Tag: gyrator bartola

6SF5 driver for 300B/GM70/813 SE Amps

Some years ago my friend Paolo brought up this lovely valve which looking at the curves seemed to be a great candidate for a driver which required large voltage swing. This indeed is needed for 300B, 211/GM70, 813 SE Amps. I build it and tested with the 300B, which is great.

The immediate challenge (and probably the reason why it hasn’t been used extensively in the past) is that has a high anode resistance as well as very low current capability. These 2 things are of course a killer for driving these demanding output valves. However, with a gyrator load and a follower stage, the reality is different

 

6SF5 driver

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Gyrator FET options (More!)

Someone had to invest and sacrifice some gyrator boards to test various lower FETs (either depletion or enhancement devices as well as TO-92 or SMD options). That was me. 

Why? Because I want to push this circuit further and find the best options as well as provide to the builders out there some other device alternatives when they can’t solder SMD components. 

So let me present you the abused test mule and the various boards under the mercy of my tests:

Boards with different FETs under test

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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:

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01a Preamp Build: UV-201a version – step 1

This is what I managed to do with a couple of hours at the workshop. Time is gold for me these days. Quick drilling and fitting the main components. You can’t get a smaller preamp than this one. It’s quite packed:

Planning the layout before drilling
Back panel with all connectors (supply and signal)
Inside the preamp
Close look at the gyrator boards
UV4 sockets with rubber dampers
Fitting a pair of brass base UV201a

Next, soldering. Yay, just looking forward to my favourite part of the build process