Best practices with tube rectifiers

[Iplayloud]

I have a princeton reverb clone that has the standby switch wired to lift the center tap of the ht winding, instead of the usual configuration. I suppose the idea was to let the rectifier warm up before applying the voltage? Is there any advantage in letting the rectifier warm up? Like for power tubes where it will increase tube life.

Also I'm about to wire a ss-tube rec switch and wanted to check if there was any problem if I wire it so the filament isn't fed while SS is selected? The rectifier keeps my chassi alot warmer and the filament supply keeps the PT a chunk of degrees higher, and I expect to use the ss most of the time.

Also, any problem with the HT voltage being hooked to the rectifier while it isn't used?

[Jazz P Bass]

Ideally you want the rectifier heated up & ready to go before the B+ is slammed on.

The only iffy about removing the filament winding on the unused rectifier (solid state mode) is what happens if the switch is thrown while the unit is on?
I would simply remove the B+ from the rectifier.

There are two schools of thought on this.
One: leave the tube rectifier on all of the time & switch in/out the ss diodes.
Two: switch the high voltage ac windings.

[Alan0354]

One concern is if you switch to SS, you increase the +B by something like 30V. If you Princeton using 6V6, that might over voltage the tube. I am not saying that is definitely bad as people over voltage tubes all the time!!!

I personally would put the SS rectifier and the reservoir cap before the standby switch. I do not like to have the rectifier and reservoir cap after the standby switch because every time you switch it on, you have a big current surge through the rectifier to charge up the reservoir cap. I still prefer the bullet proved old Fender way to have those before the standby switch so the reservoir cap is fully charged up ready to work.

I think you over think about the filament of the rectifier tube. if it draw 1A or so, it's only 6W. It's not going to make or break the PT. Your amp is warm from the other filaments and the idle current through all the tubes. What is the point of lowering from say, 60W to 54W?

[Iplayloud]

Yes, the rectifier needs to warm up before providing rectified voltage to B+.
What I was wondering is if it also needed to be warm before 'receiving' the voltage from the PT. If not then lifting the centre tap would be completely pointless? (Like in your Bad Cat schematic)
It is pertinent in my case bacause while in ss mode, the rectifier filament supply would be off but the high voltage is still hitting the rectifier.
If the rect should be warm before receiving the voltage, and should also should not be exposed to the big current surge of suddenly charging the big reservoir cap, then ideally 2 standby switches would be necessary?

JazzPBass, I don't really see what would be the problem if the switch is flicked during operation? The B+ would slowly ramp up as the rect warms up? If that is so terrible, then a stock princeton with no standby switch is giving a serious beating to it's rectifier?
By the way my present plate voltage is 380 so i have some room upwards..

[pdf64]

'Ideally you want the rectifier heated up & ready to go before the B+ is slammed on'

That runs counter to my understanding, which is that

- 'hot switching' a load (most especially capacitive) on to a tube rectifier is stressful / bad practice.

- the slow ramp up of VB+ provided by a GZ34 as it warms up is beneficial.

- standby switches are almost universally used far beyond their rated voltage.

- for regular equipment with a VB+ <1kV, the best use for a standby switch is not to use it, as it offers no benefit. For new build, consider making it a mute switch.

[Alex R]

'Ideally you want the rectifier heated up & ready to go before the B+ is slammed on'

That runs counter to my understanding, which is that

- 'hot switching' a load (most especially capacitive) on to a tube rectifier is stressful / bad practice.

- the slow ramp up of VB+ provided by a GZ34 as it warms up is beneficial.

- standby switches are almost universally used far beyond their rated voltage.

- for regular equipment with a VB+ <1kV, the best use for a standby switch is not to use it, as it offers no benefit. For new build, consider making it a mute switch.

lol yes I've always seen standby switches as something I don't really need to bother my pretty little head about. Like whether EL34s idle better at 30mA or at 35, or whether my ground reference resistors are CC or not .

[Iplayloud]

lol yes I've always seen standby switches as something I don't really need to bother my pretty little head about. Like whether EL34s idle better at 30mA or at 35, or whether my ground reference resistors are CC or not .

