45 SE Amplifier – revisited

A unique sound

If you have a small room and high-efficiency speakers, then keep reading. If not, you can still enjoy reading about probably the best sounding output valve in my view. The 45. I wrote about it few times and have to say, it’s still one of the best. Better than a 300B but unfortunately can do only 2W. You can use it in Push-Pull and is superb. Have a look at this.

Lately I had a few requests from different people about this circuit which was my main amp for few years. If I were to build it again, I’d do some changes like the following ones.

For the minimalists, you can implement the 7193 driver with SiC diodes in the cathode. Actually is a hybrid setup between 4 SiC diodes and a cathode resistor, unbypassed of course. You don’t need the SIC PCB, you can hard-wire the diodes, however the PCB simplifies the build. Each diode drops about 1V, less with small current like in this driver. The cathode resistor R3 provides the additional bias. It’s a mu-follower/SRPP circuit so no need to bypass R3:

7193 driving the 45 with fixed-bias

The operating point was found years ago upon testing and listening. The gyrator PCB is straight forward with a BF862 (or a BSH111BK with Rev08), The pair R4 and C3 provides the voltage drop for the output stage. C3 is an oil cap or film – MKP (DC-Link). You don’t want an electrolytic here as is the final cap.

The NP accoustic transformer was reviewed before. It’s a great OPT and the value for money is fantastic. You won’t get an amorphous core OPT sounding like this one at the price point.

The 7193 drives the 45 grid directly. I use Rod Coleman’s fixed regulator and the output valve has an NP Acoustic amorphous-core transformer which is great value for money.

You don’t need necessarily to heat the driver valve with DC. I do it, but isn’t mandatory if you pay careful attention to the heating wiring.

The output stage topology was discussed before at length. I’m running the 45 rather hot but works well within specs.

You need 120Vpp to drive the 45 at full tilt in A1. This is a great SE amp and is as simple as it gets. You won’t regret it and you will get hopefully the best of the DHT sound transparency and level of detail.

Improving the basic circuit

Ok, you want to improve this, don’t you? The best way to avoid blocking distortion and to improve the performance of the output stage is by adding a Source Follower to drive the 45 grid. A very simple step here. You can reduce the coupling cap (C2) value as the grid resistor here can be as high as 1Meg (or more). No issue with the grid current as the MOSFET in the source follower doesn’t grab any at DC. The 7193 driver can source/sink enough gate current to the MOSFET. The source follower provides enough current (source and sink) with the 20mA idle current at very low impedance to the 45 grid.

The negative swing can go as low as -120V so -150V is preferable to provide enough headroom. The positive swing can be 0V in A1 or we can allow some headroom for A2 operation with a +50V supply. You can get about 3.5W maximum in A2 operation if you drive the grid positively to +20V:

The 7193 has a gain of 20. This may prove to be a tad short. If you have a pre-amp then is fine. I used to run this amplifier with a 26 DHT preamp stage. However, if you want more gain, you can replace the 7193 with a higher gain driver like the D3a. Also the C3g, 6J52P, E810F, 6e5p/6e6p, 6C45p and others works really well here. Similar circuit. Only change is to dial the right anode voltage in the gyrator PCB as well as setting effectively Rk.

It’s a circuit you can play with and swap different drivers with minimum changes (just dialling anode voltage and changing Rk). You can experiment a lot and listen to the differences. I’d add to the chassis 2 sockets (one octal and one 9-pin noval). That way you have all the choices needed.

You can of course experiment as well with a DHT driver. Plenty of options. Likely you’d like to use a SUT before the DHT. Best drivers in my opinion are the 4P1L and the 2P29L. Both with low mu of about 8. You will need the SUT to deliver a gain of 5 to 10.

You won’t regret in building this amplifier. You can get the new Emission Labs 45 series. You can try the EML45B (I haven’t had the chance yet) if you need more power. It can deliver twice the output power allegedly with the 45 sound. If you have tried it, I’m keen to hear your comments about it.

Sometimes less is more. The 45 valve is a great example of that mantra. I remember enjoying several late hours of music. Playing Coltrane and Miles Davis through my Fostex FE167e was a real pleasure. The 45 is magic, please try a bit of its addictive tonic, you won’t regret it.