Biasing a cathode follower?

[Sch3mat1c]

Well ok, true that's hot switching... but rectifiers still surely find the hardest service of anything, <40uF or whatever limit or not. And they work fine.

So getting to my point, if we agree that especially class AB amps have no chance of cathode stripping, why are people worried about it to such lengths? I know I've seen threads here, vaguely recall seeing you in them somewhere, recommending soft starts...

Nope, my amps all discharge that completely all due to the tubes. Even that 5x12AX7 + 6SN7 active preamp I made, which uses a whopping 220uF on it (yep... I just said, to heck with winding a choke, I'll just weigh it down after a resistor). Although that has a few dividers, three being insignificant but the fourth is two 22ks in series (for heater bias... it's a very DC coupled circuit) which ought to help it out a bit.

Just because tubes are used at >200V doesn't mean they don't conduct at 10V. Hept'AU7, which features 60uF around the CLC filter still produces sound 20 seconds after killing power. It's really quiet and distorted but still conducting.

And the final point... Pentodes have a CCS plate curve (like transistors). Hence Zo is almost exclusively the load resistor, and slew rate is determined by the supply current. A 6AU6 at a common level of say 1 or 2mA, especially driving a triode, ought to be very slow indeed...

Tim

 

[fdegrove]

Hi,

but rectifiers still surely find the hardest service of anything, <40uF or whatever limit or not. And they work fine.

Yes...Never said anything to the contrary.

So getting to my point, if we agree that especially class AB amps have no chance of cathode stripping, why are people worried about it to such lengths?

Because people read all kinds of wrong information as far as I can tell.

I know I've seen threads here, vaguely recall seeing you in them somewhere, recommending soft starts...

If I'd recommended a soft start it would be in a different context but nothing to do with cathode stripping...
See, I told you before you had the wrong idea in the first place.

Although that has a few dividers, three being insignificant but the fourth is two 22ks in series (for heater bias... it's a very DC coupled circuit) which ought to help it out a bit.

If the filaments are biased they're probably not floating either but referenced to ground, so I assume that's how the caps are drained.

Just because tubes are used at >200V doesn't mean they don't conduct at 10V.

Provided the cathode is heated, yes they will of course conduct.

Pentodes have a CCS plate curve (like transistors). Hence Zo is almost exclusively the load resistor, and slew rate is determined by the supply current. A 6AU6 at a common level of say 1 or 2mA, especially driving a triode, ought to be very slow indeed...

The load resistor being which one exactly? g'?
Not sure I follow you on this...

Cheers,😉

 

[Sch3mat1c]

If I'd recommended a soft start it would be in a different context but nothing to do with cathode stripping...
See, I told you before you had the wrong idea in the first place.

Ok then, hoped so. 😛

If the filaments are biased they're probably not floating either but referenced to ground, so I assume that's how the caps are drained.

No, referenced to half B+. But filaments are beside the point. (Interestingly enough, they floated around there without the bias. Says something for the H-K diode!)

Provided the cathode is heated, yes they will of course conduct.

So why do you doubt they are discharging it?

The load resistor being which one exactly? g'?
Not sure I follow you on this...

Cheers,😉

All resistance in this case. If the tube had a (perfect) CCS supply, Zo would be Rp alone, some 10s of megohms depending. Rl and Rg will shunt that a bit...

Tim

 

[fdegrove]

Hi,

So why do you doubt they are discharging it?

Because I very much doubt the cathode will remain hot once you've switched off the amp....
That's how those caps keep their charge for a month or so in the amp I was referring to....
We could have discussed this for ages....

Rl and Rg will shunt that a bit...

O.K....That's what I thought you'd mean anyway.

Oh...just to make sure you don't have the wrong idea again: I have nothing against penthodes per se...
Must be that other person...again...

 

[Sch3mat1c]

Well gee, the cathode stays hot for a while... you must be around way too many DHT's to be healthy to not know that and see it first hand on the multimeter!

Tim (I still love pentodes more than you 😀 )

 

[Brit01]

Just reading through this and wondering if you could use a cascode DN2540 as a CCS on the cathode of the CF.

Or if a bootstrap is used this CCS in placement of the bottom resistor?

 

[piano3]

Can one still find DN2540's ? They seem to have disappeared. BJT cascode current sink should work well anyway if you don't mind the trouble(minimal) of biasing it.

 

[Brit01]

Can one still find DN2540's

I picked up some last time I was in Texas from Mouser a few months ago.

Also got some pnp/npn hanging about.

But the DN2540's should work no?
For the lower resistor below the bootstrap? Or forget the bootstrap and just a CCS?

 

[piano3]

There is a good article on depletion mode mosfet sinks(and many other interesting things)on Richard Sear's site; if you google "triodeguy" .Remember to read the update as he fried his original non cascode sinks.

 

[Brit01]

Thanks.

Funnily enough I was reading this last night.
Got the cascode idea from this forum.

Works very well as a plate load but not sure on the cathode.

 

[piano3]

One MJE340,one high beta BJT,or 2 if you really want to go mad,a few resistors and a zener(or an LED or so)-can't fail! MJE340 is a little static sensitive but not like a mosfet.