baudouin0,
Referring to Post # 4:
It it seems the amp screen switch will never be switched when the amp is powered.
The idea was to have extended listening in one mode or extended listening in the other mode. Switching was to be done un-powered.
The idea was not to be able to do powered switching for instant comparative A/B testing.
That requires a much more complex solution.
"You should make things as simple as possible, but no simpler" Albert Einstein.
Referring to Post # 4:
It it seems the amp screen switch will never be switched when the amp is powered.
The idea was to have extended listening in one mode or extended listening in the other mode. Switching was to be done un-powered.
The idea was not to be able to do powered switching for instant comparative A/B testing.
That requires a much more complex solution.
"You should make things as simple as possible, but no simpler" Albert Einstein.
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HOWEVER… I also should like to point out that you could use a pair of way-less-expensive DPDT switches (for each channel), if you like. After all, if you follow the golden-injunction to NOT switch when power is applied, then switching “at different times, shortly spaced” is perfectly acceptable, too. It is your amplifier and your fingers doing the switching!
I get that but there is no reason triode can't be added with the same parts count.
I used this: 1Pc 12 Pin Mini Toggle Switch 4PDT 2 Position ON/ON 6A/125V 2A/250V AC MTS-402 | eBay
to switch a pair of these: MY4NJ 12v/24v/220v Coil Power Relay DIN Rail Mounted 14 Pin 4PDT W/ Socket Base | eBay
and the input attenuation (short a resistor). The gain in UL and tetrode was close enough that the same setting worked for both modes, with triode not being attenuated.
This was years ago, mind you. I just use triode 90% of the time now.
Thanks for all the comments everyone. As I mention before, I think this idea is OK to try. I'm going to look into a cover for the switches so that someone has to physically disassemble something to get at them. Step 1 will be to test the amp in the simpler UL configuration, and then if everything seems ok, move on to a comparative test with the pentode configuration.
Also it's a 3PDT switch so that I can power off the screen grid supply when the system is in UL mode. Seemed prudent enough since the screen grid supplies are using 12.5 mA or so quiescent; I didn't see a point in using that extra current for a non-functional circuit component (when in UL mode).
Also it's a 3PDT switch so that I can power off the screen grid supply when the system is in UL mode. Seemed prudent enough since the screen grid supplies are using 12.5 mA or so quiescent; I didn't see a point in using that extra current for a non-functional circuit component (when in UL mode).
Currently your zener is to ground. That means you will have to bias the current through the zener to the maximum screen current you will take during the signal peaks to prevent the current reaching zero and the voltage dropping. The zener is high voltage - that's a lot of heat.
If you put the zener to HT and the resistor to ground, then as the screen only draws current you only need to provide say 2ma through the resistor to keep the voltage stable. That coupled with a lower voltage zener will reduce the heat dissipation in both zener and resistor.
That way you can leave the circuit on and get rid of that 3'rd pole. The only disadvantage is the screen is regulated w.r.t. HT but that is what UL and triode modes do.
There even simpler and lower power.
If you put the zener to HT and the resistor to ground, then as the screen only draws current you only need to provide say 2ma through the resistor to keep the voltage stable. That coupled with a lower voltage zener will reduce the heat dissipation in both zener and resistor.
That way you can leave the circuit on and get rid of that 3'rd pole. The only disadvantage is the screen is regulated w.r.t. HT but that is what UL and triode modes do.
There even simpler and lower power.
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Switching DC is harder than switching AC, note, and it very much depends how much DC power is available to hold an arc. DC arcs don't have a zero-crossing to quench them so its a question of the power available and the contact spacing whether an arc quenches. In current limiting resistors are present on each contact set that may be enough.
Phoenix Contact 2903676 lists a max switching voltage of 250VDC which is remarkable for this size relay. Given that I routinely switch 300VDC with relays rated for 75V, I think this relay would be perfect for switching this.
2903676 Phoenix Contact datasheet and CAD model download | Octopart
Datasheet PHOENIX CONTACT | Single relay - REL-IR4/LDP- 12DC/4X21 - 2903676
Another thing about switching the screens in and out while it's on is: in a push pull amp, the spike is cancelled in the OPT. You might hear a small click, but that's it.
2903676 Phoenix Contact datasheet and CAD model download | Octopart
Datasheet PHOENIX CONTACT | Single relay - REL-IR4/LDP- 12DC/4X21 - 2903676
Another thing about switching the screens in and out while it's on is: in a push pull amp, the spike is cancelled in the OPT. You might hear a small click, but that's it.
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baudouin0,
Take a Power Mains line voltage ranging from 117VAC to 123VAC that is a 5% change.
That is +/- 2.5% from the center voltage of 120VAC.
If 120VAC into an amplifier gives a B+ of 4 times that, it is 480VDC.
