Author: Ale Moglia

"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)

Bench VARIAC (preventing disasters)

Have you ever built a HT power supply for your amplifier, turned it on and experienced some smoke? Well if you haven’t you either have been lucky or too meticulous when testing. Either way, there is always good to have a safe mechanism to test our power supplies,  amplifier, etc. Some devices are too precious to take any risk when doing the first tests.

A Variac is a perfect device for your tests. Is actually an autotransformer for the mains. As it’s an autotransformer there is only one winding therefore there will be no isolation between mains and the output. There are many Variacs in the market. As I wanted to add current and voltage meters (see below), I decided to build my own. There are some cost benefits as well and you can build this circuit very easily.

My choice was the Indian Ravistat 2F-1: an open type, single-phase variable transformer, rated at 240V@2A. This variac gives a continuously variable alternating current (a.c.) output voltage, has a solid construction, high efficiency/excellent regulation and no waveform distortion with smooth linear output & overload capacity.

Bench variacA 2A Variac will provide at least 1.8A @ 240V (432W). This is more that what I need for my testing purposes. You may want to go for a higher power version depending on your specific requirements.

The idea of adding the current meter is very simple, but effective. The current meter will give you an early warning that something may be going wrong if the current surge is more than expected. In that case you can stop and dial down the voltage to avoid a problem.

I bought a pair of cheap Chinese AC meters. I built the bench Variac using the remainder of a floor plank which I applied a cover of button polish and clear wax. Using aluminium and standoffs I placed the heavy Variac, added the switch and mains connectors. I have binding posts for tests as well as a proper AC outlet:

IMG_3537 IMG_3538

 

Now, it’s time to progress with the amp building and start using this device when testing!

 

 

 

Cheers,

Ale

2Ж27Л / 2SH27L / 2Z27L

2J27L photo1

I was looking at my valve stock when I found the 2SH27L/2Z27L Russian universal directly heated pentode and asked myself the question of whether this could be a good candidate for a DHT pre-amp.  This little fellow was developed alongside the 4P1L after the second world war and was intended to be operated by batteries.

IMG_3530If you remove the aluminium screening case, you can discover a lovely valve inside.

 

 

 

Looking at the key characteristics we can see that:

  • Heater voltage: 2.2V
  • Heater current: 57mA
  • Anode voltage: 120V (200V maximum)
  • Anode current 1.9mA (2.5mA maximum)
  • Transconductance: 1.9mA/V

The heater requirements are minimum (120mW) so this could be easily run from batteries. I was intrigued by this valve working in triode-mode. How will perform?

Let’s have a look at the  characteristic curves first:

2j27L triode LARGEThe anode resistance is high, about 12K. The gain is also high for a DHT, so 16 is attractive for a pre-amp depending on the specific use/need.

What about its linearity? Let’s look first at the distortion at low signal levels (1Vrms output):

2J27L THD Vo=1Vrms

The distortion is very low, which is great for the preamp, however looking at the triode performance at large signal levels we can find that the linearity is not that great: 0.2% @ 10Vrms output:2J27L THD2 Vo=10Vrms

Obviously the optimal bias point for low level signal is not good for large signal as grid current kicks in and distortion goes up. Limitations on anode voltage and current may impact our capability of pushing the bias point further right in the curve to avoid grid current al large signal.

If we look to reduce the anode voltage supply we can see that distortion increases but a good compromise point is Ia=2mA, Vgk=-2V and Va=72V in which THD is 0.026% @ 1V rms.

A 70V HT supply is more attractive, however if we think about using this valve in a preamp with a cathode follower output driver like this one, then the HT supply requirements will be enforced by the latter.

Finally, if anyone is interested in playing with this valve and simulate the preamp in SPICE, here is a quick model which I haven’t tested yet:

2j27L spice model2Z27L-Triode SPICE MODEL

 

 

 

 

Improving the CX301a DHT pre-amp

An idea discussed with Rod Coleman, Andy Evans and others in DYIAudio forum which still is evolving. The purpose is to increase the output driving capability of this preamp using a cathode-follower stage. A bonus is to provide the filament bias using the cathode-follower valve filaments:

CX301a DHT preamp cathode follower

A 26 DHT pre-amp from Albert

Albert sent me recently some great pictures and feedback of his 26 pre-amplifier using gyrator as the anode load similar to my design on the 26/01a.

DSCN0734 DSCN0742Here are Albert’s notes on his 26 preamp:

“I have my 326 with separated power supply, 5U4G – 8uf PIO – 20H – 50 ASC – 30H – 50 ASC + 18uf pp feeding the  Salas’ HV Shunt V2 Regulator, then to 2 ( left and right ) cascoded DN2540 gyrators. Rod’s filament supply coupled with filament bias @ 850mA with anode @135v. I don’t have anything to check the distortion, but this is the most good sound preamp that I have finished to date. It pleases my ears so much that I have the intention to turn it on once I’m in the house. It sounds gorgerous with very good image seperation. Sound stages are wide, deep and absolute quiet. The highs are sparkling smooth with lots of details. Mids are lush and warm with admirable vocals. I’ve found it even better at the lows if compare with 2x 156c. It goes deeper with clear notes.”

 

 

Teflon sockets (Loctal and UX5)

Today received a great set of teflon sockets from Jakeband. A pair of UX5 and a pair of specially made loctal sockets for 4P1L.  The quality of these sockets is supreme and if I can say these are worth every euro you pay for them. Luciano from Jakeband takes his time to make a high-quality socket using virgin teflon, copper tellerium pins with gold coating (nickel free).

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Mingus Big Band

Fantastic night out and having the pleasure to listen to Mingus Big Band after nearly 15 years when saw them first @ Fez Cafe, New York. The brass section was sublime. I had goosebumps when listened “Fable of Faubus” and also enjoyed massively “New Now Know How” and “Pedal Point Blues”. What a great night and the best is that I will have the honour again on Saturday, their last performance at Ronnie Scott’s here in London:

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