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In Reply to: RE: So what have I got wrong this time? posted by vinnie2 on May 24, 2016 at 08:48:37
vinnie2,
OUPUT STAGE
You B+ for the 45 is WAY to high. The maximum plate to cathode voltage for the 45 is 275. Now add about 50 volts of cathode bias and 275+50 is 325 VDC for the power supply. You have 400! Further, you need a cathode bias resistor. For 35mA, this is 50/35mA and I generally use 1.5k Mills MRA-12. The bypass cap is approximately, C=1/(2*pi*R*F), so with 1.5k and f of 5 Hz, that's something like 20uF. I often use 47uF/400V Panasonic TSHA.
NOW, why did you get hear? First, the schematic you tried to copy was direct coupled. This is where the too much B+ came from. Second, you either forgot the cathode resistor which would have been much bigger or there were stacked supplies.
INPUT STAGES
You MUST remember that when you add an addition stage, that is one versus two 26 stages, you have to change the absolute polarity. You really can't make a comparison without doing this.
Each of the 26 stages inverts 180 degrees as does the output stage. The output you used is phased correctly for two stages but not three.
POWER SUPPLY
Generally, for a three stage amp you need not decouple the second stage from the output stage but you must decouple the input stage.
Do these things and then chase what others are saying....otherwise, you'll end up chasing your tail.
Hope that helps...
Follow Ups:
output
I'll have to check on the b+ fore the 45. It may be too high. I do have a cathode bias resistor, the schem is drawn wrong. As I said in a earlier post in this thread, the cap and resistor are actually below the pot not above it. Guess I had better correct the schem and repost it.
Input
This came up the first time I breadboarded this amp. I think Tre' brought it up. The preamp I am using has a single 26 tube so we decided that made things ok again.
Power supply
Are you saying I should take out the cap and resistor between the 26 and the 26 and 45 and make it direct coupled?
POWER SUPPLY
Nope. The B+ feeds the 45 and the second 26, then you decouple using a resistor and a cap to ground to feed B+2 to the first 26.
The way I have it right now I am taking the B+2 of the decoupling node for both 26's. Is that ok?
vinnie2, you gotta remember that the output stage and the second 26 stage are 180 degrees out of phase when drawing from the supply whereas the input or first 26 stage and the output are in phase. Add the difference in signal levels and this why you want to decouple the input stage. The tail will wag the dog, so to speak.
Decoupling both input and driver stages generally increases the supply impedance of the driver stage and doesn't sound as good as just decoding the input stage.
Is it necessary to decouple if the second stage is choke loaded or IT coupled from the plate of second stage?
Not sure I follow what your asking...
I'll have to take your word for it. After I got things working again (see update), I tried it both ways and was not able to hear any real difference. Of course I did not listen very long and I did not try a lot of different types of music, so it may be there. I will have to remember that if after I finish building it I want to make some mods. Besides that my ears are 69 years old now, and who knows what they are not telling me! I left it without decoupling just in case it does make a difference.
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