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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 05 Apr 2021, 13:50 
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Joined: 23 Jul 2020, 12:20
Posts: 11
Hi,
Simple Question. I have built te KT88 SE Amp from Mikhael Abdulah sucessfully four times now. I followed the Design provided by Marc (Blueglow electronic) exept for the red LED. (I use white ones which work fine)
All the Amps sound beautiful and all of them now have more then 50h on the clock.

My Question is, i would like to try experimenting with a paralell single end amp, based on that schematic.
I am unsure about the mA drawing on the Anodes? My Power Transformators are custom handwired and shed in a cube (best results ever) :

375V (0.28A) - 0 - 375V (0.28A)
6.3V (7A) - 0
5V (3.5A)

i am unsure that this is good enough for the two KT88 and the two 6N1P.

According to the KT88 Datasheet the KT88 draws aprox. 147mA and the 6N1P around 23mA (at 229V)
So, right now i roughly draw 340mA in total (for both chanels)

If i add another KT88 it would be double these 147mA? (2x 147mA & 1x 23mA)
That means i need at least 650 - 700mA for the total of four KT88 and two 6N1P

Yes i am aware that four tubes have more voltage drop than just two (does anyone know the voltage drop of just one KT88?)

Thank you your Help. I'm still learning, please bare with me.

Pero


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PostPosted: 07 Apr 2021, 08:56 
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Joined: 23 Jul 2020, 12:20
Posts: 11
No One? :-(


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PostPosted: 07 Apr 2021, 11:31 
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Joined: 06 Apr 2009, 10:08
Posts: 1748
Location: US Pacific Northwest
peroja wrote:
No One?
Perhaps because we are unsure of your desire. Do you just want to run two KT88s in parallel at the same bias point?

Mikael's design has the power stages biased to -39v. At this bias they are drawing approximately 70mA. The 6N1P is biased at about 4v and drawing about 4mA. So one channel of the current amp draws about 74mA. Two channels ≈150mA. If you want to bias the KT88s at the same level, you'll need 4*70mA + 2*4mA = 288mA. You'll also need a little margin for operation so I would recommend a power supply capable of outputting at least 300mA. Your 280mA power transformer is going to be overloaded and probably won't work very well.

You have options. If you want to keep the power transformer, you could lower the bias current of the KT88. The best method will of course depend on your desired topology. You could opt for a larger power transformer that supplies more current and/or voltage.

When you say "experiment" you need to let us know more of what you're thinking. Do you want to preserve the operating points and simply try for more power? Do you want to alter the load for different harmonic performance? Do you want to pick a bias point for a different sound? More information from you will help us provide more information.

Keep in mind, parallel stages can be tricky beast to make perform well. Parallel stages with very large power tubes (such as the KT88 or 6550) are even more tricky to optimize. Before you start to simply "experiment", you should probably make sure you have a very good idea of what it is you want to accomplish.

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PostPosted: 08 Apr 2021, 20:28 
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Joined: 23 Jul 2020, 12:20
Posts: 11
Hello Matt,

My current Power Transformers are :

375V(0.28A) - 0 - 50V - 375V(0.28A)
6.3V(5A)
5V (2.5A)

So i assume i have 560mA?
I already asked the Guy who custom builds my Transformers if it is possible to go for 700mA but when i read your Answer i guess this would be an overkill?

Yes, as i am very Happy with the Sound i just want to go for more Power. I currently run Infinity RS4B Speakers on 4 Ohm as i am unsure if they are 6 or 8 Ohm. The Power is just good enough for my Livingroom but just about. I would be very happy if i would have double the Power of what i currently have. (I guess between 5 and 8 watt)

My idea is just to add another KT88 paralell but have the BIAS separately on all 4 tubes.

Currently i have on Pin 8: 220uF/500R, so that would be on each of the four tubes
Direct Connections would be on Pins 3 and 5
As is dont use UL i have just Bridged Pin 3 and 4 with 100R. This i would do on all four tubes as well and connect on pin 4 AFTER the resistor.

I am just a bit unsure if that would be good enough? Or do i have to add R12, R13 and R14 on the second KT88 as well?

See Picture attached :)

Thanks for Helping me.

Pero


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PostPosted: 09 Apr 2021, 19:38 
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Joined: 23 Jul 2020, 12:20
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Here is my Idea. I also consider to use a 6SL7 instead of a 6N1P
What do Guys you think? Could this work?


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PostPosted: 14 Apr 2021, 06:35 
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Joined: 23 Jul 2020, 12:20
Posts: 11
I belive i am at the wrong place here.. I was in hope to find some help. i was wrong.


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PostPosted: 15 Apr 2021, 14:49 
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Joined: 19 May 2011, 05:38
Posts: 55
you will run out of grunt trying to drive 2 kt88 with a 6sl7.
May be ok parallel sections each with its own plate and cathode resistors.


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PostPosted: 15 Apr 2021, 14:50 
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Joined: 19 May 2011, 05:38
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you will run out of grunt trying to drive 2 kt88 with a 6sl7.
May be ok parallel sections each with its own plate and cathode resistors.


Sorry double post


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PostPosted: 16 Apr 2021, 04:16 
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Joined: 01 Jun 2013, 09:05
Posts: 1059
mike567 wrote:
you will run out of grunt trying to drive 2 kt88 with a 6sl7.
May be ok parallel sections each with its own plate and cathode resistors.


Sorry double post


Hi,
+1
I hear an echo :D must be room resonance.

I think your referring to Mr Miller (Miller effect) and the double loading on the grid capacitance which is even worse on triode strapping of pentode.
The tubes also need to be matched with matched bias and compensation so as time goes by the tubes will "wear" differently.
I would use active cathodes or auto bias but that has its own problems. Maybe mixed bias is another possible maybe.
There may be grid blocking:
https://www.aikenamps.com/index.php/wha ... conduction.

I notice a comment about substituting a different colour of LED in the cathode position where the "Colour" isn't important its the voltage drop and capacitance that matter and set the bias and drive etc.
So there is a bit more to it than just glue another output tube alongside the other.
There is also a change in primary impedance of output Tx as well.
As Matt says there is more to it. (just a few thoughts).

Regards
M. Gregg

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PostPosted: 16 Apr 2021, 13:31 
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Joined: 19 May 2011, 05:38
Posts: 55
I was referring to miller cap. Plate to plate feedback also places additional load on the driver tube.
The 6N1P is a stout little tube. Similar to 6922 tube.


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