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Old 18th December 2011, 01:14 AM   #21
ryuji is offline ryuji  United States
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If your whole goal is to just reduce the volume you could always put a voltage divider on the input. running your DAC at 1% is hurting resolution on your sound card as it stands by lowering it so much.

you could always also build an actual volume control on the amplifier

Reduce the amplifier gain as much as you can before tho as high gain will be quite sensitive to noise.

To help get rid of the noise, replace all your wires with shielded cabling if needed. I had to do that on both my speaker and input sides
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Old 18th December 2011, 02:43 AM   #22
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You have less gain inside the feedback loop, so less feedback - the feedback resistor could be reduced to get feedback to the original amount. Unbypassed cathode resistor on the first stage should be fine. You did adjust the voltage divider values in the paraphase splitter for the lower gain of the 6CG7?
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Old 18th December 2011, 02:49 AM   #23
GloBug is offline GloBug  Canada
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Ah I see.

Not properly I'm sure.
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Old 19th December 2011, 02:20 PM   #24
GloBug is offline GloBug  Canada
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OK the schematic is finally coming along. I have been trying to draw it in Google Sketchup, as it is what I know best. The problem was it was hard to see zoomed back because the program was putting these little line "extensions" on all the corners etc.

The problem was solved by switching the actual drawing template from "Architectural" to "Plan View", much easier to see now.
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Old 20th December 2011, 11:23 PM   #25
GloBug is offline GloBug  Canada
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So here is a schematic for better or worse. You can see the tinkering I was doing in the front end from last year. Some real funny voltages there now.

On the plus side I have a nice system figured out for drawing schematics in Google. Turn out OK for the first go. I will have to see how it looks uploaded on here, hopefully you can read the values.

I also uploaded a schematic for reference. It's basically the stock Magnavox AMP132 with the screen choke drawn in, compliments of ab00ez.

So it looks like I really ventured of with now real plan. I changed the heater wiring for the 6CG7 to replace the 12AX7 and added the extra socket for the additional 6CG7. It was about this time that the noise started. It was silent before that.

Oddly enough it sounds pretty good right now in this state, aside from the other noise and the too much gain thing.

So have a look and let me have it! lol
Attached Images
File Type: jpg PPP Schematic.jpg (317.8 KB, 90 views)
File Type: jpg Magnavox 132 screen choke mod.jpg (527.7 KB, 90 views)
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Old 21st December 2011, 12:14 PM   #26
GloBug is offline GloBug  Canada
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Sigh, maybe I will just go back to the 12AX7 in the original schematic.

Even if I bias the 6CG7's right, I'm not sure how "proper" it is for the amp. It was suggested by many as a good swap so I thought I might have some support regarding the switch.

Let this be a warning not to ruffle feathers with diyAudio moderators and their cronies.
With the exception of Kevin, the rest have completely shunned me lol.

Well maybe I can get myself out of this mess, I am going to try a few things today., I would like to continue to use the 6CG7's because they do sound nice, even in their current arrangement. Plus I have the "3rd" socket now which I don't really want to leave empty.

Last edited by GloBug; 21st December 2011 at 12:40 PM.
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Old 22nd December 2011, 12:15 AM   #27
GloBug is offline GloBug  Canada
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I can be such a whiner at times lol.

I have a drawing coming along with suggestions from the previous pages.
Now that I have the drawing it makes it easier to visualize it, as it was "all in my head" before.

-Individual RC bias, with possible pots.
-Adjust the feedback way down, I had the opposite going on with 100K vs. the 27K stock rfb. I will pop in a 10K and see what happens.
-unbypass the inverter.
-gridstoppers

If that does not work out, I might try a cathode follower front end. I looked into it more, might go this way because frankly I don't know what to do with this paraphase inverter with the 6CG7 in that spot.

There I feel better now.

Last edited by GloBug; 22nd December 2011 at 12:17 AM. Reason: -gridstoppers
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Old 22nd December 2011, 02:44 AM   #28
GloBug is offline GloBug  Canada
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Well I made some changes. too many at once I suppose.

I updated the schematic, (I had the feedback drawn in the wrong part)
-I put 33K gridstoppers on the first 6CG7.
-changed the grid resistor from 100K to 470K. (I know this makes it louder, just trying to get it closer to the way it was)
-lowered the feedback resistor from 100K down to 10K.

Now the amp squeals/howls. I only had it on for a moment, not sure if it's oscillating or if it is feedback howl. I will have to go back and change something. else. I still am going to raise the voltage back up on the 6CG7.
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Old 22nd December 2011, 03:19 AM   #29
GloBug is offline GloBug  Canada
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Nope it wasn't feedback howl, it made a god awful noise when I flipped the plate leads around. lol
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Old 22nd December 2011, 04:03 AM   #30
GloBug is offline GloBug  Canada
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OK "Davesnothere's" tip about the gridstoppers worked. I should have known better and do use them in guitar amps, it was something I overlooked because the amp was silent at one point in time without them.

Now I finally got rid of the noise again with the 33K gridstoppers.

I also changed the feedback resistor to 22K, which is what it was on the original schematic. I have tried lowered, but it made those noises.

I also switched the plate wires back to normal.

So I am half way there. I'm not a hundred percent sure, but I think the volume situation is a little tamer now.
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File Type: jpg PPP Schematic V3.jpg (298.4 KB, 37 views)
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