Anyone got any 125-150ohm 5 watt Resistors spare?

shaunf

Hey guys,

I'm looking for a couple of different value resistors in the range mentioned, and don't really want to buy larger quantities than one or 2 in a couple of different values between 125 ohm and 150 ohm According to the schematic I'm looking at, they need to be 5 watt.

I'm wanting to check and possibly reset the bias on my PA 20 amp, since I've noticed it is red plating slightly, and I suspect it's running a little hot on the bias. It runs EL84's and is cathode biased.

I'd be really grateful if any of you have any spare resistors in this range and power rating and would be willing to sell me a few.

Thanks in advance!

Shaun

Gearhead

Couple of remarks
1. It is good to stick to the schematic but 3 or 4W will also be fine for EL84;
2. There are other values available, but you're most likely to run into 120, 130 and 150 Ohm values;
3. A pack of 10 will cost you R50. Getting all three values will cost about as much as a set of strings. Why bother and get less than three packets?

shaunf

Gearhead wrote: Couple of remarks
1. It is good to stick to the schematic but 3 or 4W will also be fine for EL84;
2. There are other values available, but you're most likely to run into 120, 130 and 150 Ohm values;
3. A pack of 10 will cost you R50. Getting all three values will cost about as much as a set of strings. Why bother and get less than three packets?

Thanks for your input. What prompted my request for assistance was me searching on RS Components, and filtering the results to show me resistor values in the range I am looking for, with power ratings of 3-5 watts. Unfortunately the only options I get on RS are either 120ohm, or 150ohm. The schematic lists a 125 ohm resistor for this purpose, so the 120 ohm option is probably useless, and the only other option available is the 150 ohm. I was hoping to try to get my hands on a couple of in-between values.

Gearhead

I found 130 on RS, that's why. The current difference between 125 and 130 is within tolerance of the resistor, meaning with a bit of bad luck you might wind up with more resistance on the 125 than on the 130. Normally the 130 would be a tad less current but if that stops the redplating remains to be seen. Rather buy a 150 and if you want hotter bias, put in the old resistor in parallel.

Isn't there a local store there in Durbs?

shaunf

Gearhead wrote: I found 130 on RS, that's why. The current difference between 125 and 130 is within tolerance of the resistor, meaning with a bit of bad luck you might wind up with more resistance on the 125 than on the 130. Normally the 130 would be a tad less current but if that stops the redplating remains to be seen. Rather buy a 150 and if you want hotter bias, put in the old resistor in parallel.

Isn't there a local store there in Durbs?

Do you perhaps have a link to the 130 ohm you're talking about? The only 130ohm ones I can find in that general power rating level are blue square radial leaded ones that really aren't going to do much justice to an otherwise mostly original vintage amp.

There is a local store, Electrocomp, that I was planning on trying, its just that they are a good 60km from where I am during the day and close early and don't open on Saturdays, so they're a huge pain in the arse to get stuff from for me at least, but I suppose they're probably my best bet.

Re the red plating, I'm not sure if it is or isnt. I noticed yesterday after playing the amp for around 30 minutes, that there was a faint red glow down the centre of the plates. Amp is well behaved and sounds great otherwise, I just decided I should probably check it out. I can see that the cathode cap and resistor aren't original, and although the cap is the correct spec, when looking at the label, I'm unsure what value the resistor is.

Gearhead

Karel explained to me that running the shared 130 is a bit too much for most pairs of EL84, 150 really isn't that much less current.
I must add that the 5W is specified on most 18W schematics I have; stand corrected.
Again one of the better options is to go for paralleled resistors (just not the 150//125 like I suggested :-[ since that would give you half of what you need, dunno what's with me today >☹ ) so you can use two 3W 270Ohm to give you 6W 135Ohm.

Kalcium

There is a mantech in Durban just off stamford hill, but thats pretty far from Hillcrest, theres also A1 radio off umgeni (there're about 1km away from each other)...other than that theres Ron Tomlin in Gale Street (near berea centre)