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KT120 Oddblocks
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Author:  gofar99 [ 08 Mar 2011, 22:48 ]
Post subject:  KT120 Oddblocks

Hi Everyone, As promised I am starting a thread on KT120 Oddblock amplifiers. I have a pair up and running. Still early in the sorting out phase, but initial results are quite promising. As expected they are powerful. :o Calculated output power is 40 watts RMS (sounds like it too). I have done only minimal testing of them and will do more in the tweaking process. I will post a schematic when I am comfortable that they work up to my personal standards. My initial thoughts are they are quite quiet, low distortion, full power bandwidth (like all the other Oddwatts), good sound stage and imaging. Basically like all the others in the family just bigger. The B+ is 405 at 280 ma right now, but I believe that 450 at 300 ma is a reasonable target. Since the cathodes are about 37-39 volts above ground, this will result in total dissipation of about 120 watts it may be a bit close to max. :hot: Right now it is 102 watts and this may actually be better for the long run. I am still juggling with the load impedance to find the best match and a few other things. The driver is a 12AX7 SRPP. As this is a class A1 U/L mode there is no grid current during normal operation so there is no need for a hefty driver. For all you folks who abstain from DC heaters these amps use AC. It seems to reduce the S/N a bit, but until I run tests it is hard to say if it is significant. One thing for sure.... the KT120s are really huge by normal standards and look super.

Good listening
Bruce

Author:  HT Performance [ 09 Mar 2011, 20:59 ]
Post subject:  Re: KT120 Oddblocks

gofar99 wrote:
"...For all you folks who abstain from DC heaters these amps use AC. It seems to reduce the S/N a bit, but until I run tests it is hard to say if it is significant... "

Hi Bruce!

I´m also using AC heaters on my EL34 Oddwatt. It´s amazing how it sounds. There is absolutely no hum at all!

I think that a good component placing and to avoid high AC currents are the key, but always with short twisted wires and not flying around the chassis. So I like to use AC series heaters. :)

Great Job!
Miguel

P.S. - I also like the "Thunder Blocks" name! I´d love to see a simple picture!!!

Author:  gofar99 [ 09 Mar 2011, 21:13 ]
Post subject:  Re: KT120 Oddblocks

Hi Miguel, Photos attached of the prototypes. The little tubes are 12AX7.

I need to replace the heat sinks as they are not thermally efficient enough. There are some low cost ones (under $1.50) from Mouser I am getting that will work better. These have a rating of about 8 degrees C / watt. The CCS really needs one in the 3-5 range to keep it from getting too hot and shutting down.
Attachment:
Thunder Blocks Front B.jpg

Attachment:
Thunder Block Front B.jpg

Good listening,
Bruce

Author:  HT Performance [ 10 Mar 2011, 04:48 ]
Post subject:  Re: KT120 Oddblocks

They are huge!

They show SOME respect! :) Why did you placed them so close to each other? Aren´t there heating issues between them?
If the neighbor tube heats up the anode, can it compromise overall performance?

Cheers,
Miguel

Author:  gofar99 [ 10 Mar 2011, 18:19 ]
Post subject:  Re: KT120 Oddblocks

Hi, Yes the KT120s are huge and they do generate a lot of heat. :hot: The reason for the close placement is that I used a chassis from one of the Oddwatt Audio KT88 amp kits. A lot easier than drilling all the holes and such. For an actual build I would use a larger chassis and space the tubes out a bit more.

Good listening
Bruce

Author:  rparsh [ 12 Mar 2011, 20:19 ]
Post subject:  Re: KT120 Oddblocks

Hello, this is one project I have been waiting for ever since I bought the four KT 120 tubes! I ordered 2 Edcor power transformers, the 180-0-180 @ 500ma ones. 5 to 6 week wait of course. Should be exciting. Thanks, rparsh

Author:  rock4016 [ 13 Mar 2011, 09:47 ]
Post subject:  Re: KT120 Oddblocks

Your B+ will be too low with those transformers, unless you are going to use a voltage doubler.

Author:  poty [ 13 Mar 2011, 13:28 ]
Post subject:  Re: KT120 Oddblocks

It depends... Having such big margins with current, using solid state bridge rectifiers, full secondary and some luck you can have as much as 500V. RC filter will "eat" 30-40V, and we'll have healthy 460-470V.
Another question - will the planned 450V be enough? It seems the KT120 doesn't reveal full capability with the voltage.

Author:  rock4016 [ 14 Mar 2011, 07:26 ]
Post subject:  Re: KT120 Oddblocks

Eh, yeah, I goofed :( . Looks like you could get about 480V B+ with a SS Bridge. According to PSU Designer. I was thinking about a 180V transformer, not 360V.

Author:  cheap-Jack [ 14 Mar 2011, 10:16 ]
Post subject:  Re: KT120 Oddblocks

gofar99 wrote:
For all you folks who abstain from DC heaters these amps use AC.
HT Performance wrote:
I´m also using AC heaters on my EL34 Oddwatt. It´s amazing how it sounds. There is absolutely no hum at all!

HI.

I agree provided the AC supply is done properly.

Like my Dynaco ST-70 also used AC heaters supply. Twisting pairs of wires to the tube heaters go without saying.

What I installed addtionally something crucial is a common mode AC noise filter to heater group of EACH channel. The AC noise filtler comprised 1 bifilar handwound coil on a small wooden form & an AC film filter bridging the O/P leads of the coil. Bifilar winding is a must for any common mode noise filter.

Since the AC current is pretty large to go thru the coil, so the magnet wire used to wind the coil should not be too small. I used AWG #18, something I got in my part bin. The 2 small wooden forms were I DIY 'handcrafted' to size to fit the already very conjected space available underneath the metal chassis of the power amp.

Length of the manget wire is custom cut to provide only a small faction of a volt in order not to offset the required 6.3V heater voltage.

I need to install such RFI noise filter as the tube heaters in my phonostages are DC cells powered rechargeable by a SMPS which may emit RFI noises to the power amps. Besides common mode helps to cut down hum noise going thru the heaters.

c-J

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