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Old 13th October 2010, 09:05 PM   #1
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Default 6l6gb PP w/fixed bias?

Got a new box of tubes, lots of 6l6gb's in it, so I wanted to toss a stereo pp amp on my bench for a while. Looking for a good schematic, fixed bias is a must, as I dont like the "auto bias" system.



Thanks guys!
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Old 13th October 2010, 09:07 PM   #2
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Why not try out the Red Light District? Fixed bias with LED strings on the cathode.
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Old 13th October 2010, 09:19 PM   #3
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The Vixen.

This design has what you need: it's fixed bias, active screen voltage regulation, and was designed for the 807, which, as you may already know, was the RF amp version of the 6L6 (the five pin base, glass envelope, plate top cap for higher voltage operation/more convenient RF construction). 807s have the same sonics as the 6L6, and used right, sound very good indeed.
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Old 14th October 2010, 01:37 AM   #4
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I'd prefer the led's NOT blazing... I like traditional fixed bias.


Miles, you think your design can be cut back for the gb's? these are not the nice ones, but the ones that glow past 19w plate+screen Got that schematic on hand?


My opt's for this project are foster 35w 6.6k pri 8/16 sec. circa 1960, looks as good as the big fosters I have tucked away.
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Old 14th October 2010, 05:24 AM   #5
Yvesm is offline Yvesm  France
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My two pence:

6L6 Push Pull

You can use any 6L6xx variation w/o pain.

Yves.
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Old 14th October 2010, 10:08 AM   #6
Ian444 is offline Ian444  Australia
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I have played around with an amp that has had around five different front ends and a couple different output stages including the original that came with it, Oddwatt, RLD, Simple PP, Baby Huey and whatever else, in the end I bought new OPT's and now its just the Baby Huey diff amp with 6N1P and ZVN0545 source followers, pentode with screen regs, and LED cathode loads aka RLD. Sounds good (not fantastic, but pretty darn good), good enough that I'm leaving it alone for now. Note that none of those designs were specifically for 6L6, it was just an experiment.

Now might be a good time to ask a question of those more knowledgeable, the local feedback (sometimes called degeneration) from 10 ohm cathode resistors commonly used with fixed bias, would be around 1V per 100mA cathode current. How significant is this feedback? I guess the significance would be relative to the grid voltage that would cause a 100mA change in current. I ask this question as LED's on the cathode also give a small amount of local feedback and as a result I have not needed GNFB on some small PP amps I have been experimenting with.
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Old 14th October 2010, 10:21 AM   #7
oshifis is offline oshifis  Hungary
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Picasa Web Albums - László - 807_amplifier

Circuit diagram on the last picture.
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Old 14th October 2010, 12:44 PM   #8
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Quote:
Originally Posted by oshifis View Post
Picasa Web Albums - László - 807_amplifier

Circuit diagram on the last picture.
Whoa!

oshifis (or anyone else)
Could you please explain how the cathode grounding scheme of the output stage works?
Is that a centre-tapped OPT?

Thanks
Jeff
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Old 14th October 2010, 12:53 PM   #9
TheGimp is online now TheGimp  United States
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Use a transformer with 4, 8,16 ohm taps.

Skip the 8 ohm tap.

Ground the 4 ohm tap.

One cathode gets the normal ground connecton, the other gets the 16 ohm connection.

Cathode feedback.

One Issue I can see with this is that the cathode current will be unbalanced in all configurations except when driving a 16 ohm speaker. This means there will be some DC current flowing in the base speaker which is DC coupled. If a multi speaker system is used the cross over will isolate the mid and high speakers so no DC flows through them.

Last edited by TheGimp; 14th October 2010 at 12:56 PM. Reason: add note on dc current in speaker.
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Old 17th October 2010, 10:02 PM   #10
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I went back to SE after a few circuits were tested in PP configuration, Turns out, my tubes are not what I was told they are, as they run BETTER than my NOS GE 7027a's, whom by the way, kick ***.


I can get 100ma (per 2 tubes) out of the 6l6gb's, and have zero plate color, it starts turning orange in the dark around 130ma, so backing them down to 90ma per pair looks to be working well, iv got Alice cooper running through them right now, and cant be botherd by PP configurations quite yet, as these tubes, with 525v on the plates, 270 regulated screen, and 45ma per tube can make my office uncomfortably loud.
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