RD 2100 no bias
RD 2100 no bias |
Music Man Amps Discussion Forum » Adjustments, Maintenance & Calibration » RD 2100 no bias
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Richard Haskins (rich55) Username: rich55 Registered: 03-2013 |
I just got a RD 100 with the GD-2A circuit board. The amp sounded too distorted so I decided to check the tube bias, Across one of the 3.9 ohm I could get 25mv dialed in. The other resistor nothing across it. I swapped out a few sets of known good tubes with the same results. |
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Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
I would check ALL the driver components in that area of the circuit. Most likely the transistor just took a dump but the 470 ohm resistors right before it should be checked also. If you have to order, get a few extra. and change them both at the same time. |
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Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
PS-I would read THIS:http://www.pacair.com/discus/messages/22 /1174.html |
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Richard Haskins (rich55) Username: rich55 Registered: 03-2013 |
I am not in front of the unit but I checked the 470 resistors in circuit, it seemed as it should be. Come to think of it the transistor in question didn't have the same markings as the other one in the amp, It appeared to possibly be replaced. I'll look again but it didn't have any clear markings. That is why I was checking and checking to make sure something else wasn't the problem. I already purchased a handful of the 2n6292 transistors, they are dirt cheap. so i guess I'll go with them. |
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Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
If you have a handfull, use them but you might have to juggle them to get a couple that match up. Since you hav ethem, why not. I don't clean UNLESS they are scratchy. Not knowing what you have to clean with, some of the cleaners remove any lubricant in the pot and they make them worse. I would just juggle the transistors 'till you get two that match. They might react differently IN circuit. Mike |
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Richard Haskins (rich55) Username: rich55 Registered: 03-2013 |
Mike thanks for the input, it helps a lot. |
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Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
I actually don't see that anywhere either. Is this an export amp maybe? I don't know. I'm not familiar with the export amps. Maybe Lars will pop in. He IS. I would be curious to see a pic of the inside close up. Exactly what does the 750 connect to? |
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Richard Haskins (rich55) Username: rich55 Registered: 03-2013 |
how do you post a picture here? |
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Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
I'm guessing the 750 ohm resistor is a screen resistor for the output tubes. I've seen this in an RD-65 amp (I replaced it with the 1.5k it calls for in the GD-2 schemo). If so, you'll find that the resistor is connected between the 350V supply and pin 4 on both 6L6 tubes. |
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Richard Haskins (rich55) Username: rich55 Registered: 03-2013 |
Lars you are correct. The 750 ohm resistor I am assuming replaces the two 470 1 watt on the schematic. |
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Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Posting pics: |
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Richard Haskins (rich55) Username: rich55 Registered: 03-2013 |
Success. I installed two new transistors and I was able to bias the tubes no problem. Very stable reading across both resistors. What a difference. |
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Richard Haskins (rich55) Username: rich55 Registered: 03-2013 |
forget the dual pot. it's a GP-1 circuit board. I'll start re thinking |
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Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Pedal it. I use NO excessive gain structure in my amps. I want dirt, that's what the pedalboard is for. MM amps USUALLY don't get any kind of good sounding grit unless they are broke!~ get a BD2 or something like that, change out the grit diodes for some LED's and maybe a germanium diode or two and add some bass with a cap change or two and you have a grinder that will be CLEAN when you want it. Mike. |