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Old 22nd May 2008, 02:13 AM   #1
JayH3 is offline JayH3  United States
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Default Need help with my ST70

I built an ST70 from scratch, point to point wiring for my first project. I used new Dynaco repro transformers from Triode, upgraded cap psb from Triode, JJ Tubes, and the driver board is the DIYTube design.

Here are my problems:
1.) I had to disconnect the 16 ohm leads to the negative feedback because of horrible buzzing. It is now fairly quiet, but I would rather have those leads connected.

2.) It plays music without any buzzing, however there is a quality to the music I guess I would describe as distortion.

Any suggestions on where to look?

thanks.
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Old 22nd May 2008, 02:37 AM   #2
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Hi Jay,

The horrible buzzing you speak of sounds like oscillation. Try reversing the plate and screen wires (if ultralinear) from tube to tube. In other words, connect the wires on the upper tube to the lower tube and vice-versa. The color coding, and polarity, on aftermarket transformers is probably wrong and the improper phasing is causing the problem. It could also be something else, but start here.

You could also get the same effect by switching the coupling capacitors to the output tubes. But don't do both or you'll be back where you started.

Victor
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Old 22nd May 2008, 02:38 AM   #3
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Could the feedback have been connected the wrong way around?

The correct way (negative FB) makes the amp quieter and less distorted, with reduced gain and tighter bass.

The incorrect way (positive FB) increases noise and distortion and also increases gain excessively, which can give rise to instability and spontaneous oscillation, usually heard as a deafening howling noise. Running the amp for long in this state can cause damage.

With no feedback at all, the amp will be 'in between' - rather too much gain, with the output possibly sounding a bit distorted, with 'woolly' bass, but not unstable.

The easiest method to reverse the way the FB is connected is usually to swap over the OP tube plate connections to the OP transformer primary.
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Old 22nd May 2008, 02:47 AM   #4
JayH3 is offline JayH3  United States
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Quote:
Originally posted by ray_moth
Could the feedback have been connected the wrong way around?

The correct way (negative FB) makes the amp quieter and less distorted, with reduced gain and tighter bass.

The incorrect way (positive FB) increases noise and distortion and also increases gain excessively, which can give rise to instability and spontaneous oscillation, usually heard as a deafening howling noise. Running the amp for long in this state can cause damage.

With no feedback at all, the amp will be 'in between' - rather too much gain, with the output possibly sounding a bit distorted, with 'woolly' bass, but not unstable.

The easiest method to reverse the way the FB is connected is usually to swap over the OP tube plate connections to the OP transformer primary.
That sounds very reasonable. Are you saying I should swap the FB between the L and R channels or something else entirely?

Here is a link to the board if you havent seen this design. http://www.diytube.com/st70/diytube_st70_B.pdf
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Old 22nd May 2008, 03:22 AM   #5
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Quote:
Originally posted by JayH3


Are you saying I should swap the FB between the L and R channels or something else entirely?

No, do not cross channel anything. The second page of the instruction manual is the key.

Is one side oscillating, or both? Reverse the connections at the drive lines to the channel(s) that's buzzing.
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Old 22nd May 2008, 06:47 AM   #6
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Quote:
Are you saying I should swap the FB between the L and R channels or something else entirely?
I'm thinking of only one channel. If the primary of the OP transformer is connected 'out of phase' with the secondary, then the feedback will be positive instead of negative. The way to correct this is to change over the leads connected to either the primary (which is usually easier), or the secondary. You can try this with whichever channel has the problem.
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Old 22nd May 2008, 03:03 PM   #7
udailey is offline udailey  United States
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Sorry to butt in here. I think someone here could help me as well. I am working on a similar problem. Maybe its oscillating but I have all wires connected correctly.
Here is link to my problem
http://www.diyaudio.com/forums/showt...46#post1517246
Maybe in solving this guys problem I will be able to fix mine or vice versa.
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Old 23rd May 2008, 01:56 AM   #8
JayH3 is offline JayH3  United States
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That solved it! This thing sounds so much better than ss.

Now that I got it working with cheap caps I want to upgrade them. Should I upgrade all 10 caps or will some of them not have much impact at all? Which caps in the circuit will have the most impact?
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