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PostPosted: August 29th, 2019, 10:18 am 
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Joined: February 28th, 2013, 1:19 pm
Posts: 372
I will try over the next few days post more details on the design. I did not develop a schematic of the entire amp, but since it was modular, and based on well documented schematics, just selected values appropriate for the operating points selected.

1. PASS BA Complimentary Output stage - I just built this based on the recommended parts list in the DIYAudio.com Build Guide. I did buy tightly matched MOSFETS (IRFP240 and IRFP9240) from a reliable Ebay vendor (moniker: hotshotaudio). You can also get the same from the DIYAudio.com store for around the same price (the DIYAudio store was out of matched sets when I was putting together the amp). Also buy Keratherm pads, MUCH easier than using mica and thermal paste. I used KOA Speer metal oxide source resistors (2W/0.47 ohm/5% tolerance) and matched them, but if you want to be fancy, you can use metal foil, thick film, or whatever. The E-caps in the bias supply are Elna Silmic II.

2. Aikido stage. I have to review what I used, but I believe I used 470 ohm/0.5W PRP resistors for the cathode resistors in the voltage amplifier stage and a 150 ohm/0.5W PRP for the cathode resistor in the tail of the bottom tube of the White follower (I have to check on both of these values). Carbon comp gridstoppers throughout, as well as PRP or quality metal films for voltage dividers, ground references, etc.. Used audio quality film HV power supply bypass capactors (10 uF or so) at the board. Also used a 1100 ohm/10W wirewound resistor in series from this capacitor to the plate of the top tube in the White follower. I am using a 6N1P or 6CG7 input tube, with a 6N6 white follower, as previously discussed.

3. MOSFET Power supply - I used an ANTEK AS-4220 400VA/20V toroidal transformer for a stereo amp (300VA/18V is the base recommendation). I get a slightly higher rail voltage (24VDC rather than 22VDC), but possible to go up further. The DIYAudio Universal Power Supply board is built according to the DIYAudio.com Build Guide, and recommended parts list, only I left off the resistors between the two capacitor banks and used a choke instead (because I had them). The chokes are Hammond 159ZJ 10mH/5A. I did put in some large film bypass caps in parallel with the second bank of E-caps, these ideally should be located at the Output board. Recommend using the best, 105C, long life E-caps in the supply you can find. I believe I have some top grade Nichicons.

4. Aikido Power supply (one per channel) - This is a pretty basic preamp type supply, using a Hammond 250VCT/100mA + 6.3VAC/2A (I think a 269AX). Used a Broskie PS-7 power supply kit in a bridge configuration, replacing the supplied E-caps with high quality, larger value Nichicon at the output (kept the input cap the same value). Instead of the supplied resistor for CRC, I used a 10H/50mA (Triad) choke for a CLC supply. B+ is around 320V based on memory (need to check). Heater supply is AC, with a voltage divider of the B+ for 75VDC offset for noise.

5. Chassis - Used the 4U Deluxe chassis from ModuShop sold through DIYAUDIO.com.

6. You will need alot of 3mm standoffs, capscrews, etc. Really easy these days through Amazon.

Well, this fills in some of the details on the amp. Let me know if you need more info or would like me to check something on my build.

David


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PostPosted: August 29th, 2019, 5:15 pm 
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David, do you think these heat sinks are big enough for 25 watts?


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PostPosted: August 29th, 2019, 7:30 pm 
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What is the size, I don't have a very good sense of scale based on the picture. Also, the form factor doesn't look like it will work well for the stock boards. Looks better suited to TO-3 devices. There are 6 TO-247 devices per channel, so really tough to get them all on a heatsink like that. That looks like it could take 3 TO-3 devices. Not to say you couldn't fabricate an aluminum channel bracket to extend a mounting surface out, but the MOSFET board is about 10 inches long. My gut feel is that thermally, those together are probably good for one channel at 25W Class A.

David


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PostPosted: August 29th, 2019, 7:44 pm 
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That's the top of an ECC99 power left of the picture. They are around 9" tall, 6.3"square. Click on the pictures to see correct orientation.


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PostPosted: August 29th, 2019, 10:03 pm 
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Find the rating of the sink as specified in degrees C/watt and add about .25 degrees/watt for the case to sink interface and then add the junction to case factor of the transistor. Then multiply by 25 and you will get the temperature rise on the junction. The transistor spec will then tell you if that is adequate to keep the junction within working range. I would suggest that you stay about 20 degrees C below the maximum junction temperature for reliability. Note that the most efficient cooling is with the fins vertical and at least 1" space between the bottom of the fins and the chassis.

