Tubes Asylum

RE: Octal base driver for 300b

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This Post Has Been Edited by the Author

In Reply to: RE: Octal base driver for 300b posted by andy evans on August 26, 2020 at 00:32:54

I don't blame you for trying to avoid the bypass cathode cap as I'm also mindful of its effects. But then we also need to keep in mind that the last cap of the PSU is also in the current loop and hence in series with the signal. I guess Ultrapath is a way to mitigate the two caps in series while avoiding degeneration, but that has its drawbacks as well. SiCs are a good way to go although I haven't yet tried the ones I got some time ago (CSD01060A) for this purpose.
I took a look at the EL33 and at the OP you're proposing it might work, but it'd be cutting it close. Assuming that you will be needing around 70 Volts peak one way to drive the 300B, you'll be coming very close to drawing grid current at full power and just as importantly there will be no headroom for any high transient peaks. Unless you have very sensitive speakers (i.e >100dB/W) that's something to consider.
The other issue is the higher plate impedance due to the unbypassed cathode. Remember that in this case Ra=Rk(u+1). According to the datasheet the EL33 in triode mode has an Ra of 7K and with a 200 Ohm unbypassed resistor this will become greater than 11 KOhms. You will need to use a very good choke if it's to be driven by this impedance. The 10Y will work a little better with a choke load, but the stage gain will be about half. TANSTAAFL applies here too.

Edits: 08/26/20

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