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Old 22nd September 2004, 07:32 AM   #1
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Default beginner's ecl82

hi gurus,

I recently bought a 'junk' tube amplifier over the internet auction. after several attempts to repair, I got fed up. I salvaged the valuable parts, and built a new one based on a schematic i've found on internet (http://nmwilliam.tripod.com/ecl82.html)
Even though this is the first time i tried this, the amp sings on the first time and im amazed how sweet tube sounds.-) byebye~silicons!!

Ive attached the schematics, and I highly recommend this amp for a newbie (if i could do it, i figured any idiots could) hahaha. At first, there was a little buzzing, but i finally got rid of it because i forgot to ground my heater.

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Old 22nd September 2004, 07:43 AM   #2
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Default modifications

There are several questions i would like to ask:

1. Can I 'triode-strap' this amp? My idea is to connect pin7 to pin6 with a 100 ohm resistor. Will this work? Is this modification worth it?

2. If I want to use 'exotic' components, which part would you gurus recommend to change first? (which parts would effect the sound the most)

3. My current power Tx is 300Vac. I am worried this may be a bit too high for the tube, even though it sounds ok. I am thinking of using 2k resistor in series to bring the voltage down, but this would require a large wattage resistor. Is there any way I could lower the supply voltage?

btw, after I 'salvaged' the parts, I have leftover of 10H choke(a gem find) , 2x ecl82 tubes and 1x ecc82 tubes -)
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Old 22nd September 2004, 08:11 AM   #3
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Hi There,
and welcome to the wonderful world of tubes.

There is a thread called "is there a tube gainclone" or something and these little amps are discussed therein, do a search, this thread contains loads of links.
I currently have a variant of the Svetlana circuit breadboarded on the bench and it sounds pretty good. (for what it is) I have used junkbox parts and junkbox transformers. Follow this link, its 'figure2'

http://www.svetlana.com/docs/products/glass/6BM8.html

As for triode strapping the pentode, you will reduce the available output considerably (its only about 2 watts)

Don't worry about the B+, mine is connected to a bench supply and I have varied it between 200 upto 400VDC. To my ears the amp sounds better with a higher B+, even though you are on the limit of the curves for the pentodes.

Have fun!!!
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Old 22nd September 2004, 08:25 AM   #4
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thanks

I have browsed through the website and i wanted to built this one initially:
Click the image to open in full size.

however, i couldnt understand the notes about adjustable bias voltage, approx -25 volts. How do i adjust the bias if the 470k resistor is fixed? How did you build yours?

Thanks
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Old 22nd September 2004, 09:24 AM   #5
arnoldc is offline arnoldc  Philippines
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i believe you need to supply the -25 volts from another source (such as a bias tap on your transformer)
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Old 22nd September 2004, 09:24 AM   #6
SHiFTY is offline SHiFTY  New Zealand
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1) I wouldn't bother. Triode mode would make it too low on power for most use (<1W) You could use EL84s though.

2) I would make use of that 10H choke in the power supply. What rectifier are you using? (Tube I hope ) What is the layout of your power supply? I find that a choke in the power supply improves the sound... Apart from that, you could try some different output transformers...

3) Check the power dissipation of the PENTODE half. (CAREFULLY! READ THE SAFETY THREAD!!!)

Do this by measuring the voltage at the plate, then the voltage at the cathode. The dissipation is then the current across the cathode resistor (Vcat/R) x the voltage across the tube (Vplate-Vcat) where R is the value of the cathode resistor in ohms.

If this is less or equal to 7 Watts then no worries. If its 8 or more, then increase the cathode resistor appropriately. Use a 5W cathode resistor too...

You might want to also try building the classic Mullard ECL82 circuit, I really like it and it has stood the test of time really well...
LINK

Regarding that PP circuit, its referring to the power supply (not shown). PP circuits can be a lot trickier...
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Old 22nd September 2004, 03:26 PM   #7
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thanks shifty

Actually I was saving the choke for a bigger project in future

Nothing fancy on the psu here, just plain 4007 bridge diode rectifiers, bypassed with 0.01uf caps. This is followed by two 50uF caps, exactly as shown in the original post. I am thinking of salvaging an unused transformer to be used as choke. problem is i cant measure the inductance though.

How will the choke effect the sound apart from less noise? I read that chokes pushes the soundstage behind, is that true? My amp is almost dead quiet allready, and if can, I want this amp to be as compact as possible.
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Old 22nd September 2004, 03:36 PM   #8
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I built the SE circuit.

<img src="http://www.svetlana.com/graphics/TB/No10fig2.gif">

The 'adjustable' bias is meant to be from an adjustable source, behind the resistor.

I wouldn't go too mad with this tube, it was used as the output tube in many 1950's radios and grams and isn't exactly hifi.
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Old 22nd September 2004, 09:48 PM   #9
Fuling is offline Fuling  Sweden
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Quote:
I wouldn't go too mad with this tube, it was used as the output tube in many 1950's radios and grams and isn't exactly hifi.
I don´t really agree here. I own one ECL82 amp and one PCL82 amp, both SE, and I think this is a quite amazing little tube.
Even though I have a few amps with way fancier tubes (f.ex. 6B4G) and OPT´s, my little PCL 82 amp with TV output transformers still makes it into my system now and then without being ashamed.

Run them in triode mode and enjoy <1W of music

A great tube for beginners to play with, too. Only 200V B+ and they´re happy as pigs in sh*t!

Cheers

Fuling
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Old 23rd September 2004, 12:33 AM   #10
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The 6MB8 sounds great, comparable to any higher quality mass produced HIFI amps/receivers....but warmer However the 6MB8 is not really an "Audiophile" tube like 2A3 or 300B's etc....

I built the Lighthouse circuit with a few tweaks and designed my own PSU. I'm VERY happy with the sound quality . The dollars/sound ratio makes it a well worth project....you will be surprised how this little thing cranks.

I also build a Solo TQWP and mounted a pair of RS1194 in it.
IMHO this is a perfect match.

My neighbour (SS audio geek) was totally amazed over the sound quality and loudness of this small amp.
His jaw hit the floor and he started shaken his head in disbelief.
Needless to say he has a new found opinion on tubes!

Attached are a few pics..

http://www.flyballequip.com/pictures/6MB8front.jpg
http://www.flyballequip.com/pictures/RS1194soloTQWP.jpg

I'd be happy to share my tweeks, tips & build info if anyone wants it.

Mike
Langley B.C.

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