Dual Rectifier Roadster: Possible problem with FX Loop tube?

[Coleo]

Well I haven't even turned my amp on for two weeks and the last time I played it everything was perfect. I turn it on today and the amp has no balls, switching through the channels with the output way up, each one has a crackle and obviously sounds sick. When I turn the bypass FX loop switch on in the back it roars back to life with each channel playing perfectly at volume that is basically 100x as loud as it needs to be. Does anyone know where to start here? All the tubes were checked less than 8 months ago and a couple replaced. This is really annoying.

Thanks

 

[afu]

Well I haven't even turned my amp on for two weeks and the last time I played it everything was perfect. I turn it on today and the amp has no balls, switching through the channels with the output way up, each one has a crackle and obviously sounds sick. When I turn the bypass FX loop switch on in the back it roars back to life with each channel playing perfectly at volume that is basically 100x as loud as it needs to be. Does anyone know where to start here? All the tubes were checked less than 8 months ago and a couple replaced. This is really annoying.

Thanks

V4 is the loop tube. Replace it and see what happens. It's cheap, it doesn't hurt to have an extra (if it's not the problem), and costs less than the shop (if it is the problem).

Preamp tubes usually last a long time, but like any other mechanical item, they fail from time to time. If you've already replaced tubes and now the loop tube is determined to have failed, you may want to ensure that the loop Send control is at or around noon; lower the channel volumes a bit; and let the Output control bear the task of bringing the levels up to your desired level.

When you bypass the loop, the output control is also bypassed. If your channel volumes are set high, they will be incredibly loud in bypass, as there is no global master volume to control the signal after the loop. From my experiments, the bypassed volume settings need only a slight increase when the loop section is turned back on.

To give you an idea of a moderate setting: My Channel 3, Modern, tube rectified setting has the channel volume at about 9:30 to 10:00. If I switch to diode rectification, my channel volume sits at 9:30 or a bit below. When the Output goes past 9:30, it's freaking loud and after 11:30 or so, it starts knocking things off tables and walls, and the speakers push air against me. My Send control is a bit back from noon to match the volume differences between the on/off of the loop switch. This type of setting allows you to more easily balance the other channels with your distortion.

I hope it's that simple and good luck, buddy.

 

[deeman]

you may want to ensure that the loop Send control is at or around noon.

I bet this is it.

 

[Coleo]

Thank you thank you for the informative post. I think I have a spare pre-amp tube kicking it somewhere in the guitar room so will get to this when I return home this weekend. I know the send levels have not been touched and are at the mid position, but I will also test out your suggested settings as well as I'm always curious what different combinations will yield different sounds.

 

[Old BF Shred]

Try running a cable from send jack to return jack directly. If it works then, it's not a tube.

 

[Henweigh]

Having an identical problem to OP with a Road King v2 that started last night. Sounded fine earlier this week, turned it on last night having changed nothing, all of a sudden everything sounds super weak when the effects loop is engaged. Volumes are drastically lower and gain sounds thin. Everything sounds great when effects loop is bypassed.

I've been using only an instrument cable in the effects loop, and messing with the effects loop gain knob. Nothing makes it better. Of note... when hitting a chord hard on Channel 1 clean, the sound distorts harshly and the volume fluctuates wildly.

Thought it would be a tube issue. Went out this morning and bought a pair of Chinese Groove Tube 12AX7s. Tried switching out every one of the preamp tubes individually. No difference. I guess I'll try switching them out in pairs now.

The whole problem seems to point to the effects loop preamp tube (V5, right? That's what my manual says), but switching it out has had no improvement. Everything sounds great when the loop is bypassed, so I feel like all of the preamp and power tubes are doing their job except for maybe the one dedicated to the effects loop.

 

[Henweigh]

Tried switching out the preamp tubes in pairs with the 2 new ones that I bought today. No luck. At one point, I replaced V4 and V5, and experienced even worse problems, the amp had barely any output, even when the loop was bypassed. The problem remained that way when subbing out V3/V4, V2/V3, all the way down the line. I went back and put the 2 new tubes in V5 and V6, as that was the last configuration where things weren't COMPLETELY messed up. That brought the output back, but the original OP problem persists.

At one point when experimenting, the loop gain knob was completely not functioning, but it's back.

I currently have the 2 new tubes out, and all the originals back in what I hope to be their original spots. Same as when I started. Loop sounds terrible with just an instrument cable in it (or nothing), bypassed sounds great.

Could this be a circuitry problem at this point?

 

[Coleo]

Try running a cable from send jack to return jack directly. If it works then, it's not a tube.

