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kevin gilmore
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I like that chassie a lot ...reminds me a bit about that kids toy where you have to balance a ball through a maze using some handles on the side ....nice wood work ...did you use a router/template or did you do it be hand?

My KGST is running too hot.  I can't put my hand on the top plate for more than 3 seconds.  I did drill some holes on the bottom plate and many holes on the top.  Should I put a lot more holes on the top plate?  Or would it be better to do more holes on the bottom?

 

20150603_203522_zpswpfqhgon.jpg

 

20150603_203640_zpsyxgudszu.jpg

 

By the way, it sounds amazing when it's really hot.

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I must say, my KGST built by Geoff runs surprisingly cool.  He made it to swing 400v's and added some smart heat management.  I can lay my hand directly on the cover, over the tubes, after it's been running for several hours without discomfort.  It's warm but not at all hot.  I've been super pleased with his execution of the KGST design.  My living room stays between 75 and 80 degrees F.

 

HS

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Hi!

Finally, i got my boards populated, preliminary tests of the PSU looks good with 361v and 367v unloaded. Before i can test the AMP boards i must get some work done on the case. The reason is that i will have the CCS mounted off board.

The main reason is the a got a nice surplus case with excellent ventilation, the only problem is that its slightly narrow and the amp boards does not fit side by side. The case it rather high and the sides consists of heat sinks, so one solution is to mount the amp boards vertical on the side wall and the ccs off board on the side wall heatsink. The added bonus is improved cooling with the heat dissipated directly outside the case. 

 

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I have some concerns though, In the most natural position the tubes will be mounted horizontally with one tube over the other. They might see quite different thermal conditions with the top tube right in the way of the bottom tubes convection. Will that be of any concern?

Another unrelated question: Each psu board has two ground terminals to a common ground for both power and signal. I have some troubles finding a grounding scheme that does not introduce a ground loop. Is there a 'standard ' grounding scheme? How do you do it?

Best Regards Christian

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Regarding your tube mounting, from Radio Designers Handbook, Langford-Smith;

"Where it is necessary to depart from vertical mounting, ............................................. the plane of the grid side rods (or major axis of the control grid) of indirectly heated valves having a high transconductance and/or a long unsupported cathode, should be vertical. This reduces the possibility of filament- and cathode-to-grid short circuits and microphony in filament valves."

 

6s4a data sheet; http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_2/6S4-A.PDF

 

I don't think you would class 6s4a as particularly high transconductance, so this is probably not an issue. Don't quote me, but I think the grid plane would be horizontal in your scheme, I'll let you decide that one! Maybe someone else has mounted them horizontally? As you point out, heat could be a problem, but until you try it, you won't know with that either!

 

Is there some reason that you can't spin the boards by 90 degrees and have the tubes side by side? After all, the boards are shorter that way.

 

The other consideration is the sideways force on the socket, but then these are short and light enough for that to not be an issue, and I don't suppose you'll be bouncing your chassis up and down!  (Larger tubes sometimes have additional support.)

 

 

 

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Well, the data sheet says operating position: any, so i do not think that is any problem.

The absolute heat will not be a problem either, the case is extremely well ventilated.

My worries are that the two devises in a ltp operates at different temperatures. I am used to mounting ss devices in an ltp rather close on the same heatsink to keep their thermal drifting similar.

Do tubes have the same issues as ss?

Regarding position of the board. If I rotate the board it will be difficult to mount the ccs off board keeping the wire length short.

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Ooops, should have read the data sheet. Would have saved me some typing! I see your problem with rotating the boards.

 

I can't see you ending up with a massive temperature difference between the tubes, but even so, won't you be adjusting the balance at it's stabilised temperature? OK, if one tube's getting much hotter than the other, then it may shorten its life, but these tubes are cheap.

 

Bet you wish you'd built the KGDT!

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The tubes are cooled by IR radiation only, the glass envelop absorbs 25 to 75% of this radiation (based on other estimates om the net) so i´m guessing 50%, including re radiation. I think it is reasonable to estimate 50% radiation and 50% convection for the tubes. Only the convection are affected by the horizontal mounting. I took some help from a engineering friend who did a  quick and dirty simulation at work.

 

Based om the the above assumption and max power plus filament we arrived at a envelop temperature at approx 210 Celsius for both tubes the upper tube are cooled by hotter air but it has higher velocity from the convection from the lower tube. It all ads up being the same temperature for both tubes.