Yeah there's always a type of people that mistreat the equipment and seem to get away with it.. but it's certainly not according to such principles that anyone landed on the moon. Hence the term "best practices". Because what can go wrong, will eventually.. go wrong.

[Alan0354]

- for regular equipment with a VB+ <1kV, the best use for a standby switch is not to use it, as it offers no benefit. For new build, consider making it a mute switch.

Totally agree. Standby switch only stress out the rectifier and capacitor when turn on. There is a current surge whenever you turn it on and it's on the rectifier and capacitors. On both my build, I don't put standby switch. You take a break for over 15 minutes, turn the amp off!!! You take a shorter break, leave it on.

I think OP is way over think this whole thing. Just use a switch to switch in either rectifier and call it a day.

Ha ha, There is already a build in mute circuit.........volume of your guitar!!!

[Dave H]

On both my build, I don't put standby switch. You take a break for over 15 minutes, turn the amp off!!! You take a shorter break, leave it on.

I always leave it on and pull the input jack plug half way out so that the shorting contacts mute the amp.

[Iplayloud]

I see how it may look like I'm over thinking this.. It's very simple I expect that the amp in question will spend most of it time is ss mode and I insist on turning off the filament supply because it keeps the PT and entire amp alot warmer. I want to make this amp as road-worthy as possible. I was looking to find out if i was going to be damaging that GZ34 by keeping the high voltage windings hooked while the filament supply is off.

[Alan0354]

I cannot imagin it will get a lot warmer with just one extra tube on. Even if it is warmer, what do you care? Are you playing in a very confined room that heat is a problem? Tube last a long time and it's not that expensive. I don't see why you worry about all this.

If you are not going to use the tube most of the time, pull the tube out, tape it inside the amp. If you ever going to use it, plug it back in. It's only a few seconds to do that. You are going to cut the filament so it's not an instantaneous switch over anyway.

Follow the way Mesa is doing in the post #2 by Jazz P Bass. If you put a switch to open one leg of the filament winding to the rectifier of the tube, you should be safe. You will not put high voltage across the anode and cathode/filament.

[Iplayloud]

Yeah I'm taking it to the international space station and Neil says his dash lights dim when I turn it on. At 2 amps that rectifier filament draws like almost half of the entire wattage the PT it putting out. I'm not worried, it's just that I can wire that switch any way I want and my preference was to shut that filament off. I simply wanted to know if applying the voltage to the cold rectifier was going to cause cathode stripping or any such damage. I suppose not so I will go ahead with that and thankfully be done with this worm can question. Thx

[Alan0354]

It's the standoff voltage I am worry about only. When the tube is on and with anode to +B, the filament/cathode is only about 30V below the anode. But if the filament is off, there will be no emission and the filament/cathode will drop to 0V if the circuit pulls it down. Then you have +B drop across the anode to cathode. That's the only think I am worry. That's the reason I said look at the Mesa schematic and copy that. In that schematic, the cathode is tied to the SS rectifier output which is 0.7V below the anode. So there will be very little voltage drop across the anode and cathode.

If the filament is not on, you should not see the cathode strip. it's only when it is emitting electrons that you wear out the tube. It's the long term standoff voltage between the anode and cathode/filament that I am concern with. Even in normal tube amp with rectifier tube, before the tube warmup and start conducting, the cathode is at 0V. But that only last for 15 sec. or so, not long term standoff. So my suggestion is follow the way Mesa wiring and just add a switch on one side of the filament winding. Don't cut off both sides of the filament, or else, the switch is going to have the full +B across the contacts. I am sure the normal switch is not designed for that.

[frus]

At 2 amps that rectifier filament draws like almost half of the entire wattage the PT it putting out.

you sure?
2 amps x 5 volts = 10 W

[Dave H]

If the heat from the rectifier filament circuit is a problem you could dump the tube rectifier and add a 'sag' resistor with the 'tube'/SS rectifier switch wired across the resistor. I used to have an amp wired like that but I preferred the 'tube' setting so I removed the switch and wired it with the resistor in circuit.