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.

Jet City 333 JCA20H HSG Mod (high saturated gain) with schematic circuit diagram.

There have been a few threads on how to get  a full high gain no-holds-barred design from the Jet City JCA20H amplifier.

I find this topology does a good job of going clean-ish when the volume from the guitar is backed off nearly all the way but has more than enough overdrive to use with my strat. I have a bleed cap fitted on the guitars volume BTW.

Compared with the original first three gain stages setup of x69  x23.6 x59 the individual gains are now x55.7 x47.6 and x54.6 so no valve is pushed too hard to spec. What has happened is a attenuation between stages has been removed and the treble voicing moved to after the tone stack. If we had infinite supply voltage and valves that didn't arc then the original cascaded gain to the start of the tone stack was around a factor of 12.5 thousand, with the modifications it would be around 118 thousand.

The mods are presented as achievable without cutting tracks or removing the PCB from the chassis. Simple to try and simple to put back. From the official schematic here are the changes made.

C27, 1uF. Fit an additional 1uF in parallel to make it 2uF.
R28, 220k. Fit an additional 220K in parallel to make 110K.
C26, 20nF. Fit a 22nF 400V cap in parallel to make 42nF in total.
R25, 2K2 fit a 1uF 25V cap  in parallel.
R53 1Meg. Sort circuit this part
Gain Pot 1Meg Log. Fit a 62K or 68K resistor across the outer track connections.
R53 to Wiper of Gain pot. 1.8Meg and 120pF in series (was100k and 1nF)
C23, 470pF Short circuit this part.
R14 470K Wire 220pF and 220K in series to restore some of the treble lost shorting out C23.
R12 47k (NEG Feedback Resistor, Power Amp section) Fit 330pF in parallel to quash harshness.

And two mods from The Valve Wizard Pages
R20 100k. Fit an additional 100K in parallel to make 50K
I also fitted a neon between pins 2 and 3 as protection.

The amp also has a depth or resonance modification fitted which warms the amp in the correct place.It also has a triode switch which mellows the tone and an unbalanced phase splitter to allow 2nd Harmonic distortion in the output stage.I'll detail the output stage modifications in another post.

I found that the treble bleed at the gain pot helped establish a usable clean sound lifting the mid to treble but not the bass which often pushes the stage into grit. For now I have fitted R-3 as 100k and C-3 as 1nF. However this may change as it is still work in progress. (UPDATE: Fitted R-3 = 1.8Meg and C-3 = 120pF as final value)

Below is the response at the cathode follower with the gain pot set to about 2. The lowest plot is without a bleed modification. The green upper plot is 100k and 1nF which gives a 12dB lift beginning at the lower mid frequency. The intermediate red trace is 100k and 220pf.

Jet City JC20H output harshness

The Jet City JC20H is an EL84 based guitar amplifier which has had some criticism for sounding a bit harsh or brittle. Mine was no exception. The stock EL84s proved to be substantially mismatched and were ditched for some matched JJ Tesla. However the brittleness proved unmoveable despite many changes to the pre-amp section.

The solution was with the power amp section. A 330pf capacitor across the 47K negative feedback (NFB) resistor marked R12 in the service schematic. This removes the unnecessarily high bandwidth of the output stage, and will also increase high frequency stability. Whatever the cause, the brittleness is gone and the amp sounds a whole lot better. Roll off is 1/ 2Pi * R *C  , so 330pF and 47K will equal about 10kHz which is ample for a guitar amplifier. 

The simulation below shows the frequency response with and without the 330pF capacitor.  The peaking at 14kHz is reduced by about 9dB. Scale is 3dB per division vertically and 2Hz to 20kHz.

Pots that drive me up the wall.

I get annoyed at pots in guitar amps (potentiometers), usually the volume pots. They have two attributes I hate. Noisy over time and the inability to get from zero to low volume in an orderly fashion. 

Firstly let’s look at issue of a noisy pot, you move the volume and get great crunches out of the speaker. I got to thinking about this the other day and came up with the following plan. I’ll replace the 1Meg volume pot (log scale for volume) with a 2.2Meg stereo log pot.  I’ll then wire the two pots together in parallel, so wiper to wiper, top of pot to top, ground to ground. This will give me a 1.1Meg Log pot.  This way I have two wipers to go wrong, but I should have less chance of a disconnection occurring as one of the wipers should have a good connection at any one time. Something to give a go.

This then set off a train of thought. What about wiring just the wipers and ground connection together. This should give a modified Log response with hopefully a slightly more gentle lift from zero and a bit more granularity in this area. So a pot and a parallel sub-pot.

If I plot out the response from a theoretical Log pot and then as a “sub-pot” I get the following change plotted as the green curve. This assumes zero source impedance and an infinite load.  I do get a slight easing in the very low volume setting area, followed by a slower curve into the 60% rotation which then quickens up to meet at 100% rotation.

Now anyone familiar with a Marshall JCM800 amplifier will tell you that 0-4 is zero to full power, after this you are soon overdriving the power section. This method will give slightly more control outside of the power amp overdrive section, which is where I want it.

You can also play with the same idea with a linear pot, say the Treble tone control in a passive TMB (treble, mid, bass) tone stack. Here the parallel pot is marked as X1.

We will get a degree of log law faking but we also modify the end to end resistance which will play with the impedance sensitivity of the other controls. To be fair it isn't too far from a fixed resistance at the same point, but different enough.

A common treble value for the pot is a 250K linear.  If we substitute in a 250K lin stereo pot wired as a sub pot then the following changes take place. Note that the mid scoop is reduced, and at maximum treble boost the scoop centre frequency changes from ~250Hz to ~350Hz. There is also a more linear change with pot rotation.  You can also get a scoop reduction by just replacing the parallel pot with a fixed 50K resistor but that can’t be as much fun.

Firstly the notmal TMB circuit moving the treble control from 0 to 100% in 10% steps.  6dB / division.

And now with a 250k stereo pot wired as a sub-pot.

Feel free to play and adapt.

Roosville.