Hello everyone,
I am building a small valve amp that uses 2 x EL84 for the output stage. I will include a DPDT switch for the screens, to either select a "normal" pentode operation (screens go to HV supply) or Ultra-Linear operation (screens go to UL taps in the transformer). I attach a picture of my schematic.
My question is, would it be dangerous to switch mode of operation if power has been applied? Dangerous to the tubes, transformer, switch due to arcing for example. So, should the user select operating mode prior to applying power?
I understand that a high value resistor to the screens actually protects the tube, so seeing infinite resistance during the switch transition would not hurt the tubes. Is this true?
On the other hand, I suppose that changing the transformer current (especially during high output power condition?) could create a flyback voltage due to its inductive nature - either resulting in a dangerous arc, or in an annoying popping sound. Would this make any sense?
And, is there a way to be able to switch under load, but eliminate the aforementioned dangers? For example using a capacitor or resistor?
Many thanks!
I am building a small valve amp that uses 2 x EL84 for the output stage. I will include a DPDT switch for the screens, to either select a "normal" pentode operation (screens go to HV supply) or Ultra-Linear operation (screens go to UL taps in the transformer). I attach a picture of my schematic.
My question is, would it be dangerous to switch mode of operation if power has been applied? Dangerous to the tubes, transformer, switch due to arcing for example. So, should the user select operating mode prior to applying power?
I understand that a high value resistor to the screens actually protects the tube, so seeing infinite resistance during the switch transition would not hurt the tubes. Is this true?
On the other hand, I suppose that changing the transformer current (especially during high output power condition?) could create a flyback voltage due to its inductive nature - either resulting in a dangerous arc, or in an annoying popping sound. Would this make any sense?
And, is there a way to be able to switch under load, but eliminate the aforementioned dangers? For example using a capacitor or resistor?
Many thanks!
Attachments
It will be dangerous, especially for the speakers. The speakers shall be always connected in a tube amplifier with output transformers, and, as you rightly stated, the change of operation point will cause a popping sound at least. There could also be a spark in the contacts when switching back and forth, basically a lot of not so good things can happen.
It is never recommended to do this while the amplifier is powered up.
It is never recommended to do this while the amplifier is powered up.
Not a problem, Ampeg, Marshall, Fender and many more international manufacturers do exactly as you are.
Not an issue at all and quite common.
There is only 8mA screen grid current when biased hot so as long as your DPDT switch is made by a known manufacturer, not a fleabay/Amuzun knockoff, safety precations will be observed.
I doubt you will even hear a mild click through the speakers. On the many amplifiers I have made, I have not had an issue or a complaint from any of my customers.
Not an issue at all and quite common.
There is only 8mA screen grid current when biased hot so as long as your DPDT switch is made by a known manufacturer, not a fleabay/Amuzun knockoff, safety precations will be observed.
I doubt you will even hear a mild click through the speakers. On the many amplifiers I have made, I have not had an issue or a complaint from any of my customers.
Why not triode/UL instead, this would likely be the more interesting combination if you can tolerate 3dB reduction in output power. I built triode connected PP EL84 amps for lower powered second systems for several friends as well as myself and they always sounded very good.
I used to build amps with triode/UL option and while you could switch in operation it did definitely generate a bit of a pop when doing so. I'd recommend with sensitive speakers at least that it would probably be better to switch between modes with the amp off. It probably depends to an extent how quickly the switch transitions from one connection to the other.
Watch out for the voltage rating on the switch. Most switches are rated at 250Vrms at best, in some cases with 400VDC and significant AC voltage present on the plate circuit the insulation in the switch could break down. Make sure the switch housing is connected to chassis GND so that if the switch fails you do not get zapped and hopefully the primary AC fuse blows. (Or use a B+ fuse, again noting voltage ratings.)
I used to build amps with triode/UL option and while you could switch in operation it did definitely generate a bit of a pop when doing so. I'd recommend with sensitive speakers at least that it would probably be better to switch between modes with the amp off. It probably depends to an extent how quickly the switch transitions from one connection to the other.
Watch out for the voltage rating on the switch. Most switches are rated at 250Vrms at best, in some cases with 400VDC and significant AC voltage present on the plate circuit the insulation in the switch could break down. Make sure the switch housing is connected to chassis GND so that if the switch fails you do not get zapped and hopefully the primary AC fuse blows. (Or use a B+ fuse, again noting voltage ratings.)
It's a guitar amp and the pentode mode of operation is something I want to have for sure, the UL is the experiment.
This is why I will use an Apem 646 H/2 - one of the few switches I was able to find mentioning such high voltage tolerances in their datasheet.
Well I do not plan to do this regularly, but I figured it may be needed for the sake of a direct comparison. My speaker is sensitive loud-wise (101dB with 1W) but not fragile - actually overpowered for the application (it is a 150W one). I doubt the pop could hurt it, can't say the same for my ears, though!
I tried searching for models that have a switch onboard, but couldn't find any... Can you point me to some models that do?
Can't remember exact models at present but here is a typical UL/Pentode switch wiring using a plastic DPDT paddle switch. (https://uk.rs-online.com/web/p/rocker-switches/2877363?gb=b)