2.5% of 480VDC is 12V.
Pentode mode and Beam Power Mode:
Screen voltage regulated and referred to the cathode voltage:
If you properly regulate the Screen voltage, the change to Plate current will be:
(change in B+ Voltage) / rp. Lets use an EL34 as an example.
12V / 15,000 Ohms = 0.8 mA
If the quiescent current is set to 50mA, you have 50mA versus 50.8mA for a
power mains voltage change of 2.5%.
50mA or 50.8 mA, only a 1.6% change in plate current for a 2.5% change in power mains voltage.
Screen voltage regulated and referred to the HV:
The plate current is dependent on screen voltage, much more than it is on plate voltage.
So you should not reference the screen voltage to the HV supply.
If you do, the plate current will vary Lots more than 1.6%.
Mains voltage often change by 2.5%.
Now think of applying that change to your screen voltage; up goes the plate current.
That is the reason you need to dissipate more power in the screen regulation (unless you want to use a regulated pass element that comes from the HV, and uses a voltage reference for the regulator to keep the screen voltage constant).
Take a Power Mains line voltage ranging from 117VAC to 123VAC that is a 5% change.
That is +/- 2.5% from the center voltage of 120VAC.
If 120VAC into an amplifier gives a B+ of 4 times that, it is 480VDC.
2.5% of 480VDC is 12V.
Pentode mode and Beam Power Mode:
Screen voltage regulated and referred to the cathode voltage:
If you properly regulate the Screen voltage, the change to Plate current will be:
(change in B+ Voltage) / rp. Lets use an EL34 as an example.
12V / 15,000 Ohms = 0.8 mA
If the quiescent current is set to 50mA, you have 50mA versus 50.8mA for a
power mains voltage change of 2.5%.
50mA or 50.8 mA, only a 1.6% change in plate current for a 2.5% change in power mains voltage.
Screen voltage regulated and referred to the HV:
The plate current is dependent on screen voltage, much more than it is on plate voltage.
So you should not reference the screen voltage to the HV supply.
If you do, the plate current will vary Lots more than 1.6%.
Mains voltage often change by 2.5%.
Now think of applying that change to your screen voltage; up goes the plate current.
That is the reason you need to dissipate more power in the screen regulation (unless you want to use a regulated pass element that comes from the HV, and uses a voltage reference for the regulator to keep the screen voltage constant).
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I don't disagree with any of that.
My point is that the UL and triode modes the screen voltage are referenced to HT not ground. It all depends on what screen voltage you wish to have and the problem will get much worse at low screen voltages. Of course if you operate at lower screen voltages the bias will need adjustment for the same plate current. In my amp (6550) you would need to run about 25ma through the zeners to cope with the peaks. Say the screen is at 350v thats 8.75W in the zeners per tube x8 in my case. Maybe if you want it to be ground referenced then an NPN transistor (collector HT, emitter screen, base - voltage reference) would be a better option.
Interesting maybe using a potential divider from the UL connections and 2 NPNs to each screen, or zeners might be a way of running the UL mode with higher plate voltage. Have to try on LT spice.
My point is that the UL and triode modes the screen voltage are referenced to HT not ground. It all depends on what screen voltage you wish to have and the problem will get much worse at low screen voltages. Of course if you operate at lower screen voltages the bias will need adjustment for the same plate current. In my amp (6550) you would need to run about 25ma through the zeners to cope with the peaks. Say the screen is at 350v thats 8.75W in the zeners per tube x8 in my case. Maybe if you want it to be ground referenced then an NPN transistor (collector HT, emitter screen, base - voltage reference) would be a better option.
Interesting maybe using a potential divider from the UL connections and 2 NPNs to each screen, or zeners might be a way of running the UL mode with higher plate voltage. Have to try on LT spice.
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Certainly there’s more than one way to skin the cat when it comes to a screen voltage regulation. You guys have thrown out some good ideas. Power demand for this subsystem is certainly a concern but i have thus far spec’d Zeners and current limiting resistors with power ratings 2 x what they will dissipate in normal operation.
Unfortunately, the minimum current at the zener voltages dictate that this part of the system uses a bit of current. My thought here is that the zeners aren’t much for regulation beneath the zener current, that part of the current voltage curve isn’t much better than a resistor.
Spice modeling indicates that this additional current demand shouldn’t stress the power supply too much but I still don’t think it’s prudent to run unnecessarily.
Unfortunately, the minimum current at the zener voltages dictate that this part of the system uses a bit of current. My thought here is that the zeners aren’t much for regulation beneath the zener current, that part of the current voltage curve isn’t much better than a resistor.
Spice modeling indicates that this additional current demand shouldn’t stress the power supply too much but I still don’t think it’s prudent to run unnecessarily.