davolk wrote:I just acquired a used Nomad 100 watt 1X12 after a buddy of mine had a 55 2X12 that sounded awesome. I got the 100 watt because of the 50 watt option for smaller venues. I re-tubed the power section with MB 6L6s but the preamp tubes are probably the originals. Channel 1 sounds ok, but has a constant crackling that I think might be a dirty pot. Now for the bigger problem: Channels 2 and 3 don't sound good at all. I describe the sound as muddy and cheap. I play a USA Wolfgang which should pair fairly well with this amp. I sent MB an e-mail asking what they thought and here is the response I received:
"Well- it is tough to say what the "cheap, muddy sound" may be, since it is so subjective... I can tell you that the NOMAD series by design has a very THICK (muddy) tone... not a whole lot of clarity between notes on channels 2 & 3. It wasn't designed to be as articulate as a Rectifier. Now, with that said: it may just be that the preamp tubes are old & need changing to breathe some life into your tone... but again, that is hard to call via email without even hearing the amp."
So that was not much help. Does anyone have any ideas on how to correct this? Is it possibly as simple as a new set of preamp tubes? I don't mind the "thick" or "heavy" tone, but it really sounds like a cheap solid state amp.
Now get a schematic...they are posted here and there.
This update stiffens the power supply.
It is designed to eliminate a few problems associated with muddy sound, humming noises, and other things that may affect a guitar player........
At point "A" (follow along thru) there is choke, then there is 6 resistors in series, 220, 365,365,365,365,365....after that - there is a single 10uF cap, change that to 22uF.
At Point "B" leave the 33uF stock.
At point "C" there is two 10uF caps in parallel, change both those to 22 uF.
At Point "D" there is 10uF cap. Change that to 47uF.
We come to point labeled "360V" This is really point E...not labeled as such on schematic.
At point "360v" there is 10uF cap. Change that to 47uF.
And all these caps, I would like 105C, 5mm lead spacing, 450V, 10,000 hour life rating. I get them from digikey.
Then for relays,
I take them all off and install sockets.
Once the sockets are installed, it's easy to change a relay, in about 5 minutes or so.
This eliminates removing the circuit board to change a relay. Instead, I just plug a new one in, or test an old one.
I can also leave a space, under the relay socket
to accommodate a wire tie.
Now test your relays, and make sure that in both positions, you are getting 1 ohm or less contact closure.
Should be 0 ohms. I test them with a 6 volt power supply to switch the coil on or off.
Replace any bad relays, of course.
You need your relay contacts to be very tight, of course.
Just one last thingie....
I would like you to install a 68K metal film resistor on the input jack, in series with the ferrite bead, to pin 7 of V1.
The guitar is no longer connected to pin 7 V1, there is a resistor in series now....
The next issue is the transformer power connector.
This is a white plastic connector that connects transformer secondary to the main board.
I would like you to take that connector off the board, and solder the wires directly to the board.
So that being said,
This is not the amp to "learn on."
Have the updates installed by a qualified person, with the professional tools.
Circuit boards are easily damaged by inexperienced persons.
WTF does all this do?
FIRST, let me make it clear...if you test with a sine wave and oscilloscope, you will never find this problem.
You have to test with A GUITAR.
Yes, that is correct, test with A GUITAR. Your sine wave generator is futile.
There is actually a low frequency oscillation in the power supply.
This oscillation is triggered by playing a note or a chord on the guitar.
When this oscillation occurs, it causes the grid of V4A to be driven slightly positive, temporarily..
When this occurs it causes V4A to temporarily distort, a "cutoff distortion."
This cutoff distortion is the MUD you are hearing.
A. muddy sound, undesirable distortion
B. sometimes a "buzzing noise." when a note is played only. This is temporary ripple in the output of V4A...
C. Sometimes a "tremolo effect." sound cuts in and out.
By stiffening the power supply, the low frequency oscillation will cease to occur.
the amp sounds considerably cleaner
the highs are noticeably cleaner and brighter
much less muddiness, less undesirable distortion...
buzz sound is gone.
Can crank amp up much higher, without oscillations. Tone is coming thru unhindered.
Now, it sounds like it should sound. New Amp.