Switch output tubes by switching filament voltage?

[cyclone]

I've searched here and elsewhere, I haven't tried this because I'm worried about burning up expensive tubes, but if I have an OPT with dual primary taps (8K and 6k6) is there any reason I can't put four output tubes in (2x6v6 and 2x6bq5) wired up to the pair of winding taps and switch tube types with a DPDT on the filament supply to the output tubes?

It's a fixed bias amp of my own design with no NFB. I may put one pair of tubes on cathode bias to simplify switching and bias. I'm not sure if I need to switch the output of the PI or if I can parallel it, I'm sure there are other considerations???...Thanks in advance!!

Pete

[R.G.]

Switching off the heater voltage does turn off the tube, converting it from a working tube to a set of oddly-shaped, non-conducting wires. It also increases the parasitic capacitances attached to the other, working tubes. That may or may not be a problem in a given circuit, but it's worth remembering.

You could do much the same thing by opening the cathode connections of the "off" tubes, or connecting them through a high value resistor, say about 10K when off.

I think it's unlikely that leaving both sets of heaters on will harm the tubes, but it might be too much of a load on your power transformer, so it's possible that opening the heaters would be good. Depends on the PT rating.

[überfuzz]

I'm pretty sure I've seen adaptors doing the same thing, i.e. no mods needed to test the stunt you described. I don't know the purpose of this mod, but how about sniffing around for this adaptor and see how big a difference the tube model does in your amplifier.

[Gregg]

You could do much the same thing by opening the cathode connections of the "off" tubes, or connecting them through a high value resistor, say about 10K when off.

This is how it's usually done. For example check out some Mesa schematics.

[Chuck H]

Yes. It seems to me that lifting the cathode prohibits any conductivity WRT load. So I would think that lifting the cathode renders the same "set of oddly-shaped, non-conducting wires" R.G. discussed regarding cold filaments. I could ne wrong on this and am willing to accept corrections At face value this seems preferable to switching the filaments on and off for a few reasons.

[Mick Bailey]

I worked on an amp with a pair of 6L6 and EL34 which switched the cathode connections. No untoward effect when one pair was 'off'.

[R.G.]

Yes. It seems to me that lifting the cathode prohibits any conductivity WRT load. So I would think that lifting the cathode renders the same "set of oddly-shaped, non-conducting wires" R.G. discussed regarding cold filaments. I could ne wrong on this and am willing to accept corrections .

Nope, you're right. Funny shaped antennae.

The only bad thing with leaving both sets of heaters hot is that the PT may not be able to provide all that heater current.

[Tom Phillips]

Pete,

You didn't ask this but I offer something else for you to consider. Specifically, do you know that switching between the two different tube types and the different taps will give you two specific sounds that you will like? It is certainly true that amps that use these different tubes do produce different sounds. However, it's not just the output tube type that makes the difference. The design variations and setup of the other circuitry in the overall amp including the speaker contribute significantly to the final sound and they won't change when you flip the output tube selection switch.

Your planned approach is interesting and such things are part of the fun of building your own design. However, I have found with amps that include multiple mode options (both custom and commercial products) that the owner usually ends of using one of the major option choices all the time. Given that, maybe you would end up with your best liked sound by experimenting with the two tube types (even use each tap with each tube) and then optimize the rest of the amp to really dial in your preferred tone.

Just food for thought.

Cheers,
Tom

[Chuck H]

+1
If the only change in a circuit is a power tube type, and the different tube has at least a similar impedance requirement, the tonal difference is pretty small. It's there, but it's small. IMHE there is a little character to different tube types (that becomes a lot more clear when clipping) and it's always been enough for me to just evaluate an amp and choose a tube type I like best for THAT amp. With the same preamp, OT and speaker I can't even imagine the difference inspiring me to think something like "Hey, I should put in the EL34's for this track."

[pdf64]

For the sake of completeness, should the issue of cathode stripping be dealt with?
My thinking is that the cold pair of power tubes would be subject to voltages in excess of 2 x VB+ on their plates, possibly significantly in excess when overdriven due, to inductive spikes?
Hence the conditions may be reached, if only momentarily (though perhaps repeatedly), in which the cathode stripping mechanism may take effect.
So it may be beneficial for the hazard to be assessed, if only so that it can be dismissed as inconsequential.

