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Rob11966 18th October 2007 12:52 PM

KT88 Minimum Mounting Distance
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.


traf 18th October 2007 01:11 PM

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.

Brett 18th October 2007 01:11 PM

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.

stoolpigeon 18th October 2007 01:13 PM

Hi Rob,

I have a friends TVA-1 which uses KT88 spaced at 4 inches.


resident 18th October 2007 01:39 PM

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!

richwalters 18th October 2007 02:59 PM

The old Bell & Howell cine projectors used KT88's that were so crammed in with other components to be nearly touching. And they still ran on.

Rob11966 19th October 2007 03:14 AM

That's all reassuring news. I will proceed with my design. I liked the account of the projector but I presume it had some enormous fan cooling it down?

gingertube 19th October 2007 03:37 AM

2 Attachment(s)
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!!!


Rob11966 19th October 2007 10:50 AM

Hi Ian,

Yes, I do have a copy. To be honest for some of the issues that I have encountered, I have found the experience of this forum a little more practical - not to say that I haven't found 'Building Valve Amplifiers' useful of course!



resident 19th October 2007 01:03 PM

Ian you're totally right.
It's better if the tubes are at 90degrees each other!

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