NAD & some issues I could use some help on

[lonewolfsx]

Hey everyone, I just picked up a 2008 QR in the white plexi head box. I'd rather have it in black, but it's about how it sounds to me anyway. When playing the amp at home, I am having an issue where the treble and aggressiveness faded out and back in. I've replaced all of the preamp tubes and tried a different set of power tubes that I know are working from my Marshall head. My amp does not have the effects loop volume but is the model with the clean channel separate eq. I've cut the amp to 50w on the back and removed the outer pair of power tubes, and run the amp into a rack EQ and used the level on that down to rule out the amp volume knob as the issue. Basically, I can stand there and play the same power chord over and over and it sounds amazing and has a lot of brightness and presence, then something will cut out (no popping or anything though) and all of the treble frequencies fade out as if someone has thrown a blanket over the speakers. The gain and volume remain the same. If I keep playing, eventually the treble comes back in and it sounds normal again. Ruling out tubes, guitars, and cables as the problem does anyone know what this could be? Or what the solution was? I suspect a cracked solder joint somewhere but I haven't owned the amp long enough to open her up yet.

Any help appreciated, thank you

[splawndude]

Welcome to the board lonewolf!

Congrats on your new QR

Sorry you are having trouble.

Obviously, any serious troubles should be reported to Scott or an authorized Splawn dealer. That said, we might be able to help a little bit but you've already done some of the basic troubleshooting that I would do. I'm not an amp tech either by the way just so you know. Here are a few things and I apologize up front if you've already checked/addressed.

Can you run amp standalone, no FX loop, rack, or footswitch?
You mentioned you took the tubes out of your Marshall and put them in QR - what are they exactly?
Can you spray the tube pins with contact cleaner (like Dioxit)?
When you run at half power are you also dropping the ohms in half?
Have you biased the amp or checked the bias?
Are you comfortable inside an amp that could have potentially lethal voltages stored in the filter capacitors?
What happens if you run the amp and presence set to '0'

[gtrjunior]

I was gonna suggest checking the bias also. I was having volume fluctuations with mine. And the bias was way cold. I know you said the volume was consistent on yours but I feel it's worth a check anyway.

[shaja]

Welcome man. I would do what Splawndude and Gtrjunior said, but you should send Scott a mail, he usually responds very fast.

[lonewolfsx]

Thanks, I'm glad to be here! I've been lusting after some hot rodded marshall tone for a long time. About a year into playing I tried out a late model Marshall 2205 JCM800 and absolutely loved the tone... and perhaps my ears weren't as practiced then because I've played JCM800's since and still can't quite get the tone in my head. I'd always discounted Splawns as too expensive for me but I was able to get this one very inexpensively as the store that had it refused to ship and they are located in a pretty remote location so I'm sure they had trouble selling it. I was looking at Laney AOR's and JCM800's, and a few others but the splawn is much better sounding than either of those to me. I also have a JCM2000 TSL with some light mods that I liked but it doesn't have any thickness and while I like it for mid-gain sounds the high gain left a lot to be desired to me and that is my focus most of the time.

to answer splawndude's questions:
-I can run the amp with the fx loop and footswitch disengaged and I get the same behavior. Sometimes the treble won't fade out until a few minutes of playing but it inevitably does and the amount of time it is not present fluctuates from a few seconds to a few minutes of playing. Sometimes the amp starts up with the muffled sound and the treble frequencies come in after a few minutes of playing.
-I haven't sprayed the pins with contact cleaner but I will pick up some dioxit and try this out. I've been meaning to get some anyway for my other amps so now is a good time anyway.
-I am dropping the ohms on the speaker from 16 to 8 when running in 50w mode. It's a 16 ohm cab so this should be correct.
-I have not biased the amp or checked it, but I am capable and will certainly do that.
-Definitely. I have modded both my Marshall TSL and a Jet City JCA22, only adding some switches to change different values of caps and resistors but enough that I feel comfortable that I won't kill myself.
-The amp feels quieter with the presence set to zero but I'm sure part of that is my ears being more sensitive to treble frequencies than bass. When the amp is in its "muffled" mood turning the presence and treble up does add some more of those frequencies in, but it still feels like the top is chopped off, as if everything above about 4khz is gone. With my rack EQ, I should be able to adjust those frequencies and it changes nothing even if I max the sliders from 5Khz-20Khz because it had no signal in those frequencies to begin with.

I did do some further testing by running the amp out from the FX loop and directly into my audio interface and a cab simulator. This cuts out the power amp entirely as a possible problem, and I think I can rule out the fx loop as a problem too because I get the same behavior whether or not it is engaged. So guitar -> amp fx out -> computer was my test and for a while I didn't have any issues... but then it started again and I could clearly hear the upper frequencies disappear for a bit then come back, and this repeated for probably half hour of playing the same few power chords over and over again so that there was constant sound through the amp. With this information, I'm thinking it must be a cracked solder joint somewhere in the preamp side of things. I'm going to take the chassis out to bias anyway, so I'll look around and retouch anything that looks weak, but if I have to I will take it in to a more qualified tech.

Side note though.. it still sounds pretty great even in muffled state. It loses some gain and aggressiveness so I don't want it all the time, but it'd be great for a smooth solo for a few Journey songs haha. Looking forward to getting this all sorted out because I planned to sell my TSL to fund the purchase but I won't part with that amp until I'm sure this one is replacing it fully functional. I will email Scott Splawn but I read in two other threads on this forum with similar issues to mine that he doesn't have time to repair or give advice on used amps so I'm trying not to bother him until I have done as much as I can on my end.

Thanks again for the advice, and of course if anyone else has experienced this I'm still open to input. It will take me some time to get the amp out to bias as I basically work 7 days a week.