• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Got a big output trafo I want to use.

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Jump to Latest
lawbadman

lawbadman

Member
Joined 2003
Hey guys,


I now have in my possession an output transformer with a 4.5k primary capable of 100watts.
I want to take advantage of the full power available with this transformer but 100watt designs are very scarce so I am left with 50-60 watt designs. My goals are (in order) power output, clean sound, low cost. I dont have a budget as yet since I already have the most expensive part, but I will not be using exotic parts.

Now, there are sooooooo many designs to chose from I really cant decide. For about 3 weeks now I have been scouring the web and this site for ideas and clues but I have not found a lot that helps.So I have compiled a short list of possible builds.
If any of you have built any of those on my list please comment and let me know how stable the amp is and how it sounds.


http://www.drtube.com/schematics/radford/sta100.gif
http://www.audiofanatic.it/Schemi/Tipo/Valvole/finali/pic_finali_PP/6550PP_6SN7.jpg
http://www.audiofanatic.it/Schemi/Tipo/Valvole/finali/pic_finali_PP/KT88PP_6AN8_dynakit_mkIII.jpg
http://www.webace.com.au/~electron/tubes/Circuits/achpwill.gif
http://www.triodeel.com/gec100w.gif
Not sure about the primary impedance of the output transformer for this: http://www.triodeel.com/m&atva1.gif


Comments and suggestions are very much welcome.
I really need some direction an which to build.




Thanks all


Lawrence
 
kmaier

kmaier

Member
Joined 2007
It's true that you can push a pair of (real) KT-88s to 100 watts, but they take a beating and won't last long.... back in the day they were cheap to replace, not now. Suggest you revise your goals... reliability, sound quality, power, cost.

If you want 100-watts nowadays, you would be better to use a quad set of new issue T-S 6550 outputs, but you'll also need a lower P-P load to get the best from them. Just my $0.02.

As for schematics... I would suggest a LTP phase inverter... if funds allow, a high-quality current source on the common cathode pair. Would also provide for both AC and DC balance to get the best performance from it. As you're finding out... you have numerous options for circuitry. Some of the older ARC designs were quite good... a bit complex however.

Regards, KM
 
kmaier

kmaier

Member
Joined 2007
Hi Badman,

The last schematic is almost identical to the modified williamson in your first post which is from the old Acrosound brochure... and was later printed in an old Dyna brochure sporting the latest (Dynaco) OPT from David Halfer, both dating back to the 50's.

I doubt there would be much difference sonically between them. I personally prefer a long-tail pair (LTP) phase inverter over the cathodyne used in the Williamson design. As you're not planning to build that soon, scan the net and look some for other designs. Here's one from Isao's site... uses an EF86/6267 as an input amp driving an ECC82/12AU7 configured as a LTP:

http://tube-amps.net/images/KT88_PP_01/KT88_Schematic.jpg

My old 50-watt design done back in the 80's used a 6072 as the input amp driving a 6FQ7 for the LTP and had both AC and DC balance on the output tubes and a custom current reference module in place of the LTP cathode resistor. Happy hunting.

Regards, KM
 
kmaier

kmaier

Member
Joined 2007
lawbadman said:
I saw that circuit, but I have heard that the 12A_7 tubes were no good for hi-fi, ture or not?

Are there sonic differences between the LTP and the cathodyne why you prefer the LTP?
Click to expand...

Well, it would appear that many of the 12A*7 tubes have received a bad rap in the latter years. Amazingly enough, the recordings done 5 decades ago, which are still considered to be superb, were done with audio gear full of them, who knew. Despite public opinion, I still consider the 12AU7 an excellent tube and still design with it today.

Regards, KM
 
kmaier

kmaier

Member
Joined 2007
lawbadman said:
link does not work.....
Click to expand...

Oops... edited. If you like, I can convert to PDF and email it. It will look much better. I also have the other 2 dwg files which are the power supply (all solid-state, hey, it was the 80's) and the even the current modules which were all custom designed with solid-state components available back then.

Regards, KM
 
lawbadman

lawbadman

Member
Joined 2003
I think I like the Hashimoto circuit. The impedance of the output is 5k. Being that mine is 4.5k, would that put too much of a load on the KT88? What about adding an extra pair of KT88 so as to ease the load on just one pair and also to get more power???


KM, what current sources are those that you are using in your amp?
 
kmaier

kmaier

Member
Joined 2007
4.5K is a nice load for a pair of KT88s in push-pull. I would not worry at all. I also wouldn't go parallel push-pull either, just personal preference and the added cost of a matched quad plus additional DC balance circuitry.

I would suggest you stick to a single pair and live happily for 50-60 watts as it's at most a 3dB decrease from 100 watts and will certainly be more reliable.

There are two modules I custom designed for the amplifier. One I referred to as a Current Source Module for the input stage which requires a voltage reference from the power supply and the second I referred to as a Current Reference Module for the LTP phase inverter. This one was expensive to build... has exceptional bandwidth, dynamic range, exceptional accuracy and is temperature compensated to boot. The CSM is set for 2.8ma of current and the CRM is 10ma. Both could be replaced by resistors to reduce the cost.

I'm not sure all of the components are still available as it was about 20 years ago when I designed them.

Regards, KM
 
kmaier

kmaier

Member
Joined 2007
Wieslaw Lipowsk said:
>>Here's one from Isao's site... uses an EF86/6267 as an input amp driving an ECC82/12AU7 configured as a LTP:>>

I failed to see a rectifier for the high voltage/plate in that schematic. Where can you get B+ of 525 VDC from?
Click to expand...


Look closer... also read the description. There is a single diode used as a half-wave and it rectifies the negative side, not positive as in traditional designs.
Regards, KM
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Top Bottom