1000a power resistors

larryderouin

I'm VERTICAL and Breathing...most of the time.
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I've been running mine for about 4 years now and noticed that the tops of the 30W 600oHm and 6Kohm Power resistors (these are the two monsters stuffed in between the power transformer and one pair of 7591's) are chipped and the ceramic has burn marks on it. I'm worried about them going into a thermal meltdown as I use the 1000A about 6-10 hours a day. right now I have a fan directed at the back, but the heat given off by them is like a blast furnace. It can't be good for the 7591's and the transformer. I've got Aluminum HVAC Duct tape on the one side of the transformer to reflect some of the heat from it, but obviously that won't work for the tubes.

I'm thinking of replacing these with something higher wattage to help with the heat given off by them (they are hotter than the 7591's @ 35ma).

I looked thru mouser and found the Vishay RH-50 series chassis mounts. Only thing is the closest to 6K is 6.2Kohms. And the only 6Kohm 30 to 50 watt is an ohmite (non stocked and 6 week wait time). As this feeds the tuner sections (drops from 420V to approx 100V) will the 200 extra ohms drop the voltage too much to the tuner. I'm no good at the math for this stuff. So I need someone to figure it out for me, please. I'm no good with letter math (flunked Algebra).

I figure I can mount them directly to the chassis in the middle in front of the 6AN8's, with Heatsink grease between them and the chassis (one big heatsink) like the newer versions of the 1000A does. Or mount them on the divider plate, between the front and back of the chassis. I think either will work ok as a heatsink for them. They should run cooler than what is there now as they won't be running close to flat out, maybe 60%.


This was taken when I got it and before cleaning. The Resistors are between the lower right 7591 and the P.T.
100_1149.jpg

Excerpt from the schematic. R179 and R181 are the ones discussed here.
Sansui 1000A Power supply mods.JPG

thanks

Larry
 
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Sure seems like it would be a lot better if you could have a big choke to replace some of that 600 ohm R and another PS Trans. to take care of the 100V to the tuner. Sure more $ plus you would need a small chassis, but I bet sonics would be better plus less heat back there as the stock PS Trans. should run cooler also.
 
Avionic; I didn't think about them in parallel. Good idea. I could use that divider plate as a heat sink for the 12k's and the chassis itself for the 600 ohm.

Shadow; If I touch the opposite end of the transformer it's relatively cool compared to the side with the tubes and the R's. Current thinking is if I move the resistors away from their current location, it'll help keep the tubes cooler, along with the trannie temps being pretty much the same all over. Plus if I get higher rated wattage resistors, they won't run as hot.
And I want it as stock as possible.

Larry
 
Read your post, one thing to try.

Remove the divider from the chassis and drill a neat pattern of 1/4 inch holes in it, in the space where the OP tubes and the resisters are, and place two small computer fans on the back where the OP tubes are (fans blowing OUT).

This mod. will really cool down the 1000A. :yes:

Mark T :music:
 
Thanks Mark. What I'm worried about is the chipping/cracking of the resistor bodies up by the Mica washers. Eventually they are going to give out completely and I'd rather proactively replace them now, rather than have one short out and have ot rebuild the whole power supply(or at least a good portion of it).

I've already got 2 3" fans pulling out from in back of the OP tubes with the case on. I can feel the resistors getting loose in the mounts(grating and physical looseness). when I rotate the tubes between it and my 800-C when I rotate the units around.

I'll give the "holy divider" some thought also.

Larry
 
Larry, Not sure if you remember me but you sent me a schematic of the 1000a sometime back. Much appreciated. My 1000a four pot has had one of those resisters replaced by a prior owner, in what clearly was a half a$$ job. I have not got to the resto yet, but will be very interested in how you deal with this issue as I will need to address those resisters on my restoration in the future. Please keep us posted on how what you come up with. Thanks again!
 
wweigle; I went with avionics idea of 2 12K parallelled (you'll need 4 total) anda 600ohm same type only 50w). Got them in mail y'day. I've got my TA-600 on the bench right now, so I'll have some time to think about how I want to mount them.

I'll keep updated.

Larry
 
Schematic somewhat misleading on power levels.

The 600 Ohm section of the large power resisters need not be 30 Watts when used as a SEPARATE part, a 10 Watt "sand" resister will work.

The reason (I think) they rate it at 30 watts is because it is PART OF another resister that gets VERY hot and it needs a rating of 30 Watts to work and not burn out.

Being a separate part, the Wattage is less (I had a gassy tube go bad suddenly while testing the set, and it took out the 600 Ohm section fast!)

The solution was to put a 600 Ohm 10 Watt resister "below deck" and measured less than 10 Watts power dissipation.

