament wires twisted tight, are any of the preamp tube grid wires (pins 2 and 7) laying on, Or too close to the filament wires?
Are the B+ wires feeding the recto twisted as they should be?
Try a known good non humming preamp tube as this could well be your issue!
With your meter set for low ac volts how much ripple do you read at the power supply side of the preamp tube plate load resistors ?
Does the center tap of the power transformers filament winding have a solid ground ?
I list these in no set order, just a bunch of things to check out!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Eating through Power tubes, selmer little giant MKIII clone
First , i thank you for your really fast answer.
I can answer clearly yes to all your question except for the one and "easiest" about tubes. Indeed, all is ok and i read and check all the connections ( 3 times in fact! Lol) but my ez80 is very old not sure about it.
I ve already burnt a el84 ( because of cathode and voltage to high! I m ashamed!).
When the guitar is plugged no more noise and it sounds great ( i made the pin 9 and 1k resistor mod)
But now flash appears at the + side of both 32uf caps from time to time. Really strange.
I can answer clearly yes to all your question except for the one and "easiest" about tubes. Indeed, all is ok and i read and check all the connections ( 3 times in fact! Lol) but my ez80 is very old not sure about it.
I ve already burnt a el84 ( because of cathode and voltage to high! I m ashamed!).
When the guitar is plugged no more noise and it sounds great ( i made the pin 9 and 1k resistor mod)
But now flash appears at the + side of both 32uf caps from time to time. Really strange.
Re: Eating through Power tubes, selmer little giant MKIII clone
I should precise that the flash appears shortky after i connected pin 9 to a 1k 10w to its original place:the + of the second 32uF cap
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Eating through Power tubes, selmer little giant MKIII clone
Do you mean pin 9 on the rectifier?
Pins 2 , 6 , 8 and 9 are not used on a EZ80 for anything, in fact they may all be internally connected within the tube?
Pins 2 , 6 , 8 and 9 are not used on a EZ80 for anything, in fact they may all be internally connected within the tube?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Eating through Power tubes, selmer little giant MKIII clone
No, sorry, i m not clear in my explaination.
At first, i connected the pin 9 of the el84 to the 32UF cap using a 1k 10w , then i give a try, and the sound was really great, really more organic !!! But after one moment i saw flashes lights appearing just at the jonction of that same 32uf cap. i stopped the amp then decided to change the cap and remove the 1k 10w resistor at the same time. then i start the amp again but flashes lights were still here.
i tend to suspect the El 84 , why? Because the plate tension is still the original one (276v), because the cathode resistor is just at 230r instead of the 270r you adviced above. so i think my el84 is just dead?
Just like Beyouth , i will wait to change the cathode resistor and the anode voltage before changing my el84 one more time.
do you see any other explanation ?
About the hum/grounding problem, it totally stop when the guitar is plugged and doesn t do any "tic" if i touch a metal part or not, no special reaction except that there is no more hum ( i ve shielded the amp female jack input to prevent any loop...)
At first, i connected the pin 9 of the el84 to the 32UF cap using a 1k 10w , then i give a try, and the sound was really great, really more organic !!! But after one moment i saw flashes lights appearing just at the jonction of that same 32uf cap. i stopped the amp then decided to change the cap and remove the 1k 10w resistor at the same time. then i start the amp again but flashes lights were still here.
i tend to suspect the El 84 , why? Because the plate tension is still the original one (276v), because the cathode resistor is just at 230r instead of the 270r you adviced above. so i think my el84 is just dead?
Just like Beyouth , i will wait to change the cathode resistor and the anode voltage before changing my el84 one more time.
do you see any other explanation ?
About the hum/grounding problem, it totally stop when the guitar is plugged and doesn t do any "tic" if i touch a metal part or not, no special reaction except that there is no more hum ( i ve shielded the amp female jack input to prevent any loop...)
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Eating through Power tubes, selmer little giant MKIII clone
Anytime you see a arc at a connection point like that filter and resistor of yours, you have a bad solder connection!
And yes, I think you have a shorted out EL84!
And yes, I think you have a shorted out EL84!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Eating through Power tubes, selmer little giant MKIII clone
Just bad solders ?! Shame on me!!
Thanks!
Thanks!
Re: Eating through Power tubes, selmer little giant MKIII clone
Thanks for your help!
Here is the result:
http://youtu.be/6eZQgOTiJME
To reduce anode voltage, i will add a 500ohm 5w cement resito just after thr ez80, it should work.
Here is the result:
http://youtu.be/6eZQgOTiJME
To reduce anode voltage, i will add a 500ohm 5w cement resito just after thr ez80, it should work.