After working for many months in refining the 26 DHT preamp, it was time to go back and revisit my 01a. I always liked this valve, it has a superb tone which probably is due to its thoriated tungsten filaments. Challenge is though, you need to find a good pair of CX301a as they tend to be microphonic. This is why many discarded them, I found them unique in tone and clarity.
As I removed my Salas HT shunt regulator from the HT supply chassis, the raw power supply now comes up to 280V with no load. The preamp has a cascoded gyrator which has a good PSRR, so decided to test it as is, so only had to adjust the bias point by setting the right voltage on the gyrator. The anode current was set to 3mA for minimum distortion. Yes I know I’m running the CX301a at a higher anode voltage than permitted, but it works fine. Problem is my bias resistor, so the filament bias can be optimised. Ideally the resistor should be around 15 ohms, but I had the 51 ohms around which worked fined with the Rod Coleman regulators I had as well and allowed for some filament starvation (circa 245mA). The bias voltage is then 12.4V which is not ideal for maximum swing at minimum distortion. I will change it in future. THD at 5V rms output was only 0.04% and at 1 Vrms just 0.01%:
Distortion plot:
It may be referred as the poor man’s DHT preamp. However, getting hold of a good pair of CX301a may be not that easy. I’m currently using Sylvania CX-301a, which are really nice.
My thoughts:
- CX-301a is a superb valve and its tone is very distinct amongst other DHTs
- It can be microphonic, though it can be minimised with rubber dampers and filament starvation
- A very linear stage providing as low as 0.01% THD @ 1 Vrms
- Cascoded Gyrator in mu-follower can provide a very low output impedance which provides a better bass response
- The low current capability of this valve is a challenge for driving long cables. I have less than 1m in my setup and I can say that cannot distinguish any degradation compared to other DHT preamps (e.g. 26 with OT)
I really like the open frame build of your preamp. It looks like the sockets are just resting in the retaining rings, is that the case here? Also, where did you find your rubber mounts for the tube sockets?
Hi John,
The rubber standoffs were sent to me by a friend. Check the 26 preamp thread in DYIAudio where you will find the details.
Cheers,
Ale
Where did you find the aluminum angle material that you used for the frame? It reminded me of the old erector set beams that we played with as kids. I think it looks awesome in your project.
Hi John,
I got them from Thomas Mayer in Germany (vinylsavor). They are great and provide so much flexibility that is like playing with Meccano!
Cheers
Ale
Hello All,
I agree with you all 26 is a great tube for line stage if you listen to vocal only. The mid come out warm and thick, but is missing some detail (This is what I hear). However 301A line stage is difference it gives flavor and detail cross spectrum and this is what I like.
Hello,
I agree somewhat with your comment. I also found the tone of the CX301a fascinating and very detailed. Unfortunately it doesn’t have the grunt to drive my current amplifier so I should be looking to add a source follower or similar to provide output drive needed in my case
cheers
Ale
Could you use a single 10m45s as a current source for the 3ma plate current instead of the gyrator circuit that you have in the above schematic?
Hi John, I haven’t used myself an 10m45s, but I guess it could work as minimum current is 1mA on that device. This will be suboptimal however. I explained the reasons why in my latest 01a preamp design blog entry.
I am planning to request my friend 01A or 26 Preamp. Suggestions welcome especially for avoiding hum/microphonics. I have bought /already have some tubes. I have 1 pair of Type 26 Sylvania, 1 pair of 226, 5Y3 and 12Ax7 for 26 and Type CX-301A and Type 80 for 01A.
Do you think 12AX7 is necessary for 226? My friend wishes to use it. For 01A, he plans to use only CX-301A and Type 80.
Also he wishes to use coupling transformer for 26 and coupling capacitor for 01A. I do not understand the rationale. Kindly advise.
Please advise some Dos and Don’ts. Thanks for suggestions.
Hi Kiran,
I’m sorry but I can’t advise on a design based on which valves your friend is looking to use. I assume he knows what he’s doing. Good luck.
Ale