If you want a secondary fuse then one in the CT is fine.
Heater supplies should not float, so the EZ81 heater secondary needs to be connected to something. Connecting it to ground puts a high voltage across the heater-cathode insulation - which is designed to cope with this, but may fail. Connecting it to the cathode puts a high voltage across the heater secondary insulation - which may or may not be designed to cope with this, possibly depending on the age of the transformer and exactly what the datasheet says. Using a highish value resistor as Osvaldo suggest would be a useful precaution.
Heater supplies should not float, so the EZ81 heater secondary needs to be connected to something. Connecting it to ground puts a high voltage across the heater-cathode insulation - which is designed to cope with this, but may fail. Connecting it to the cathode puts a high voltage across the heater secondary insulation - which may or may not be designed to cope with this, possibly depending on the age of the transformer and exactly what the datasheet says. Using a highish value resistor as Osvaldo suggest would be a useful precaution.
Not necessary, but in my own experience, recommended.
I use a 47 or 100KΩ and a 1µF 400V from pin 5 to gnd. Also, nor the C nor the R are mandatory, but I use them, as in the DOS schematic.
This resistor does't conduct any current, so a 1/8W is sufficient, but for commodity I use ½W.
This is my circuit.
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I've never seen anybody worry about fusing the heater supply. A fuse on the primary side of the transformer should cover you unless you are worried about damaging an irreplaceable vintage power transformer I wouldn't bother.
For safety reasons I'd add bleeder resistors across the HT capacitors.
For safety reasons I'd add bleeder resistors across the HT capacitors.
hi
see below my ps as it is now
i started the preamp yesterday for the first time and took some measurements
i am geting 308V not 260V
so the plate V for the 12b4 is 116V not 90
i want to add a bleeder resistor, although the caps are bleeding when i close the amp (?)
if i add a bleeder resistor
1- will it drop the b+ ? ( before i change the 1K resistor )
2- were to place it for better regulation? C1 C2 C3 all of them?
3- how can i calculate the values of the resistor for better v regulation, W and R ?
so many questions ....
thank you all
see below my ps as it is now
i started the preamp yesterday for the first time and took some measurements
i am geting 308V not 260V
so the plate V for the 12b4 is 116V not 90
i want to add a bleeder resistor, although the caps are bleeding when i close the amp (?)
if i add a bleeder resistor
1- will it drop the b+ ? ( before i change the 1K resistor )
2- were to place it for better regulation? C1 C2 C3 all of them?
3- how can i calculate the values of the resistor for better v regulation, W and R ?
so many questions ....
thank you all
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tnx DF96
i just read somewere that placing aBR on a the choke cap can help, but didnt quite understend
the caps are bleeding when i close the amp so i dont worry about safety but i still check them with a meter and a screwdriver before handling the amp
i will change the 1K R to try and lower the B+ just because i have one ready...
i just read somewere that placing aBR on a the choke cap can help, but didnt quite understend
the caps are bleeding when i close the amp so i dont worry about safety but i still check them with a meter and a screwdriver before handling the amp
i will change the 1K R to try and lower the B+ just because i have one ready...
I suggest two variants to the schematic:
1) remove the bridge diode and the led,
2) place a neon lamp in the +B to gnd, via a 200KΩ resistor.
This way, the lamp will indicate when +B is present, and also save the cost and space of the diodes. Also, neon lamps shut down at about 60V, indicating that +B is sufficient low to play in the equipment soon.
1) remove the bridge diode and the led,
2) place a neon lamp in the +B to gnd, via a 200KΩ resistor.
This way, the lamp will indicate when +B is present, and also save the cost and space of the diodes. Also, neon lamps shut down at about 60V, indicating that +B is sufficient low to play in the equipment soon.
tnx Osvaldo
its a neat idea
i will look for this lamps localy or order some on line
the bridge diode was salvaged from a small charger and was all ready installed
i can have both, one for B+ and one for heaters
Surely, but it is useless to have two pilots in the same set, or not?
I use them (neon lamps) as zeners for voltage references in electronic tube regulators.
hi again
as I wrought before I started the preamp and the B+ was a bit high 308V
so I replaced the 1K resistor with a 1.2K and then problems started…
sparks & smoke from the positive terminal on C3 100Uf/450V (not 500V like in the schema)
I replaced it back to 1K but still S&S from C3
Replaced the cap with a 47Uf/400V so I got S&S from positive terminal on C2
I thought maybe the current is to high so I tried putting limiting resistor between the cathode and C1 then the fuse on the CT of the HT blew
When disconnecting the smoothing circuit and measuring from the rectifier I get 292VDC and the from TR 635VAC
The chock is 10H/80ma with 200R DC
To my understanding probably too much current?
May be need smaller caps < 47uf ?
If smoke&sparks came from a cap can I use it for further testing's or its dead?
your help please
as I wrought before I started the preamp and the B+ was a bit high 308V
so I replaced the 1K resistor with a 1.2K and then problems started…
sparks & smoke from the positive terminal on C3 100Uf/450V (not 500V like in the schema)
I replaced it back to 1K but still S&S from C3
Replaced the cap with a 47Uf/400V so I got S&S from positive terminal on C2
I thought maybe the current is to high so I tried putting limiting resistor between the cathode and C1 then the fuse on the CT of the HT blew
When disconnecting the smoothing circuit and measuring from the rectifier I get 292VDC and the from TR 635VAC
The chock is 10H/80ma with 200R DC
To my understanding probably too much current?
May be need smaller caps < 47uf ?
If smoke&sparks came from a cap can I use it for further testing's or its dead?
your help please
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