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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

PP CFB toroidy OPT

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I may be a bit late to the party, but this looks super duper good!

I have some experience with toroidal output trannies from my KT66 project (schematic and photos here). I like those toroids very much, but they need a bit of care to make sure the DC bias currents are as symmetric as possible. The conventional auto bias or fixed bias might not be the best solution here, because the tubes will drift after a while, which will give an imbalance of the bias currents and therefore saturate the toroid. I used a Tent bias board to keep the balance, but I guess there are other ways to do that, too -- just make sure you have some thingy taking care that the bias currents are always nicely symmetrical!

Will this become a kit? Or will you provide boards and a BOM? I am already making up excuses to build this...
 
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Will this become a kit? Or will you provide boards and a BOM? I am already making up excuses to build this...

The driver pcb is already prepared for additional autobias board.

I had no intention of selling boards and at the present moment no piece available for interest. It is possible that I will change my mind. :)

PP1-DRIVER-pcb2.jpg
 
Guys, help me understand theese distortion profiles. I measured two output tubes , KT88EH and KT77JJ. Conditions were the same, B+ 400V, Iq 55mA, global feedback 10-12dB, 10W into 4R5 (Class A operation). As you can see KT77 has better performance. Got me surprised the KT88 triode mode THD , it's worse than UL.

KT77JJ sin1k 10W@4R5
UL 0.15%
TRIODE 0.07%

KT88EH sin1k 10W@4R5
UL 0.28%
TRIODE 0.5%

PP1 KT88EH 10W 4R5.png PP1 KT77JJ 10W 4R5.png
 
Hi LKA,
Did you consider a differential cascode with E88CC and a CCS tail? @ 350V, 100K anode load and 47V on the upper tubes grids you can do +50dB of gain use less parts and still swing 140Vpp and you can use the extra gain for more global Negative Feedback that will help a lot with lowering output impedance, frequency response and lowering THD.
 

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Conditions were the same, B+ 400V, Iq 55mA...

The condition may be optimum to KT77 but not to KT88. KT88 may need some other Iq to be most linear. Try to fine tune. It may also be that those KT88s are not well matched and require different drive voltage. Try to fine tune this too.

global feedback 10-12dB...
To have constant (same) GNFB with UL and triode, require that you adjust P3 for both modes. Did you do this?
 
"It may also be that those KT88s are not well matched and require different drive voltage"
I bought them as matched quad from toroidy.pl

"To have constant (same) GNFB with UL and triode, require that you adjust P3 for both modes. Did you do this?"
I know, but I did not adjust the P3. The difference is less than 2dB.

measured OLG an CLG at 1kHz
UL OLG=32.4dB CLG=22dB Feedback=10.4dB
TR OLG=29.7dB CLG=21dB Feedback=8.7dB

KT77 behaves very well at 55mA and 35mA too.

PP1 KT77JJ 10W 4R5 bias55mA.png PP1 KT77JJ 10W 4R5 bias35mA.png PP1 KT77JJ 25W 4R5 bias55mA UL.png PP1 KT77JJ 25W 4R5 bias35mA UL.png
 
in my case (global nfb approx 10dB)
KT88EH, CFB+UL is better than CFB+TR
KT77JJ, CFB+TR is better than CFB+UL
KT77JJ is better than KT88EH
(talking about measured distortion profile)

Simulation is very close to reality in case of KT88 (if proper model used).

PP1-simed-thd.png
 
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