Going DHT end to end
As previously mentioned, I played around with the 46 driver. I love it sound and is a great valve. However, there were 2 reasons that pushed me to switch to the 47. Firstly, I wanted an extra tad of gain. Secondly, I have a nice stash of RCA 247 globe which measure extremely well. I’ve been reserving it for a 47 PP amp with local feedback (a la Pimm) and hopefully will get to in the future. Anyhow, the 47 in triode mode has a mu of about 8 which in combination with the SUT, gives me good gain to drive my 300B. After tweaking on the bench the stage for optimal swing and distortion performance, I ended up with the following circuit:
Ok, there is a catch. I use a low output impedance DAC which I then control via software the volume. If you want to add a volume potentiometer then you have an issue as will impact the frequency response. If you want to implement this, you will have to use a cathode follower to drive the LL7903, something like this will work perfectly. An AVC would be an option. I’m using Slagle’s AVC which is superb.
Taking the 47 (or 46) around 200V and 30mA gets the best out of it. You also need to watch out for some old valves as they may struggle to provide full swing at low distortion and (more importantly) generate too many unwanted odd harmonics at high voltage swings. You can check this with an FFT view at the mu output of the hybrid mu-follower.
I will not explain again why I personally use the mu-follower, however there is a great advantage here. You can use avoid a bypass capacitor at the cathode resistor which is great for this higher Vg valves. As filament bias becomes unpractical here (I’ve been there done that), the hybrid mu-follower wins here, and sounds great!
There are a few compromise adjustments made here as have maximum 200mA DC capacity at the HT, so dialled the 300B down to safe levels. Also I’d reduced about 1V the filament regulator raw voltage to help with the heatsink dissipation. Other than that, the amp is the same as previously shared:
If you look at the driver performance, it’s great to see that it can do about 0.12% for 200Vpp output. Mainly H2 and the relative H2 level is -58.1dB. Not many drivers can do this. Only few DHTs. Best I could get with IHT is about 0.2%. Anyhow this is great performance. Same flat distortion measured up to 30kHz, the driver has enough drive current. At LF (below 100Hz), the distortion of the LL7903 dominates and brings it up to 0.3-0.5% at 20Hz. Still very good:
Ignore LF noise as my bench is noisy and get 50Hz plus byproduct creeping in as induced noise into the system.
Looking at the frequency response you will see a very nice performance up to 26kHz. I tested with a 220nF/510K output network for coupling out the mu output:
There is a resonance above 100kHz which will be tamed by the output stage, so I won’t bother on it.
Here is a 247 in action:
Ok. The modular build of the 300B allowed me to swap easily the D3a driver board for the 47 one. Only a few changes needed to adjust the filament raw supplies and I was ready to test:
Sound impressions
Ok, I’m a DHT lover and was right to get back to 100% DHT. The D3a is amazing driver, don’t get me wrong. However, the 46 and 47 brings a level of extra detail I was missing slightly. Also, removing the resistive volume control and getting back to Dave Slagle’s AVC was a perfect match. I think having only 2 valves in the signal path pays off big time. The dynamics are there, strong, fast amplifier. the detail is unique, as you can only get with DHTs. Clearly, I’ve tried many option and keep going back to DHTs. Bass is deep and powerful. Man, I love this amp and can get loud if you push it!
Going full DHT has its toll. The power supply complexity can be daunting but worth it. A lot of iron for such a simple signal path. Well, that’s the DHT reality. I highly recommend this setup. If you can’t get the 47, there are plenty other DHTs to consider. I’d go for a VT25/10Y or 801a. Mainly a lot of the mid-mu DHT can work here. The circuit is very easy to adapt. You only need to change the cathode resistor and dial the right anode voltage on the hybrid mu-follower board. That’s it. Nothing more.
I have some EML20a valves around which I didn’t like much as preamps. they pick up him easily but they are design for this job. Also the EML30a can be a good option. I would also test my ER801a as am loving to get back so thoriated-tungsten filament sound back into my system.
Anyhow, I’m enjoying thoroughly the sound of this amp. Will keep this for a long while.
Ale, I not sure how you use the the slacker avc, – in lieu of the LL7903 or in front of?
Hi Ale,
Way to go…bravo! I’ve tried both 46 and 47 as outputs in a 24A/45 Loftin-White amp that I built…I keep returning to the 45…even have replaced the street 45 with EML meshes.
A brave step for you, and it looks very impressive in your photos…wish I could listen. 🙂
I’m curious about which Slagle Autoformer volume controls you use and about where you place them?
Best,
RC
Hi all,
I should have been a bit more clearer. The AVC is at the input before the LL7903.
