Thumping in V6

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Thumping in V6

Postby Heath » Thu Mar 12, 2015 3:10 pm

I'm a relative newcomer hoping for a little help. I did the basic build with Edcors and the zener mod, just following the directions.

Most of the problems I've encountered I've been able to solve by combing through this forum: feedback was wired incorrectly, bias was too high so I put in 12V zeners and 200 Ohm pots, some humming, etc So thanks for a great forum and knowledgeable people willing to help out.

My voltages were all within range except for
V3-V6 pin 7 was ~400 (but I read in someone's post that a high B+ was OK, so I didn't worry)
V3-V6 pin 9 was ~402

and

V2 pin 1 was 256
V2 pin 6 was 247

The amp sounded good (except for a little hum that I was living with until I got it fixed) and I was giving it a trial run. After I had been listening for a while, I heard a thud, thud, thud and V6 started glowing brighter and brighter. I shut it off.

I double checked all resistor values and they are correct.

If anyone has any advice for me that would be great. I want to get back to listening to all tube goodness.

By the way, Shannon, the hole in your FPE file for the on/off switch doesn't fit the switch in the mouser BOM. No biggie, but I thought you'd like to know. It is sized for a smaller switch.

Heath
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Re: Thumping in V6

Postby Shannon Parks » Wed Mar 18, 2015 1:24 pm

Heath wrote:My voltages were all within range except for
V3-V6 pin 7 was ~400 (but I read in someone's post that a high B+ was OK, so I didn't worry)
V3-V6 pin 9 was ~402

and

V2 pin 1 was 256
V2 pin 6 was 247


These voltages are OK.

Heath wrote:The amp sounded good (except for a little hum that I was living with until I got it fixed) and I was giving it a trial run. After I had been listening for a while, I heard a thud, thud, thud and V6 started glowing brighter and brighter. I shut it off.


Send pics of the inside (large is good). Presume a safety ground to the chassis, and then a single wire from PCB ground to chassis should fix this hum issue. Also shielded input wiring.

Verify that your diodes are still OK with a DMM. If they're OK, then I'd restart the amp and closely monitor the bias voltages, particularly V6 which may have thermally run away. What were they biased at?

Heath wrote:By the way, Shannon, the hole in your FPE file for the on/off switch doesn't fit the switch in the mouser BOM. No biggie, but I thought you'd like to know. It is sized for a smaller switch.

Heath


So noted. Thanks!

Shannon
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Re: Thumping in V6

Postby Heath » Mon Apr 20, 2015 8:04 pm

Thanks for getting back to me. I replaced a diode and biased everything to 350 mV. It's working fine, but I can still hear a little hum when nothing is playing.

I think I followed the grounding scheme carefully, and I used shielded cable for the inputs.

Here are links to pictures of the undercarriage.

https://www.dropbox.com/s/f70q9qaeankzp4j/pic1.jpg?dl=0
https://www.dropbox.com/s/f3a94c572yuagko/pic2.jpg?dl=0
https://www.dropbox.com/s/r2sbklc903wlfak/pic3.jpg?dl=0

The electrical tape is a temporary situation until I figure out how best to route those cables.

Some more info: I tried a ground lift plug, didn't work. When I shorted the inputs, there was no hum. When I connected both inputs from the preamp there was hum.

I appreciate all of the help.
Heath
 
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Location: California

Re: Thumping in V6

Postby Shannon Parks » Wed Jun 17, 2015 4:45 am

All your wiring looks good, Heath. Since you get it hum-free with shorted inputs we can assume the problem is there. First verify that the RCA connectors are not shorted to the chassis and that the plastic washers are doing their job. You can just unscrew the connection at the board and then use your DMM from the shield to the chassis. If they aren't shorted (one or the other or both), then I think we need to just shorten those lengths. Cut those wires done and route directly across the bottom of your amp from the RCA to the front connector. This will decrease the loop size by a factor of two and will fix the hum pickup.

Shannon
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