8 hours ago, Shawn said:

What specific differences do you notice after adding the 100 µF caps?

Higher gain and in my opinion better frequency response. I haven’t tried other than the 100uF electrolytics. With the capacitors the amplifier is an absolute favorite.

I haven’t really followed up on this for a while, but today I did a quick LTspice simulation of the 2A3 stage.

With a constant plate current of about 23.8 mA, a 3100 Ω cathode resistor looks like a reasonable bias point (compared to 3300 Ω previously). The operating point itself is stable, and plate dissipation comes out to roughly 7.7 W, well below the 15 W maximum for a 2A3.

The resulting plate-to-cathode voltage (Va-k) is around 329 V, which is clearly above the classic 2A3 datasheet limit of 300 V. Based on older/NOS 2A3 specs, this would be outside the safe operating range. Some modern production tubes (for example, the EH 2A3) are rated for a higher Va-k (up to ~360 V), so they may tolerate this operating point without issue.

Just wanted to share the simulation results. Please share any 2A3 running data points in reality.

On 10/27/2025 at 11:48 AM, JoaMat said:

Completed servo controlled grid voltage.

@JoaMat
I have a quick question regarding the servo.

Once the servo is in place, does that mean the cathode resistor and its bypass capacitor are no longer needed, and the cathode is effectively tied directly without local bias components? Or are they still retained in some form?

Thanks.

I tie cathode direct to -HV, -400 V in my case. Works with EL34, 300B, 2A3, EML 20B-V4 and EL156.

The datasheet for my Electro-Harmonix 2A3EH Gold says 450 V maximum plate voltage.

50 minutes ago, JoaMat said:

I tie cathode direct to -HV, -400 V in my case. Works with EL34, 300B, 2A3, EML 20B-V4 and EL156.

The datasheet for my Electro-Harmonix 2A3EH Gold says 450 V maximum plate voltage.

Thanks a lot, JoaMat. Much appreciated.👍

That confirms my understanding.

 

Did some work on the auto-switch filament board today.

It’s based on the same schematic as before, but with additional rectification and filtering added this time. The mounting holes and overall dimensions are kept identical to my previous filament board, so it can be dropped in as a direct replacement without any mechanical changes.

Someone told me D3a is a good tube,

Did this in LTspice…

Modified Megatron – one less capacitor in signal path and no feedback. Works in LTspice. That's always something.

Did some testing today on the auto switch filament board.

It seems to correctly detect 2A3 300B so far. For the test, I directly soldered the cable onto the 300B filament pins. I know that’s not the best practice, but it was quick.