5E5 build with switchable fixed/cathode bias?

[stephenhartley]

Hi

I've started my latest build which is largely based on a 5E5 pro (which is cathode based). I want to put in switchable fixed/cathode bias - can anyone direct me to a schematic/layout that shows this? (I've got a Hammond PT with tap for fixed bias)

Also, I'm using the same faceplate as my previous build which only has room for one switch - is it possible to put on/off and standby on the same on/off/on type switch & if so can anyone direct me to a schematic/layout?

Thanks in anticipation.

Stephen Hartley

[bob p]

I could never consider buying books that have such crappy drawings, but here's an example:

Cathode-Bias/Fixed-Bias Switching in Tube Power Amplifiers

[bob p]

you can buy an on-standby-off toggle switch. they're called progressive toggles.

[stephenhartley]

you can buy an on-standby-off toggle switch. they're called progressive toggles.

Great - thanks for that - I'll look into it

[loudthud]

you can buy an on-standby-off toggle switch. they're called progressive toggles.

The common center off switch is called ON-OFF-ON. The much harder to find progressive switch is called ON-ON-ON. You will sometimes find them in industrial supply places like Mouser, Allied, Newark et al. They can also be used to select 4-8-16 ohm taps. If you can't find one, a center off switch could be used, but obviously center is Off and you have to move the switch from one extreme to the other to go from Standby to Play.

[bob p]

I used center off toggles to go from Standby to Play before I learned about the progressive type toggles.

[tubeswell]

You can do this with a DPDT switch. The Rk/Ck is permanently wired to ground and one of the switch poles bypasses the Rk/Ck to the cathode, with the opposite pole throw not connected to anything

The other set of poles toggles the bottom end of the grid leak resistors either to ground, or to the bias supply. When this switch pole is toggled to the bias supply, the other set of poles obviously has to be toggled to the Rk/Ck bypass.

[mhuss]

I prefer the DPDT solution as well because you're not heating up the series limiting resistor when you are in cathode bias mode as well as hard discharging the cathode cap. I imagine there would be a pop either way.

--mark

[tubeswell]

Here's a picture of what I meant

[stephenhartley]

There's another schematic somewhere that's slightly different, but |I used the one that you've put above to work out how to actually wire up the switch - took me a couple of hours of head scratching but finally figured it out. I bought a progressive switch off e-bay

[Randall]

Thanks for that drawing tubeswell, I've been wondering how to do this. Pretty tidy method. One question, why do the grid resistors get switched to ground in cathode bias mode?

[tubeswell]

... why do the grid resistors get switched to ground in cathode bias mode?

Because that's where they should be. The grid leak resistors are there to enable any surplus positive charge that would otherwise build up on the grids during normal operation to be replenished with electrons, thereby keeping the grids at their proper bias voltage with respect to the cathode. In fixed-bias mode, you want the grids at the desired -ve bias voltage, hence they're connected to the bias supply. In cathode-bias mode, you want the grid at the appropriate voltage w.r.t. the cathode (which sits at a +ve voltage). And because the cathode in cathode-bias mode is typically set at the desired +ve voltage w.r.t. ground potential, then the grids should (all other things being equal) be at ground potential.

Of course, you can have combination bias, in which case the grids are at some -ve voltage and the cathode is at some +ve voltage w.r.t ground potential.

However, anything less than pure fixed-bias mode is going to have less gain, because (all other things being equal) the plate-to-cathode voltage is greatest in fixed-bias mode.

[stokes]

I used tubeswells circuit in my own Boogie amp years ago just to see.Its pretty basic,if you just look at a schem of a fixed bias and one of a cathode biased output its pretty easy to figure out.Tubeswell saved ya some head scratching.