K-12 kit, buzz in R channel

[mnr102]

Hello,
I'm wondering if anyone's had the particular problem I'm having w/ my K-12 (rev G) kit amp, or if anyone can give me some debugging ideas:

Overall, I'm very happy w/ the sound of the amp. L channel is very clean. However, when I turn down the music or when there's no input source, there's a buzz in the R channel. It's always there, and its volume & frequency is independent of the volume knob. That is, it's always at the same freq & same volume, regardless of where the volume pot is. Also, I tried swapping input RCA cables & also swapping speakers. The buzz always stays in the R channel. I've also swapped sets of tubes, and even bought a seperate set of tubes. Buzz still stays in the R channel.

Thanks much,
Mike

 

[gmilitano]

Could you post a picture?

Look for the speaker output or signal inputs to be close to an AC source such as the mains, HV or heater.

 

[mnr102]

photo

Thanks for the ideas. Unfortunately, I worked that angle already. I moved speaker & input & transormer wires around, but it had no effect. I also played w/ a metal spatula, creating transformer shields with it, but that too had no effect. I'm very convinced it's some instability in the circuit. I think going thru the signal path w/ an oscope & then throwing caps across resistors is the only idea I have left.

-Mike

 

[gmilitano]

Look to see if the chassis of the pot is grounded to the board. There is a note about this in the instructions provided. Try the input signal after the pot. My pot did not last very long.

I found that with my S-5 Electronics K-12M Tube Amplifier Kit the snubber capacitors as proposed by Voltsecond
helped clean up some of the hiss.

You can also try the transformer rotations as shown on the voltsecond page.

 

[angsuman]

Make sure your AC wiring is twisted tightly I used to have a problem with the pot as well. At the lower and upper ends it would cause very low frequency oscillations in one of the channels like a 5hz pop in the woofers. I added a 0.1 uf capacitor across the HV inputs which did help to a degree. However I found the best solution was to increase the feedback by lowering the value of the feedback resistor a bit (I can't recall the value at the moment).

 

[mnr102]

yup, oscillation at ends of pot

Thanks to GM for the suggestions. I'll try these.

And Angsuman, yup I have the oscillations at the end of the pot spins too. It's high F on one side & low F on the other. I'll look into lowering the feedback R value. So, if that improves stability, what's lost in the process? Bandwidth, I'm guessing?

Thanks,
Mike

 

[angsuman]

Interesting, yeah that's exactly the problem I had it was with the pot. I'm not sure if its a bad pot or if those ends of the pot lead to instability. I personally found the sound was improved in that it had lower distortion. I'm sure you've found turning it up results in pretty bad distortion. The only trade-off really is the fact that you will need more signal level to get it louder especially if you're using something like an Ipod or a computer connected to it. My CD player is fine with tis 2V output. I added a 70k resistor in parallel to the feedback resistor to lower its resistance. You might want to experiment for the best setting.

 

[kvk]

Anyone know what kind of volume pot the K-12 amp has?

Is it a stereo or mono?

 

[mnr102]

It's a stereo pot. 6 leads, 3 for each channel.

 

[kvk]

It's a stereo pot. 6 leads, 3 for each channel.

Ohms?

 

[mnr102]

Sorry,don't know off the top of my head. If I think of it, I'll measure it next time I'm near it. The schematic may list this.

-Mike

 

[Jeb-D.]

I got an earlier version of the kit that had a few board errors. One of the errors was that the hole they supplied for grounding the pot was actually tied to the input signal line of one channel rather than to ground. You may want to check yours for the same.

 

[mnr102]

Thanks Jeb,
I'll check that out. That would explain why only one channel of my amp has noise. Here's to hoping it's that... an easy fix.

-MIke