B+ is actually truely 390~400v but the ratio will still make do? looks like 100v at the center of the divider. is that what is wanted?
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never really asked my friend why it got its b+ that way, a guess is the at7 will contribute its own current swings to the primary of the transformer, thus improving gain slightly?
Hi!
Select it such to have the minimum voltage difference between heater and cathodes. Is this just supplying the pre tube or all heaters?
Thomas
Select it such to have the minimum voltage difference between heater and cathodes. Is this just supplying the pre tube or all heaters?
Thomas
Ok,
Here are a few thoughts.
The 12at7's are getting the B+ filtered by the o/p Tx. The 12AU7's are connected to the B+ supply (possibly with ripple).
I would think the 12at7's will be modulated by the driver tube. (just my thoughts.)
If this is the case if you take the 12au7's from the same B+ point they might be quiet. However I do not think this is a good way to do it.
If this proves correct than I would filter the B+ better and put a dropper resistor and another cap for supply to the driver and pre amp tubes.
I have never seen this done before!
I would be interested to hear if anyone is familiar with taking B+ for driver from the power tube anode?
Regards
M. Gregg
Here are a few thoughts.
The 12at7's are getting the B+ filtered by the o/p Tx. The 12AU7's are connected to the B+ supply (possibly with ripple).
I would think the 12at7's will be modulated by the driver tube. (just my thoughts.)
If this is the case if you take the 12au7's from the same B+ point they might be quiet. However I do not think this is a good way to do it.
If this proves correct than I would filter the B+ better and put a dropper resistor and another cap for supply to the driver and pre amp tubes.
I have never seen this done before!
I would be interested to hear if anyone is familiar with taking B+ for driver from the power tube anode?
Regards
M. Gregg
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all my heaters are connected to the 12.6v supply, the 807's heaters are wired in pairs
i don't think its reasonable to ask for the output transformers to source that much current. your thoughts of the OT adding filtering to my B+ got me thinking a bit on that tho. transformers will be seen as an inductance to the circuit. so maybe i should add an inductor to my b+ output that leads to my 12au7's
oh yea, and taking power from the output transformer. thats exactly what the UL tap does! same goes for running a pentode in 'triode mode'
before i had my power supply completed, my friend suggested just as a quick and dirty test to see if all was well, he suggested powering tubes off the UL tap as it would inherently drop voltage. but i have a feeling that feeding my pre's from my OT's would add feedback, and possibly oscillations?
before i had my power supply completed, my friend suggested just as a quick and dirty test to see if all was well, he suggested powering tubes off the UL tap as it would inherently drop voltage. but i have a feeling that feeding my pre's from my OT's would add feedback, and possibly oscillations?
i don't think its reasonable to ask for the output transformers to source that much current. your thoughts of the OT adding filtering to my B+ got me thinking a bit on that tho. transformers will be seen as an inductance to the circuit. so maybe i should add an inductor to my b+ output that leads to my 12au7's
I dont think that using the o/p Tx to filter supply to the driver is good. What I am saying is that there is a difference with the B+ on the 12at7's and the B+ with the 12au7's they are not the same supply!
And this might be the difference! I agree I would not source power for the pre via the o/p tx.
think my train of thought on inductance from the transformer is filtering my at7's supply, and thus i should consider putting another unductor before my au7's is on the right path? heck, a inductor would add a little voltage drop, which they could use anyways, since they are running slightly out of circuit spec anyways?
Hope this has given you some ideas as to what may be happening! It will be interesting to see if this is correct! I still think the first cap is a bit small. Let us know what you find.
Regards
M. Gregg
i actually intially built my circuit with a 47 uF primary electrolytic cap. i calculated using circuit impedance and equations elsewhere that said i think between 8 and 12 uf was more ideal. and the hum with my old speakers got reduced to nearly nothing with a 10uf USA military PIO!. however, i just upgraded to much more sensitive speakers i just made. and hum is back to haunt me
found the equations:
my amp is 260~300ma at a B+ of 400v so
400/280mA = 1428 ohms, 1/10th of that is 142.8
C=1/(753.6x142.8) = 9.29 uF
seems to make a lot of sense to me. given how common 10uF input caps are in amp circuits. something to experiment with, i suppose
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heres another observation i have noticed. if i hook my meter up to the chassis, and hold the other probe in my hand, i can measure a 60 hz signal which seems to be close to the same as what i found on the heater supply neutral to chassis/earth ground earlier
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hold the probe in my hand, as in the plastic part
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hold the probe in my hand, as in the plastic part
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807 max heater with respect to cathode is 135v, au7 is 100v, and at7 is 90v, and 100v for 12ax7 is 100v dc so i need to aim for 60~80v then for my divider then?
so, the divider should use nearest value to 23k ohms to obtain 75v as 100k+23k, so a 20k~24k resistor, whatever is avail at shop should do the job.
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whats the capacitor for? grounding AC signal?
i have a large quantity of various capacitors. is there any math to the selection of that capacitor? maybe i have a more ideal one
just from memory, i have some 1 uF 250v, 47 uF 500v,
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