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 Post subject: Re: My KT88 Build
PostPosted: 06 Sep 2017, 21:00 
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Joined: 30 Oct 2015, 07:44
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Thank you very much. Something reasonably pleasing to look at, serves a purpose and sounds great :-)


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 Post subject: Re: My KT88 Build
PostPosted: 07 Sep 2017, 04:58 
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Joined: 01 Jun 2013, 09:05
Posts: 599
Nice build,

Just a question,
You obviously have, but where and how have you bonded the metal top plate to the supply Earth?
What kind of bottom cover have you used if metal is that also Earthed?

Just interested, nice joints on the enclosure. Yes its a PITA when you try to sand hard wood imbedded in softer material.. :D
The bit you don't want to sand always grinds down first!

NB the link you provide for the schematic does not seem to show a discharge resistor for the power supply, if you don't have one its a good idea to fit one! Just be careful not to exceed the tube rectifier current. I would put it on the second PSU cap after the choke.
Remember the discharge resistor is a high voltage part depending on B+ voltage, In my builds I use 470K 5watt Kiwame but its build dependant vs how fast you want to pull the B+ down after power off.
(The discharge resistor can run warm so fit in open space).

Just might be of interest.

Regards
M. Gregg

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 Post subject: Re: My KT88 Build
PostPosted: 07 Sep 2017, 06:33 
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This is just for interest,

Regarding PSU discharge:
Many people rely on the current draw of the power tubes after power off to pull the B+ down, however if for any reason the heaters fail or the amp is powered up with the tubes removed the PSU will stay up at a voltage for quite some time!
(I had a shock on a guitar amp the next day after power down) In an extreme situation my friend got thrown off a TV set that had been off for a week but the cat-strap had been removed. But that's just an unusual situation.

Test before you touch anything>>me looking inside a friends tube preamp, yes it was that resistor I changed, look this one here..ZAP! :D It had been off for half a day.

Blown fuses in PSU rails can also cause this situation.
Some people use LED's in series with discharge resistors so they can see the B+ come down.
There is a sonic impact with discharge resistors which is least noticeable before the choke in the PSU, however with tube rectifiers the lower the charge pulse the less stress it puts on the tube.

The last thing we want is the system powered down and a child lifting the amp with an open base and copying daddy putting their hands on the floating B+. At least if there is a path to discharge the longer its off the lower the voltage will be. This extends to pulling tubes and putting hair clips in the sockets or animals jumping on the equipment (hence the tube cage) but damage limitation is probably better than nothing.

Regards
M. Gregg

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 Post subject: Re: My KT88 Build
PostPosted: 07 Sep 2017, 18:57 
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Thank you M. Gregg,

The chassis top is grounded. It's a bit difficult to see from my photos but if you look closely, there is a stud installed at the back of the plate. The metal surface was masked off before priming and I did treat it with an anti-corrosion product (No-Ox) just in case the steel and the stud decide to make a galvanic mess. The stud has ground lug, a star washer and a nylon lock washer on top to keep it from backing out. It's then tied directly to the ground pin on the IEC connector at the back of the chassis.

I'm still working on the bottom cover. I had considered another simple metal plate with some ventilation holes (again with a ground stud to the IEC) but, honestly, once I put this project away, I'm thinking metal bottom might be over-kill and besides, it already weighs a bunch. I'm considering a simple hardboard bottom with ventilation holes. I'm also working on some ideas for legs or supports to get it up one-half to one inch or so. Possibly some very short square tapered legs.

I do thank you for bringing up the bleed down resistor...in my haste to get something operational I did completely forget this. Thanks for the reminder! It was impressed upon me many many years ago to always assume it's charged up and can bite. I didn't think much about the possibility that someone else might crack it open to have a look one day.

M. Gregg...just curious...with your approach to a bleeder resistor, what sort of bleed down time in your mind is a safe one? I'm thinking something close to a minute or so. I think this because I imagine what it would take for someone to lift this beast, turn it over, tear off all the warranty will be voided stickers and take the bottom off I welcome your opinion or anyone else' for that matter.

My max charge will be something near 450 to 500 VDC. After the supply stabilizes it's at about 415 VDC. Capacitance is 100uF. My wild guess would be a 500K resistor should bring voltage down to a safe number in approx 1 minute. My math is a little rusty but I think that's about right.

Again, thank you for the thoughts, comments and feedback. Very much appreciated.


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 Post subject: Re: My KT88 Build
PostPosted: 08 Sep 2017, 05:04 
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Regards the bleed down time,

The most important thing is that there is a bleed down path ie resistor.
The time is a trade off because adding the resistor also adds a load current to the rectifier tube.
So you might over load the rectifier by trying to bleed down with a low resistance.
I would use a 470K 5 watt the 5 watt is over kill but you don't want a failed resistor so over rate it.
Remember the resistor working voltage is VERY important it needs to be higher than the B+ voltage or it will flash over and short the supply. So as long as its down over about 3-4 mins remember at half that time you will be down to about half supply, the tubes should be drawing current anyway. The other thing a bleed resistor does is prevent dielectric absorption recharging the PSU, this happens when people use a fast discharge probe then remove it, then the caps seem to recharge.

Obviously you can bleed down faster with a diode bridge because you can use a lower resistance without drawing over current through the rectifier.

Regarding the bottom plate I would use aluminium light weight with some vent holes, the reason I don't use hardboard etc is because the aluminium will conduct heat out of the enclosure, hardboard will insulate the heat and increase the temp inside the equipment.
I find thin checker plate quite good but each to their own preference. (Pick)

Regards
M. Gregg


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 Post subject: Re: My KT88 Build
PostPosted: 08 Sep 2017, 06:50 
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I guess I should add this link:

https://en.wikipedia.org/wiki/Dielectric_absorption

Regards
M. Gregg

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 Post subject: Re: My KT88 Build
PostPosted: 08 Sep 2017, 07:23 
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Joined: 30 Oct 2015, 07:44
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Great info,

I thank you. Great read on the absorption thing. I remember a bit of this from my younger days. Back then all that theory was just "theory". Some of those things taught in school are finally coming full circle to reveal some useful meaning.

That bottom plate looks great...I'll have so see if I can source some light weight sheet material like that. That's spot on for what I was hoping for.

Again, thanks!


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 Post subject: Re: My KT88 Build
PostPosted: 17 Sep 2017, 18:23 
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Joined: 30 Oct 2015, 07:44
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I surely hope that I haven't bored everyone but I had a couple last photos I thought to share. I have added the bleeder resistor for safety...I've also added a bottom cover which has been grounded as well. The feet are simple blocks of walnut that I split then laminated some purple heart between for a little visual interest. Today I placed it into a cabinet where for the entire day it has been on and cooking away. I think I'll call it finished.

Image
Image
Image


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 Post subject: Re: My KT88 Build
PostPosted: 31 Oct 2017, 08:10 
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Joined: 22 Jun 2009, 10:36
Posts: 15
beautiful build but I was wondering about that cabinet, doesn't it get pretty damn hot?


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