White Cathode Follower Quandary

[cu6apum]

Broskie says that the load current must be within 1/5 of plate current of the chosen valve. Ecc88 has 15mA, whilst 35V/2k5 is 14. Where am I wrong?

 

[cu6apum]

Edit: oops, sorry, WCF, not BCF. 🙂
Is there any voltage swing limitation in VCF? I read somewhere that it's low voltage schematic (hence headphone amp as main application) and as I can see the input signal must not exceed the offset of the lower lamp to avoid clipping. In BCF there's no such question, as the lower lamp is just a CCS.

 

[Merlinb]

Broskie says that the load current must be within 1/5 of plate current of the chosen valve.

He does? Seems kind of arbitrary...

Ecc88 has 15mA, whilst 35V/2k5 is 14. Where am I wrong?

That's 14mA peak to peak, or 7mA peak, which sounds reasonable. Or just use two ECC88s in parallel.

 

[cu6apum]

14 mA peak. In worst case. But this is related to classical CF, not pushpull. The second supplies such currents easily, but what about the voltage swing? See my edit above.

 

[Merlinb]

Is there any voltage swing limitation in VCF? I read somewhere that it's low voltage schematic

What schametic are you talking about? Can't you use any voltage you like?

 

[cu6apum]

What schametic are you talking about? Can't you use any voltage you like?

The same old WCF, nothing unusual. See Tube CAD Journal: Tube regulators part 2, July 1999 for example.

 

[Merlinb]

Is there any voltage swing limitation in VCF?

Obviously the voltage across the lower triode must not attempt to swing too low or the lower triode will clip. Imagine there is 100V across the lower triode at idle; you want 17.5Vpeak output, so the anode will swing down to 82.5V as its anode current increases by 7mA. I haven't checked, but that sounds pretty easy for an ECC88.

 

[dirkwright]

Let me revive this topic a bit.
Hi.


The question is - will the White scheme help, or it won't? Most White followers are designed to drive headphones, with hundreds of millivolts on the output. I need some dozen volts. On other hand, the lower the output impedance is, the better the output stage is driven. If I implement classical Broskie follower loaded with a CCS, the second tube must have quiescent current not less than 40mA (5x load current in worst case) - ie, to be a real samovar like 6AS7.

Thanks for thoughts!..

My thoughts are to put this into a simulator and see what you get. With the high voltage of vacuum tubes, you should have no problem driving 12V into a relatively high impedance.

 

[cu6apum]

I think I'll end up with this, yes. 12v is fine even looking at the schematic, but 70v (+-35) is not so obvious.

 

[Merlinb]

I think I'll end up with this, yes. 12v is fine even looking at the schematic, but 70v (+-35) is not so obvious.

Ack, I thought you said 35V peak to peak! Sorry. Yes, 70Vpp is more taxing, though still possible with a high enough HT.

 

[wa2ise]

Anyway, the solution to your problem is simple. Just add a dummy load that is present when using it as a line stage, but is switched out when you plug phones in...

Radio Shack used to have 1/8 inch headphone sockets that had internal switching you could hang the dummy load resistors (a pair of 33 ohm resistors) and you could have RCA jacks wired in parallel with the resistors. These jacks should be fairly easy to find at various online places.