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 Post subject: Re: KT120 Oddblocks
PostPosted: 16 Nov 2017, 01:45 
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Joined: 12 Sep 2013, 22:15
Posts: 22
Hey Everyone, thinking I'll put together a pair of these. just looking at the schematics and want to confirm a few part specs

for the 25R balance pot, is a 5W pot sufficient?

The first filter in the PSU schematic says '2x 100uf/500V' but there's only one cap shown on the schematic, are these connected in series to give 50uf/1,000V, paralleled for 200uf, or is it suggesting using a box electrolytic for the two 100uf caps either side of the 100R resistors?

Is 5W sufficient for the two 100R resistors in the power supply?

can anyone recommend a decent heat sink for the LM317HV's? preferably something that can come from Mouser.
My KT88 Oddblock builds function pretty well as a heater in winter (and a torturer in summer), so I'm concerned these things are gonna kick out some real heat. Is it worth having the heat sinks on the exterior of the chassis? these amps are going in my workshop, so they don't need to be too pretty.

cheers
Josh


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 Post subject: Re: KT120 Oddblocks
PostPosted: 16 Nov 2017, 15:41 
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Joined: 04 Jun 2008, 20:59
Posts: 3649
Location: Arizona, USA
Hi Josh, 5 watts is fine on the pot. The 2 100uf are in parallel, you can use a 220 if you want, just 2 of the 100s were less costly than a single 200. The 100R resistors are 5 watt in size. They act as a fail safe in case of tube failure. It is a sort of hidden feature in the amps. If a tube fails it might not blow the fuse. At least not soon. To minimize the damage as tubes do fail on occasion the excess current will cause one or the other resistor to fail followed by the second one. You actually do not want close tolerance resistors. That way one will get a little bit more heating and cook. I originally thought that perhaps I would use one 82 or 91 ohm resistor and one either 120 or 110 in parallel. This would guarantee one would fail first. But with typical 10-20 % sand resistors one will usually be a few percent different from the other and do the job well. BTW in this mode of failure it is likely that the LM317 has already failed due to over voltage caused by the tube failing. Since the LM317 will usually fail in the shorted through mode, this provides the excess current that will make the resistors fail and save both transformers and the rectifiers. You could fuse the circuit, but the initial current inrush would tend to make it fail regularly and be a real pain having to replace all the time. I don't offhand have a part number for the heat sinks. But the two LM317s in the KT120 amps dissipate between 10 and 13 watts. This value will vary a bit depending on the B+ applied to the tubes.

Yes the amps are "heaters" The KT120 versions use about 245 watts each. Being a class A design most of the power is changed into heat.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 17 Nov 2017, 21:19 
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Joined: 12 Sep 2013, 22:15
Posts: 22
Brilliant, thanks for that Bruce, I'd better check those resistors in my home amps, I had a TAD KT88 run wild a few weeks back, managed to hit the power switch after about 3secs, but have noticed it's slightly noisy now (audible from about 12inches) but thought it might be the old tubes i'm using while waiting for a replacement.

As for the heat sinks, I'll just go with something big and efficient, I'm gonna go extra large on the chassis for these amps.

cheers


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 Post subject: Re: KT120 Oddblocks
PostPosted: 18 Nov 2017, 16:15 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, They should be really quiet if you built them like the project plans. I was using a pair of the KT120 mono blocks into speaker with 96 dbw sensitivity. No hum even with my ear to the grills with the inputs grounded. In the system (with a modded Forewatt driving it) there was ever so slight tube hiss. Most is from the preamp as it has the gain control at its input. You get a tad bit more noise that way (audible at about 1 inch) but far less likely to over load the preamp and it runs the tubes in a more favorable range (0-2 volts output) than if the volume control was on the output side.

I use heat sinks that are about 2 inches by 3 inches by 1/2 inch made of extruded aluminum for these amps. They have no identification markings. More is better naturally.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 27 Nov 2017, 10:15 
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Joined: 23 Nov 2017, 11:14
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An output transformer question by someone contemplating a non-kit build. My apologies if this has been covered, but my search didn’t reveal an answer.

Is the specified Edcor OPT, model CXPP70-MS-3.5K, a special order variant of the standard model CXPP70-3.5K (no MS designation) available on Edcor’s website? If so, what’s different, and will Edcor sell a pair direct to consumer? Would I expect an upcharge?

Thanks.


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 Post subject: Re: KT120 Oddblocks
PostPosted: 27 Nov 2017, 15:35 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, It is the same one. They used the MS designation in the past to dessignate multiple secondary taps. Since there is only one CXPP in this size they all have multiple output taps.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 09 Dec 2017, 08:54 
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Joined: 16 Oct 2017, 12:25
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Hi
I have a pair of 1650t hammond OT and I'm wondering if they would work in this amplifier
the impedance is a bit low at 1900, is this too low or can I get away with it?

Thank you


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 Post subject: Re: KT120 Oddblocks
PostPosted: 09 Dec 2017, 14:12 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, For class A operation in the tish design with KT120s 3200-4000 is the proper range. Lower (or higher) will cause a loss of ouput power. Your transformers are suited for class AB operation.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 31 Dec 2017, 07:53 
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Joined: 07 Mar 2017, 03:14
Posts: 63
Hi Bruce,
In your kit, you put power and output trannies front to back, not side by side on the back. Which tranny is on the front? Which one is on the back? Why did you choose that layout? Does that layout cause any increase on the s/n ratio?

Happy new year to all.. :sing:


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 Post subject: Re: KT120 Oddblocks
PostPosted: 31 Dec 2017, 18:08 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, The output trannie is close to the front. We wanted the transformers to be as far apart as possible as we noted they would couple a bit. After all they are rather substantial and handle a lot of power. Since the amp has only modest gain having the output near the driver does not make any difference. As for S/N issues...there are none this way. The ones I have measured all run better than -90dbv (that is db below 1 volt output) with input shorted. They are really quiet.

Good listening
Bruce

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