 

So based on a 10 min simulation done by two idiots without any knowledge, there is no problems....

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Tubes don't really mind a bit of extra heat, unlike transistors - there's a reason they are called thermionic valves.  Cathodes are  heated to around 425-600 degrees celsius, so what difference does a few extra degrees in ambient temperature make?  

 

If you don't believe me, do a Google search for Audio Research D250 amplifier picture - you'll see rows of 6550 tubes mounted horizontally with one row above the other - and those tubes generate a lot more heat than a couple of 6S4As.

Edited by JimL
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Thank for you replies!

 

I have learned two things. 1. The tubes will have similar temperature. 2. It doesn't matter.

 

Still wondering about the grounding scheme though, with two grounds on the PS ant two in the AMP board it is easy to create ground loops.   I´m thinking about using the output terminal on the PS as a star ground for signal and and power, connecting only one ground lead to the AMP board. That star ground will be connected to chassis and earth. XLR Pin 1 will be connected to chassis.

 

Any thoughts.

 

B.R Christian

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I think kevin described how he implements ground in the dynatwo thread, might want to check out the last few pages for that

I don't really follow the best practices for this, I've found that the only thing that makes a significant difference is whether I ground the xlrs properly or not. for me this means grounding pin 1 and the ground tab to chassis.

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Thanks JoaMat.

 

Yes, it looks very similar. The difference is that i my scheme the amp grounds and the chassis star ground are connected at the same terminal at the PS. Question is which GND to use on the AMP boards. I'm thinking the GND at the HV and output terminals, but i am not sure.

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Now I have a good plan for the ground and case layout. Unlike my other projects, this amp will get at really nice case, hopefully I will get the time to finish it this weekend, pictures will follow. 

 

I have a spare EI transformer and the next projects is growing in my head, JimL:s SRX. When going through his excellent documentation (thanks JimL) i cant stop thinking that his cascode CCS make sense for the KGST as well and I will probably make room in my case for a future cascode CCS based on Kevins new circuit on an add on board. Is this a good or bad idea?  

 

B.R Christian

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Yes there is a boards for it, but I already have a populated boards from the first group buy and want to make use of it. I already plan to mount the  CCS of board so it would be easy to make a add on board with Kevins new css and connect it to B+ and pin 9. I would then remove the resistors not needed. I will get the amp running unmodified but I will mount the 10m90s so it i have room for the add on board if I want to.

 

I also wondering abut the procedure and measurements for adjusting the cascode.

 

B.r Christian

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You can adjust the cascode without hooking up the high voltage power supplies, by using a 15-20 volt supply (2 nine volt batteries in series will do).  Connect the positive terminal of the battery to the positive PS terminal on the board and the negative battery terminal to the end of the 100 ohm resistor closest to the 100 ohm trim pot.  Then put a meter across the 250 ohm resistor and adjust the trim pot until you get 2.5 volts, which corresponds to 10 mA for the current source.  If you can't get to 2.5 volts, decrease the fixed resistor to 200 ohms,  and adjust the trim pot until you get 2.0 volts across the 200 ohm resistor.  If the voltage is always higher than 2.5 volts for the 250 ohm resistor, replace it with a resistor around 300 ohms and adjust the trim pot to correspond to 10 mA.  

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Forgot to mention, that this is apparently the first KGST build using the boards with cascoded CCS from MWL168 GB.

/S 7 july 2015

 

 

 

Began building my KGST friday, starting with the PSU (using the one from KGSSHV).

Powered up the PSU HV sunday. All seemed honkydory ....hit +/- 351V ....and stable using 10k loads ....had it running for an hour or so, no problems.

Powered up the LV +/-15V ...then I got the crazy idea of checking the BIAS ....480V ....what?? hmmm ... found my fuck up ....had mixed up some zeners.

After replacing those, I got 581V.

 

Then the amp boards .... assembled pretty quickly ....it is a lot easier than the KGSSHV onboard in my opinion ...anyways ....finalized them this evening, and powered both up.

Apart from starting out having -350V on the output and then in 30 secs or so quickly falls it seems stable. Adjusting the balance/offset it sits at 0-5V at all outputs ...

 

Here's a few photo's

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OBey4GV.jpg

 

 

Hfv9NnT.jpg

Edited by sorenb
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