3B7 DHT Preamplifier

An enthusiast blog reader (Paul Prinz) implemented a nice version of the DHT preamplifier using the gyrator PCB but for the 3B7 triode. The 3B7 triode has a pair of DHT triodes on the same bottle. It has a high mu for a DHT (about 20) but with that it comes the higher anode resistance. This was a drawback when implementing a choke or transformer loaded stage due to the high anode resistance (there is no free lunch am afraid). However, with the mu-stage, this doesn’t become an issue and we can get the most out of this valve using the gyrator load. 

Although I tried the 3B7 in the past, I proceeded to get it out from my valve stash and trace it again. Here is a nice set of a Sylvania military NOS one:

Connecting the two triodes in parallel helps to reduce the anode resistance to about 6KΩ, the transconductance is doubled to about 3.8mS. The μ doesn’t change and should be about 18-20.  

As this valve pair will operate at relatively high current then it can drive any stage. This is a great option when you need more gain compared to the classic 01a/26/4P1L stages. 

Here are the curves for the pair of valves in parallel:

 

The valve is very linear, as all DHTs. 

If we want to implement the gyrator load design with the PCB, here’s a simple one:

 

The design is very close to the 01a preamp Gen2. You can easily adapt the 01a to work with the 3B7. A loctal socket and a probably higher HT supply. I think you will be able to get away with 250V if the drive signal is low. Otherwise 300V is recommended. 

Performance is very good and according to Paul, it sounds even better than the 4P1L. Will have to build and judge it!

Happy New Year!

Ale

8 Replies to “3B7 DHT Preamplifier”

  1. The 3B7 reminds me of the 1J6G, a double DHT with high mu. The thing I notice is that it dies seem to draw grid current – so you need a source that can drive that and I suspect the quality of the results will depend on whether the source you drive it from can supply that.

  2. Hi Paul, yes and probably because is designed for A2 operation. Looking at the data sheet it can be seen that grid current is about 500uA for +1V.

    I did a quick THD test this morning and was disappointed to see 0.1% at just 1Vrms output. Need to look into this to check whether it’s the valve or anything else. I was biasing it about -3.5V so it should be fine from a grid current perspective.

  3. Awesome!
    Will a grid stopper Be necessary with the 3B7?
    Looks like plate resistance could be low enough paralleling two tubes for a transformer coupled stage.

  4. Hi Jason, i used a grid stopper with my 3B7. Better safe than sorry. As it is a wideband valve to be used up to 100MHz, it is a good idea. I used a value of 750 Ohm, but i guess that anything from a few hundred Ohm upto 1 kOhm should be ok. regards, Paul Prinz

    1. Thanks for the info Paul. Also what is your preferred operating point? Are you exceeding the 180V plate spec as Ale has?

      1. Hi Jason, Sorry that i did not respond so quickly. My operating point is 20mA, and Uk=5V (4.3V and 5.7V), and Ua=215V. So Uak = 210V, yes it’s a bit over the top, but i don’t encounter any issues. I increased the current from 14mA to 20mA, and i hear a difference. Music starts to be more present, it comes alive, nice dry base. I used the BF862 before, but the Idss was around 14-15mA. Ale sugested 20mA, so i found and i am using the BSH111BK. I am enjoying it! Cheers, Paul.

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