I hope I drew these schematics correctly and its not a stupid question. Which configuration makes the "best" hybrid bridge rectifier, A, B or C? I have a "hunch" of which I like best, but no science behind it. I found schematics in my stash to be varying.
A: NEG (diode/diode) - POS (tube/tube)
B: NEG (tube/tube) - POS (diode/diode)
C: NEG(tube/diode) - POS (tube/diode)
A: NEG (diode/diode) - POS (tube/tube)
B: NEG (tube/tube) - POS (diode/diode)
C: NEG(tube/diode) - POS (tube/diode)
'A' is preferred because the two tubes can share the same heater supply, and you can use rectifier tubes with shared cathodes.
What is the benefit of using such a rectifier configuration, outside of its soft start characteristic which the tube provides?
If you use a typical tube type power transformer with a center tapped HV winding AND you ground that CT to get a positive and negative supply for a tube amp, then A is the only useful choice. The Tubelab TSE operates this way, and you absolutely want that negative bias supply stable before the B+ comes up. Many DHT's like the 300B can begin to conduct within a few seconds of filament power application. They are faster than a 5AR4 or a damper diode. A SS rectified negative supply ensures that bias is applied to the grid before the B+ supply comes up to avoid an uncontrolled start up.
What is the benefit of using such a rectifier configuration, outside of its soft start characteristic which the tube provides?
I have several "flat pack" 1:1 power transformers 300ma 120V:120V I want to utilize them in series for 240V out, but it won't have a CT so need a bridge, very compact. And I wanted softer start benefit too.
"but it won't have a CT so need a bridge, very compact. And I wanted softer start benefit too."
Error, I wouldn't have a CT if the transformers when the flat packs in series are unequal voltages, which is another reason as I do have other voltages I can combine. In the example I gave I would have a CT at the point I hooked them in series.
If you use a typical tube type power transformer with a center tapped HV winding AND you ground that CT to get a positive and negative supply for a tube amp, then A is the only useful choice. The Tubelab TSE operates this way, and you absolutely want that negative bias supply stable before the B+ comes up. Many DHT's like the 300B can begin to conduct within a few seconds of filament power application. They are faster than a 5AR4 or a damper diode. A SS rectified negative supply ensures that bias is applied to the grid before the B+ supply comes up to avoid an uncontrolled start up.
Thanks, been thinking about how to solve this timing problem in the cheapest way using that dual rectifier... When the amp is powered up, all the other tube filaments will be started and the bias will be present. But the filament to the rectifier would get a "delay on make" relay inserted into its 5v AC filament supply. This would make the rectifier behave like a "contactless" HV DC switch when the filament warms, no? Additionally the HV will reach the circuit "softly" as the filament warm-up time adds to the delay. That tube is replacing many components it seems!
Is there anything incorrect about this circuit? Do I need to tie the upper filament leg to the cathode maybe?
In a typical 5AR4 the heater is already connected to the cathode. In a 5Y3 or 5U4 the heater IS the cathode as they are directly heated. The 6BY5 does have two independent cathodes and an isolated heater with a 450 volt H-K rating so the cathodes can be at up to 450 volts of B+ while the 6.3V heater is grounded or lifted to some intermediate voltage that suits all the other tubes in the circuit. The 6BY5 was originally intended for an oddball TV design that used a PAIR of damper tubes. They were seen in some HiFi designs in the 1960's. Most individual damper tubes also have a fairly high H-K breakdown rating in one direction only.
The rectifier tube gets the most abuse in an amp and will have a short unhappy life if not properly aligned when manufactured. That poor tube must feed all the hungry tubes in an amp while it is still half asleep, fill up all the empty filter caps and withstand a good deal of reverse voltage and peak current at the same time during power up. There was a period of time around 2010 - 2011 when the current production suppliers sold bad batches of 5AR4's that would flash over in an SSE on initial power up. Builders often had to go through two or three new tubes before finding one that didn't spark out. Once found that tube usually lived for several years in the amp. If the cathode coating is not perfectly uniform there will be excess current density at the thin spot in the cathode coating because it heats quicker. If the cathode is not concentric with the plate there will be excess current flow where the cathode is closest to the plate. This will often cause an arc or flash over. The old 5U4's and 5Y3's could spark at switch on then perform properly once warm. If a 5AR4 sparks once it rarely recovers. It will continue to arc until it or something else blows, hopefully a fuse.
As a kid who dragged home every dead AA5 radio he found, I learned that the dead tube was usually the rectifier, not the audio output that is running at 100% of its rated plate dissipation or more.
I'm not so sure that the rectifier tube would like having all of the other tubes hot and ready to draw full current while its heater was slowly warming. I have never actually tried using the heater of the rectifier tube for a switch, or a time delay. What I have done is to use a mosfet in series with the ground side of the HV power supply (HV winding CT or negative side of the bridge). The drain of the fet gets tied to the CT or bridge, the source goes to ground, and the gate gets a PWM pulse train from a PIC chip or dual 555 timer. You power up with a zero pulse width which increases to 100% (5 volts DC) over the desired soft start time.
The rectifier tube gets the most abuse in an amp and will have a short unhappy life if not properly aligned when manufactured. That poor tube must feed all the hungry tubes in an amp while it is still half asleep, fill up all the empty filter caps and withstand a good deal of reverse voltage and peak current at the same time during power up. There was a period of time around 2010 - 2011 when the current production suppliers sold bad batches of 5AR4's that would flash over in an SSE on initial power up. Builders often had to go through two or three new tubes before finding one that didn't spark out. Once found that tube usually lived for several years in the amp. If the cathode coating is not perfectly uniform there will be excess current density at the thin spot in the cathode coating because it heats quicker. If the cathode is not concentric with the plate there will be excess current flow where the cathode is closest to the plate. This will often cause an arc or flash over. The old 5U4's and 5Y3's could spark at switch on then perform properly once warm. If a 5AR4 sparks once it rarely recovers. It will continue to arc until it or something else blows, hopefully a fuse.
As a kid who dragged home every dead AA5 radio he found, I learned that the dead tube was usually the rectifier, not the audio output that is running at 100% of its rated plate dissipation or more.
I'm not so sure that the rectifier tube would like having all of the other tubes hot and ready to draw full current while its heater was slowly warming. I have never actually tried using the heater of the rectifier tube for a switch, or a time delay. What I have done is to use a mosfet in series with the ground side of the HV power supply (HV winding CT or negative side of the bridge). The drain of the fet gets tied to the CT or bridge, the source goes to ground, and the gate gets a PWM pulse train from a PIC chip or dual 555 timer. You power up with a zero pulse width which increases to 100% (5 volts DC) over the desired soft start time.
- Home
- Amplifiers
- Tubes / Valves
- Newby question: Hybrid bridge rectifier