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Old 11th August 2011, 09:53 AM   #1
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Default 6l6/12ax7 schematics please

hi everyone,
my name is mike, i come from a small country called new zealand-

i am currently planning to build a 6l6 push pull stereo power and pre amp using 12ax7s in the preamp and driver stages and have had real trouble finding a schematic that has been tried and proven using the above combination.
i have come across a great little pre amp circuit from here: Fun With Tubes
which looks nice and simple and i was planning to insert it into the schematic for the "oddwatt 225" schematic, 6SL7 SRPP / KT77 Push-Pull Class-A Ultra-Linear DIY Tube Amplifier
however the 6sl7 tubes are very hard to find where i live, yet there are plenty of 12ax7s around.
can some of you gurus please advise me on wearther or not it is a good idea to substitute the 6sl7 units for the 12ax7s? i have been reading in a few different places that the 12ax7 is too weak to properly drive a 6l6- is this true and if so are there any other options for driver tubes?
i have built a couple of small guitar amps in my time but this stereo amp is a first for me.
i plan on winding a toridal transformer for the supply side myself and having the audios made locally by a well regarded winder- i am an electrician by trade so i understand most of the basics, but there is a few that i dont.

thankyou in advance for your help, you all have a great site here and it feels great to be apart of it!

mike
ps


i dont know if it is relevant or not but this amplifier is going to be for my mothers new stereo- she once owned an old harman kardon tube amp many years before i was born, and has said shed like to have another now she wants to get a decent stereo, but cannot afford the price tag of many of the better units, so i said "instead of buying something plastic, put your money into a decent cd player and a good pair of speakers and ill build you the amp!" it only needs to be around 25-30w per channel, so the oddwatt 225 looked like an ideal victim!
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Old 11th August 2011, 10:40 AM   #2
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Hi Mike and Welcome to diyAudio Plenty of tube people here to lend a hand!

Tony.
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Old 11th August 2011, 11:32 AM   #3
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thanks mate

Quote:
Originally Posted by wintermute View Post
Hi Mike and Welcome to diyAudio Plenty of tube people here to lend a hand!

Tony.
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Old 11th August 2011, 12:21 PM   #4
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Bruce mentions that you can sub a 12AX7, you'll need to adjust some of the resistors. I'd email Bruce and ask him. He's a great guy and will be happy to help.
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Old 11th August 2011, 12:30 PM   #5
K.A.B is offline K.A.B  Sweden
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Hello Mike,

I dont no if you want an SE or PP amplifier but here are some schematic

6L6 single ended tube amp | Tube amps DIY
http://www.triodeel.com/ac6l6280.gif

For starters but take a look on googles picture side 6l6 SE - Sök på Google there are more schematics

Regards
Anders
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Old 11th August 2011, 01:39 PM   #6
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that PP schematic will definately need more gain from a CD player output, and with 12ax7s you probably only need 1 tube, (single gain stage and cathodyne phase inverter. are you looking for cathode bias or fixed biased power tubes?
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Old 11th August 2011, 06:39 PM   #7
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I recently built the 'Poddwatt', designed by Bruce Heran, and it is a great little amp.
My advice would be to go with the 'as designed' version rather than trying to re-work a design to match the tubes you have on hand. Bruce mentions that the '12AT7 and variants' don't sound as good as the 6SL7 and 12SL7 tubes; I'd pay attention to that before going to the 12AX7.
When you look at the total cost of an amp project (chassis, transformers, hardware), the tube cost is not that large unless you opt for 'silly-grade' tubes.
I know shipping is an issue for you, but tubes are light and shipping costs are pretty low. I buy tubes from former USSR places often.
Just to put things in perspective, the preamp tube costs (at ESRC in FL, USA) are quite low- 6SL7 @ $7, 12SL7 @ $4.
Bruce mentions that the 6L6GC is best for rock, for other genres the KT77 ($40/pair at thetubestore) is the way to go.

If you have an active 'ham' aka amateur radio community in your area, you may find a source for tubes at reasonable prices. Used tested tubes can be fine performers, if you can get them at a decent price.

PS- If you want 25wpc, then PP is the route to go. What sort of speakers does your Mom have?
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Old 11th August 2011, 10:57 PM   #8
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You might find this interesting...

Magnavox 6L6 amp?! a.k.a, IT WORKS! (plus pics!) - AudioKarma.org Home Audio Stereo Discussion Forums

Many classic tube amps used 12AX7s to drive 6L6 class tubes with good success so a bit of studying of these older circuits may be helpful. Sure there are better drivers but it can be done so why not.

HH Scott 99D for example.

Click the image to open in full size.
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Old 13th August 2011, 11:57 AM   #9
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thank you all very much for the info! i have decided to go with the same preamp with 12ax7s, but shop overseas for the 6sl7 units to use as the drivers and i will probably go with the kt77s, taking the authors word for it they are better for clasical and other types of music- i personally love the tone of the 6l6, but i have to take into account that this uit wont be used for rock n roll all the time.

once again, thanks for your info! i have a few projects on the go as well- one is a phono preamp that is due for completion tomorrow, will post some pics in another thread soon!
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