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In Reply to: RE: Octal base driver for 300b posted by andy evans on August 25, 2020 at 01:58:28
>>I'd prefer a low bias voltage to avoid a cathode bypass cap if possible<<
Leaving the cathode un-bypassed will raise the tube's plate impedance significantly. While this might be okay for a voltage gain stage, it's not a very good idea for a tube acting as a driver and even more so if it's driving a DHT. I would reconsider that idea if I were you.
A good 300B driver tube that's been used by quite a few is the 6N7 where the two sections are run in parallel.
Basically I hate cathode bypass caps and avoid them wherever possible, e.g. using filament bias or SIC diodes. But with a high inductance plate choke it's been possible to use a cathode resistor of e.g. 200R under certain conditions. For this you need to put as much current as possible through the tube and also have a low bias voltage. And of course a low plate resistance helps. It's not impossible if you choose the right tubes. I'm using a 46 right now with a 200R cathode resistor and a 180H plate choke. A 10Y works as well.
I'm just now building a 300b with a EL33 driver, and that can take higher currents and has a low bias voltage. Mu of 20 in triode. Wondering if there's anything similar - maybe the 6AG7/6P9. It's a mu of 20 and up I'm looking for.
You can get very large gain from a CCS-ed pentode. And low output Z thanks to the mu-output. Tune the gain with a resistor between plate and ground. Gary Pimm is the first I heard writing of this, and unfortunately I have not spoken to him in forever.
CCS tuned for the idle current and the resistor current.
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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 if you can live with that. TANSTAAFL applies here too.
Thanks for the reply - I hadn't read that the anode impedance in triode was 7k. It's true that a plate choke with good inductance will be needed. I have the following:
Hammond 157G - 40mA, 30H
Hammond 126B - 30mA, 44H
NP Acoustics amorphous - 15mA, 136H
If I plan on -5v bias then the cathode resistor and voltage a-k would be
Hammond 157G - @40mA, 30H - 125R - 230v a-k (this is max for EL33)
Hammond 126B - @30mA, 44H - 166R - 200v a-k
NP Acoustics amorphous - @15mA, 136H - 333R - 170v a-k
The Hammond 157G would be more comfortable at 35mA, -5v, 215v a-k resistor = 143R
Pondering which of these to use - could try them all. The other one I'll try is an EL84. Not octal, but 2.2K anode impedance in triode, operating point 175v a-k, -5v, 35mA. Cathode resistor 143R. I'd try this with the Hammond 126B. I also have some 6S4 I could try.
Glad you have these chokes to experiment with. Just to elaborate a bit regarding my comment on the quality of the choke, it has more to do with the shunt capacitance / leakage inductance characteristics rather than a high-ish inductance value. Same criteria that govern good IT performance apply here as well.
For this reason I suspect that the 157G will not be a good match as it's designed to be a PSU filter choke where wide bandwidth is a detriment and not an asset.
If you're going to entertain noval tubes like the EL84 you also might want to look into the 6197 / 6CL6 in triode. You can bias it with a 150 Ohm cathode resistor and it will be drawing 30mA with around 170V on the plate. At the specified OP it has a gain of over 20. The best part is that it has a plate impedance of 1.9KOhms
The 6197 with 5 x CSD01060A SiCs connected in series at the cathode, should bias the tube at the above mentioned OP with the added bonus of retaining all of its gain and the low impedance due to the avoidance of current degeneration.
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