Cathodyne + EL34

[FREYES_7]

Hello All,

Just found in marketplace a matched pair of svletana's EL34 for a good price, very rare find in my country,

I'm planning on building an AB763 like amp, but the 50W, these tubes in push pull configuration yield to, seems to be too much power for me,

Maybe a cathodyne PI and a triode mode switch option would do the trick,

The amp will be used as clean jazz amp, and pedal platform

Has anyone tried these before I find out the hardway? hehehe

[Carlsoti]

The only cathodyne split EL34 amps I know of are Orange OR80/120 and their derivatives. A lot of their signature overdriven character comes from the assymetry of the cathodyne when driven hard. I'm sure they clean up a bit if you don't punish the tubes. I'm not sure how much less power they make in triode mode, but the datasheet should give some indication. Honestly, one SE EL34 with a really efficient speaker could probably carry most jazz gigs.

[pdf64]

To obtain a lower power output, with reference to typical 50W 2xbig octal push pull amps, reduce the HT voltage and increase the load impedance.
Consider Marshall's Studio series; about 15W by using a 250V HT, cathode bias, and a 6k OT.
With the significant benefit of much reduced wear on the valves.

[FREYES_7]

The only cathodyne split EL34 amps I know of are Orange OR80/120 and their derivatives. A lot of their signature overdriven character comes from the assymetry of the cathodyne when driven hard. I'm sure they clean up a bit if you don't punish the tubes. I'm not sure how much less power they make in triode mode, but the datasheet should give some indication. Honestly, one SE EL34 with a really efficient speaker could probably carry most jazz gigs.

Cool hehe I'll check the OR80,

I also thought about an SE EL34, but kind of worried about the price the OT would have 😂 (and its weight), I haven't investigated about it tbh tho

[FREYES_7]

To obtain a lower power output, with reference to typical 50W 2xbig octal push pull amps, reduce the HT voltage and increase the load impedance.
Consider Marshall's Studio series; about 15W by using a 250V HT, cathode bias, and a 6k OT.
With the significant benefit of much reduced wear on the valves.

Cool! Will check this schematic, sounds like a great solution, was thinking about using 350V, but I'm all in about the benefits of reduced wear in tubes hehe

[HotBluePlates]

... pair of ... EL34 ...

... 50W ... seems to be too much power for me ...

What amount of power do you think you want?

A different approach would be, "How loud should the amp be at output tube distortion? What speaker will be used?  What is the speaker's sensitivity?"

Someone wanting distortion at a shouting-level might say distortion should happen at 85-90 dB SPL.  But then they use a Celestion speaker with 100dB sensitivity with 1 watt applied.  So their actual desired output power at distortion is 0.1 watt:

   dB SPL = 10 log (Power Output / Refernce) + Sensitivity
   dB SPL = 10 log (0.1 watt / 1 watt) + 100 dB SPL
   dB SPL = 10 log (0.1) + 100 dB SPL = -10 + 100 dB SPL = 90 dB SPL

"Twice as Loud" is roughly a 10 dB SPL increase, and happens at 10x Power.  So 1 watt is "twice as loud" as 0.1 watt, 10 watts is "twice as loud" as 1 watt, 100 watts is "twice as loud" as 10 watts.

____________________________

If you start off answering the question "How Loud?" then you know how much power should be delivered to the speaker.

You can always rig an amplifier to pull less-current through output tubes than "the typical amp."

Current pulled through the output transformer primary impedance create a voltage-drop across the primary impedance.
Voltage-drop x Current = Power Output

To obtain a lower power output ... reduce the HT voltage and increase the load impedance.

Or just reduce the supply (really screen) voltage.  The result is the output tube(s) pull a smaller current.

If the goal is "Less Output Power" we could just leave the output transformer primary impedance non-optimized (meaning do not increase the load impedance).  The result is Voltage-Drop across the output transformer primary impedance is smaller, so Voltage-Drop x Current is smaller, and Power Output is smaller.

I know that you know these things, and only point it out for others.

____________________________________________

... Maybe a cathodyne PI and a triode mode switch option would do the trick ...

IMO, Triode Mode sounds dull and un-exciting.

As for a cathodyne phase inverter (or any phase inverter type), you do not select it based on desired power output.  Instead, you design the output stage then look at the bias voltage of the output tubes and compare that to the available supply voltage for the phase inverter.

   - The Bias Voltage implies a required drive signal: 
      - Tubes biased at -18v grid-to-cathode need at least 18v Peak to drive to full clean output power.
      - Tubes biased at -48v grid-to-cathode need at least 48v Peak to drive to full clean output power.

      - Cathodyne phase inverter needs a supply voltage >3 times the required peak-to-peak output voltage, because it has to develop 2 output signal plus leave plenty of voltage across the tube.

      - Other inverter-types might make use of a smaller supply voltage, or deliver a larger output signal from a given supply voltage.

      - The right/functional choice of phase inverter depends on available supply volts, and how mig a drive signal must be delivered to the output tubes.

      - Whether or not you need output section negative feedback may influence your choice of phase inverter.

      - If you want dirt at low power/volume levels, move away from the "clean cathodyne" to the "less-clean paraphase inverter" (which also performs easily with smaller supply voltages).