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Old 14th September 2010, 07:38 PM   #11
20to20 is offline 20to20  United States
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Quote:
I'm lost...
1. You had a working prototype. You had known good components.
2. You tore it apart and rebuilt it.

Go back to a working prototype build and then make one tidy-up at time. Or undo each step backward toward the prototype build, testing each backstep until it's working again.

Also, you cannot expect each tube variant to perform the same in the circuit. Those 12xx's have different amplification factors. Use the scientific method in troubleshooting by making only one change at a time between tests.

Don't rule out tiny solder splashes or bridges.
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Old 14th September 2010, 07:40 PM   #12
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Click the image to open in full size.



Imagine that schematic, minus 1 of the 807's, R6 was eliminated, trimmer on the bias supply is now 50k, input into the bias supply is now 20vac (had no need for the amount of bias a 72v supply gives) the single 807 when running operated @ 32ma cathode current.



Resolved the issue, 3 min's on the phone with Anatoliy solved the mystery! (it was a miracle the amp worked at all)


Anatoliy, you rock! Next time you come to las vegas, I have a treat for you.
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Last edited by 807tubenut; 14th September 2010 at 08:08 PM.
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Old 14th September 2010, 08:45 PM   #13
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Thank you! I don't use an alcohol.

Here are curves you can use:
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Old 14th September 2010, 10:04 PM   #14
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Having looked at the schematic (saved on my computer, loaded into Paint, zoomed "large size" 400% so I can read the small font in low-contrast colors) and followed your description of mods I'm pretty sure I now see what the problem was (it appears to have everything to do with excessive signal loading of the 12AT7 output through the 807's bias circuit - even in the original circuit, R6 looks to be three orders of magnitude lower than it ought to be), but since you say you "resolved the issue" it would be good if you actually told us exactly what it was. A simple "I had done this instead of that" would be great.
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Old 14th September 2010, 10:22 PM   #15
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I was going to suggest to replace 470K resistor by 100K one, it would be safer for NOS tubes.
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Old 14th September 2010, 10:50 PM   #16
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Quote:
Originally Posted by benb View Post
Having looked at the schematic (saved on my computer, loaded into Paint, zoomed "large size" 400% so I can read the small font in low-contrast colors) and followed your description of mods I'm pretty sure I now see what the problem was (it appears to have everything to do with excessive signal loading of the 12AT7 output through the 807's bias circuit - even in the original circuit, R6 looks to be three orders of magnitude lower than it ought to be), but since you say you "resolved the issue" it would be good if you actually told us exactly what it was. A simple "I had done this instead of that" would be great.


In the supplied schematic,I don't see a plate load resistor on the first tube! (12AT7) (I'm assuming the 100K is feedback,and the "265V" is the plate supply for the 12AT7.)
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Old 15th September 2010, 12:16 AM   #17
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No, the 100k is the plate load resistor, B+ feeds the 12at7 in this configuration.


100k would likely be ok, the reason the designer used the 470k was to drop off some of the radical voltage his 70v bias supply would produce, I went with 20vac for the supply, and only needed the 4.7k resistor to do its job.


The fix to my amp, was as simple as taking the 270v B+2 wire off the anode of the 12at7, and letting B+1 power the tube, properly. since I was defeating the anode load resistor.




Oh, and the working voltages of the amp are as follows.

560v B+1
270v B+2
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Old 15th September 2010, 12:19 AM   #18
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Quote:
Originally Posted by benb View Post
Having looked at the schematic (saved on my computer, loaded into Paint, zoomed "large size" 400% so I can read the small font in low-contrast colors) and followed your description of mods I'm pretty sure I now see what the problem was (it appears to have everything to do with excessive signal loading of the 12AT7 output through the 807's bias circuit - even in the original circuit, R6 looks to be three orders of magnitude lower than it ought to be), but since you say you "resolved the issue" it would be good if you actually told us exactly what it was. A simple "I had done this instead of that" would be great.


Cant do much with the schematic size, that's how It came to me from the guys website.
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