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Old 11th April 2006, 05:51 PM   #1
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Cool Need some help for 6L6 amp

Hi everyone !

I`m kind of new in the wonderful world of tube audio amplifier... but as summer come, I would like to build a 2 channel x 25 Weff hifi audio amplifier. I got valve amplifier book of Morgan Jones, so you can make some reference in it....

What I would like to have in my amplifier :

- 2 channels
- Around 25 W per channel, should be enough
- 2x 6L6GC for ultra-linear push-pull (so 4 in total), with cathodyne phase splitter
- I already got 2 Ei gold elite 12DW7 / ECC832 triodes (so I would like to use them)
-I have place on a chassis for 2 output trans (For the output I think I should go with the hammond 1650F) and 1 power trans for both channel, its realy easy for me to get some hammond...

I`m almost certain with all those contraint it`s possible to build something nice. So as I`m not super familiar with tube circuit desing, but I got a good base in electronic, can some one help me by sending me a circuit with thoses contraint, with the way both channel separate the input and the power line.

And if you have some recommendation to do it`s apreciated too.

Thank you for taking the time to answer me !
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Old 11th April 2006, 08:59 PM   #2
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Using a cathodyne splitter will probably mean using a Williamson-like design. If you apply global negative feedback with that circuit, you will have two coupling caps within the FB loop, which can lead to low frequeny instability unless you use an output transformer of very qood quality. I doubt if a Hammond meets the requirements. But why impose these restrictions in the first place? There are other options, such as a Mullard-style circuit with its LTP splitter and direct coupled first stage, which would be inherently more stable.
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Old 12th April 2006, 01:19 AM   #3
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The Dyna 35 used the 7247 to drive EL84s - the 6L6 needs a bit more drive, but it should be do-able. I myself would choose a long-tail splitter with anything but 6V6 or EL84.
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Old 12th April 2006, 09:12 AM   #4
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Hi, thank for your inputs,

Quote:
But why impose these restrictions in the first place
Ok, maybe I`m a bit too stiff, but at least I would like to get :

-Preamp tubes : 12DW7 ( ECC832), because I all ready got those
-Trans : Hammond, is easy to get and look very ok for me
-Chassis : I can go up to 12`` by 12 `` without extra cost
- And 2 channels wouldn`t be too much, with power supply on board

For the rest like output tube (as long they are not 300$ a pair i`m very flexible, same as phase splitter but I think push pull is for me.
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Old 12th April 2006, 11:25 AM   #5
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I had to look up the 6L6 data sheet and the Hammond 1600 series data sheet, but I would have thought you should be using the 1650H to get 6k6a-a and allowing you to obtain 25W with an HT voltage of 360V. Obtaining the required 45Vpk-pk swing from each output of a concertina phase splitter with only 300V or so of HT is asking a bit much, so I would echo previous posters' comments that a long-tailed pair is the way to go.
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Old 12th April 2006, 02:52 PM   #6
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JJ or EH 7591s would work fine with a concertina splitter, and if you're buying output tubes anyway...

The power amp section of a Fisher receiver or integrated amp is just what you're looking for. Simple, 25-30W. Lower the plate and cathode resisistors of the splitter to 39-47K, lower the 7591 grid resistors to 150K. Change to cathode bias to make it even simpler.
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Old 13th April 2006, 01:03 AM   #7
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Guitvinny,

IMO, Tom Bavis' suggestions are EXCELLENT.

The 'U7 section of the ECC832 makes a better phase splitter than a section of an ECC83 does because of its higher gm.

Cathode (self) bias will not let you get close to cutting the 7591s off. However, since you are looking for only 20+ W. from a PP pair, it's not an issue. Also, near Class "A" operation avoids the cross-over distortion bugaboo.

While SED 6L6GCs are pretty nice, the 7591 is (IMO) a better HIFI tube. Fisher and Scott got a LOT of "milage" out of the type. You can too.
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Old 13th April 2006, 03:12 AM   #8
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Hey thank for all thoses input !

This is realy reorienting me !

- Now for the tranny power supply, in witch range of power should I get ? (around 350 - 0 - 350 ?)


- For the phase splitter should I go for long-pair ? Cathodyne ? .. else ?

- Is there any similar shematic for this ?

Thank again !
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Old 13th April 2006, 07:03 AM   #9
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Quote:
Originally posted by guitvinny
Hey thank for all thoses input !

This is realy reorienting me !

- Now for the tranny power supply, in witch range of power should I get ? (around 350 - 0 - 350 ?)


- For the phase splitter should I go for long-pair ? Cathodyne ? .. else ?

- Is there any similar shematic for this ?

Thank again !
With a 350-0-350 power tranny, you are probably looking at about 40 watts in Ultra Linear or about 55 watts AB1-PP with a 6L6. I would imagine the 7591 would have a similar output at those voltages.

If I were designing this amp I would probably go with a LTP PI, although cathodyne would probably work just fine too.

-Adam
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Old 14th April 2006, 07:00 AM   #10
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Guitvinny,

If you are determined to use those 12DW7s, Tom Bavis told you the way. "Lift" Avery Fisher's topology. Fisher 500C Schematic

If you want to use a differential (LTP) splitter, you need twin triodes with "identical" sections. The "El Cheapo" project's 12AT7 phase splitter/driver would be DANDY in front of a pair of 7591s, as the 7591 is slightly easier to drive than 'AQ5s are.

"El Cheapo" Signal Schematic "El Cheapo" PSU/CCS Schematic

Read ongoing "EL Cheapo" saga here.

IMO, a reasonable choice in O/P trafos for use with PP UL 7591s is the Hammond 1650H.

If you SS rectify the B+, 300 VRMS from the power trafo should be quite satisfactory. If you tube rectify, 325 VRMS will take of the larger forward drop.
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