Cheap Current Source Drivers

[harry]

... as found on Merlin's website.

The Valve Wizard


I'm trying to adapt this to use with a Type E tank (800 ohms, instead of 1.4k - requires 3.1mA drive current instead of 2mA) and an ECC81.

According to this site:
TDSL Tube data [ECC82]
the ECC82 has an anode resistance of 6250-7700 ohms.

TDSL Tube data [CV4024]
whereas the CV4024/ECC81 has an anode resistance of 11000-16500 ohms.

(I'm not sure why there's this variation?)

To work out what the resistance of a paralleled pair is, is it as simple as halving this quoted figure?


The reason I ask is that the reverb signal is very quiet (yet if I bang the tank it is very loud - so the recovery's just fine! I'd say it's the drive at fault) and perhaps I need to better match the load rather than just copy Merlin's example.

[Steve Conner]

It's a fundamental limitation of triodes. There are three parameters describing a triode: the amplification factor (mu), the transconductance (gm), and the plate resistance (Rp). But a triode design only has two degrees of freedom. So these three parameters are always bound together in such a way, that if you know any two, you can calculate the third.

The upshot of this is that triodes with higher mu must also have higher plate resistance, and vice versa. If you want more drive to your tank, you have to choose the ECC82/12AU7 and accept that it needs more grid drive to do its job.

I'd recommend the SRPP circuit that Merlin shows further down the page.

[harry]

Thanks Steve.


I'm using DC heaters so I can't use the SRPP - especially with an ECC81 as the heater-to-cathode max is 90V I think.

It's annoying - the drive's almost there! but not quite...

[Steve Conner]

No reason why DC heaters can't be elevated with another DC voltage.

[harry]

Ah, of course - you elevate the ground reference as well!

[mooreamps]

..... except you may notice most of the more experienced custom builders do not use the SRPP ; but rather will drive the tank transformer with a small power tube like the 6V6. For a guitar amp, the SRPP has a glassy brittle solid state sound to it, and I would believe it would just ax the tone from a tube amp.


-g