Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: johngovan on September 30, 2018, 09:38:43 am
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I am really grateful to Tubenit for this wonderful ODS Amp. I made a clean preamp of his DMars ODS but i have some problems encountered. My preamp has nasty breakup.
I crancked up the gain at max and i strum the string moderately hard and there's a crackling noise or somehow a form of saturation that is not pleasing to the ear. Is sounds like a broken speaker. I will post an audio clip soon so anyone can distinct my problem.
I hook different guitar. Same problem
I hook different cable. Same problem.
I hook it in another power amp. Still the same.
I hook it in my audio interface. Still the same.
Do you have any idea what is the main problem? Thanks for future help. :)
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https://soundcloud.com/john-govan-426812247/amp-problem
here's the sound clip of my problem.
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Got a schematic? What is your audio interface? What signal level does it expect?
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Here's the schematic sir Sluckey. My audio interface is Focusrite Scarlett solo. I hook the preamp there and it is not clipping my interface.
i also hook this preamp into my guitar poweramp, i still have the same problem.
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I'm confused...
First you said "I hook it in my audio interface. Still the same. "
Now you say "I hook the preamp there and it is not clipping my interface."
Which statement is true? Or are they both true?
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I crancked up the gain at max and i strum the string moderately hard and there's a crackling noise or somehow a form of saturation that is not pleasing to the ear. Is sounds like a broken speaker.
this is the issue i have sir. I'm sorry for my statement that's somehow confusing. I'll try to clarify it.
I plugged my guitar into the input of D'mars ODS clean preamp and the output of the preamp goes to the input of my Audio Interface.
i set the output level of my preamp to make sure it is not clipping the input of my interface.
I set the preamp gain at max (Not the output level)
When i strum the string from normal to hard it creates a crackling or somewhat a nasty breakup as the sound goes away. I posted the soundclip link above to best describe my problem.
Hope i've constructed my problem more clearly. I'm sorry, english is not my first language. thanks for consideration.
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Got a schematic? What is your audio interface? What signal level does it expect?
"Focusrite Scarlett solo" docs:
https://focusrite.com/usb-audio-interface/scarlett/scarlett-solo
https://customer.focusrite.com/sites/default/files/focusrite/downloads/31524/scarlettsolo2ndgenugenv1.pdf
"Inst" input is rated for a huge 3V signal. More than almost any guitar, but may be overloaded from a tube preamp.
Interface impedances are *not* specified. Usually when they give you Mike, Line, and separate Inst, the Inst is 1Meg; but the specs do not confirm that.
If it isn't just over-driving the next box, then the next question is: voltages at tube and FET pins. Is some stage way off the proper voltage and distorting too easy?
"Crackling" also suggests "chop-sticking"-- with power on and connected to amplifier, *carefully* poke internal connections with a dry wood or plastic stick. The free chopsticks from the Chinese restaurant are suitable. If poking a connection makes a crackle, that connection is bad. Sometimes a connection is "good" for small signals but breaks-up on large signals.
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PRR gave good advice on "chopsticking it".
There are some guide for trouble shooting here:
https://el34world.com/Forum/index.php?topic=21999.0
Did you try several different tubes?
Can you post voltages and a photo of the preamp, please?
With respect, Tubenit
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This is the picture. I tried to poke thr wires in it but no luck at all. I also hook it up in my guitar amp power amp and i still get the same problem so i know it is not my interface. I cannot measure the voltage as of now becuase unfortunately my DMM is not working. My new DMM is on its way. Thank you sirs for helping me. I really appreciate it. This preamp sounds awesome. I know it that there's something wrong in my build.
This picture i've posted is my current tweaks to eliminate the problem. I change the value of plate and cathode resistor but still no luck. I also change some caps value and it is still there. I also tried changing the tube but it is still there.
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I think this is more clearer
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In your photo, the pin 6 plate resistor appears to be "burnt" or over heated?
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No Sir. It is just a shadow. When i try to lower the gain. About 9 to 10 o'clock, i dont hear that problematic sound. But when i passed that level, i hear some and as i increase the gain it gets louder and louder. Then i try to increase the treble level and it becomes more worst. I really love this preamp because it sounds so big and loud. But that's the only problem i have.
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Hope somebody can help me solve my problems. I tried to rewire it but it still has that. I tried different tube. I also try to make a marshall JMP master volume preamp and has the same issue too. I use an smps power stepup powersupply for the heater and the B+. I only made the preamp not the whole amp. I will show voltages of my build.
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I will show voltages of my build.
Great, that may help us.
Is there anything your changed from the original D'Mars ODS preamp design? What mosfet are you using?
Any chance you've drawn a layout for that board that we can take a look at to make sure it follows the schematic?
Is it correct that the line out goes to a 1/4" jack?
The "crackling" noise that sounds like a broken speaker was how I described an amp that I worked on that had a bad resistor. The resistor was brand new, but actually had a broken wire inside the enameled body. What happened was it would test fine for continuity, but when in the combo cabinet, the vibration of the speaker would cause the wire inside to open and close giving me a crackling noise and distortion. I eventually found it by very gently tugging on the resistor leads and noticed one side of the wire moving in and out.
Having said that, if you are running it into an interface audio into your computer, I can't imagine that being the problem?
It looks like you have this on a PCB board. Are all the traces solid with no shorting in them?
My best guess is there is a cold solder joint or bad connection somewhere? My second guess is a bad cap somewhere? I've had a couple of smaller value caps be bad or go bad early. I'm referring to 500p or less in value. My 3rd guess is the socket isn't making good contact with the tube pins?
with respect, Tubenit
BTW, I agree with you that your sound clip gives an impression of a "big and loud" full tone (with the exception of the crackle).
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On occasion when needed, I've followed Sluckey's excellent advice in "divide and conquer" & used insulated alligator jumper cables to bypass sections
of an amp design until I pinpointed exactly where the problem was occurring.
That approach has helped me numerous times when I wasn't able to find anything that seemed obvious.
I've posted a couple of examples below. Your challenge is attempting this with a PCB board as they typically are not as sturdy as a turret board for example.
With respect, Tubenit