KT77 Pushpull integrated amp

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quadtech
Ist häufiger hier

#1 erstellt: 25. Dez 2008, 07:50

I completed my KT-77 Pushpull integrated amp. It is kind of based on a Dyncao ST-70 design, using toroidal transformers.

As posted on another thread, I put up some pics here -

http://picasaweb.google.com/prasadb/TubeAmp#

Long, perhaps boring description....

I got the driver board and some components quite cheap from a diytube member. It is the VTA board designed by Roy Mottram as an upgrade for the Dynaco ST-70 and uses three 12AT7 tubes for both VA and PI. PI is a Schimdt type long tail pair. There is a trimpot onboard for each output tube to adjust bias voltage.

Power supply and soft start PCB was made by me using eagle and toner transfer. It's the crude looking board on the left in the pics. I used a CL-80 current limiter to slow the power on inrush current on the toroidal PT.
I considered using a GZ34 tube rectifier, but finally went with a solid state design using two UF4007s in series in each leg, and have CLC filtering.

The potted, shielded toroidal transformers (OPTs and Power) are made by Trafomatic of Serbia, purchased in the US from Vista Audio. I was lucky that a friend could bring them. They weigh a ton.
My friend also got some other stuff like binding posts.

The tubes are a quad matched JJ KT-77. The amp is switchable between triode and Ultralinear modes.

For the chassis, I spent many hours on cardboard mockups to get it right. Size 15"x12"x2.5". Got it made with 1.2 mm MS, at a sheet metal workshop here in Hyderabad. The owner is an autocad whiz and laughed at my crude diagrams. He put everything into autocad which helped to clean up the design, and his workers did an amazing job on it. Normally, he will not do small jobs as he has his hands full with jobs for defence and other stuff, but did this as a favour for me. In fact, it took him 3 weeks to find time to make this.

The amp is very quiet and sounds great (I lack the vocab for all the usual descriptions, review ishtyle). I am testing with some cheap drivers for testing, but once my FE206 cabinets are done, I will know how it really sounds.

The only downside is that it is so heavy, my back aches if I need to move it. The final soldering after the transformers were mounted nearly killed me!

Amp_Nut
Inventar

#2 erstellt: 25. Dez 2008, 08:50

SUPERB Job !

Clearly, a labour of love, and I am sure that you will be suitably rewarded with the sound, and the knowledge that you built the best you could.

Where are you located ? I am in Mumbai. Any possibility of hearing it ?

Cheers !

Savyasaachi
Inventar

#3 erstellt: 25. Dez 2008, 08:52

Very nice..love the metal work

quadtech
Ist häufiger hier

#4 erstellt: 25. Dez 2008, 09:01

Where are you located ? I am in Mumbai. Any possibility of hearing it ?

I am in Hyderabad. Right now, I don't have proper speakers,
but I plan to start work on fostex enclosures soon.
AKS said to expect at least a month for each box,
and I have some limitation on how much time I can put in.
So I expect around March. If you are in Hyderabad around
that time or later, do PM me.

Labour of love - very true. It took over an year
to learn the stuff, get the parts etc. Many times, it felt
easier to give up in favour of an easy to build gainclone.

Prasad

Arj
Inventar

#5 erstellt: 25. Dez 2008, 10:40

Prasad,
How are the JJ KT77s in your opinion ? did you get a chance to compare them with any of the other El34s ? (I believe they are mutually compatible)

i had heard reports of major quality issues in it . how do you get them in India ?

Manek
Inventar

#6 erstellt: 25. Dez 2008, 13:09

I've have read about problems faced with the Ei tubes...jj stuff I thought was quite stable....except for one kind of power tube which they made were not close to the spec of original and quality sucked. I don't remember which tube but I doubt it was the kt77 or el34.

Manek

quadtech
Ist häufiger hier

#7 erstellt: 25. Dez 2008, 16:57

A friend of mine from the US got me the JJ KT-77s along
with the transformers. I do not have any other tubes, so
did not get to do any tube rolling yet. But the
KT-77 amp sounds perfectly fine to me (not blessed with any magical hearing though).
Would have been good to have an oscilloscope, to check out
the square wave response.

They say the early KT-77s had issues, especially with
very thin pins. But JJ sorted those issues.
It seems those early batches caused a lasting bad impression.

To be on the safe side, it is better to connect the
UL taps to pin 4 via a 100 ohm 2W resistor. Definitely
recommended if using SS rectification. I tested with
and without these resistors and see no difference, so
they stay in the amp.
See this for more info -

http://www.audioxpre...ia/Gillespie2544.pdf

sivat
Stammgast

#8 erstellt: 28. Dez 2008, 07:03

Congrats Quadtech, your product looks impressive. Wish you were in BLR.

aks07
Stammgast

#9 erstellt: 13. Jan 2009, 09:14

Stunning fit and finish ! I am sure they sound even better.

For next project go P2P

Amp_Nut
Inventar

#10 erstellt: 13. Jan 2009, 09:50

KT-77 amp sounds perfectly fine to me (not blessed with any magical hearing though).
Would have been good to have an oscilloscope, to check out
the square wave response.

Tube amps often yield miserable Square wave responses...

Talking of O'scopes... last Saturday, saw this Really neat Solid State, 20 MHz 2 Channel DIGITAL Chinese ( ? ) scope.

Only a few Inches thick... Colour display, with USB interface to store screeen shots ( i guess )... Rs 20K !