Introduction: DHT madness

I’m not going to dwell on DHT sound. I’d rather say that if you’re looking for a stellar DHT candidate, the 4P1L beats them all. It’s dirty cheap, reliable and sounds amazing. You can go any route you like, it’s your own decision of course. However if you’re looking for a 100% DHT amp to build, here is an interesting example for your consideration.

I’ve tried 4P1L in many topologies. The advantage of its low filament requirements is that you can implement it in filament bias and simplify the circuit significantly.

The gyrator driver using the PCB I designed recently, can be used to avoid iron and have an excellent first stage and make this Russian Amp in steroids: 4P1L driving 4P1L.

One minor caveat around 4P1L in excess. I have found (as well as many others) that if you use too many 4P1L stages (e.g. 4P1L line stage driving a 4P1L-4P1L amp) then it will sound a bit harsh in the treble. i suspect this may be explained due to the H3 component level when triode-strapped. I’d rather limit the number of 4P1L stages to two. You’ve been warned.

The other great thing about the 4P1L is that is quite consistent between samples and easy to match pairs. Also in PSE mode you can drive it to full tilt with only 20Vrms and achieve up to 5W in class A1 with a pair of valves.

The circuit

This is a variation of what a I have built in the past. I haven’t built exactly this amplifier, however I’m sure that it will perform as well as my previous versions.

The topology of this amp is dead simple. It has a first stage (driver) mu-follower with a gyrator load. The driver is coupled by a capacitor (C2) into the output stage (PSE) formed by a pair of 4P1L. These are also with filament bias. All DHTs have a Rod Coleman regulator which will allow to extract the best of the DHT sound with no compromise:

The beauty of this design is that you need only one HT (330V) and of course the filament supplies which are unavoidably the complexity of any DHT circuit. The gyrator is based on a J310 lower FET to allow the first 4P1L to run at a very healthy 30mA and ensure there are no slew rate issues in driving the output stage. The top FET (e.g. DN2540) will carry the heavy load of dropping the voltage about 100V and therefore will need a heatsink to dissipate the 3W of heat. The filament bias of the first stage will burn about 10W on the resistor so you ought to have a 20W wire wound part here. The output stage filament bias is more demanding with 16W burnt on each resistor. I used Dale’s 10Ω/5Ω/3Ω parts to make the equivalent filament bias. These Dale NOS wire wound sound great but check the DIYAudio 4P1L PSE thread as there are several recommendations around filament resistors.

How much output power? Well, with a 3K transformer, you will easily get 3.5W or so. That is plenty of power for a single ended stage if you have high efficiency speakers. The maximum achievable is 5W. However, the H3 spectrum is higher due to grid current. I prefer the sound of the 4P1L PSE up until 3 or 4W. To get the most out of the 4P1L you can drive it hard to 300V, it will work fine:

If we bias the 4P1L pair at 300V / 30mA each valve, then the bias needs to be around -26V. The filament resistor is either 40Ω or 35Ω depending in which leg of the filament you place the filament bias due to the voltage dropped by the 4P1L filament.

I used an LL1623/90mA but the LL1623/60mA version should do fine (in fact, any nice 3K or 5K SE output transformer should work fine here). If you want more power you can use a 4P1L triplet and distortion will drop 1% and you can achieve higher output power. If you want less distortion and you don’t need 5W peak then you can wire the output transformer in 5K:8 and you will get 2.5W @1.5%. I  liked this impedance ratio, sounded really nice and clean.

With 18.5Vrms required to get the maximum output power and a 4P1L driver with gain of approximately 9, you need 2Vrms to get all the juice out of this amp. No further preamp is needed if you’re driving this amp with a conventional CD or similar source. A volume control is needed at the input though.

Well, hope you’ve enjoyed this post and hopefully I have temped you to explore yourself into the DHT world and build a nice 4P1L PSE amp!