So I swapped a spare 12AX7 here for both V5 and V4 and same problem. But when I connected a cable from send directly to return, it came back to life!

So I guess the next thing is to isolate when the problem in the FX circuit is coming from. Any thoughts here? Would it come from the chain of effects themselves or is this likely a circuit board issue in the amp ?

Thanks for the discussion here everyone.

 

[Coleo]

So i put the entire pedal board in line before the input jack to see if maybe there was a bad pedal or power connection and everything worked fine. There are only 4 pedals and they all did what they were supposed to do, so that leads me to believe that the pedals and effects board connections are fine.

So....onto the amp I guess.

 

[Henweigh]

Alright so, I got this fixed, hope this post offers some sort of insight for you too Coleo. You mentioned that it came back to life when you connected a cable from the send to the return. I'd already tried that, tone, volume, and dynamics were terrible with a cable in the loop or nothing in the loop. However, your post got me thinking... the Road King has 2 effects loops. I only use effect loop 1, and haven't touched loop 2 in months, but figured it was worth a shot to try plugging a cable into the send and return of loop 2. As soon as I tried that it came back to life, and remained fixed after I took it out. Everything sounds fine with a cable in loop 1, in loop 2, or nothing at all.

All I can figure is that for some unknown reason, the amp was picking up something I believe in the send jack of loop 2, as if a cable were plugged in there. Weird. I was getting a little bit of noise that comes and goes, some random distortions, even with the guitar's volume knob rolled down to 0 on higher gain settings. But last I was playing, they had gone away.

So, with your problem... are you only having it when you have the loop engaged but nothing in the jacks? You said it sounds fine with a cable plugged in. How does it sound with your board in the loop?

I feel so bad for my amp. Tried swapping so many tubes today, so much powering on and off. Ugh.

 

[yianni54]

Hey Guys,

I cant say im having the same problem but on my Roadster if I go into the effects with a pedal im getting alot of noise. I even put my ISP g string in the loop and I get npise until of course i make it go away with the noise gate but even then I can still hear the buzzing trying to get through.


Additionally, and I m not sure if they are related but even if I am on channel one clean and I turn the reverb on, im getting buzzing, almost sounds like gain as If I was on ch. 4.

Any thoughts?

For a while now whemn I run my POD HD 500X through the fx loop I just couldnt get it to sound clean. I was getting noise and clipping.

I bought the amp in September so I doubt I need new tubes or could i?

 

[Coleo]

Alright so, I got this fixed, hope this post offers some sort of insight for you too Coleo. You mentioned that it came back to life when you connected a cable from the send to the return. I'd already tried that, tone, volume, and dynamics were terrible with a cable in the loop or nothing in the loop. However, your post got me thinking... the Road King has 2 effects loops. I only use effect loop 1, and haven't touched loop 2 in months, but figured it was worth a shot to try plugging a cable into the send and return of loop 2. As soon as I tried that it came back to life, and remained fixed after I took it out. Everything sounds fine with a cable in loop 1, in loop 2, or nothing at all.

All I can figure is that for some unknown reason, the amp was picking up something I believe in the send jack of loop 2, as if a cable were plugged in there. Weird. I was getting a little bit of noise that comes and goes, some random distortions, even with the guitar's volume knob rolled down to 0 on higher gain settings. But last I was playing, they had gone away.

So, with your problem... are you only having it when you have the loop engaged but nothing in the jacks? You said it sounds fine with a cable plugged in. How does it sound with your board in the loop?

I feel so bad for my amp. Tried swapping so many tubes today, so much powering on and off. Ugh.

I only have one FX Loop on the roadster so I don;t think that will work for me. Basically, there is a problem in the FX circuit somewhere. When I put a cable from send to return, it operates just fine, lots of power and sounds as it should. When I hook up the peddle board to the send and return in the loop, then nothing is fine. With the cable going from send to return, I put the entire efx chain of about 5 pedals infront of the input jack and all effects were working normally.

Stuck

 

[Old BF Shred]

On some (if not most or all) of the Mesa amps they use a return jack that has a switching tab that engages when a cable is not plugged into it.
If that contact point develops any corrosion, you would get the symptoms you describe.
Usually you can open the amp to get to, and clean, that contact. Worse case, the jack needs replacing, but cleaning will usually do it.

If you try this, be very careful as even an unplugged amp can store dangerous voltages on the power supply capacitors.

If you are not comfortable doing this, seek help from someone who is. :wink:

Good luck!

 

[UnderJollyRoger]

I was going to suggest the same thing ... you can try plugging and unplugging a cable into the FX Loop a few times to remove corrosion before actually opening the amp ...