[Chuck H]

I'm not sure that it applies if the cathodes aren't heated at all. If the tube CAN'T conduct, how can the cathodes strip? I honestly don't know about this though.

[R.G.]

It's always worth worrying about, even if it is a footnote, as you say.

Switching off the heaters does allow the cold-cathode condition to exist, and the tubes turned off do experience peaks of as much as a bit below 2xB+. The answer to whether this would allow a slow, creeping, integral-of-the-hgh-voltage-times cathode stripping to happen is a solid - I dunno.

Maybe. Could happen. It would be quite slow, probably maybe.

Opening the cathodes keeps this from happening, as the cathodes then float up with the plates, held by the plate-cathode capacitance, small as it is.

Leaving the cathodes hot but no current flowing is OK if there's not much voltage between cathodes and plates. Leaving them hot with voltage on the plates can cause "sleeping sickness", which is not cathode stripping, but is a similar illness. They found this in the days of vacuum tube computers. Sometimes a tube which is held off for long periods has an electrochemical change in the cathode oxides that makes the cathodes slow to respond to signals.

Switching the plates to open is better for long term tube life, no question. It's just dangerous for the people involved because of the high voltages. There may not be a huge difference, but it's there. It's also possible to turn off a pentode by dropping the screen voltages to ground? -Vbias? as the screens are effectively a second set of grids but with low gain.

It is possible to get around some of the issues with either high-isolation relays or high-side-driver circuits for power MOSFETs. Both of these would isolate the plates from high voltage and stop the murky issues of sleeping sickness and cathode stripping from even possibly-maybe happening, and protect the delicate humans working with the circuits. That leaves only the issue of the current drawn by two always-on sets of heaters.

The relay solution will be expensive and difficult to source. The MOSFETs version is easier. In today's motor-drive world, it is common to turn MOSFETs on the high side - several hundred volts up from ground - on and off with a "high side driver". This turns on/off a MOSFET, usually N-channel, which is in series with the + voltage but with no DC connection to ground on the gate side. Most of these involve a capacitive up-verter driven by the high frequency switching of the commonest of these circuits, but an LED-Photovoltaic driver exists which will turn on a MOSFET by turning on an LED, like a normal optoisolator but on voltage-steroids. I think you could do something with MOSFETs and LED-PV isolators to open the plates effectively and not affect normal operation.

[Gregg]

In YJM100 they used MOSFETs to lift the cathode however I'm wondering whether a 900V MOSFET is not an overkill or they just used what they had in stock.

[Merlinb]

Leaving the cathodes hot but no current flowing is OK if there's not much voltage between cathodes and plates.

Leaving the cathode heated is the worst option. As long as the cathode is hot, you are burning off barium, so the tube is ageing just as much as if it were doing hard work. Interface resistance (sleeping sickness) also still grows whether there is anode voltage or not, whereas there are no deleterious effects that result from a cold cathode with anode voltage present.

Switching heaters is definitely the most efficient (simple) option.

[R.G.]

Leaving the cathode heated is the worst option. As long as the cathode is hot, you are burning off barium, so the tube is ageing just as much as if it were doing hard work. Interface resistance (sleeping sickness) also still grows whether there is anode voltage or not, whereas there are no deleterious effects that result from a cold cathode with anode voltage present.

Merlin, I'd like to read up on those items. Can you post a reference to the issues of burning off barium and sleeping sickness growing whether the anode is powered or not?

Just off hand, the barium on the cathode is not a layer of barium metal, it's barium oxide, and in some tubes strontium oxides (IIRC), so they're not burning in the sense of being oxidized more, I think. If the barium is leaving, is it maybe leaving because the vapor pressure of barium oxide at heater temps is large enough to boil off barium oxides into vapor? That seems counter intuitive.

Most of the stuff I've read about cathode chemistry and wear says that the erosion of the barium is from the high-velocity impacts of residual gas ions driven by plate voltage. That certainly slows down when there's no voltage to drive it.

And interface resistance was noted in what little I could find as an effect from no-current operation under voltage. I'd like to read up on the effect happening when there's no voltage.