The positive feedback encouraged me to record this second video. Unrehearsed rambling on the CX-112a preamplifier. Hope this is useful to many of you and would love to hear your feedback.
Tag: DHT Preamp
Hybrid mu-follower boards in SMD
I’ve been using these boards for many years now. Haven’t offered them as is required for anyone to be experienced well enough in SMD soldering to get these done effectively. I have a reflow oven so work at scale is easier this way. I’ve been doing a lot of SMD board design and building over the past few years though.
On a busy time regardless, I found the space to build these four commissioned boards for D3a drivers like the ones I used on my 300B amplifier. There seems to be a lot of interest and build work done around this design, so happy to see this happening.
CCS in power supplies
The use of CCS in HT power supplies is well known, however generally misunderstood why it can be a good addition to some circuits. There is an excellent article from Gary Pimm which has been lost and luckily I found it on my archive. Here it is in case you haven’t read it. Worth it as a refresher or for anyone who is new to the subject:
Continue reading “CCS in power supplies”DHT Supply PCB
I took me a couple of years but eventually found the time to build the BOM and a simple documentation guide for the DHT LT supply PCB. I’ve been using this PCB for a long time on most of my DHT preamps and in my current 300B SE Amplifier.
It’s a 16 x 8 cm board which can be stacked for a stereo DHT stage. Enough flexible is provided for all sort of filtering combinations (either choke to cap input supplies).
Some may say is a killer, but I add a common-mode choke to suppress HF noise from mains. I design my supplies with a no-compromise thinking and best possible components (e.g. including split-bobbin main transformers), chokes and capacitors to deliver the lowest noise possible to the filament supply.
This board doesn’t replace the filament regulator (e.g. Rod Coleman regulator), it’s just the raw supply circuit needed before them.
I will sell the remainder of my stock of these lovely ENIG finished PCBs. You can order them here.
DHTs and cathode bias degeneration
A few years back I went down the path of exploring different DHT in pre-amp stages for sonic improvements on detail and dynamics. I explored all sorts of different DHTs as have posted in the blog and many more I never got around to write up about.
801a preamp build
I wanted to share a nice review provided by Lionel Borcelot on his journey in building the VT-25/10Y/801a preamp as shared previously. I think this is a good example on what to expect and how to adapt/reuse existing components at hand to get the most out of this design. Bravo Lionel, excellent job.
01a Low Gain DHT preamp (finished)
It was a half-hour job to mod my Mule preamp by swapping out the UX-120 and introducing the 01a (CX-301a in fact), add the resistor divider to deliver low gain on this preamp:
I set the valves at 115V/4mA. The Source Followers are running at 15mA and the resistor divider is actually 100KΩ/33KΩ (Kiwame) which is what I had at hand. It worked like a charm.
The 01a is driving my Slagle AVC into the 300B amplifier. I can say I can hear now the subtlety of the thoriated-tungsten filaments of the 01a with the extra level of gain I was hoping for. The amp sounds very dynamic at high level and with the clarity of the 01a.
This is indeed a clear winner. I will leave this preamp on my system for a while to confirm further impressions. For now, I just simply love it.
01a Low Gain DHT preamp
How many times I’ve been asked “how do I lower the gain of the DHT preamp?” many builders love the DHT sound and yet don’t really need that gain. Of course when you use a transformer load, step down is the obvious choice. And it works really well when you invest in a very good quality OPT.
However, when you use DHT with high rp, generally most of the low current ones, you face a tough challenge with regard to frequency response.
With the hybrid mu-follower topology (aka gyrator) we have the problem that the gain is fixed to nearly mu.
I’m in love with the DHT sound and in a preamp a 01a and the 801a/VT-25 DHT Preamp Update preamp cannot be beaten in my view. They all deliver a gain of 8 with a hybrid mu-follower topology.
This can be overcome in the following way: Continue reading “01a Low Gain DHT preamp”
UX-120 DHT Preamp into 300B
My friend Rob was right. The UX120 sound is unique. Perhaps is due to its cylindrical anode, thoriated-tungsten filament. Anyhow, he was also right that they pick hum and are extremely microphonic!
Well, not a candidate for a preamp indeed. However, with a low mu of over 3, it just added an extra gain kick into my 300B system:
The modification of the Mule was quick an easy. Fitted back in the UX4 sockets, adjusted the SiC array to 6V (5.8V drop) on filament bias. The hybrid mu-follower boards were adjusted down to 85V and 7-8mA. They are DC coupled into a pair of Source Follower boards biased at 15mA
Also I fitted the copper screen board on the back as I had in my Ba/Aa preamps. This works perfectly and when grounded there is no hum picked by the valve. No need to cover the valve at all.
The sound is amazingly good. Level of clarity and detail which is unique of an DHT. Very similar to the 01a sound in my view. Just great. I had to put the volume pot at the output as the microphonic noise is terrible. Yet, level is set to be usable (very usable) in my system. Love this sound indeed.
The 300B still is driven by the D3a. I will experiment with them in the future as have few things to do before changing them.
VT-25/801a/10Y DHT Preamp with SiC filament bias
For quite some time now I wanted to share one of the variations of the famous VT-25/10Y pre-amplifier. As usual due to work, business travel and family duties, I’ve struggled to find the time to update the blog.
Continue reading “VT-25/801a/10Y DHT Preamp with SiC filament bias”