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  #1  
Old 26th July 2007, 08:41 PM
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john & Jake the dog john & Jake the dog is offline
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Default 84 heater mod

Hi,
I've just completed the heater rectifier mod in the Kel84, many thanks to Terry(systemx) who very kindly sent me the required bits.
Very fiddly to make bearing in mind that nothing can be allowed to short.
There is a lot of room backwards of the circuit board and I feel that it could benefit from a slight re-design laying it horizonaly rather than verticly.

The amp works though, but I will have to have a think about how it can be laid out a little neater.the 84 is a neat amp and this is a little Heath Robinson in my mind, I put it together remember,
I am not sure which amp is shown on the FAQ but that confused me as well, not knowing which amps have circuit boards apart from the 84.
rectifier1.JPG
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Old 26th July 2007, 08:52 PM
systemx systemx is offline
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Default Re: 84 heater mod

Well done, I ment to tell you that the resistor value is correct at 1 ohm. the amp in the photo looks like an 88, the fuse blowing bit also applies to an 88/6550 I think, I had no probs with mine. Yes it is a fiddley layout, but I hope you are pleased with the results.
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Old 26th July 2007, 09:02 PM
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Default Re: 84 heater mod

Hi Terry,
You did mention to me that 1r was correct, thanks again for the bits as without them I wouldn't have done this for an awful long time.
I'll be able to listen extensively in the morning to see if I hear anything different.
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Old 30th July 2007, 10:20 PM
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Default Re: 84 heater mod

Hello John,
Anything to report on this mod? I was thinking of doing this one but i have not seen any posts where anyone has described any changes/improvements to sound quality made by it.

Regards, Phil.
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  #5  
Old 30th July 2007, 11:41 PM
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Default Re: 84 heater mod

Hi Phil,
Funny mod this, I can't put my finger on it but something has improved.
Listening today there was change in realism I think, hard to explain as I just can't pin it down, speed possibly, notes starting faster and cleaner.
My advice would be to do it and see what it does for you, if you can describe it please let me know
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Old 30th July 2007, 11:53 PM
bikerhifinut bikerhifinut is offline
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Default Re: 84 heater mod

Quote:
Originally Posted by john & Jake the dog View Post
Hi Phil,
Funny mod this, I can't put my finger on it but something has improved.
Listening today there was change in realism I think, hard to explain as I just can't pin it down, speed possibly, notes starting faster and cleaner.
My advice would be to do it and see what it does for you, if you can describe it please let me know
Do you think a lowering of noise due to cleaner dc to the signal valves is helping with low level detail? Ie low level signals not being masked by noise induced into the cathodes from the heaters? So you hear the start and end of each note properly and more of the decay?
Its a theory thats all.
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Old 31st July 2007, 12:51 AM
systemx systemx is offline
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Default Re: 84 heater mod

John and Jake.
Next step the coupling caps maybe. I would not recommend the shunt volume untill you are happy with your speaker choice, the 5.5db loss maybe too much at the moment. I did try running all the valves on the mod (the diodes are rated at 5A) for a while, but there was no improvement over just sig valves, to be honest, I did not expect any.
On reflection the feedback adjustment turned out to be a step backwards, while it opened the sound stage a bit, the bass from the WD25t's was less controlled, almost loose. Replaceing R10/11 and R12,13,14,15 did not affect performance

Terry.

Last edited by systemx; 31st July 2007 at 09:48 AM.
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Old 31st July 2007, 09:47 AM
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Default Re: 84 heater mod

hi,
yes that's probably it, fits the theory I think.
Hi Terry,
coupling caps definitely, I've started, so I'll finish
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Old 31st July 2007, 10:57 PM
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Default Re: 84 heater mod

Hello John,
I've done the mod, but not on my KEL84, on my phono stage. Partly because i thought any difference might be clearer because of the low signal levels but mainly because the PSU of my phono stage is built on tag strip so much easer to mess with! I have to agree with you, the effect just to start with is not that obvious, but after a few minutes i became convinced i was hearing more low level detail. A lowering of the noise floor sounds right, even though there was no obvious noise there before. I think my 84' will get this treatment as well.

Regards, Phil.
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  #10  
Old 1st August 2007, 12:12 AM
systemx systemx is offline
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Default Re: 84 heater mod

Phil
I think the diodes have a lot to do with it, the idea that it stops switching noise going into the tx sounds more likely.
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