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Green77 17th April 2012 07:17 AM

Hi all!

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

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

TonyTecson 17th April 2012 08:23 AM

fixed bias gives more power and better bass in my EL34 amp, the circuit is very much similar to yours.....

Green77 17th April 2012 08:44 AM

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...


TonyTecson 17th April 2012 08:58 AM

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...

Green77 18th April 2012 05:24 AM

Yes, i know. Do you have your circut to post?

TonyTecson 18th April 2012 07:08 AM

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......

Green77 18th April 2012 07:47 AM

Thanks. Will look at it..

roline 18th April 2012 11:18 AM

1 Attachment(s)
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.

Green77 18th April 2012 12:00 PM

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?


roline 20th April 2012 11:27 PM

1 Attachment(s)
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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