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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DHT Phono Stage Test

High gain stage with DHT

Some time ago a colleague  (Shawn Fox) contacted me to find out whether I could test some rare high-mu DHTs. I didn’t have them in my stash, so he offered to send them across for testing. He was quite keen to find out the performance with a gyrator load due to the particular characteristics of the DHT in question. The valve in question is the CX-340. There isn’t much information about this valve am afraid and coincidentally, Thomas Mayer (Vinyl Savor) wrote not long ago a review of this valve.

Tracing the curves, the first step

The high anode resistance as well as the low anode current in which this valve operates makes it a real challenge to implement successfully. Hence, this is why the gyrator load plus an output follower stage comes into play as the best companion for this valve. Before we look into the circuit itself, I submitted the 40 valve to the mercy of my tracer:

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2P29L DHT Preamp

The Mule saga continues and it was time to modify the 3B7 preamp and to test the 2P29L valve. This was quite easy as they both have loctal sockets. I had to modify the Rod Coleman filament regulator to set the filament current down to 120mA. Then a bit of wiring work, and in less than an hour a new preamp was ready. Job done, this is why I built the Mule:

The circuit is quite close to my original design time ago. I modified the filament resistor to use an existing Russian NOS wirewound part I had in stock and suit this preamp quite well. Added grid and screen stoppers as well:

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3B7 DHT Preamp

The VT-25 DHT Preamp is now part of my system as I do like it a lot. Therefore the Mule was vacant for a new experiment. It was the time for the 3B7 valve. This was introduced to me by my friend Paul, who has implemented it using the gyrator and is very happy with it. The 3B7 has a pair of triodes in the same bottle. I wired them in parallel as well as the filaments, which have same current specifications as the 01a. 

Most of the mid-mu DHTs tend to be microphonic. This one is an exception, although some singing comes out of them, its very minor and not impacting the stage. 

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Gyrator PCB Hack: final Enhancement Mosfet design

I evolved my previous design here, thanks to the help of Rod Coleman and fruitful discussions with him.

There is an option to improve the design by bootstrapping the top MOSFET to avoid using a bias Zener and allow the bottom device to have a constant VDS. This can be achieved by double bootstrapping the FETs. Here it goes:

Similar design as before. Only difference is that R7 is used to create the bias of T3, and thanks to the bootstrap of C2, the bottom FET (T4) now operates freely regardless the swing. D1 is needed to protect T4. R7need to be adjusted considering the output voltage expected as well as the maximum VDS before D1 starts to conduct.

There is an stability challenge and it can be addressed as Rod Coleman clearly points it out, a “guard ring” :

The other pro trick is the guard ring: this will dramatically reduce problems of dc-drift, if the PCB surface gets contaminated, e.g. when soldered with some old or poor-quality solder. Or damp air, fumes etc. It’s a conductor (pcb trace) around the high-impedance network formed by the 10M resistors. A staggered-pinout version of the TO220 is needed to implement it, as the TO220 is the hotspot for leakage (B+ of drain to the 10MΩ-driven gate!).

If there is a leakage path, it leads only to the guard ring, which is only a few volts away from the intended bias – rather than if the leakage can reach ground or B+, which would drive the circuit crazy. Connect the guard to a low-Z source – the Output in this case.
 
Anyway – I hope it is useful in some way!
(Rod Coleman)
 
How well it performs? Here you can see – no guard ring here, just adapted standard PCB for testing purposes:
Not bad at all with 3MHz bandwidth. However, considering the circuit complexity, I much rather stick to the depletion version which performs much better in my view:
 
Nearly 5.7MHz under same conditions!
 
Cheers, Ale
 

Gyrator hack: Enhancement MOSFET option

Happy Easter to all! (whatever you celebrate, doesn’t matter, it’s always good to have some days off)


I have my preferred gyrator setup which includes a top (depletion) MOSFET IXTP08N100D, which has a unique high VGS(th) which helps improving the performance of the bottom FET, in my case the BSH111BK. The combination of both is superb and they do measure (and sound) superb. The frequency response is flat until 3.4Mhz (-3dB). Yes, a high bandwidth amplifier, so you need to be mindful of this when using high gm/gain valves. I read somewhere people complaining that gyrator “oscillate”. Well they don’t, however they create a high bandwidth amplifier which is therefore prone to oscillate if you don’t take the right measures. If you don’t know what you’re doing, it will oscillate for sure, you have been warned.

Ok, if you can’t get hold of (any) depletion MOSFET as the top device, there is an option, a la Gary Pimms.

The circuit can be tweaked slightly, as can be hacked the PCB (I can show you how if you’re intending to use this circuit)

Here is the design:

 

The main difference is that D4 provides a stable reference voltage (18V) which ones you subtract the VGS(th) of the top MOSFET (typically 2-5V) then will give you enough headroom to allow the bottom FET to operate under low output capacitance due to higher VDS. This is the common limitation of the cascoded pair of depletion devices. You can’t get more than 2-3V.  As the top device forms a “cascode” with the bottom, it also limits the maximum voltage possible to the drain of the bottom device. The protection zener of the bottom device can be removed to ensure maximum swing. This stage can do 20Vpp easily. C5 provides some filtering to the zener noise, which is very low. I can’t see an issue at the driving levels in place. 

The protection zener (D2) for the top device is needed unless the MOSFET comes with a pair of back to back as some do.

There are multiple options for the top MOSFET. I like the (nearly EOL) STP3NK60ZFP which is a FP TO-220 device, very handy for heatsinks and high voltage and comes with the bonus of the protection zeners. The best option is the AOT1N60 and also the easier to get hold off FQPF2N60C. 

So, the performance is great. You can get flat response up to 2.1 Mhz. Here is a snapshot with my buffer which limits to 1.5Mhz:

 

 

However, my prefered stage can do 3.4Mhz under same conditions!

 

L3-3 valve tester repair 

I’ve been working on repairing  a fantastic L3-3 valve tester. It arrived safely, albeit not working the anode regulation circuit. I replaced the pots for either 10T (for grid regulation) and the screen and anode pots (1M). The regulation of the anode isn’t working, so after some time of a frustrating troubleshooting, I decided to open a nice Ale and listen to Herbie Hancock on LP. Happy Friday!