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Old 6th September 2009, 04:01 PM   #21
Salas is offline Salas  Greece
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Quote:
Originally Posted by puginfo405 View Post
pardon me,the pictures did not upload the first time, so here it is.
Proceed with those builds, they will help you make tidy and clean wiring. Then you will be able to determine if there are issues or not.
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Old 7th September 2009, 02:28 AM   #22
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Hey! You wonder why you have hum! The pics say the whole story.

Don't even bother to fix it in this style. Seriously.

++++++++++++++++++++++++++++++++++++++++++++++++++ +++++

Hi Salas, i dont quite understand the deviation here. maybe you can share with me on things which are deemed wrong here. the parts on the board have been moving around with no effect on hum. the TXs are now in the wrong position but have been also in the 90deg off position with no effect. maybe they are far apart for induction (1ft away).

the caps are shown here below, so there are caps used. the dual start is also there!

cheers
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Old 7th September 2009, 03:13 AM   #23
Salas is offline Salas  Greece
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When you have a kit amp based on an unknown (to us) pcb design, wired with no dressing in a long distance on a plank, using wires and alligator clips going around, its futile to test it and decide that you are having a hum issue or not and due to what. It can be anything. Amps don't get tested or debugged like that. Put it together in some enclosure or tower in a comprehensive manner, so we can start following it. Assuming that the boards have no issues themselves, then some clear pictures in its final form can help us help you. I don't want to dissappoint you, but this is a very early, very hasty stage to draw conclusions about what you have there. Even if we solved it at this stage, it will be a totally different situation when you put it together in its final construction, and then another issue could arise. Take a picture of pcb's underside before you mount it, in case it is going to be useful when you finish the test build.
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Old 7th September 2009, 06:08 AM   #24
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Quote:
Originally Posted by Salas View Post
When you have a kit amp based on an unknown (to us) pcb design, wired with no dressing in a long distance on a plank, using wires and alligator clips going around, its futile to test it and decide that you are having a hum issue or not and due to what. It can be anything.

ahh i get it now. yes this was meant to be temporary. so i will proceed with the first build with the frames/permanent wiring and take go from there.
1 - have put in the 220k grid resistors on 6550 stage
2 - trying to match the GE50 cct with the JA30. due to the complexity in the stage between AX (input) and AU (invertor) in GE50, a separate pcb with parts will have to be done and then wire patched into the main board (at cut sections) where applicable.

keep you posted thks.
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Old 7th September 2009, 06:21 AM   #25
Salas is offline Salas  Greece
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Good luck!
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Old 3rd October 2009, 04:31 AM   #26
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hi all, just finished the mod using the GEC50 cct given by Salas. now the board looks like below with a smaller board attached atop. the main objective is to reverse the AU and AX from the original ctt. amazingly, with properly soldered wires, GNDing and wire dressing, the humm has disappeard despite components mounted in very close proximity.

made some changes to suit the pcb tracks and also existing components.
input stage : its now 12AX7 !!! B+ at 320Vdc. has an additional 1k ohm resistor on the incoming input line and all else is same.
invertor stage : its now 12AU7 !!! B+ at 327V, did not use the balance trim pot but used 2 matched 47k ohms resistor on the anode line to B+. all else is same.
power stage : same 6550 with B+ at 428V, cathode at 40V and screen at 425V. changed all resistors - 10k at grid not used but only a 220k, cathode resistor is 500ohms // 220uF cap and screen resistor is 100 ohms.

all components in the dotted red rectangle between the AX and AU are on the small pcb. the pcb is connected back to main board via short wires.

current problem : unable to crank up volume. there is music coming out but in very small volume level (very soft). the music is clear to a point and further volume incremental only distorts the whole thing. connecting the NFb resistor changes the sound to morse code like sound. pls help thks
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Last edited by puginfo405; 3rd October 2009 at 04:33 AM.
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Old 3rd October 2009, 09:48 AM   #27
SY is offline SY  United States
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Sounds like you've got an oscillation. The "Morse code" sound may well be blocking distortion.

1. Get a scope. Working blind, your chances of getting a complex circuit working properly are minuscule. A good used 100MHz scope will cost you less than the output transformers. You're trying to fix a car without socket wrenches.

2. Did you remove the grid stopper resistors from the output stage? You have an X through them. If so, put them back in. They're there for a good reason. All stopper resistors should be tight to the grid pins.

3. Make a DC voltage map, i.e., measure the voltages to ground at all plates and cathodes to see if the basic circuit parameters are correct.

4. DC heaters are a waste of time in power amps. If the heater leads are tightly twisted and all else is right in the circuit, you'll be able to keep hum more than 90dB down.

5. Double-check that the feedback you're applying isn't accidentally positive.
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Old 4th October 2009, 04:17 AM   #28
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hi SY,

2 - the X means the resistor is not there but the grid pin has a direct track connection to the 0.5uF. also it T's off to a 220k resistor and then GND. is this OK? just that it does not follow the actual cct. if the 10k must be there then the track must be cut to insert.

3 - i made a voltage map as suggested:
12AX7 B+320V - anode(top)=198V, cathode(top)=2V, anode(bott)=198V, cathode(bott)=2V.
12AU7 B+328V - anode(top)=149V, cathode(top)=5V, anode(bott)=148V, cathode(bott)=5V
6550 B+430V - anode(top)=423V, screen(top)=425V, cathode(top)=40V, anode(bott)=422V, screen(bott)=425V, cathode(bott)=40V

any abnormalities observed? the screen is higher than the anode in the 6550, is that OK/safe? datasheet says max 400V on screens.

4 - yes DC on 6550 is not needed.....agreed.

5 - lets leave nfb out for now till the basic problem is off.
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Old 4th October 2009, 12:22 PM   #29
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The grid-stopper was there for a reason- stability- and shouldn't be removed.

You're still in the limits for the screen voltages- the spec is really "screen to cathode" not "screen to ground," though that may not be obvious. So those voltages all look OK.
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Old 4th October 2009, 04:49 PM   #30
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ok then i will install the 10K grid stopper and see. just to back track, its the idiotic cct below which im trying to rework to the GEC 88-50. it never had the grid stopper there.
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