AL1200 RECAP: Any opinions on ohmite wirewound resistors for HV bleeders?

[357]

I'm recapping my AL1200 and I checked the resistors and they are all 49.xx Kohms and within .5 of an ohm. 2 look like they got a bit hot as the lettering is faded.
I planned on replacing them and since they are wirewound, I got new wirewounds but 10w instead of 7w

I was just wondering before I pull the trigger, does anyone have any bad experiene with ohmite? these are 20J50KE "E" standing for ROHS compliance.
Do you think I'll be ok with these?

thanks

 

[357]

http://www.ohmite.com/assets/docs/res_20.pdf?r=false

 

[357]

Do I need to worry about inductance on the bleeder resistors? I can understand for the parasitic suppressors but the power supply too?

 

[Beetle]

Do I need to worry about inductance on the bleeder resistors? I can understand for the parasitic suppressors but the power supply too?

Bleeders normally only see DC; the "wirewound" resistor might measure a bit of inductance but not enough to worry about at RF, let alone 60 Hz.

 

[357]

 

[357]

 

[357]

Seems to be working ok.
The caps fit perfect

 

[Captain Kilowatt]

Absolutely nothing wrong with Ohmite. In fact, Ohmite is/was pretty much the industry standard. All the broadcast transmitters I worked on had Ohmite 100K 100 watt wire wound bleeder resistors in the HV power supply section. Globar is another quality name in high power resistors but I have seen them used more in high power dummy loads than power supplies. Globar makes some really good carborundum resistors that are non-inductive and suitable for loads in the HF spectrum.

 

[357]

OK thanks.
The reason I'm asking is I tried to put as much research into the parts I could and many suppliers are bone dry for parts and I seen the price go up on what they did have almost daily.

I couldn't find any data on the "crc" resistors that were in there. I did see lots of images of the RCD 170 series and while it was too late because I already ordered the ohmite, the RCD 170 series mentioned low inductance and a main feature which got me worried. data: https://www.rcdcomponents.com/wp-content/uploads/2022/01/100-Series-Datasheet_RevI.pdf

So that got me worried that there must be a reason the went for the inductance feature

 

[nomadradio]

Wirewound resistors have one advantage in this application. Their resistance value won't drift with age. A series string of caps like this uses the bleeders to equalize the voltage so it's divided evenly across each capacitor. The inductance they show is not relevant. This is a DC circuit and it just won't care.

Back in the day you would see carbon-composition resistors used for this task. Bad juju. They are famous for changing their internal resistance with age and mileage. But they were the lowest-cost option, and designers would use them with the knowledge the warranty would be long expired before they could cause any mischief.

Carbon-film and metal-film resistors are both perfectly suitable, and likewise don't exhibit a drift problem in my experience. And they're a lot easier to find in high resistance values.

But yeah, I would consider the wirewound part the "premium" option, but not really a whole lot better than the carbon/metal film resistors. Just use one that doesn't run too hot. Heat stress makes more difference than the choice of component type alone.

73

 

[357]

thanks Nomad.
That helps me relax a bit. I am seeing 105C off these resistors with normal use (the ones to the back the fronts are getting more airflow).
Hopefully thats normal. I was hoping 10w would run cool, I guess not

 

[nomadradio]

Assuming 400 Volts across each bleeder, this should dump 3.2 Watts in to it. You would think that would keep the temp a bit lower, running at one-third of its rating.

Probably won't hurt anything for the first five or ten years, anyway.

73

 

[linearone]

If you wanted the resistors to be cooler, you could have jumped down in size by a lot, and gone up to 100k, so what if it only takes an extra few seconds or a minute to bleed down.

 

[linearone]

No load plate v is around 3600 right? With 8 caps that's 450v each.
With 50k bleeders you should drop each cap to 6.25v or so for a total of 50v across the bank in about 71 seconds. Each resistor will dissipate 4 watts.
A minute is fine. If you could stand to wait two minutes you could use 100k and they will diss 2w .

 

[357]

changing the resistors to 100K 3 Watt Metal Oxide. This was a very bad idea. They should be 50K 7 Watt wire-wound, HV resistors

I seen that online so I stayed with the 50k
I muffed up mounting them a bit close to the board...thats kinda bugging me but I dont see any signs of heat damage yet.
I guess they bleed voltage but they also equalize the voltage across the caps.