Resistor question

[ChrisinMO]

I just bought my second JMD-1. (Both used but good shape). I really like these amps. My question is: the one I just got, I was checking the bias and noticed someone had paralleled a second resistor over r48 and r49 with what looks like the same as the original resistor. Does anyone know why anyone would do this? I've read about a bad power tube taking one of these resistors out and that they act kind of like a fuse. None of them "look" burnt. Can anyone tell me why this is like this is what it does to the amp circuitry? The amp only will bias to around 78-79 and I run out of adjustment. I'd like to go to about 74 ma. Do these extra resistors have something to do with this? Thanks?

[Anitoli]

R48-49 are the 1ohm resistors used for biasing the amp. This method is based on ohm's law of " one mv across one ohm = one ma" .
When two like value resistors are paralled their resistance is halved, like wise if two resistors are in series their resistance doubles. I cant tell you what they were trying to do, if you readings and calculations are based on a one ohm resistor then you need to get those off of there and verify the original resistors are at one ohm so you can get an accurate bias adjustment. This is probably why you cant get the bias ma that you want.

[ChrisinMO]

R48-49 are the 1ohm resistors used for biasing the amp. This method is based on ohm's law of " one mv across one ohm = one ma" .
When two like value resistors are paralled their resistance is halved, like wise if two resistors are in series their resistance doubles. I cant tell you what they were trying to do, if you readings and calculations are based on a one ohm resistor then you need to get those off of there and verify the original resistors are at one ohm so you can get an accurate bias adjustment. This is probably why you cant get the bias ma that you want.

So should I just take the paralleled resistors off and make sure the originals read 1 Ohm across on each one? Then bias as usual?

And thank you for your response!

Chris

[ChrisinMO]

I checked it. Set my meter on 200 Ohms and it read 10 Ohms. (After I removed the two in parallel). I'd guess at some point the original resistor was blown and they couldn't find the 1 Ohm resisters so they got as close as they could by paralleling 2 -10 Ohm's to make a 5 on each side. I have the right 1 Ohm resisters I ordered a while back. I think I will replace what's there with them. 1 Ohm, 1watt, 1%, metal film.

[Anitoli]

Yes, take them off then verify the value of the original resistors. If cool then set your bias as normal. When run with two like that the resistance will be lower than expected and the measured voltage drop will be less than anticipated. So the tubes are probably alot hotter than they should be because the reading is going to be slightly lower.

[Anitoli]

I checked it. Set my meter on 200 Ohms and it read 10 Ohms. (After I removed the two in parallel). I'd guess at some point the original resistor was blown and they couldn't find the 1 Ohm resisters so they got as close as they could by paralleling 2 -10 Ohm's to make a 5 on each side. I have the right 1 Ohm resisters I ordered a while back. I think I will replace what's there with them. 1 Ohm, 1% metal oxide.

5 is a little too much.

Put the new one ohm resistors in and you should be alright.

[ChrisinMO]

I checked them and they were about 1.4 Ohms each. I put them in and the amp biases right and sounds great! I have both sides set at about 72 ma. Thanks so much for your help! I can think I know what to do, but it sure helps when someone that really knows tells you what the outcome should be. LOL! Thanks again!