Relays for rectifier switching and power tube cathode lifting

[Buck455]

I'm trying to bake in a couple of functions from the Mesa Lonestar to my latest build. Specifically, I want to have the tube/diode rectifier switching capability as well as the ability to cut power by lifting the cathodes of two of the power tubes.

I was wondering if anybody has any experience with either of these and if so, can you recommend any relays suitable for this applications.

Thanks all!

[R.G.]

I do. Switching high voltage DC with relays is hard, largely because of the formation of stable arcs which burn and weld contacts.

There are some GREAT relays that will do the job. They're called MOSFETs. An 800V to 1kV power MOSFET will do the job nicely, and are intended for this use.

Another trick which makes this easy to do is to use an LED to PhotoVoltaic MOSFET driver. These things have an LED on the control side, and a voltage-generating photovoltaic stack plus turn off circuit on the output side. They generate about 8V when the LED is turned on, and they isolate by about 4kV.

So you can use two N-channel (cheap and easy to get) high voltage MOSFETs as switches, and only have to run 5-10ma through the LEDs from a low voltage switch or control setup to turn them on and off. Be aware that there is a body diode inside MOSFETs which conducts the reverse direction, and plan for it. But for switching rectifiers and tube cathodes, this works well. I've done both applications, although not in the same amp.

[exclamationmark]

You can also get reed relays good for up to something ridiculous like 35kV. The only caveat is that the maximum current draw is only 2-3A before they weld themselves together, which can easily happen upon startup, or switching due to the inductive/capacitive nature of well... everything inside a guitar amp. Also, they're 20 or 30 bucks a piece. If you want to go down this road, be sure to include current limiting resistors.

Personally I would go the MOSFET route (plus the experience to learn something new is always nice). They make nice power switches.... now if only I could find how to work the damn things as signal switches!

[Gregg]

Mesa Roadking II has rectifier and preamp plate resistors switched and if you see the guts of that amp you'll see the same type of relay everywhere (except for the speaker relays where larger currents are switched). There are "regular" DPDT relays that are rated at 250VAC/DC at 1-3A that obviously can handle the job. I guess if you parallel two sections of a DPDT relay that's even better.

[exclamationmark]

An interesting thing to note about 'normal' relays is that they're rated MUCH higher at AC than at DC, since AC typically crosses the '0' point several times a second, extinguishing any arcs. Some relays even wait for the crossing point to appear and avoid arcing this way (not really sure how - but I recall reading it somewhere). Usually they're rated up to about 30v DC before pitting starts to happen from small arcs. Certainly they will work, but lifetime might be compromised. The lifetimes for these things are typically specced in the millions of operations, so a 95% reduction in lifetime might still outlive all of us!

[R.G.]

if only I could find how to work the damn things as signal switches!

Got you covered. You put them source to source, and figure a way to make both gates positive with respect to the joined sources to turn them on.

This is easiest to do with an LED to photovoltaic gate driver. IRF, Panasonic, Toshiba, and possibly others make LED-to-Photovoltaic drivers, including a dual. The dual does the gate drive job for a signal switch. There are commercial MOSFET output SSRs that do exactly this.

There is some small amount of distortion, which is probably why you haven't seen them flogged about for audio. I think Steve Connor has messed with the discrete version, from one of his posts. Easy enough to make high voltage/high power switches this way, even speaker switches.

If you're OK with switching lower, signal style voltages, use the H11Fx series of LED to photo-FET. These things make very quiet switches, but are voltage/current/power limited in what they can switch. They switch from infinity to a few hundred ohms, so if the load after them is larger than a few K, the loss is unnoticeable. And photo-FETs don't have the distortion that MOSFETs do.

[mooreamps]

I'm trying to bake in a couple of functions from the Mesa Lonestar to my latest build. Specifically, I want to have the tube/diode rectifier switching capability as well as the ability to cut power by lifting the cathodes of two of the power tubes.

I was wondering if anybody has any experience with either of these and if so, can you recommend any relays suitable for this applications.

Thanks all!

leave the cathodes alone ; and switch the instrument signal instead...

-g

[Steve Conner]

Sure, that will work great when it comes to switching between tube and diode rectifiers.

I don't think either of these positions is too demanding of a relay. You can arrange the rectifier relay so it doesn't have to break the full B+ voltage, or if it does, it can break it as AC, which is easier.

And likewise, when switching cathodes, the tubes themselves will help to cut off the cathode current.

The last relays I bought were Omron LY series. They are quite hefty. The voltage rating of a relay can be increased by getting a 2, 3 or 4 pole one and putting the contacts in series, this splits the arc up into several pieces and helps it to go out. This trick also works with ordinary switches: I like to use it in tube amp standby switches.

MOSFETs would also work. As RG mentions, I once tried making a speaker protection relay out of some back-to-back MOSFETs, driven by a PVI5080 or suchlike. The circuit can be found in the IR application note. It worked well enough for hi-fi use, introducing about 0.005% THD at a 100W power level.

But the test amp developed some weird (audible) HF distortion in one channel, and I never got round to figuring out whether it was my heretical speaker switch or something else. It's currently taking a holiday in my junk pile

I've also used the H11Fx series, but the voltage and current ratings are extremely limited: maybe 30V and tens of microamps. The only real use I've ever found for them was controlling a precision integrator in a piece of lab equipment.

You can get MOSFET-output solid state relays like the circuit I discussed above, but all in one package, with ratings up to 400V. I've seen people use the smallest, lowest current ones for tube amp channel switching in place of LDRs. They're still a bit too expensive to be using them 8 at a time for high-voltage "digital pots", and they also have a bit too much capacitance in the off-state.

[R.G.]

It worked well enough for hi-fi use, introducing about 0.005% THD at a 100W power level.

I remembered that number wrong for some reason. I thought it was more than that. At 5 millipercent (!) it's a fully usable technique for MI amps.

[Buck455]

Thank you all for the quick and helpful replies. I'm looking into using an isolated mosfet as suggested for lifting the cathodes and rectifier switching. I'm also going to try a triode/pentode switching scheme to have 3 "power" levels. I'm a bit uneasy about all the different opinions on "pulling" two of the four power tubes without switching speaker impedance, but Mesa has been doing it for years. So I'm going to assume today's transformers and tubes are capable of the impedance mismatch.

Thanks again and happy holidays to all.

[R.G.]

Don't sweat the impedance mismatch. A 2:1 mismatch in either direction can be considered to be a tone change, and not dangerous as long as the original OT was not already on the hairy edge of overheating in "normal" operations.