Next episode in my Akai/Roberts rebuild project... (Thanks again for the help so far.) First order of business (no actual modding has yet occurred) is to install bleeder resistors to prevent myself from getting zapped. Here's the power supply schematic:
(note there's a misprint: should be C1A and C1B, not C1A and "C2B")
I don't see any bleeder resistors in the schematic. (There's no heater voltage from B+ elsewhere in the amp, either.) Initially I was going to put a (say) 470K/2W resistor from the 195V point to ground to discharge all four caps. What I don't understand is how the choke might complicate this. I know bleeders are also used to provide a minimum load to the choke, but frankly this part went over my head.
Something is also telling me that if the discharge current falls below the minimum for the choke, it won't properly discharge C1A with the bleeder on the other side of the choke. Am I just misunderstanding this/making things up?
Any suggestions on the placement of the bleeder(s) would be terrific.
-Erik
P.S. I'm also thinking that it should have at least a fuse on the B+ (the only fuse is on the mains) but that requires actual measurement of the circuit to find the right value, yeah?
(note there's a misprint: should be C1A and C1B, not C1A and "C2B")
I don't see any bleeder resistors in the schematic. (There's no heater voltage from B+ elsewhere in the amp, either.) Initially I was going to put a (say) 470K/2W resistor from the 195V point to ground to discharge all four caps. What I don't understand is how the choke might complicate this. I know bleeders are also used to provide a minimum load to the choke, but frankly this part went over my head.
Something is also telling me that if the discharge current falls below the minimum for the choke, it won't properly discharge C1A with the bleeder on the other side of the choke. Am I just misunderstanding this/making things up?
Any suggestions on the placement of the bleeder(s) would be terrific.
-Erik
P.S. I'm also thinking that it should have at least a fuse on the B+ (the only fuse is on the mains) but that requires actual measurement of the circuit to find the right value, yeah?
You're thinking of a choke input power supply. This is a capacitor input power supply. You can put the bleeder wherever you like, since it is only a trickle-drain of current. 1M or 470k in parallel with the first cap is the usual arrangement.
According to the schem the quiescent current is 50mA (from the voltage drop across the choke), so you'd probably be OK with a 250mA or maybe 315mA fuse (or less if this is a class A amp).
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It can go at the inductor output, if properly rated. You can use for example, a 100k, 2W or 200k, 1W bleed resistor.
If it is placed later in the power supply, the DC voltages will be affected because of a divider action with the 5k and/or 50k resistors.
Are you sure that there is no resistive path to ground in the audio circuit that would serve this purpose?
nice thread!
An externally hosted image should be here but it was not working when we last tested it.
Yo Erik,
When the AC power is turned off the tubes still continue to draw current for several seconds as they cool down. This action is usually enough to drain the filter capacitors. But if you want to feel content you can install a bleeder resistor most anyway. I'd use around 100K from the 265 volt point. And don't be concerned about the choke. With power off it's action is just a 200 ohm wire wound resistor.
When the AC power is turned off the tubes still continue to draw current for several seconds as they cool down. This action is usually enough to drain the filter capacitors. But if you want to feel content you can install a bleeder resistor most anyway. I'd use around 100K from the 265 volt point. And don't be concerned about the choke. With power off it's action is just a 200 ohm wire wound resistor.
For me C1A is best C2A and C1 left alone,has the geatest ripple current.Like that there is no common conductor with the others and doesn't heat-up his neighbur.
The bleeder in front of the choke, like that the currents in the other components remains the same.
If you use the bleeder current for a LED there is at the same time an indication that the HT is present.
Mona
The bleeder in front of the choke, like that the currents in the other components remains the same.
If you use the bleeder current for a LED there is at the same time an indication that the HT is present.
Mona
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