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In Reply to: RE: Finally ........... it's done. posted by deafbykhorns on May 31, 2017 at 04:45:32
Don't know how to determine the power output. The schem is attached.
Follow Ups:
Put a 8 ohm resistor in place of the speaker.
Drive the amplifier with a tone generator.
Connect a scope across the resistor.
Measure the AC voltage across the 8 ohm resistor just below clipping.
Voltage divided by resistance equals current.
Current times voltage equals watts.
Now you know the output power just below clipping into a 8 ohm resistor.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
How do I tell when it is "clipping"?
The wave form will "square off" at the top and at the bottom.
Increase the output of the tone generator until it clips then lower the output of the tone generator until you don't see any clipping.
Then measure the AC voltage across the 8 ohms resistor and do the math.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
So "clipping" is a most descriptive name for it then. Thanks! I will give it a try.
Well done Vinnie!
However I do have a question about the voltage drop from 475 to 350 through the FS030. It does seem to be too much. How much current are you passing through the transformer?
ray
Thanks. Actually now that you mention it, I did not check the B+ again going into the opt, so it may be a bit lower. That was from a earlier version of the amp and I am not sure it is still correct. The 350 coming out was actually about 340 vdc when I checked it last night, so the B+ going into the opt may be lower too.
I think when vinnie shows 350vdc at the plate he means plate to cathode not plate to ground.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Actually the 350 vdc (now 340) was measured between the end of the resistor next to the cap on the 813 and ground, so it should be the same as mearing between the plate and ground. I just realized I really should have checked and written down all the operating points before putting it in place in my system. I will have to try and measure them where it's at I guess, as it is a heavy sucker and the less I move it with my bad back the better.
Edits: 05/31/17
Why is there a 200 ohms resistor between the output transformer and the plate of the tube?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Tre
I changed my reply below. It was incorrect.
Actually there isn't. The opt is connected to the cap end of the resistor. It is G2 that has the resistor between it and the plate. It's part of the triode connection for the 813.
Edits: 05/31/17 05/31/17
OK.But we're back to the question of why you are dropping so much voltage across the output transformer.
What is the DC voltage at the top of the output tube's cathode resistor?
Knowing that will tell you how much current the output tube is drawing.
BTW Even though the 200 ohms resistor is connected properly it still needs to be down at the tube socket under the chassis, not up by the plate cap.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 05/31/17
Perhaps you misunderstood the recommendation. When triode strapping, the resistor is placed between the plate and G2, not between the OT and the plate.
My reply was incorrect and I edited it. Please read the corrected version.
I see. I was going by the schematic, which shows the resistor between the OT and the plate.
I didn't notice the pic with all the extra wires connected up by the cap. All those should really be moved below deck.
As Tre mentioned, the resistor should be soldered as close as possible to the G2 pin. I presume the other wire coming off the caps of the driver tubes is going to the 0.1 coupling cap. That can be moved underneath too. Just connect it to the same place as the other end of the wire going to the cap and remove the wire. The result will be much cleaner looking and safer.
Please see my "update" post above. I believe there is a reason I can't do that.
The 200 ohm resistor (grid stop resistor) goes between the screen grid and the plate, with the resistor itself as close to the screen grid pin on the tube socket as you can get it.
You want the lead, from the resistor to the grid pin on the tube socket, as short as possible.
The length of the resistor lead from the resistor to the plate connector is not important.
The wire from the output transformer goes directly to the tube's plate connection.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Sounds like I need to change the position of that resistor and the connection to the cap. Thanks
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