Doing SS rectification improved sound in Class A. Help me understand why.

To put it short>

I have a Reisong A10 which by now is heavily modified with mods largely based on Skunkie's design. I also replaced the output transformers to new ones from Toroidy which makes a massive improvement.

I wanted to experiment with adding SS rectification instead of using the 5AR4. The goal was to increase the B+ and keep the bias current the same by installing a larger cathode resistor on the EL34.

The result of this is largely improved sound, and I do not know why.

The EL34's run on the same current (about 70ma each) before and after SS rectification.

What I notice in sound: With SS rectification the mudiness in the audio dissappears. As if I can look into the music. The bass has cleared up and is very consistent. High frequency response impoved. I can have very very subtle detail wich are mudded before. I can reproduce this constantly.

Can someone help me understand why this might happen?

I have built my test setup in such a way that I an easily switch between the cathode resistor values needed for SS and Tube rectification. I measure the same voltages as Skunkie does with her Reisong a12 mod, except B+ gets an additional 20 volts by switching to SS rectification.

5 answers0 replies

5 Comments

Skunkie

Oct 13, 2023

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Serveral things could be at play: one, you increased the voltage across the tube which IMHO usually results in better sound in a tube amp. I design most of my amp to run the tubes at fairly high voltage. The other, as you noted, taking the load of the rectifier heater, which is right at 2A, makes the power transformer run cooler and increases it's ability to supply voltage to the B+ rail. On a SE amp, the rectifier tube itself should not impact the sound. If changing it to another type or brand does, than the power supply has issues as you have found out. So my best guess here is you have raised the voltage across the tube and at the same time removed some VA load from a marginally large enough power transformer, all good things! Would you mind sharing with everyone what cathode resistor you used? Thanks for this update!

Yarach

Oct 13, 2023

Replying to

Must have been the powersupply struggling to keep up then.

I did some measueremnts. Turns out toe 70ma current is what I get when using the tube rectifier.

Measurements with 5AR4:

Voltage: 317v (plate-to-cathode)

Cathode resistor: 220ohm.

Voltage across resistor: 15.5v

Current: 70Ma

Measurements with SS:

Voltage: 337 (plate-to-cathode)

Cathode resistor: 330ohm

Voltage across resistor: 20 v

Current: 60.6 ma.

So while the bias is cooler with SS rectification ( I will increase the bias this evening), it still yields better better results. This would in part explain the cooler power transformer as the currentdraw is lower in that case in my amp.

EDIT:

Before wanting to lower the cathode resistor I did some measurements after 60 minutes of power-on. Some values have slightly changed.

338v plate-to-cathode

22v across the 330 ohm cathode resistor. This yields something of around 67mA plate current which gives me around 90% dissipation. I might try to get to 100% in the future.

Edited

Skunkie

Oct 14, 2023

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Replying to

I get 85% across the plate for a UL tube, it would be 90% across the cathode. Honestly, I would leave it there if you like the way it sounds. You could measure the voltage drop across the G2/screen resistor to figure out the screen current, and subtract that from the total current to find the actual plate current. I;m using this calculator to make it easier. https://robrobinette.com/Tube_Bias_Calculator.htm

Yarach

Oct 15, 2023

Replying to

That calculator is very handy indeed. Glad it exists as I learned a lot from that website. Only gotten into tubes and amp design past year in part thanks to you haha.

Would you be interested in the circuit design as I have it right now with measurements? Maybe it yields some useful information for a future YouTube video. The "Reisong a12 revisited" or something haha. I also extensively tested with Triode mode vs UL and different kinds of feedback.

Let me know.

Yarach

Oct 13, 2023

Also note: When using the 5AR4 the power transformer gets around 45 C in temperature. A little bit too hot to touch. After going SS it barely hits 35 C.