Monday, 30 January 2012

JCA20H with EL84 Triode / Pentode Switch and schematic circuit diagram.

Though the JCA20H is _only_ 20watts, you have to take account of how loud 20 watts is, how you define 20 watts (some define it at 1% THD, some at 5% THD, it makes a big difference) and how efficient your speaker is. I decided as it's a simple mod to put in a triode switch which simply drops the output power to about 10 watts. However I found that the triode mod has develops a much smoother tone which I like. A fly in the ointment is that I also unbalanced the phase splitter at the same time, so two variables at the same time. 
However looking at the spectrum for Pentode and Triode shows that the harmonic balance shifts and that the change in tone is not just perception and that switching a EL84 output stage between Pentode and Triode is not only an output power change. (unless the phase splitter modification has also contributed)

The initial test setup was to keep the input stage as clean as possible and then drive the output stage in pentode configuration to 5% THD into a 8ohm resistive load.

The normal Pentode spectrum (500Hz sine input) looked like this.

Pentode 5%THD into 8 ohm.

When switched to Triode mode the THD  increased by 1.2% and the upper spectrum changed structure.

Triode mode.

With the input gain set at maximum and clipping in the pre-amp you can see how the different operating modes modify the tone. In Pentode mode the dominant harmonic structure from the overdrive is clear.

In Triode mode again the upper harmonic structure produced from the preamp is modified.

 In the final version I fitted R7 and R5 (470R) as 1k5, 5 watt.  C-8 Helps keep the supply stiff, R-6 brings the phase splitter value from 82k to 50k which skews the output balance and allows the output stage to generate second harmonics (ongoing trial) C-6 reduces the power stage bandwidth to about 10kHz. The depth mod is from the web to boost low frequency.

JCA20H HSG output stage.
I did play with the idea of having SW1 as two separate switches so I could run one valve in pentode and one in triode in lieu of messing with the R46 value in the phase splitter but I didn't get around to it. You do loose some output power in changing the R46 value, but it is negligable.


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  2. Just a small question, you mention you set the final value of R7 and R17(470R) to 1K5/5W.
    Is it effectively R17 in the PS you change from 1M to 1K5 or am i seeing this again wrong? Thanks in advance!

  3. The text was in error and is now corrected; R17 is now changed to R5 which will now make sense. Thanks for noticing this. The reason for the 1k5 change was that I had two 1k5 5W resistors with long leads in my spares box that I could solder directly from the board to the switch using some insulation tube over the bare resistor lead. I recorded it as this was the value fitted when the spectrum was measured. Both 470R and 1k5 are valid. The phase mod (R-6 130k) didn't really make a huge difference. However I was surprised how much better the amp sounded in Triode, I did this on an EL34 50W output stage and there was no perceptible change in tone.

  4. Ok, thanks for clearing that up. Nice i you have a spectrum analiser to toy around with :-).
    Ever tried heating the tubes with dc? and if so what where your results?

    1. I have not gone to the trouble of DC heaters so I can't comment. If you look at my post on how to build a pickup analyser for peanuts you will see that the spectrum analysis was performed with a freeware programme and the computers sound card. It also has an internal frequency sweep generator and a scope. It is a really nice piece of software, not entirely bug free, but still very nice to use.

  5. Just a little Sign from France :)
    Thank to present this fabulous amp!
    I think this amp is the last one created with really good circuit .
    And Simple Circuit .
    And Easily Modieffable .

    Do you think it's not better to had a Master Post Phase Inverter or a Power Scalling ??
    And if yes ..Have you got the schematics file ?
    I could upload my schematics and you can say if it's OK..

    Thanks and Best Regards !

  6. I have not tried a post phase master volume Being in the power section feedback loop it may possibly have issues depending on the gain of the power stage and how close the master volume control is to the power section. I think power scaling is now well proven though maybe overkill on a 20W amp. I really like the tone of the EL84 in triode mode.

  7. hey i happen to like triode modes also and the cap over the resistor is great. would you mod me a jca 22h? please? is my contact

    1. JCA 22s have the SLO circuit, which adds the OD channel to the regular channel. The problem is that one channel has an even number of gain stages, and the other an odd number. Though the SLO channel's signal is very much larger than the regular channel, the OD waveform is still being modulated with a sort of "reverse waveform" (ignoring the products of nonlinearities, noise, etc.). I guess if you like that, go for it - it is a source of "distortion" (of a sort).

      The JCA20H is a more pure sort of beast..

  8. Running the pentode/triode switch split is a brilliant idea! BTW, running one pentode and one valve strapped as a triode works - in this amp, at least. It is a little noisy, especially running with attenuated nfb.