6S45P

The 6S45 is quite a nice tube. It's high mu makes it an excellent all-round tube combined with it's low Rp. I tested it at 145V on the top of the choke and biased at around 30mA (Rk = 62 ohm). This raised bias voltage (compared to standard parameter of 150V/40mA/-1.5V) allows for a larger input signal before the tubes runs into cut-off. This makes it more suitable for real-life use such as preamps or input stages in poweramp.

I did some tests with both RC and choke loading to see the difference in numbers. The tests I ran are as follows:

Frequency range: apply an input as to give a 1Vac (0dB) output level at 1kHz. Then dial in the generator to see where the roll-off is at -3dB.

Square wave response: take a 0dB square wave output (1V) level at 1kHz and see what happens at other frequencies.

Maximum output: See what the maximum RMS output is at 1kHz before the sine wave shows visible deformation.

Important note for the 6S45P

During testing with the choke I noticed some weird fizz on my scope read out. When trying to find out what was causing it my heater power supply went into overload and starting swinging wildly. The next thing I noticed was the filament acting as a flasher and me getting a sh-sh-shock from the wire coming from the choke that I was holding. Nice thing I have a nice big off-switch on my socket bench so I saved the circuit and my meters from blowing up.

My guess is that the tube started oscillating when I was testing the freq. range in the MHz region. For this reason I added the 1K grid stopper. The resistor does have some effect on the high frequency range as it lower the high roll off from over 1MHz down to 670kHz. You'll be doing your pets a favor as well :)))

I suggest that this tube not be used without a grid stopper in the circuit. It doesn't cost a whole lot but can save your setup!

RC-Loading

Freqeunce response between 2Hz and 675kHz

Square wave response for the low end, very good.

Square wave response for the high end, very good up to 100khz

Maximum output signal is about 50Vrms before clipping on the grid (positive) occurs

LC Loading

Frequency response between 5Hz and 420kHz

Sqaure wave response for the low end, no problem down to around 30Hz

Square wave response for the high end, so-so at 100kHz

Maximum output signal is 70Vrms before clipping on the power supply occurs

Bypass capacitor

After removing the 3300uF cathode bypass cap the low frequency extreme drops (or rather rises) from 4Hz to 16Hz.

Using a non-"plate load" choke as a load

For those on a budget...

This is taken using a 10H supply choke. Bottom frequency is expectedly lower due to the lower induction, but also the core is into saturation dropping it even further. Expected roll-off in this circuit would have been expected around 15Hz with a 10H choke.

At just under 188kHz the signal level starts to drop rather quickly and noises start coming out of the choke. The choke used is a 10H/120mA supply ( ! ) choke, pretty good but as you can see not really recommended for audio use. The bandwidth is so-so in the high region, but not much in the lows (<50Hz). This is to be expected however, otherwise it would have been called a plate-load choke ;)))