Mark T. :music:
 
Thanks Mark. I'll put that away for future reference. I had already ordered the 30W sales and they came in Tuesday. I just finished up the rough work.

Removed the separate 6K and mounted 2 12K in parallel on the backplate between the tubes and the can caps. I had forgotten I had cut it down to allow for air flow. So only had room for the one pair.

The combo set I ran the wires up to the dial backplate, drilled a few holes and mounted them. Then soldered all the wiring to each set. Still have to take pictures and hook it up and test it out, but running the wiring against the schematic is right. Maybe later today after "HONEY-DO's". Gonna have to get some liquid electrical tape and paint the connections too. They are out in the open with the top off.

Larry
 
PICTURES'

I brought it up on a variac with a DBT plugged in (150W bulb). Everything fine. and it works. Ran about 15 minutes and the resistor bodies got warm, not hot. I added heatsink compound between the resistors and the panels they are screwed to.

Underneath. The blue, Black, Green twisted bundle are the new leads for the 600oHm/6KoHm PKG. Blue to front side of 600 and 6K. Green to back side of 6K and black to backside of 600oHm.
100_4259.jpg

The 6K group and 600oHm on the DIAL Back panel. Left enough slack to remove panel for dial work if needed.
100_4260.jpg

The Separate 6K group. There is about 1/16th" between the resistors and the power transformer.
100_4261.jpg

Overall on deck. I used 4x40x1/2" screws & nuts. Dremeled the ends of the screws to gain clearance behind dial panel. The connections still need liquid tape on them to insulate them, but other wise its finished.
100_4262.jpg

I suppose there are numerous ways of doing this. This is just one. If there were more room underneath, and the resistors were smaller, I'd a done it there, but....

Larry
 
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Fired it up this morning and ran it for an hour. Input Voltage from the wall 115VAC. Checked all my voltages at the outputs. 429VDC plates, 399 screens, all tubes adjusted to 32ma draw. Seems to me that the plate and the screens should be higher, but the manuals and all are on the old hard drive for the laptop. I put in a new 80gb Hitachi y'day and haven't got finished moving over everything. It works fine tho and I'm happy with that for now.
 
I ran it for a couple hours y'day. It takes about 20 minutes for the voltages to stabilize @ their peaks. Plate voltage 476-8, screen voltage 426-9.
I get 102V at the tuner. Which is very close to what I had before.


I did notice something about the combo resistor. The schematic shows a 600 then a 6K in series, with a cap in-between. BUT!!!!!! I measured the sections of the combo resistor after removing it. The 600oHm section was 615oHms, and the 6K section was 5.32K. The replacements are 602oHms and 5.96Kohms for a better fit.

The input lead is the blue wire. 600ohm output is black, and 6Kohm output is Green.

On the original resistor (using the new wiring)INPUT Blue - -| -Black- 600oHm output | Green -6KohmOutput. Formatting doesn't really lend itself to this but the verticals
represent the parallel connection between the BLUE Wire and resistors.

I drew this out on Paint to show how i did it.

1000a power resistors.JPG


The Screen voltage didn't change appreciably as it's always varied between 426 and 429.

The way I figure it, the 6Kohm has a slightly higher input voltage, but with the difference in ohms between the original and the replacement set, the output is pretty much the same. about 3 more volts at the tuner.

Larry
 
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I've been running it 4-5 hours a day since. Temps are WAY DOWN around the POWER TRANSFORMER and 7591's next to it Closed up with the top on and with no fans. I DID use some Aluminum faced HVAC DUCT Sealing Tape on the Transformer surfaces that faced the 7591's and it seems to help with keeping the transformer temps lower. The new resistors don't get 1/2 as hot as the originals, so THAT in itself is a BIG PLUS. 50W dissipation vs 30W or even 10W. And these old used up test 7591's I'm using haven't sounded this good for a long time. I've go them running @ 32ma for about 14.5Wpc. And they are all showing below minimums for gm and emissions on my testers. With individual bias adjustments (old 2 pot converted to 4 pot), bias voltages are running between -13V for the worst one and -15.5 volts for the best one. So they are pretty well shot, but good enough for testing purposes.

Gonna steal the EH's out of my 800-C and run them a little harder than normal in it to see what it's capable of. 41ma @ 470v is 19.27W per. I'll probably try them right about 37-38ma which will give me between 17.39w and 17.86w per tube and play a tape of the 1812 overture (complete with West Point Cannons!!!).:music: That should wake up the old farts in the neighborhood. :yes:

Larry
 
Thanks for posting this Larry - I found it very interesting. :smoke:

You have made a really neat and thoughtful job of it :thmbsp:

AND you've had some great help along the way :yes:
 
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Thanks John. Just thinking back 5 years ago when I got this monster, I didn't know a coupling cap from a Metal Oxide Resistor. And was scared as hell when I started the recap on this receiver as it was the 1st one I'd ever done. Some of the work may look a bit on the shoddy side to some, but it's been a NO PROBLEM since except for the heat. And I DID and STILL DO have some great teachers here on AK. But a loooonnngg way to go yet.

