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PostPosted: 27 Jul 2009, 15:38 
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Hi, I have not had the opportunity to try JJ KT88s so I have no comparisons. The "harshest" of the tubes I have tried seems to be the Sovtek 6L6WXTs. They are still relatively new so they may work out after all. I would have to check the actual spacing on the chassis (both prototype and later kits) to see the spacing, but I believe it is close to spec. I would not worry about the spacing as long as there is adaquate ventillation in the area. A fan should not be needed.

I rate the amps at the point where distortion gets close to 1%, not at the full power level. However, the gain is not actually a function of that. KT88s require around 30 volts of drive to go to full output and the KT77s and 6L6s about 20. So since the actual circuit gain is mostly a function of the SRPP it is relatively independent of the output tube. So, it will take a bit more signal to drive the KT88s to full output (which is greater than the KT77s max). For example it will take about 1 volt to drive the KT77s to full output, the same signal will yield very close to 15 watts with KT88s. To get full output of the KT88s it will take about 1/3 more drive or around 1.3 volts. These numbers are approximate, but fairly close to actuals.

For harshness, I don't recall if you were the diyer that had the NFB circuit reversed. If it wasn't you write back as that can cause the amps to be harsh sounding.

Good listening
Bruce

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PostPosted: 28 Jul 2009, 02:08 
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Hi Bruce, it was me who had the reversed NFB, yes, so that is fine now. When I said ‘Harsh’ I only meant the JJs in comparison to the GLs. Maybe ‘bright’ would be a better word. The amps (especially with the GLs) sound fantastic though, so I’ll probably not try KT77s for the time being. I think it’s time to sit back and enjoy them for a while before tinkering any more! I think the next project will be a tube preamp to go with these, but I need a break to save some $$ first!

Photo of completed project attached:

Cheers,


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PostPosted: 28 Jul 2009, 08:55 
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Very nice build, Bright would probably be a good word. You can tame it a bit if needed by changing the cap in the feedback loop to a slightly higher value. Perhaps a 1500pf. The danger here is that the overall balance from highs to low would change. It seems that the preference for tube sound is leaning toward the GL KT88s and JJ 6L6GCs. Those who like the 77s seem to listen to music with a lot of detail and not as much bottom end. I like both so I am constantly swapping the tubes.

Good listening
Bruce

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PostPosted: 28 Jul 2009, 11:52 
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Congratulations cannst77, the amps really look great!
Cheers

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PostPosted: 28 Jul 2009, 12:05 
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Hi, You peaked my interest so I ordered a quad of JJ KT88 blue glass tubes to see how they compare to the others. If nothing else the blue glass will be really nice looking next to the blue Edcor trannies.

Good listening
Bruce

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PostPosted: 29 Jul 2009, 01:38 
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Bruce

Can the heaters be powered from the 6V secondary tap? If so, then the 12V SMPS is not required.
If I rectify the 6V tap and use it for the heaters what change would I need to make to the power supply?


Regards


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PostPosted: 29 Jul 2009, 06:34 
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Robert,

The Edcor transformer does not have enough current available at that 6.3 VAC tap. If memory serves, it was rated at 3 Amps and you need a minimum of 3.2 Amps just for two KT88s. Rectifying it to DC will not help, you still draw more than the transformer can safely deliver. KT77s would just sneak in a 2.8 Amps, but you still have the driver tube and 6NO30. The driver tube needs .3 Amps, so that alone tips you over your power budget. It is good practice to design with some headroom rather than push the limit of any component.

Additionally, if the transformer already runs warm/hot in the Odd Block design, tapping additional power from the 6.3 VAC taps just increases the core's temperature even more. I personally don't like things to run hot if they don't have to.

You could always add a standalone filament transformer to the chassis if you wanted, but I doubt the cost of the transformer and its associated circuitry would be cheaper than the SMPS supply.


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PostPosted: 29 Jul 2009, 09:55 
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Hi, Yes you can. I don't recall how much current is available from the trannie but allow for 4 amps to run a pair of KT88s about 3 for KT77s. The rectifier needs to be able to take the inrush as well. I use a 10,000uf cap after the rectifier and .1uf polys on the tube heaters as snubbers. The amp may have a slight increase in hum. I use a similar scheme in the kits but at 12 volts. You will probably have to put some series rectifiers following the filter as the voltage will be a bit high. These need to be high current types and will generate a lot of heat. At 4 amps each one will dissipate about 2.8 watts. They need air. That is part of the reason for the 12 volts vs 6. The current is half and the heat then is half. Shoot for 6.0 volts. 5.9 is fine and anything up to about 6.1 is OK. Going above that seems to cause some tubes to be wander thermally and will mess up the balance between them.

Good Listening
Bruce

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PostPosted: 29 Jul 2009, 15:04 
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gofar99 wrote:
Hi, Yes you can. I don't recall how much current is available from the trannie but allow for 4 amps to run a pair of KT88s about 3 for KT77s. The rectifier needs to be able to take the inrush as well. I use a 10,000uf cap after the rectifier and .1uf polys on the tube heaters as snubbers. The amp may have a slight increase in hum. I use a similar scheme in the kits but at 12 volts. You will probably have to put some series rectifiers following the filter as the voltage will be a bit high. These need to be high current types and will generate a lot of heat. At 4 amps each one will dissipate about 2.8 watts. They need air. That is part of the reason for the 12 volts vs 6. The current is half and the heat then is half. Shoot for 6.0 volts. 5.9 is fine and anything up to about 6.1 is OK. Going above that seems to cause some tubes to be wander thermally and will mess up the balance between them.

Oh, maybe I missed something. Edcor specs the XPWR105 power transformer at 6.3 VAC @ 3 Amps. Is that the transformer he is using?


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PostPosted: 29 Jul 2009, 16:16 
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Hi, No you didn't miss anything. The original Oddblocks use a SMPS to provide the 12 vdc. The 6.3 transformer winding isn't used. I can be used comfortably for the 6L6GCs and KT77s but is maxed out on the KT88s. The actual kits use an OEM transformer that has slightly different specs. 12 v not 6 and more current. This particular version is not available directly from Edcor. It makes the kits easier to build. In spite of the more difficult build, I personally prefer the SMPS as the DC is clean and well regulated. Also any noise on the dc is much easier to filter out. There are numerous SMPS that will work and the costs are low. Typically in the $15-20 range. The noise performance of the kits vs the ones with SMPS is virtually identical so both methods are viable alternatives (the transformer version has very slightly more hum and the SMPS a very slight increase in hf noise - both still typically below 1mv).

Good Listening
Bruce

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