Pentode drivers are very interesting. You can get excellent results out of them. Here is my version of a differential pair using pentodes. I want to try them out in the 45 PP design I wrote previously
People may get scared with the above diagram thinking: this is way too complex. In fact for me, it’s not. Using the PCBs I have it makes it easier and simple to build.
The differential pair is formed by V1 and V2. They have a tail CCS which sets the shared cathode current. Each valve has a gyrator PCB and a resistor to ground from the anode which sets the gain. The anode voltage can be adjusted by the gyrator but will not make a big difference on the anode current of the pentode (think of a flat anode curve). Remember the gain is defined by gm and Ra. However, if we vary the screen voltage we can adjust minor differences due to unmatched valves.
Regulating the screen is very good to keep distortion down. The additional complexity here is to create a resistor divider to adjust slightly (and independently) each screen voltage. The voltage reference from VR1 is isolated byR3 and R4 to avoid oscillating VR1 by adding additional filtering capacitance as C3 and C4. The SF boards are used only to provide MOSFETs driving the screen current to each valve.
You can simplify this circuit of course. I will try this and experiment a bit.
Hi Ale, wouldn’t a differential cascode be slightly less noisier? Cascodes don’t have partition noise.
What gain are you expecting to get?
Regards,
Silviu
Hi Silviu.
There are multiple ways to skin a cat as they say. The purpose of this driver is to swing many (many) volts as a differential driver for a PP output stage, so pentode noise isn’t an issue. Surely a cascode will be less noisy.
For a different application I’d go for a folded differential cascode as there are several advantages in the circuit. Actually Rod Coleman has some boards. I built a pair of folded cascoded using his boards and an E810F triode strapped. Noise is very low and gain is very high (50dB) on a single stage. distortion is very low and PCB very stable, highly recommended. But that was for a RIAA stage, not a driver.
What i like from this topology is that you can balance the circuit very well (in theory at least) with playing with the screen voltage and anode voltage settings (with a trimmer). This gives you flexibility on the matching requirements between valves. Also you can apply nicely “plate-to-plate” feedback (a.k.a. Schade) and isolating the output from the feedback node by using the mu-follower output to drive the output stage.
The differential cascode is more difficult to balance properly as the gain is gm*Ra. So you will have to implement a balance trimpot before the CCS and between the two cathodes of the lower cascode triodes to influence Ia and therefore gm.
Haven’t built a differential cascode stage so can’t give any practical advise here.
Cheers
Ale
Ale,
I use an E88CC differential cascode with a CCS tail with resistor plate load and I must say I am very impressed. It swings 150Vpp per side with less then 1% THD using unbalanced tubes (I just got my own tube tracer just one week ago and need to find some time to start pairing tubes). I also tested it with choke loads and the gain went sky high.
Would you say the hybrid mu-followers would further increase the gain?
PS: would there be any chance to build a +60dB gain MC phono head amp only with tubes? LTSpice says yes but reality bites 😔
Regards,
Silviu
You can achieve 200Vpp with less than 0.3% with either a hybrid mu-follower or a folded cascode. 1% is very high in my view. More importantly the harmonic spectrum is nice in decay from H2 to H5.
I have a 70dB RIAA stage with 2 folded cascode stages. First is a jFET one and second is the E810F stage I talked about.
you wont get the SNR with a valve am afraid suitable for MC RIAA. Hybrid is the way to go unless you want to pay top dollars for a high-quality SUT.