Tag: Gyrator

ELROG ER801-A Review

I’ve been listening for over a week the Elrog ER801a prototypes. I can say that I’m very please with them. Before I make any further impressions about their sound, let me start from the beginning.

I purchased last year from ELROG (now Deutsche Elektronenröhren Manufaktur GmbH.) a pair of these valves to be used on my VT25/10Y preamp.  I do not own any 801a, but have a good stash of NOS VT25 and 10Y valves. I love this preamp, probably my favourite with the 01a.

Thomas Mayer offered me to send me a preliminary set of prototype valves in advance so I can submit them to the mercy of my tests. That sounded like really good fun and also I thought that the folks out there would be very keen to see the results.

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GM-70 SE Amplifier

I keep getting requests about SE amp designs using the 6SF5 driver. Here is what I have come up with for the 6SF5 driving the GM-70. With only one stage you can drive the GM-70 to full power and get 18W out of this lovely transmitting valve: Continue reading “GM-70 SE Amplifier”

Another DHT Preamp

Another DHT Preamp on the way with Rod Coleman regs. I will do a A-B comparison with some Tentlabs modules, it’s an interesting experiment to undertake. The preamp has the classic gyrator Rev07 and Source Follower boards…

Valve test rig with fixed bias

My test rig for DHT/IHT stages (and even Pentodes) has evolved over the years. Lately I settled with some nice modifications to allow testing the majority of valves I have. I use a modular socket system, nothing fancy and can add/remove a source follower stage at the output. There is also a screen regulator in case pentodes are submitted under the mercy of the jig.

Here is the simplified diagram. I added a nice fixed bias supply formed by a SMPS PCB board which delivers up to 400V, however the output is dial to about 100V. Then I use a Swenson Regulator to knock down the noise by about 100dB. A simple pot provides the voltage needed between 0 and -100V. It can be tweaked for whatever range you need. The pot is 20T wirewound so allows a fine adjustment on the bias:

For the curious builders, here is the rig mounted on a piece of floor board:

 

300B Un-bypassed Rk Bias Line Stage

Many times I get emails from DIY Audio builders who embark on building a DHT preamp when they don’t need gain, but instead what they need is a simple line stage to drive their amplifiers and interconnect cables effectively. Then they come back asking: “can I reduce the gain of the 01a or 4P1L preamps?”

For those who don’t need the gain, here is an interesting idea which brings together several design decisions which makes the DHT sound to its best. The challenge with many of the best sounding DHTs of low-mu is that is very hard to implement with filament bias. I’ve done a driver with a 46 in filament bias which was a crazy idea. I could turn of the heating with the amp running! It was a nice experiment though. With exception of the 71a and some other few DHTs, if you’re looking for good anode current and low ra, you’re in trouble. The 300B, 45/46, 50 and some other variations can’t be used in filament bias.

Subject to your religious beliefs in audio, you may not want to add a capacitor in the cathode, like me. I won’t dive into this discussion which is pointless as is a personal decision. If you continue reading this is simply because you value the sound difference in the DHT without a capacitor bypass in the cathode. Keep reading then…

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Flexible CCS board prototype

I’ve been prototyping a flexible CCS PCB. The intent is to provide a cascoded FET pair with some interesting features:

  1. The lower FET can be multiple devices depending on the choice of reverse capacitance and transconductance. These include jFETs and depletion MOSFETs like the 2SK170, J310, BF862 and of course DN2540. For this purpose several pads are provided for SMD devices as well as TO-92 ones, just like the gyrator PCB. A protection Zener diode between drain and source can be soldered when using low VDSS devices.
  2. There is either a string of trimpot plus a resistor to set the CCS current manually during test given the variance in the FET parameters.  There is also an option to put a fixed resistor.
  3. There is a mu-output connection provided.

The board is very flexible and can be used for multiple purposes:

  1. shunt regulators (including VR valves)
  2. Anode load for phono preamps, drivers, LTPs, etc.
  3. LTP tail CCSs

I’ve been running some tests with excellent results.

If there is interest, I will run a batch of PCB to offer to the DIY community. 

Cheers

Ale

4P1L / 4П1Л Siberian Gen4 in Screen mode (part 2)

Last time I wrote about the 4P1L in screen mode. It was great to see some DIYAudio member (Blitz) to post about his work on the 4P1L with screen as anode. I call it screen mode but probably is incorrect.

His post about G3 structure remind me to post this, I have tested it but never blogged about it. Yes having G3 as part of the anode structure will increase conductance and will form a nice “mesh” anode. Here it is how I implemented:

The pin 4 (G3) is now connected to 3 (G2) to form the anode. I reduced the anode voltage down to 110V to get 10mA. It could be increased, sure within the Pd limits.

The response is very good:

Here you have the distortion at 1kHz:

How does it sound?

Well, I wrote about it before. The 4P1L is one of my favourite valves. In this mode it sounds great, with a particular clear detail in the treble. I like this valve and will play it for some time to get further impressions.

 

Ba DHT Preamp (Part II)

I’ve been posting not very frequently lately. This is mainly due to lack of time and the level of business travel which reduced to nearly none the time available for DIY audio. 

Nevertheless, the scarce time always pays off. It’s incredible how selective I have to be in order to prioritise which project I should work on. The list is long though. 

Last time I did a quick exercise on the Ba DHT based on the curves I traced and the LTSpice simulation. Well, you always need to build and test in order to check against simulations. The result is, that you may need to adjust and learn from your practical experiences. 

The Ba (like the Aa) are tricky to use. They pick up any electrostatic induced noise. You don’t need even to place your hand close, the mains noise is induced already in its plate. This force you to shield these valves if you want to use them. Am afraid, that is what it is. My friend Rob (DHTRob) warned me, thank you.

The circuit I posted here, had to be readjusted.  Distortion was way too high. The operating point wasn’t good enough. You’d normally get inclined to run the valve as hot as you can, but I was wrong here with this one.

Continue reading “Ba DHT Preamp (Part II)”