Here is the write up from long time ago:
https://www.bartola.co.uk/valves/2015/04/06/avc-finished/
I am assuming you did plate curves for the 47 in triode mode, would really appreciate if you were to share.
Hi Robert, I’m travelling on holidays at the moment. I think I uploaded the 47 pentode model I developed time ago, search around in the blog. You can triode-strapped in spice and generate your own curves that way. Model was very accurate as far as I remember.
Hi Ale,
I came across your thread from someone selling a dht preamp on WAM. I will not pretend to understand the system (no basic electronics knowledge whatsoever), however I do know I love the sound I got from a CJ valve pre into my Quad 405.2 (too much gain and not the right imoedence match but still was amazing), so my question is…can a dht preamp work with an SS power amp? Would there be any benefit of using a dht preamp over any other valve preamp option, e.g. ec883? Thanks in advance.for any insight, suggestion you may be able to offer.
Darrell
Hi Darrell,
Unfortunately the answer to your questions will be it depends on the system configuration. The DHT sound is unique and people generally chase this route due to its transparency and level of clarity in the timbre. Generally the SS amps are design to handle a line level input signal, therefore a preamp isn’t needed. If you are looking for the DHT sound, then you may want to consider building (i.e. getting someone to build it for you) a low-level DHT preamplifier.
Good luck and hope this helps. Cheers, Ale
Thank you Ale.
I am intrigued by what a dht may bring to the system’s sound so will see who I might be able to support with this.
Just one thing, I am confused by the comment that ss amps are designed to work without a preamp. The 405.2 is designed to work with a quad 34 preamp. It can’t be connected directly to request cd or other source, hence me looking for a valve preamp to replace the 34.
Many thanks.
I am really looking for your comparison of 801A vs 47 in this position as the 801A has as well a gain of 8…rp is maybe 4k…is the 47 much lower in rp ? I always prefered sofar the sound of a real triode to a pentode in triode mode, but I love your experiments / new ideas.
Hi Frank. The 801a will require a much higher HT for the level of swing needed in the amp so isn’t an easy drop in without adding an extra HT supply. Perhaps I’ll do it in the future, don’t know. I’m very very happy with the sound of the 47 in triode mode. Good choice.
Hello Ale,
congratulation for this project 300B : really stunning !
My SE 300B tango is a toy 🙂 Your amp explore the 300B thoroughly, that is fine.
My question : you say you have explored de 46 tube ; did you listen it on this amp ?
Is it possible to use a 46 with the same values ?
Thanks.
Bruno
Hi Bruno
If you read my blog you will see that I stated that 47 and 46 are very different valves electrically. I love the 46 yes, but as a driver in this topology I needed high mu. The 47 works better, although you can adapt the circuit for the 46.
Not same biasing point though. A VT-25 or 801a is a great choice alternatively.
Cheers
Ale
Thanks Ale,
sorry, I had not seen the link, very interesting.
This 47 is also a beautiful tube (I play with some UX 245 in SE) ; as you, I love the DHT’s (amp & préamp).
The way you implement the 300B in SE amazes me (never seen before).
These 8W amps really express themselves on very high efficiency speakers, specially horns.
Your site is really exciting !
Cheers.
Bruno
Curious, to avoid needing a TVC, instead of the LL7903, could the LL2746 (only 18ma), (primary in parallel and secondary in series so 0.5:4) be used between the gyrator and source follower to get the required gain ?; would that remove the need coupling cap on the gyrator ? ; not seen any examples like this so perhaps it wouldn’t work or would result in poor performance.
Hi Dave,
You can avoid the TVC if you are using a digital volume control. I’m currently using the Raspberry-Pi based “HiFiBerry” https://www.hifiberry.com, which is very easy to build and configure. It works seamlessly and the DAC sounds pretty good.
If you add an IT with step up after the driver, you will need a coupling capacitor in this design. Better to change the driver into a transformer coupled one.
I’m currently working on a 801a driver which has the LL7903 (1:8) at the input to give me over x60 gain with a 1:1 transformer coupled as anode load. The 801a is in fixed bias and will drive the 845 output stage (also in fixed bias hence the use of same arrangement for both stages). The performance and sound is great as well.
I may switch this design to 801a and test it in the 300B as is a great driver with similar gain needs
cheers
Ale
Hi Ale,
I really don’t know how I stumbled upon this page 🙂
Entropy perhaps…
Well, do you happen to have a schematic for the daunting PSU for this refined and elegant monster
Thank you
Hi Michelangelo,
Thank you. All of the PSU diagrams are available under the 300B SE Amplifier section. Have a look around
cheers, Ale
Hi Ale,
congratulations for your interesting blog.
Have you traced the curves of the 47 in triode mode?