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KT88 Minimum Mounting Distance

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Hi Folks,

Need some advice. I am just making up a chassis for a KT88 70W power amp (Dynaco MK III) with 2 KT88 output tubes. I am having a spot of bother with the layout. For various reasons that are not worth getting into, I can only get about 3 inches from center pin to center pin. The factory specs recommend a minimum distance of 4 inches. With my layout there is at least 3cm (sorry about the mixed units) between the tubes even between the widest part of the glass envelope.

What do you guys think - is this far enough apart for heat dissipation (the tubes will only ever be running at 75% of there maximum possible output even at full volume) or do I need to do a major redesign? It seems to me that many of the commercial amps space the tubes much closer than the tube manufacturers recommendations.

Has anyone had any personal experience?

Thanks in advance for the advce.

Rob
 
My old Beard P101 operated quite happily with about 10mm between each of the 8 KT88s. My current Primaluna makes do with about 8mm or about 2.25in between centres.

I would have thought that provided you have decent ventilation top and bottom to allow a convection current (air not elec!) to be established you should be fine.
 
Should be fine. It seems I can barely get a sheet of paper in between the O/P tubes on my Fisher receiver and they were original factory when I got it and sounded fine. I use one bottle diameter between tubes as my minimum in my own hifi designs, and add forced air or convection cooling on the same centres for pro/MI gear.
 
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Joined 2004
Do it! 3cm are fine.
Some guitar amps place the tubes really close too!
It'll be great for the tubes to have the same temperature! haha
.... I just remember this from one guy that was placing the tubes really close for this reason! Matched heat! :D

I don't mean it!
 
Rob,
Do you have a copy of Morgan Jones "Building Valve Amplifiers"?

In there it discusses how the heat from tubes like the KT88 and similar is mostly thrown out 2 sides, which are the flat sides of the tube anodes. If you have the KT88s less than 4" apart then at least make sure the anode flats are to the side and not in the line running through the tubes.

In the picture attached I think that means that the holes you see in the outermost structure should be in the line through the tubes. That is the heat will mostly be radiated from what is the left and right sides of the picture.

Others: Please correct this if I've got that wrong!!!

Cheers,
Ian
 

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Hi Ian,

This sounds important to me but I am not quit6e sure that I understand. I am mounting the tubes vertically and in a line next to each other. Am I correct in assuming that the tubes are mounted so that pins 4 and 8 run along this line so that looking from left to right the tubes would be mounted in such a way that looking from the front, the far left side of each tube would be pin 4 and the far right would be pin 8 and that this arrangement would direct most of the radiated heat forwards and backwards?

Cheers,

Rob
 
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