That sink looks similar to the Delta 441 sink I used on the Cherry Bomb that has a thermal impedance of .6 C/W. Lets just guess that that is the impedance of your sink and add .25 plus .75 C/W for the case to sink and junction to case interfaces. That gives 1.6 C/W. Multiply times 25 and you get a 40 C rise in the junction above ambient. Assuming your ambient will not go above 130 F or 54 C the junction should be around 94 C max. Normal operating max temp of a silicon junction is 125 C but that can vary. Check the spec sheet. If the max is 125 the 94 is the expected max it would still give a reasonable safety margin. BTW, the sink even at lower ambient temperatures will be uncomfortable to the touch.


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PostPosted: August 29th, 2019, 10:39 pm 
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Stuart,

The heatsinks are a bit larger than I thought. The BACO boards are 10" long, but the distance between the outside edges of the two outer MOSFETS is 8-1/2". So the heatsinks are just barely long enough for the devices. What I recommend is get a couple of feet of some 1/4" thick aluminum angle, maybe 2" x 3", but the long leg being enough to clear the heatsink. Then rip cut/machine the shorter leg of the L so that it fits inside to the center web of the heatsink. Using the existing holes, drill and bolt the angle to heatsink, using thermal compound. It will probably help to smooth and flatten the surface in contact with the heatsink (via sanding through grades against a flat surface like a table saw bed or surface plate. Of course, drill out the holes for the MOSFETs prior to mounting. Once you have the output stage up and running, and starting with a low bias, increase the bias over time until the MOSFETs are around 50 to 55C. Use a temperature probe on your multimeter (if you have this feature). Once that is set, you are safe. I think it will likely work just fine, though it will be ugly. Just put a cage around it.

David


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PostPosted: August 30th, 2019, 6:58 am 
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Joined: January 14th, 2015, 11:15 pm
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Take a look at post 1724 and some of the following posts on this large version of a MOFO.

https://www.diyaudio.com/forums/pass-labs/313649-build-mofo-173.html#post5661773

If size and weight are a problem I would look at fan cooling. Some of them are apparently very quiet these days.

https://www.amazon.com/ARCTIC-F8-PWM-PWM-Signal-Configuration/dp/B00H3SWJ24

On a separate note, my health is improving and I am working on a 6SK17-V - powerdrive - 45 amp. Never used 45 tubes before which will be interesting. There has been progress on my wife's resident visa for Australia and I drove all the way from North East to Bethesda and back yesterday for her medical exam. It reminded me how I hate driving in big cities now. After 4 - 5 hours driving, I slept really well last night.

ray


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PostPosted: August 30th, 2019, 10:08 am 
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Joined: March 12th, 2013, 11:12 am
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Glad to hear you are feeling better. If tubes can be generalized as having an intrinsic family "sound", I would say the 45 is the best sounding tube overall I have ever heard.


ratbagp wrote:
Take a look at post 1724 and some of the following posts on this large version of a MOFO.

https://www.diyaudio.com/forums/pass-labs/313649-build-mofo-173.html#post5661773

If size and weight are a problem I would look at fan cooling. Some of them are apparently very quiet these days.

https://www.amazon.com/ARCTIC-F8-PWM-PWM-Signal-Configuration/dp/B00H3SWJ24

On a separate note, my health is improving and I am working on a 6SK17-V - powerdrive - 45 amp. Never used 45 tubes before which will be interesting. There has been progress on my wife's resident visa for Australia and I drove all the way from North East to Bethesda and back yesterday for her medical exam. It reminded me how I hate driving in big cities now. After 4 - 5 hours driving, I slept really well last night.

ray


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PostPosted: August 30th, 2019, 7:07 pm 
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David, thanks for the angle aluminum idea. I hadn't come up with a more elegant solution.

Tom, thanks. I do know how to do heat sink calculations. I don't know the rating of the sinks I have. They are twice as tall as the Wakefield sinks you are using and an inch or so larger in each of the other dimensions. It might be reasonable to assume 0.3°C/W. Just not sure.

Probably okay for 50 watts each, or less with longer mosfet life.


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PostPosted: August 30th, 2019, 7:35 pm 
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Stuart,

You seem to be set with the heatsinks you have, a little bit of creative energy and you should be able to get something working. However, I ran across a source for large flat heatsinks similar to what I have on my amp chassis:

https://www.mpja.com/15-3_4-X-7-13_16-X ... 3306%20HK/

They are not necessarily a bargain ($60 each), but reasonable without ordering China or Italy.

David


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