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Old 17th December 2012, 12:49 AM   #51
roger57 is offline roger57  Canada
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Quote:
Originally Posted by Salas View Post
Sense and force look for the nearest return point that the consumption node refers to. If you have a sub star for the 4 cathode resistors, take those lines there, and tie input separately where the PSU that feeds the reg returns.
That's a bit tricky. I designed a "bias PCB" that's on the back of my amplifier. It contains a fixed resistor and a pot in series, both bypassed by a 220uF electrolytic. Each tube has a bias control. The output transformers are from a Sansui AU-111 that used an 8% cathode feedback tap winding. They come out of the transformer as an orange and white lead. One of them ties to ground internally. The transformer also has a separate ground wire. I used the transformer's ground wire to the star ground point, as the original AU-111 did this as well. They must have had good reason to do this (not tying the orange or white lead to GND), so I did not want to change.

In short, the ground post is the closest point for the cathode resistors, via the transformer. Everything, even the power supply ground coming in, is tied to the ground post. My thinking was that if all grounds return to this common point, then everything should be equalized. It must work well because the amplifier has a -89dB SNR which it would not attain with poor grounding. This tested on an HP8903B audio analyzer, in calibration.

I confirmed about an hour ago I may have toasted more than 1 power tube.
This evening I will replace all the power tubes with a used set, and retest using the Maida on screens and the SSHV2 on the driver/splitter. I will then try the 1uF cap on the SSHV2 screen regulator, along with the higher cathode resistors. I'll keep you posted Salas and thanks for the help.
If nothing else, this may be helpful information for someone else trying the SSHV2 in the same application.
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Old 17th December 2012, 02:19 PM   #52
Salas is offline Salas  Greece
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It can take a 100uF output cap with 0.2-0.3 Ohm ESR as your before config too. 1.8R position be a short in that case. Limits the reg's bandwidth but keeps the flat extended low Z.
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Old 17th December 2012, 03:50 PM   #53
roger57 is offline roger57  Canada
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Quote:
Originally Posted by Salas View Post
It can take a 100uF output cap with 0.2-0.3 Ohm ESR as your before config too. 1.8R position be a short in that case. Limits the reg's bandwidth but keeps the flat extended low Z.
Salas,

I have again discovered a problem where I can only point to me

During the course of installing the new regulators, I updated the pre/driver PCB. There are "unmarked" connection points on the PCB where the coupling caps are connected. During reassembly I inadvertently swapped on one channel those tie points. No wonder the cathodes were going a bit crazy. I had to think about where this exact incident had happened before. I remembered that during my amplifier bench prototyping 3 years ago, I did the same thing. Once I recalled that, I looked for a swapped connection and discovered it. I had a very late night, last night. I should have realized this much sooner...I went as far as re-connecting my old power supply regulator PCB and once seeing the same thing was happening, knew it was something DUMB!!!!

After reconnecting the SSHV2's, I'm happy to say that your SSHV2 is STELLAR driving the screen load! Absolutely no issues, even with 100 ohm screen resistors. The cap is a 0.47uF (did not have a 0.33uF)

So now the only concern is spot heat on the IRF840. (see attached picture) The other PCB is the DC filament regulator w/cathode reference and fan supply. The chassis has a low volume, super quiet fan to move air through it from the bottom out the right side.
After running it for about 15 minutes with the amplifier playing, the IRF840 is measuring about 65C to 72C, measured directly on the metal tab of the device with an IR tool. I'm thinking this is a bit high, and would like to get it down. I'm presently using a mica insulator with thermal compound, and it's mounted on a flat aluminum surface That heatsink is heavy; the fins underneath extend at least 3cm from the backplate.

I'm thinking of trying a higher thermal conductivity silver bearing paste, but this might not do much as the thermal conductivity of the mica insulator may be the limiting factor. Do you know of a better method to use?

Is 70C a bit high for the IRF840? In your experience, what should the maximum ever be for reliable operation? Going by the data sheet it appears the de-rating is significant past 70C.

Gary
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Old 17th December 2012, 04:24 PM   #54
Salas is offline Salas  Greece
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Its customary you get a troublesome mishap but all works for you soon. Happy end is what matters.

65-72C with 1C/W RthetaJC device like the 840 is not too taxing if it does not burn more than 10 to 20W on average. It breaks at 125C internally, but less average means more life in all electronics. It should get its max standing temp on idle signal. How high it goes for 840 case temp if you leave it alone without signal? Say for 30 mins. Look for German Keratherm insulators. Red. Watch out for some conductivity with Arctic Silver and the like, this is HV. Some state they are zero conductivity, others state some degree.
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Old 17th December 2012, 05:23 PM   #55
roger57 is offline roger57  Canada
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Quote:
Originally Posted by Salas View Post
Its customary you get a troublesome mishap but all works for you soon. Happy end is what matters.

65-72C with 1C/W RthetaJC device like the 840 is not too taxing if it does not burn more than 10 to 20W on average. It breaks at 125C internally, but less average means more life in all electronics. It should get its max standing temp on idle signal. How high it goes for 840 case temp if you leave it alone without signal? Say for 30 mins. Look for German Keratherm insulators. Red. Watch out for some conductivity with Arctic Silver and the like, this is HV. Some state they are zero conductivity, others state some degree.
Customary?? I hope it's a custom that ends pretty soon!!

This is the thermal paste I was referring to:
Noctua.at - sound-optimised premium components "Designed in Austria"!

It's NON-electrically conductive (I mentioned silver by mistake) so I'll give that a try. Those thermal pads you mention aren't carried here. The best alternative I could find is this:
http://ca.mouser.com/Search/ProductD...ualkey532-4170

4170G, made by Aavid. They are aluminum oxide and appear to have a higher thermal conductance than the mica. I'll order them and see how they work out.

I will also try the test you are suggesting; I'll idle the amp with no signal and check the temperature after 30 minutes. I'm probably over-worried, the maximum device temperature (Vishay) is 150C, and I'm not half way there yet. Although your sentiments are exactly mine on the issue of heat: in a case such as this, cooler performance results in better "in-spec" performance and longer life.

The real report I want to post is...the sound. Hopefully I can do that in a day or 2.

Last edited by roger57; 17th December 2012 at 05:32 PM.
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