KEL84 Triode/Ultra-Linear Switch

[BL21DE3]

Hi Folks,

In my Kel84 rebuild project I'm looking to put in a switch to enable me to choose between triode and ulta-linear operation of EL84 output valves.

However my problem is that from the original WAD schematic the UL taps run at 310V DC and I can't find many (if any) suitable witches rated for >310V DC operation. Having looked around the web I've seen various 'rules of thumb' stating that you can use 250V AC rated switches at 1.41 - 2x their rating for DC operation (i.e a 250V AC switch could handle ~350 - 500V DC). I am dubious of this and was wondering if anyone on the forum had experience of using AC rated switches for DC operation? Or can anyone recommend a suitably rated DC switch?

My other thought was to lower the voltage of the UL taps, but I'm unsure of what effect this would have on the amplifiers performance.

Regards,
Ewan

[Ianm2]

hi ewan, whilst your at it, wire it for pentode as well, consensus was this was the best sounding

[BL21DE3]

Ok, still need to find a switch first though

[Clive]

I'm not sure of the switching circuit....does it reference ground? If not the switch won't see the full voltage.

[BL21DE3]

Clive,

The switch would sit between Pin 9 of the EL84 output valve and the UL tap of the KEL84 output transformer. It would provide connections between Pin 9 and the UL tap (which is at 310V DC provided through the output transformer UL winding); Pin 9 and Pin 7 of the EL84; Pin 9 and the B+ line (which is at 285V DC from the PSU section). Not sure if any of those connections reference ground, but I would hazard a guess that the B+ and UL taps might?

Regards,
Ewan

[Clive]

Hi Ewan, it sounds like the PD across the switch is about 25V, before you assume it's safe I'd prefer someone like John C, Rich etc to offer their view.

[BL21DE3]

Clive,

Thanks for the info and I share your reservations about the safety aspect.

Regards,
Ewan

[BL21DE3]

Slight change of direction on this now, having dug around the net a bit more it seems that finding switches capable of high voltage DC switching is a bugbear of many audio DIYers. But in this serach I came across suggestions to use relays to do the switching.

So my thoughts now are to use relays rated to 350-400Vdc and control them using lower voltage switches powered from the heater supplies (this is of course based on my limited understanding gathered from the net of how to implement relays for switching).

So would this work to implement my triode/UL/pentode switch?

Regards,
Ewan

[Richard]

Hi Ewan,

In general terms about the switches there would be the issue of how much current and voltage was passed across the contacts. As Clive suggests this would be in "difference" terms of the voltage on one side of the switch compared with that on the other. In resistors you'll see it termed "limiting element voltage" or similar. The other issue with the max voltage though would be the insulation limit to ground as the switch will be fixed to an earthed chassis or touched by an earthed person. Switches may be rated for AC or DC and it might be correct to suppose that a switch rated AC would be OK for 1.4x that voltage in DC due to the peaks of the rms AC voltage but I'd be careful.

I'd go a different route. It's a very quick job to move the wires with a soldering iron to make the 3 connections for UL Triode and Pentode. However, the conditions of operation for the op stage will change dramatically and will probably be nowhere near optimum for the second two. The TX load, bias, and feedback for instance. Whilst the idea of being able to run in any of 3 modes at the flick of a switch seems attractive, I would honestly save your efforts with switches or relays, solder the connections instead and have a listen. It will tell you immediately if you stand to gain anything from your labours.

Rich

[BL21DE3]

Hi Richard,

Thanks for the reply. I think I'll heed your advice and do my switching by soldering iron .

Thanks,
Ewan