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zafira1981 11th October 2006 01:56 AM

My new project, 2xKT88 60W amp

I need to share the emotion...... I just boutgh the tubes for my amp project... it will be 2xKT88+12AU7 for output stage and 1x12ax7 diff amp (with solid state current source). I didnt finish the schematic... I have to make some questions to a teacher of my college... as soon as I had it, i will post it :D

meanwhile... some pics!!

look at the tube sockets, they're ceramic - gold! :D they look really nice...

Bas Horneman 11th October 2006 06:35 AM

Good luck with your project! And please keep the eye candy coming! :)

zafira1981 25th October 2006 08:16 PM

Some news!!!!

My output power transformer will be:

input: 5kohm (400H ) with 43% ultralinear taps, output: 6ohm
4,5kg M5 iron (dont know how is in english "grano orientado")

PSU choke: 10H 250mA
PSU transf: 370+370 V

For the tube heaters I will use 6.3v DC regulated... I need to find a good circuit, maybe with a LM317 / 338.

the schematic will be something like this:

but with a diffamp with 12AX7.

andRookie 26th October 2006 10:35 AM

I am builing the exact circuit you are refering to, with EF86, ecc83 and EL34.

I cannot answer these questions to their full extent, as I in my project depend on guiding from more competent persons. But:

Are you sure 370 V B+ will give you 60 watt out with kt88?

Are you sure a choke rated at 250 mA can deal with the current needed for 60W out?

In my project I am aiming at 30 - 40 watt out. My B+ is 460 volt and my 5H choke is rated at 300mA.

zafira1981 26th October 2006 11:54 AM

Are you sure 370 V B+ will give you 60 watt out with kt88?

nono, I will use 370+370 transformer so the B+ will be somethin like +470V

Are you sure a choke rated at 250 mA can deal with the current needed for 60W out?

mmmm... I'm not sure, the guy who builds my output trans told me that I have to use that 10H 250mA choke. I will take note of that you said, thanks!!!!

reenberg 26th October 2006 02:36 PM

This is from the Byrith book right ?
I'm collecting parts for the project he describe in his book right now.
I don't think you need regulated power for the heating in an power amp, only a pre amp is wort that much trouble.
I will stick with a regulated power supplyfor the el34 model.

And here is the link to the book

zafira1981 26th October 2006 02:43 PM

yes, it is from that book!! a friend emailed it to me :D

Now I have to modify the schematic to use KT88 and a 60W output trans.

I'm newbie with tubes, so it will take some time... affortunately, I have a teacher in my college that knows all about tubes and will help me today :D

kmj 26th October 2006 02:51 PM

hehe, this seems yo be a popular project, I myself have just collected most parts for the same amp. I will use 6550 instead of EL34 thou.

As stated earlier, just keep it comming!

have you guys noticed the differences between the schematic in the appendix and the one in the article?
Different values on caps and some that are present in one isn't there on the other.

SY 26th October 2006 03:34 PM

This is a variation of the Mullard circuit. There's a very nice deconstruction of the design in "Valve Amplifiers."

May I simplify things a bit?

First, if you pay some attention to grounding and layout, there's no reason you can't use unregulated AC on the heaters. None of my power amps use DC heaters, and they all are dead quiet.

Second, if you're using a current sink in the second stage cathodes, the plate balance adjust is unnecessary. Just use matched plate resistors and matched grid resistors for the output stage (they're part of the diff amp's load) and the CCS will force things to be balanced.

richwalters 26th October 2006 03:41 PM

400H for o/p tranny is alot of L.......Is it a toroid ? I
noticed 390K output stage grid leaks......this is way high for KT88 and especially for 6550's in fixed bias and no HF phase correction in 1st stage anode.
Those KT88's in the packets..wait before you use them...I say get design going with EL34's and study the data sheets of the KT88 and 6550C.
Svetlana spec for 6550C suggests 200k absolute max for grid leak and I'm using not even half that in my fixed bias designs.
Variants of KT88's are also sim.

Anyone else flogging ECC83's as phaseplitter drivers for KT88's ?


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