Grounding?

for Dynaco Mark II/III/IV and DIY PP monoblocks

Postby TomMcNally » Mon Nov 17, 2008 10:41 pm

I think Min is passing me ! I am going to build a pair of Poseidons
in a week or so, for a guy that found my website, should be fun.
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Postby Blair » Tue Nov 18, 2008 9:40 am

LOL!!

I can build them that fast, but cannot build a quiet amp that fast!

As much as I hate to do it, I may have to take this amp apart and rewire it. Does the bias circuit typically produce hum? I'm not too sure what else can be done here though.

I was going to fully isolate each channel by completely dividing the grounds per channel. I can do this, but the only obsacle is the bias tranny. Since I am using a half wave rectifier and only one leg of the tranny, can I simply use the CT for the ground establishment and then Y off the diode to complete the bias circuit? If that does not make sense, let me know.

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Postby TomMcNally » Tue Nov 18, 2008 9:56 am

Did you try a filament center tap ? It's important.
The only other thing I see in your pictures you have
lots of dissimilar things bundled together ... AC and
audio, etc. You have the audio input leads running
through all kinds of other wires.

I'd troubleshoot by doing this:

remove all wiring to the input terminals and short them

remove the feedback connections for testing purposes

make sure the main filter capacitors are grounded at

the negative end of your diode bridge ... directly ... no long wires

try to determine if the hum is being radiated into the
circuitry magnetically from the power transformer
or chokes

The proximity of the chokes to the output transformers
could possibly induce hum right into the transformers

I doubt the bias has anything to do with it. Make sure all
external devices are disconnected when you are listening
for hum ... just the amp with shorted inputs and your ear
against the speaker
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Postby Blair » Tue Nov 18, 2008 10:11 am

I'll give all of this a try. With the hum I have, there is no need for the ear against the speaker Yellow_Light_Colorz_PDT_07

It sounds like a KISS concert when there is no music.

Thanks for the troubleshooting tips!

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Postby Blair » Tue Nov 18, 2008 9:58 pm

TomMcNally wrote:Did you try a filament center tap ? It's important.
The only other thing I see in your pictures you have
lots of dissimilar things bundled together ... AC and
audio, etc. You have the audio input leads running
through all kinds of other wires.

I'd troubleshoot by doing this:

remove all wiring to the input terminals and short them

remove the feedback connections for testing purposes

make sure the main filter capacitors are grounded at

the negative end of your diode bridge ... directly ... no long wires

try to determine if the hum is being radiated into the
circuitry magnetically from the power transformer
or chokes

The proximity of the chokes to the output transformers
could possibly induce hum right into the transformers

I doubt the bias has anything to do with it. Make sure all
external devices are disconnected when you are listening
for hum ... just the amp with shorted inputs and your ear
against the speaker


This is where I got this evening. I thought moving those driver filter caps up towards the boards and shortening the leads may help eliminate some hum. I also pulled out the solid copper wire between each sides ground to isolate them. The only issue I'm having is figuring out how to run the bias to both channels without creating a bridge between the channel's grounds.

Image

With the center tap for the filaments, can it be anywhere on the filament windings? It would be easier to put on the bottom KT88 on each side. I would think electrically it would not make much difference.

I did not get much further into rewiring. I was out of good solder. The stuff I have is the no lead stuff and it is horrible!

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Postby Ty_Bower » Tue Nov 18, 2008 10:16 pm

deicide67 wrote:
TomMcNally wrote:I did not get much further into rewiring. I was out of good solder. The stuff I have is the no lead stuff and it is horrible!

Off topic, but I've been worried about this for a while. All my solder is leftover from when I was a kid, but I recently touched spool while finishing my Mk3. I'm genuinely concerned about the whole RoHS thing, and whether my new solder is going to suck now.

What are my options?
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Postby Blair » Tue Nov 18, 2008 10:35 pm

I don't know about the "lead free" stuff, but Radio Shgack still sells decent solder and there are a few "high silver" solders out there. I just try not to breathe it and use the leaded stuff.

I don't know how old you are, but your old solder may be older than I am. I'm only 29.

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Postby Blair » Sat Nov 22, 2008 9:53 am

I finally got the amp quiet, or at least quiet enough that my SS preamp introduces more noise than the amp makes on its own. There is a very slight 60hz hum, but I would have to have an intimate relationship with my midwoofers to hear it. It is now as quiet as my Yaqin which almost replaced this guy. Now they are both operating well, it is time to A/B test them.

I took the entire grounding scheme out and rewired the entire amp with a star ground on everything including separating the bias from the cathode resistors, etc. It made about 10 grounds under the hood, but it is quieter. I also made a CT for the filaments which was the most marked improvement. I almost though I made a mistake and blew a fuse initially because I just hooked it up with alligators before I soldered it in. Anyway, I just wanted to thank Ty and Tom for their assistance on this and saving my tubes from meltdown! I need to buy a beater quad if I'm going to build anything else like this. These SEDs are a bit steep for testing with.

I'll take a pic in a little bit.

Thanks again!

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Postby Ty_Bower » Sat Nov 22, 2008 4:28 pm

Glad to see it's working. Yes, the beater quad is a good thing to have around. I think I got these Shuguangs for something like four bucks each, shipped included. They go in first to see if the smoke comes out. All things considered, they're actually fairly sturdy tubes.

Image Image
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Postby Blair » Sat Nov 22, 2008 5:38 pm

I used to have a quad of sovtek 6550s I bought off ebay for $40 shipped. They sounded great really, but they were no big deal if I smoked them.

Seeing everyone else building monos is making me want to build a pair, but I'm trying to quit this nasty habbit Yellow_Light_Colorz_PDT_06

Thanks again Ty,

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