Triode mode questions

[jeffdavison]

"IF" I use a dpdt switch to go between modes (yes I am aware NEVER switch while the am is on), can I use a switch that is AC only? I have read about use of AC vs DC rated switches in use and that high DC voltages cannot be tolerated well in AC only switches as the contacts can arc shut in one position, but that was while current was flowing. Seeing how the switch contacts won't be made or broken while the amp is powered up....will the switch use be OK?

Another triode mode question is...is the same value resistor (100 ohm)used if KT88 tubes are used? I have seen info on the ST70 but not the MK III.

Thanks

JD

[mesherm]

I just installed two DPDT switches in my Universal amp to do exactly that, switch between UL and triode mode. I used a Shin Chin p/n R13-25B1-05 (Mouser 112-R13-25B1) it was rated 6A @ 250VAC. It had a plastic bat handle which I pried off to make it harder to switch by accident. They work fine. I checked the specs beofre I got them and they looked OK up to 500 VDC. I didn't want to use a miniature switch. I wanted more room around the solder terminals.
I used 100 ohm resistors on my triode KT88 amp, no problem. In fact, on that amp I have each 100 ohm resistor in series with a UF4007 diode so that only DC can pass to the screen grids.

[jeffdavison]

I've got my eyes on a 4pdt thats rated 10a @ 250vac. I want to use a single switch to do both channels at the same time. The body is aprox 1.25" x 1.5" and has solder lugs. I want to place it between th 2 bias pots (I'll have to widen the distance between the pot to make room) so the distance from the switch to the tubes is about 3" away, so the wires will be vwery well insulated.

JD

[EWBrown]

As long as the switch can handle at least 450VDC (breakdown voltage) between the contacts and the switch's body and mounting, this should work fine.

The actual DC voltage potential difference between the contacts for each pole will be fairly low, presuming "common" to the G2, and the other contacts to plate and SG tap on the output transformer.


The main thing to remember -even though you have stated thos before, is to avoid flipping the switch when the amp is powered on.

If you can locate a decent three-position, four-pole switch, then the choices can be Pentode mode, UL mode , or Triode mode. This starts go get a bit complicated, but should be feasable.



VTL (Vacuum Tube Logic) uses this same approach on many of their amps, I've seen it done with their MB450s (which use 8 6550s or KT88s in PPP). There is around 600VDC pplate voltage, and they use what looks like a fairly cheap plastic body and "bat handle" DPDT switch for each monoblock.

Another VTL PPP amp (with 4 6550 / KT88s per channel) uses two switches per channel, in what they call a "tri-tet" mode. With this arrangement , one pair of the tubes can be run UL, the other pair can be run Triode, at the same time, on each channel. THis gives three choices, all truode, all UL, or half and half.

Locating your 4PDT switch between the bias pots would be a good choice, or perhaps on the back of the chassis.

/ed B in NH