Onyx Melody help

mick_t

Active Member
Howdy,
I'm relatively ignorant of tubes but love my Onyx Melody PC3 despite the issues I've had with it. Recently, however, I've been hearing a crackle (almost like tubes sound when you first turn them on) in the louder passages. I just biased the amp (it was running a bit cold) and now it's a little worst.
Is it time to start flipping tubes? Where do I start?
 
UPDATE:
I got tired of waiting for a response so I pulled out some old tubes and changed one kind at a time, biased the amp, then tested it for a short minute. It continues to be fuzzy. I did not have spare output tubes so did not test them.
I also checked all cables and they are pushed in and firm so....
All I can think of is changing one cable at a time and seeing if that's the issue... or changing the output tubes....
Any thoughts?
 
You'll need to give a clearer description of your problem. Is it an intermittent crackle or is the sound continuously fuzzy?
 
Its continuous...
You know what it sounds like when you first turn on a tube amp while it warms up for a second? It sounds like that but much more subtle.
 
Does it come from both sides at the same time or is it just left or right.

If it is one side, you can swap output tubes and see if the issue follows the tube.

I leave my pre muted until my tube amp warms up so I am unfamiliar with the sound you describe.
 
A correctly working tube amp does not make any sounds as it warms up, so I don't know what sounds you are hearing. Is it coming from the speakers? That is, is it an electrical noise? Are you sure you are biasing your tubes correctly?
 
Hmmm... ok. I suppose I'm not describing this well... forgive me.
When I first turn on either of my tube amps (I also have a Jolida hybrid amp that uses a 12ax7 only for a preamp and solid state otherwise) if music is playing, the sound coming from the speakers slowly gets louder for a second while the tubes turn on and begin working. During this short time the music is fuzzy and generally horrible sounding, but only for a second. This is the sound I am referring to, but am aware that all this could possibly be confusing.
The sound from the speakers is fuzzy only very subtly, but much clearer when I go through a Yamaha solid state amp, so I know its the Onyx. Unfortunately it comes through both speakers.
 
I'm pretty sure I'm biasing correctly, though one side is slightly low... (supposed to be 1.15 and for some reason I can't get it above 1.05)
I can record it tomorrow and will post it in the morning
 
What you're describing is distortion and the fact that you can't bias one side is an indicator of a problem.

Here is a link to the information about the amp and schematic. It may be that someone here can talk you through the fix. http://onix.pbworks.com/w/page/7224976/SP3#BiasingTutorial.

Unfortunately this schematic is an early one with a bias arrangement that is only one per side, whereas yours has one bias pot per power tube. Also, the schematic doesn't give voltages. I have this amp too, but mine is packed away at present.
 
Yeah I'd call it distortion. Its only one half of one side that I can't get to bias properly, so I'd guess one output tube. Does that mean swapping the tubes should work or does it imply something more technical?
I had a filament go out last year and had it fixed... cost way too much.

... I love the sound and experience of the Melody... I'd recommend pulling it out of its packaging and using it or leaving it in its packaging and shipping it to me
 
Just a wee bit more info! :D

Did this just start happening? Does it play fine when warmed up? Is it coming from both speakers?
 
Yes it did just start... slowly has been getting worst over the last week or two. I turned it on and let it sit for two hours to properly warm up and it still sounds distorted. From both speakers...
I biased it over and over and currently have all four pots at 1.15. Now it sounds worst then ever.
 
Not interested in investing $80 if it won't help, but would tube rolling the output tubes into Tung-Sol 5881 help?
 
I don't think tube rolling will help, unless you have some known good tubes that you can try.

I have had a couple of problems with my Melody over the years. First a cap across the power cord failed and the service agent said just cut it out of the circuit (this was a long time ago). Then I had a biasing problem which turned out to have been caused by a burned out resistor. This was visible when I removed the bottom cover. If you haven't had a look underneath, this might be the time to do it. If you do, post a picture of the view from below.

I'm not sure what you mean 'had a filament go out'. Was this a faulty tube, or a failure of the filament heater supply? Have you powered the amp with all the tubes visible? Are they all lighting up?

p.s. I'll get my amp out of the box at the weekend and take a few pics which may help you and others to do some basic checks. But in the meanwhile, you can look underneath and check that the two fuses are still good. When I get it open I'll try and figure out how the bias method works.
 
Last edited:
Some pics. Looking again at the amp, if yours also has those red resistors, I think any of them could be suspect.

The second and third pictures show the boards with the two bias controls for each side. You could trace the wires from those controls to the resistors to see if any have burned out.

The fourth pictures shows where I had to replace one.
IMG_0655.JPG IMG_0656.JPG IMG_0657.JPG IMG_0658.JPG
 
HI again Mick. I've been thinking about my amp and I remembered that I also had trouble with the 33 ohm resistors that go across pins 1 and 6 of the big 5881 tubes. These go from the cathode (pins 1 and 8 are linked) to ground and if these resistors go bad you can't bias the amp. Check all four of yours and replace with 3W resistors if any are bad.

Let us know what you find. You will see that I think all the those red resistors in the amp are poor quality.
 
Thanks for your photos...
I opened it up and see nothing out of the ordinary... no blown resistors or fuses. Below is a picture of what I had worked on last year.
15163142460181730580623.jpg 1516314289371875544095.jpg
 
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