46 driving 45 – SE Amp

My favourite valves together

Recently I revisited a beloved amp, the SE 45. This time I will share a more orthodox design without sand in play. Surprised? Well, I love lots of iron as well and here is a design I’ve been playing around for some time as I have all the components at hand. 

Driving the 45

Driving the 45 isn’t an easy task. You can get outstanding performance with the gyrator (hybrid mu-follower stage) and nice triode stages (or triode-strapped pentodes) like 4P1L, 6J52p, D3a, C3g, 6J49P-DR, 6C45p-E, etc. If you want to go full DHT, then you have a challenge. The gain of DHTs is relatively low and unless you want to go crazy and spend money on an Emission Labs’ EML 20 or 30 valves, you will need to resort to a step up transformer to obtain the gain needed if you want to avoid more than 2 stages.

 I looked back into my favourite list of valves and obviously the 46 came up at first thought. I have used it before with great success in my 814 SE Amp. However, implementing filament bias with that valve was crazy. I will not do that again, therefore a more sensible approach would be either fixed bias or cathode bias. 

Let’s look at the 45 driving requirements first:

45 SE Amp with 5K6 loadIn my experience, you can get the best out of the 45 by loading it with +5KΩ and biasing it at 300V/30-34mA. The actual load with a 5K6Ω transformer will be closer to 6KΩ if you factor in the primary and secondary winding resistance. Either way, you’re looking to a 120Vpp driving requirements to get the 2W out of this stage. So you driver has to easily deliver 150Vpp with very low distortion. This rule out many options of course. I’m not going to dwell around this as it has been covered extensively by many people out there.

So if we look at the 46 in triode mode, you will only get a gain of 5. How do we achieve the voltage gain needed? My recommended way is to include a step up input transformer which also provides a galvanic isolation and load the 46 with a 1:1 (or 1:2) interstage transformer. I have used before the LL7903 which can do 1:8 with lower distortion at high signal levels. It sounds really nice and is a fantastic transformer. Recently I reviewed a great input transformer from Dorin Bodea here. This can do 1:6 but I assume that if you ask Dorin, he can get you a  1:8 version.  You can implement this design with 1:6 though.

With a 1:8 input transformer you will get about a gain closer to 40 overall. This means that with just over 1Vrms you can drive the 45 to full tilt. 

A lot of iron

So here is the initial design, you may critique this. That’s great. There is plenty of iron and oil caps for the fanatics:

46 driving 45 – DHT SE Amp

The design is minimalistic and classic. The input transformer drives the 46 directly and has a Zobel network to compensate the HF response. The 46 is cathode bias with a 910Ω resistor and a 45μF oil cap. The choice of the interstage is the Lundahl LL2746. This nice transformer has proper primary inductance for this valve as well as can be wired on 1:2 or 1:1. In my case I prefer the 1:1 stage. It performs really well, I have tried it with the 4P1L to achieve a high gain stage before, see here. The operating point is reflected below:

46 driver

The 46 is biased to 30mA and has enough headroom to deliver the +120Vpp without getting closer to grid current and with very low distortion. 

The output stage is fixed bias. I will use Rod Coleman’s fixed bias regulator. He has recently released this and I’m in the process of completing my tests. The secondary of the LL2746 is loaded with a 220K but i suspect that a proper Zobel network may be needed to flat the response. Need to test on the bench and optimise this a bit.

The LL1623/60mA is my favourite 5K6:8 transformer I have used with both 45 and 2A3/6C4C stages.

The HT supply is being reused from my 4P1L PSE amp, but can be adapted as you need.

So if you have high efficient speakers and want the best sound of the first Watt, then you should look into this amp.

I’m interested in hearing different experiences and comments about this amp. I’m sure everyone has a view.

Happy New Year!


4 Replies to “46 driving 45 – SE Amp”

  1. Dear Ale,
    I’m trying to read your schematic and am having trouble to understand why Ck is connected to HT? Would you mind to give a hint?
    Btw, I’m using your Gyrator schematic with success @ 240V, 190mA by paralleling J310’s and using an IXTP3N50D2. My application for this is a Gyrator-loaded 2SK82 (SIT; although a Transistor, it features triode characteristics) driven by a -also Gyrator loaded- 4P1L (followed by a necessary SF).
    All the best for 2017 and many thanks for sharing!

    1. Hi Ulrich,
      Happy NY to you too!
      Glad to see you have used successfully the gyrator board for the SIT amp. Recently I tried an SMD MOSFET (see posts) which can do 250mA and with best frequency response than the previously tried. You may want to look into it as it will give you lower output impedance and best response than the J310.

      The cathode bypass capacitor tied up to HT is called “ultrapath”. Do a google search and you will get a lot of information around it. I recommend reading Tubecad entries on this as well as Thomas Mayer’s (aka VinylSavor)

  2. Dear Ale,
    thanks for your fast answer! Ohh, Ultrapath via that Ck, ok, that’s it, didn’t recognize at once. Sorry.
    I furthermore already saw you post regarding that high current SMD Mosfet! Unfortunately SMD soldering is not really the enthusiastic part of that hobby to me. But I’ll give that part a try when implementing the pull-push version of that SIT amp.
    Many thanks and hope you’ll have enough “free time” for your favorite interests,

  3. Happy new year Ale. I am using 300B SET (IDHT driver) in an active system, with SET just on the mid/treble compression driver, but 45 has always been of interest to me; even a 45 would barely be putting much effort into driving just the compression driver. Looking forward to seeing the distortion of DHT driving 45 output amp. The only one I recall is from this thread: http://www.diyaudio.com/forums/tubes-valves/267747-distortion-expected-se-dc-coupled-26-45-amp.html

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