Chasing that sound
The 45 DHT is probably one of the best sounding valves out there. In fact, I have struggled to get a similar level of detail and timbre in a 300B or 4P1L output stage. Even my 814 SE Amplifier (which was class A2 and had thoriated-tungsten filaments) couldn’t replicate that sound. I posted time ago my incarnation of the 45 single-ended amplifier here. The main challenge with this valve is that it can only put out there nearly 2W, not more. With its 10W anode dissipation, you will struggle to get more juice from it in class A at a low distortion level.
However, if we look at a push-pull amplifier with the 45, we can hopefully retain the timbre characteristic of the valve, despite it won’t be a single-ended one. Well, I love good PP amps, so why not?
If you read carefully the datasheet, you will find that you can extract a lot of power from this valve in A2 operation. Precisely, AB2 in PP mode. Up to 19W from a pair! Wow, that’s impressive. I’d be happy with 10W on my Alpairs. Having enough headroom is good from a dynamic response point of view. You won’t probably need more than 3-5W on average.
Tracing the 45
I had at hand a pair of Sylvania 45 which I submitted to the mercy of the eTracer. The one shown below measure well above 80%, so it’s a good reference. Nevertherless, I’m aiming on a fixed-bias design, so providing the pairs are matched and measure decently well, then it should be fine.
Here are the lovely curves of the 45:
Creating the Spice model was easy. You can download it from here: 45 DHT Sylvania model.
The output stage
The suggested operating points from the datasheet are pretty obvious. At the end it will come up to what OPT you have at hand. Somewhere around 3.2KΩ – 4KΩ Zaa is preferred. Below is a snapshot of how much we could get out of the valves with a 4KΩ transformer:
Of course it doesn’t match the datasheet recommended point, it’s fairly close. With 240V/28mA on the anodes you will need a bias of about -46V. The grid needs to be driven pretty hard up to close 200Vpp to achieve full power. The grid will also sink about 20mA of grid current during A2 operation. Please ignore the THD estimate from the tool as it’s wrong.
Simulating this circuit in LTSpice with an LL1682PP (which is 8.8KΩ Zaa), can deliver 10W at 1.1% THD, mainly H3.
Below is the circuit I’ve been toying with for some time:
The circuit is very simple. The output stage has a DC filament regulators (e.g. Rod Coleman) and a mix of fixed bias with filament bias for better stability. As we need to provide the A2 grid current, a source follower is mandatory here. The grid bias is generated from a Swenson Regulator PCB (or a Rod Coleman fixed bias board) and adjusted independently per board to ensure there is balanced anode currents.
The grid voltage can peak to 60V so at least 80V are needed on the top MOSFET of the SF PCB. Likewise, the grid can go as negative as -140V so -160V is needed as the negative supply.
Building the above in separate chassis provide some flexibility in terms of the drivers used. I was going to use a differential pair of pentode valves in a hybrid mu-follower configuration. That is with a gyrator PCB and a tail CCS. very simple and yet effective. It can deliver sterling gain and linearity for 200Vpp operation. Similarly, triode-strapped pentodes like D3a, 6J52P or similar would work very well here.
Lots to work on, just wanted to share initial ideas on this design. Probably will inspire others.
10W 1% H3 with a 45 PP
that’s nice !
Hi Ale,
I have gone the other route with the 45. The reason being that NOS tubes are becoming scarce in really new condition and current production is expensive. Driving the 45 into A2 or AB2 will ask for high peak current. If one uses that power tube life will be shorter. So my last take on the 45 PP amp is Class A1 with just few volts swing into positive grid possible. Wide-band IT coupling with medium source impedance. This will easily allow light A2 operation but not full swing. On the other end the higher source impedance will make the amp clipping very soft which is desirable without the nasty blocking distortion caused by RC coupling. Can use fixed bias but also self bias like Olson KARNA amp. I have just designed a 10K output transformer and have another design for 12K plate-to-plate. The target output power is in the 5.5-6 W pure class A. Of course the plate voltage will need to be higher in the 300-325V range. No problem if plate dissipation is 10W or less (30-33 mA depending on plate voltage). I will do the first test of the 10K transformer in another amp I am completing. It’s a classic Williamson using the 6V6 in triode mode for about 6-7W class A. This is as close as possible to the original so despite the fact it’s low distortion without fbk I will use some. I also have another design for 16K plate-to-plate transformer to be used with 6C4C. Again plate voltage will be higher than usual at 360-375V. This is no problem for the 6C4C that is specified for 360V max. In theory I should get a Zout of about 1.5R without fbk….I have started to look into this high primary load power stages with the PCL86 where I have used a 16K OPT to get 10W in class A in pentode mode.
Ciao Paolo
This is great feedback, I appreciate your experience. I remember years ago our discussion about this circuit. Yet, haven’t implemented it.
I won’t need more than 5-6W myself, so don’t expect it to run too often in A2. I have several pairs of Sylvania 45s. I will make some tests. Can always back it down to A1 though.
I have a nice pair of NP Accoustic amorphous PP OPTs I will use for this.
Would be great to meet up again at some point. Hope everything is well.
Cheers
Ale
Yes it would be great to meet again. Still need to bring all my NOS 45’s over here but that has proven difficult when traveling with the entire family. Next October I will travel alone…for sure I should get the 6v6 amp and the TABAQ speakers posted soon to UK and we can organise something. The 3″ TABAQ in the picture I sent you some time ago sounds lovely, impressive bass for such a minuscule driver but it is clearly limited in max SPL. The 4″ with the TB 1320SIF is the way to go and will definitely make it. Really easy to build if you can get the wood cut properly.