New Output Transformer for the 01a / 26 pre-amps

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New Lundahl LL2745 OT pair arrived today in the post. Very excited, Thomas Mayer kindly send me this pair for trial. These are specifically designed for the low current and high anode resistance DHT used in pre-amps (e.g. 26, 01a, CX112, etc.). Wiring and primary details are similar to the LL1689. Hope we can get a final datasheet in September 🙂

These are gapped at 8mA. So the primary inductance is about 200H in theory (160-180H in practice according to Thomas). The labels are incorrect. This is 2×2.8:4×1, providing the option of wiring it as 5.6:1 if both primaries windings are in series and all secondary windings are in parallel.

Will rebuild the 26 or 01a preamps and do some test.

I would like to build a quick breadboard with the 01a to do a comparison against the gyrator loaded preamp I’m using at the moment.

45 SE Amplifier upgrade

Replacing driver for 7193 valve

Well, after nearly 12 months of playing relentlessly my 45 SE amplifier, had an unexpected failure in the power supply (passive regulator) which forced me to do maintenance to the amplifier. It was a great opportunity to remove the 6J5 driver and do a quick swap for the greatly respected 7193 (i.e. military version of the 2c22)

45SE Amplifier upgraded with the 7193 drivers

Bias point remains unchanged: 7mA and 260V for maximum swing and minimum distortion. Need to look at my notes, but I remember that I was something around 0.30% at 100Vpp driving the 45 (which is not an easy load for anyone).  Driver configuration was not changed, so had the 7193 now loaded with same DN2540 single transistor gyrator and mu-follower output for lower impedance. The valve was biased at about 8V with an LED array.

7193 in action

As a test, played the fantastic Symphony No. 3 from Henryk Gorecki….

 

VT-25 / 10-Y triode curves

VT-25 triode curves 

 

 

 

Everyone loves this thoriated-tungsten DHT valve. I’ve only used it in a preamp and was hooked with its sound. Really warm and nice. Downside is, it’s very pricey these days and also is quite demanding from a filament perspective.  You can check the characteristics here.

For those who like testing their designs with LT SPICE, I produced a model which matches really well the traced curves. Would like anyone to use this one, to drop me a note with any feedback 🙂

 

 

 

RCA10Y VT25 spice model

Here is the VT-25 spice model. Let me know how it works for you!

 

4-65a HT +300V supply

First breadboard of the 4-65a SE amplifier. Will start this long project with tuning the HT supplies. Will need a +600V, +300V and +100-150V supplies to be stacked.

4-65a 300V supply

Here is a simple breadboard using metal rails over an old wooden frame I had around. Oil caps will be glued temporarily and damper tubes will be fit later:

+300V HT supply (breadboard)

 

A snapshot of first test….

First tests

4П1Л/4P1L THD sweet spot

Playing around today with the 4P1L chap found a very good operating point where distortion is minimised at maximum anode power disipation:

  • Va=250V
  • Vg = -21V
  • Pa=9W (7.5W anode + 1.5W screen)
  • Ia=35mA
  • THD measured at Vo=+22.22dBu (10Vrms)

Interesting to see this valve swinging beautifully at just 0.027% THD….

4P1L THD minimum distortion