| Author | Message | ||
     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. I turned my attention to the transistor emitter for the 3.9 ohm resistor. The diode test and ohm test on the transistor seemed a little flaky but nothing to indicate it was clearly bad. I checked as many components in the tube section as possible. I compared the results to a RP amp I have, tube sections seem very similar. every thing seemed fine. My question is can a bad tube ruin the transistor (I am sure that is possible, anything is, right)? I want to make sure when I put in a new transistor it's not another component causing the failure. I removed the transistor in question. After handling the transistor the collector leg broke off, believe me I wasn't rough on it so I have to replace it anyway. Any insight on what else I should check or suspect if this is a problem that might be common to Music Man amps. Should I swap both transistors now? I ordered 2n6292. This is the first Music Man amp that I have had issue with out of five thanks! | ||
| 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. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
PS-I would read THIS: http://www.pacair.com/discus/messages/22 /1174.html about the variances in those transistors. Mike/. | ||
| 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. In order to match a pair of transistors up could I check the hFE on my multi meter, it has a setting for that and choose the closest ones or is that not going to determine anything? while I am at it ....the pots seemed ok not scratchy but I was going to clean (quick spray) them while I am in there or should I leave well enough alone,? Thanks again!!!!!!! | ||
| 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 | ||
| Richard Haskins (rich55) Username: rich55 Registered: 03-2013 |
Mike thanks for the input, it helps a lot. The good news is my order of transistors got backed ordered so I canceled and reordered with 2n6488 transistor, My RP amp has the 2n6488 in it after a closer look, so the proof is in the pudding, I'll install those now. One thing I can't find on the schematic is in both amps there is a 750 ohm wire wound resistor on a terminal strip in between the tubes and the big caps under the can. On the RD amp this resistor has a chip missing from the coating, I can see a little of the windings. I don't know if it got too hot or what but i am going to replace that too. What do you think? | ||
| 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? | ||
| Richard Haskins (rich55) Username: rich55 Registered: 03-2013 |
how do you post a picture here? | ||
| 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. Cheers, Lars | ||
| 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. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Posting pics: type this (backslash)image{Text description} (put the backslash in for real!) in it's entirety into your message for each pic you wish to post. If you post more than one pic, hit enter once or twice between each line. After you hit submit, it will prompt you to download your pic from it's location. Mike | ||
| 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. It's a RD100 The first channel doesn't have much of an overdrive with the gain cranked. The second channel is over the top with the gain max out. My RP65 with the gain turned up pretty hard has more of the distortion I would like out of the first channel of the RD100. I just started to ponder what to modify/change to maybe get a little more gain out of the first channel of the RD100. Looking at both schematics the hard gain I believe is archived with the use of a dual pot on the gain. I was kind of hoping of changing a resistor and not do crazy surgery ................. Any thoughts would be appreciated. | ||
| Richard Haskins (rich55) Username: rich55 Registered: 03-2013 |
forget the dual pot. it's a GP-1 circuit board. I'll start re thinking | ||
| 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. |