Larry
 
It's been 6 months now since the resistor change out. I'm still using the old "test mule 7591's and they are holding @ 32.5ma. Transformers are relatively cool compared to before. Overall heat from the unit feels to be less. The 6K/600 ohm combo up front is about 110* for the 12Kparallel and 100* for the 600. The 6K in back is about 130*F.

Transformer temps after 1 hour is about 100* for the Main , 100 for the center output and room temp on the outer output. The only real hotspots are the tubes.

This is with the top off.

With the top on temps go up somewhat but not nearly as much as it did before the mod. Transformer temps after 3 hours closed up are 120* for the Main and center output with the outer output about 110*

Overall heat output from the top is still not what it was before the mod even with the top on.

I've cut the fans from 2-3" against the back, to a 6" table fan about a foot away when cover is on. Nude....No fan.

With the top off it puts out about the same as my 800-C. So that makes a substantial difference.

Larry
 
Since the last post I purchased a QUAD of TUNGSOL 7591 Re-Issues from Jim McShane about 4 months ago. As I have 2 800-C's(one of which is in an Executive console) I put them in the Executive for a month and tested. Then I put them in the SANSUI in place of a quad of EH7591's.

BOY! This thing is a COOL RUNNER now. Running the tubes @ 34ma, with 478 to the plates and 456 to the screens. Bias voltage is -19.5vDC. The tungsols tend to run "HOT" on the bias voltage compared to the EH's or old stock. The EH's I have are less than 5 years old, matched quads, and test the same as the day they arrived. To get 34ma on the Sansui, the bias voltage is usually around -16.6 to -16.8 for the EH's. This is typical of the 2 quads I have here. If I bump the ma down(up) to 29ma, the EH's show -20.5 to -20.8v. The Tungsols at the same ma require -22.5 to -23v.

Sound quality is improved. On the FISHER's it's readily apparent, but kind of hard to break out individuals or instruments. On the Sansui the totality is just overwhelming. Everything is enhanced, tightened up, (folded, spindled, and mutilated), to the point of feeling you are at a live concert, except for the lack of people.

Operation of the 1000A with the Tungsols and the upgraded power resistors and their locations make for a much, MUCH cooler receiver. The biggest problem with heat was those resistors. Now the transformers don't get above 105*F-120*F closed up(case on, no fan, 4 hour run). The case still gets very warm in spots (directly over the tubes) but not hot enough to char a porterhouse steak. The Front of the top gets very warm now from the resistors, but nothing to really worry about. The Tungsols don't generate as much heat as the EH's (bottle size) which is great for the trannies. I've kept the HVAC Alum tape on the trannie cases to keep radiant heat from the tubes to a minimum. I do believe this accounts for at least 10-15* differential on the tube side of the trannies (especially the center one.). I'd strongly recommend anyone with a 1000A needing new output tubes to go with the Tungsol's vs. the EH's. The EH's are a great 2nd choice, even with the large bottle. Strongly recommend that NEW 200K grid return resistors be installed. Also add a 100ohm 1/4W resistor to each tube at the screen. And a 10ohm 1/4w fusistor on the cathode pin to ground. These 3 "mods" will keep any 7591 healthy, and minimize any arcing from the screen, will shut down the tube and the transformer to that tube should the cathode "fusistor" open.

Now I have to get another quad of the Tung-Sol's to put in the 800-C.
 
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That's a great idea. I think I may have to do something similar with my 500a as you are right, she gets REAL hot. Looks like I can get the same resistor, but in 6.2kOhms so I might have to seriously consider doing this!
 
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Which is why I paralleled 2 12K to get 6K on the resistors. Get it as close to the original values as you can. The 1000A can be sensitive to tube and voltage changes internally. It doesn't like 6DT8's in the MPX, but they are ok in the muting section and the RF Front End.

I'm running it 8-10 hours a day everyday now. The Tung-Sol 7591A's are the best tubes i can get for it, short of a virgin set of Westinghouse 7591A ......(and if you have a set of those I've got a bridge in brooklyn I can seel ya real cheap). Original bottle size, good sounding and so far dead on stable. They run "hot" with regard to bias voltage. Meaning to run 32ma, I have to dial back the bias voltage to about -22v vs. -18v to -19V on the EH's. The EH's have never run at 32ma at anything more than -19v.

Still need to get some liquid tape to cover the connections at the resistors. One of these days.

Larry
 
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