If you need gain and good drive, our friend the pentode is there. However, with the high anode resistance, it’s hard to implement as a driver. With a resistor load you get good results, but not optimal. The gyrator load (as a hybrid mu-follower stage) brings a good option to the pentode driver. The workaround to the high gain of the stage has been cleverly addressed by Gary Pimm. Here is just a brief summary of how to implement it:
The circuit can be explained easily. The pentode (U1) is loaded with the gyrator (g1). The pentode screen has a stable voltage (vs) which is provided by the voltage regulator (U2) and the CCS formed by M1+R2. You can implement the screen voltage source that best suits you. Anyhow, the input is provided to the grid (g1) and the grid resistor (Rg) provides ground reference. The cathode resistor (Rk) is un-bypassed. Quite unusual for a pentode. The thing is, we have gain to spare, but thanks to the gyrator, the output impedance of the stage isn’t mu times the Rk. Hence we can afford adding this resistor which also linearise the stage thanks to the negative feedback introduced. Ra is required to provide a stable output and limit the gain. The gain is therefore Gm times the Ra, Gm is degenerated due to Rk (unless you bypass it). Ra could be also be placed in parallel with G1, but as Gary Pimm well explains, it’s better to have it referenced to ground to improve the power supply noise rejection (PSRR).
The output is take from the mu output of the gyrator. The load is connected here. If you need all the gain from this stage you can bypass Rk or better replace Rk with a series of diodes (SiC) or LEDs. Whatever you please.
This stage can be a great driver for a SE stage. Like a 300B. A 4P1L will work brilliantly here. As most of the Russian pentodes.
Also if you want to go further, you can implement a pentode output stage and provide plate to plate feedback (a la Schade) and create a fantastic amp. Michael Koster and Anatoliy have covered this topology at length in DIYaudio, check it out. If you elevate the cathode of the output stage you can DC-couple it. Great stuff and sounds amazing, I did implement this with my 814 SE Amp.
As you can see, a very flexible stage, thanks to the gyrator. Once again, a very handy topology to use.
Cheers
Ale
Ale
A question about Schade feedback, in the pentode loaded circuit where do you recommend connecting the plate to plate (Schade) feedback? At the plate of the input pentode or at the mu-follower output. As I see it both have issues: The mu-follower is low impedance and you want high impedance for Schade to work. The pentode’s plate has the gain setting resistor Ra in parallel to the plate. Ra acts as a voltage divider which would effect the Schade feedback. Any thoughts.
Good point Anthony, I should have clarified it further. You don’t want to connect the feedback resistor on the low impedance output (mu) as it won’t work effectively given the low impedance presented by the feedback resistor. If you want to do plate to plate feedback the connection should be to the anode. You should either remove Ra and provide the gain setting by the feedback resistor, which is more difficult to achieve as you will be more restricted as gain and feedback will be both dependant on this resistor. Or you have to couple the feedback resistor with a cap. The DC gain of the stage is unaffected and determined by Ra. In AC, feedback is provided via the feedback resistor.
Cheers
Ale
Again, revisiting my reply. You actually don’t need to coupling cap. The feedback resistor in DC will be significantly bigger than Ra (for reasonable levels of feedback). You can then connect it directly to the anode of the pentode. The DC gain of the pentode will be established by the gm (degenerated or not) and RA. However due to the feedback loop, Rf is reflected in parallel with the Ra as Rf/(1+A) given the output valve acts as a trans-impedance amplifier. The gain A in this case is the open loop gain (or mu of the output pentode). This in essence reflects the feedback action as the resulting gain of the pentode driver will be decreased significantly as the value of Rf/(1+A) will be lower than RA. Bear in mind that we should expect at least an open gain of 100 or more for a high gm pentode driving a 5-6k OPT.
I have simulated this topology which works really well. What would be interesting to see is whether is worth the addition of the gyrator load instead of a simple resistor, I tried that in the past and the simple plate to plate feedback works like a charm. The advantage of the addition of the gyrator will be on providing low impedance and current drive to the gate of the output pentode. This of course could be achieved by a follower. However, with a single resistance the PSRR of the pentode will be lower than the gyrator as well.
Will have to build and listen to it!
Cheers
Ale
Hi Ale, I love your work, Can these Gyrator boards get round the Pentode driver – 300b fixed bias problem, where the drivers anode resistor wipes out the 300b’s 47k grid resistor, they still look parallel to me, so I’m not sure. thanks John
Hi john, thanks for your comments. I’d not implement a 300B without a follower and have DC coupling between follower and grid.
If you want to use the pentode with gyrator load you can add a follower coupled with a cap between mu follower output and gate. there you can add a 1Meg fixed bias resistor at the junction of the gate and cap. A 33/47nF cap should suffice. Alternatively, a DC coupled gyrator will required stack supplies like my 814SE amplifier.
Thanks, Ale