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Old 9th May 2020, 1:21 pm   #1
Chris Wilson
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Default Collins 30L-1 grid bias resistor change

I have a Collins 30L-1 HF amplifier and have found one of the carbon composite grid bias resistors is cracked. They should be 47 Ohms, 1 Watt, carbon composite, but sometimes general opinion is to replace them with 2W jobbies. I ordered some before reading that 2W ones are often preferred, and ordered metal film ones from RS. When they came they are the size of 1/8 Watt leaded type, and NOTHING like the circa quarter inch diameter, one inch long things in it from the factory. I chickened out and had a look what else I had and found some 5W planar resistors in 47 Ohm, 5W.

I THINK these should be OK, but need reassurance please!

These are the ones I am thinking of using, and the schematic is linked here:


https://uk.rs-online.com/web/p/throu...stors/8644096/

https://www.qsl.net/yb0ah/homebrew/30l-1/page7_3.html

The resistors are R21 through R24.

Thanks, I hope all here and their friends and family are well and have avoided the virus!
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Old 9th May 2020, 2:17 pm   #2
Radio Wrangler
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Default Re: Collins 30L-1 grid bias resistor change

Have a look at the datasheet for the planar resistors and check 1) the voltage rating, and 2) do they need heat sinking of some form, or what's the rating in free air?

Modern resistors have got a lot smaller for a given power rating, but ye cannae change the laws of physics, captain, so they run at much higher surface temperatures. If this is a problem, due to insulation nearby and other meltable stuff that could come into contact, you have to pick a higher power rating to get the body surface area up to that of the old part, to stay at the same surface temperature.

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Old 9th May 2020, 2:21 pm   #3
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Default Re: Collins 30L-1 grid bias resistor change

These are my resistors of choice for valve sets. They're rated at 500V.

https://uk.rs-online.com/web/p/throu...stors/7078817/
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Old 9th May 2020, 2:28 pm   #4
turretslug
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Default Re: Collins 30L-1 grid bias resistor change

Presumably, the low-inductance characteristics of carbon-comps are important here where they're being used as grid stoppers rather than actual grid-bias resistors? Planar types would be better than spirally-deposited/trimmed conventional rod film resistors- that's assuming they don't use a deposition/trimming pattern that is significantly inductive.
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Old 9th May 2020, 2:33 pm   #5
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Default Re: Collins 30L-1 grid bias resistor change

Grid stopper resistors then. I wasn't sure what grid bias resistors were and assumed they were cathode resistors. I should have checked.
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Old 13th May 2020, 1:00 pm   #6
Chris Wilson
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Default Re: Collins 30L-1 grid bias resistor change

Thanks, got the recommended ones from RS yesterday and fitted them, and it still works! That's always a good bonus!

One resistor, the cracked one, measures 37 Ohms, the other 3 low, but within 4 Ohms of 47 Ohms. What effect would the 37 Ohms one have been having had I left it in or was unaware? Thanks again, stay careful!!
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