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Old 17th April 2012, 07:17 AM   #1
Green77 is offline Green77  Sweden
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Default PP6550

Hi all!

I will start building this project now and have some questions.

Click the image to open in full size.


Has anyone here on the forum built this? How does it sound?

I have built four PP EL34 and KT88 amps. But all have had fixed-bias.
Is there a big difference sonically between cathode and fixed bias?

Thanks //Daniel
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Old 17th April 2012, 08:23 AM   #2
AJT is offline AJT  Philippines
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fixed bias gives more power and better bass in my EL34 amp, the circuit is very much similar to yours.....
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Old 17th April 2012, 08:44 AM   #3
Green77 is offline Green77  Sweden
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Ok, thanks.

Since i'm using a 300b SE today that doesn't have much bass that's one of the resons i want to build a new PP.
Maybe i should go for fixed bias after all...

//Daniel
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Old 17th April 2012, 08:58 AM   #4
AJT is offline AJT  Philippines
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very well, please bear in mind that with fixed bias, you will need to check your amp once in while to see that bias has not drifted...
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Old 18th April 2012, 05:24 AM   #5
Green77 is offline Green77  Sweden
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Yes, i know. Do you have your circut to post?
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Old 18th April 2012, 07:08 AM   #6
AJT is offline AJT  Philippines
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yes, and it is not much different from yours, although for input i used a russian 6J9, in pentode mode and 6CG7 ltp phase splitters....

more like a Harman Kardon Citation 5......
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Old 18th April 2012, 07:47 AM   #7
Green77 is offline Green77  Sweden
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Thanks. Will look at it..
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Old 18th April 2012, 11:18 AM   #8
roline is offline roline  United States
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I tried to mod in paint to show what I've done on an amp.

A different connect on the input tube, G2 resister 390 ohm to triode connect it, screen to cathode.

Constant current sink on the LTP, the trimmer pot is not necessary for 5% resistors that you measure and match. I've used the Russian ECC99 here, great driver tube. This is worth the effort to do! Mosfet configurations also will work.

I like a solid bias on the output tubes so I use adjustable fixed bias. I like the low end of the music where you can feel the speakers breathe.
For some power transformers missing the 70 volt tap, I use a small 6.3 to 110 100ma transformer backwards connected to a 5vac tap. That will give be about -90 volts DC. I use a voltage divider with pots to derive an adjustable neg bias range from -20 to -50VDC to tame the output tubes to 45 -50ma. If the B+ is around 500V, I increase the neg range, so -30 to -60VDC.
I did not add this to the markup, you should be able to do this.
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Old 18th April 2012, 12:00 PM   #9
Green77 is offline Green77  Sweden
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Thanks. I will go for fixed bias.
Isn't 240K a bit high where b- is connected?
And what resistor value did you use in the ccs?

//Daniel
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Old 20th April 2012, 11:27 PM   #10
roline is offline roline  United States
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I redrew one of my amps with a EF86 on the input and tried to match your circuit. It has Schade feedback on the outputs that can be left off. I also use a damper tube as a B+ delay device in the power supply, that can be converted to tube instead of solid state or just leave it out, no delay. I put blue leds under some of the tubes, power on indicator etc... that also can be left off.
I simplified the bias on the LTP, and it needs the heaters to be elevated in voltage due to the LTP cathode to heater can't exceed 100V....
I use larger coupling caps, PIO russian caps are nice, I prefer .3 to 1u. I like large caps in the B+ feeding the output transformers for the low end...
It was drawn for the iron that I had on hand at the time...
Food for thought...
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Last edited by roline; 20th April 2012 at 11:34 PM.
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