The 300B amplifier journey continues as planned. A bit lower than expected, however small steps being made. Recently I mentioned about the 46 driver.
I made some progress on the LL7903 step-up transformer which is wired on 1:8.
Tag: 46 DHT
300B SE Amp: 46 Driver (Part I)
And time arrived to start experimenting with the different drivers. Why? Well, the whole point of the latest modular design is that I can easily (I hope) make quick changes and experiment.
The D3a driver board can be replaced with a new one. In fact I will be building a pair of stacked boards (which are in essence ground planes) to hold the SUT as well as a pentode screen bias regulator and the driver board.
First incarnation will be on the 46 DHT. However, drilled the board to fit either loctal (e.g. 4P1L, C3g, etc.) as well as noval for other 9-pin drivers I have in mind.
The build process is fast. I have the ground plane PCBs, which I drill (M3) to fit multiple plastic hex standoff to hold either PCBs or different turret/2mm connectors to plug in/out the board as needed. This LEGO-like approach is amazing, very happy with the flexibility and speed of work I can get with this.
Stay tuned.
814 SE Amp: upgrading the gyrator loads
The upgrade made to the 814 SE was quite simple, I replaced the gyrator carbon 4M7 for a Holco one and the mu resistor for a Mills 820R MRA05 5W. The capacitor was finally reduced from the outrageous 1uF Mundorf Mcap EVO Oil Gold to an 220nF 450Vdc Mundorf Mcap EVO Silver Oil.
The main change was the capacitor and proved to have no loss impact on the bass. I have recently made the OT change so cannot comment on the overall impact in the sound of these changes. I can say at this stage that I cannot notice any big change. It sounds as good as before!
I’d have expected a noticeable change by reducing the size of the cap, but in this case I didn’t.
4-65a SE Amp: first driver test
After completing the last power supply, I finally did some real tests on the 4-65A SE amplifier. Given the DC coupled design it is a bit tricky to do the initial calibration. I had to set the operating point of both 46 DHTs by adjusting the anode voltage through the individual gyrator load presets. Also had to balance at the same time the Salas Shunt current and output voltage to the desired levels. After playing a while with it I managed to stabilise the Salas shunt regulator.
Set the 46 to drive the output stage to 200Vpp with a 3.7Vpp (1.33Vrms) input. That is a gain of approximately 54. Here is the distortion profile:
Breadboard is really quiet with the 50 and 100Hz noise below -95dB. It’s great to see the nice 46 Super Silvertone performing only 0.09% at 200V peak to peak!
Results are promising, just need final tweaks to 600V supply and then hook the 4-65a!
Ale
4-65a SE Amp: 46 filament supply tested
Finished the second channel and tested the filament supply. The filament array which is formed by two paralleled pairs of 20W 10Ω wirewound resistors gets hot as expected. The array temperature is about 110-126°C at an ambient temperature of 24°C. The anode of the 46 gets to 49°C after 20 min of use and the heatsink stays at 42°C whereas the regulator TO-220 transistors are about 45°C. There is about 30W dissipated on each array. Yes I know, a lot of power but the filament bias is hard to beat in terms of sound in my view.