Number 5 is alive

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Modding - Stage II

After hopefully successful completion of the first modding stage it´s time to bring our boy to the age of puberty. And like humans, puberty bring less control and a rawer voice to our junior.

This is the part where most of you were waiting for, but let me do some introduction first.
In this part of the project the junior gets a second driver stage. This will get us a good overdrive independent from the distortion of the power stage and will increase the overall volume... far above bedroom level.
A handicap of this is the circuit and and the amp are driven to their limits; and maybe beyond.
Especially the output transformer shows first signs of weaknesses and tends to distort more, especially at deep frequencies. A new output transformer can fix this, but more about that later.

The power amp shows its limits too when the amp is used at full volume. At low frequencies the circuit gasps for power and the amp starts to „pump“. I don´t know the specifications of the transformer used and don´t know if the transformer or the rectification is to blame for this.My experience tells me it´s the transformer.

This isn´t a serious problem, but should be kept in mind. With full gain of the new additional gainstage and master set to about ¼ to ½ this amp gets really loud. With master above half it gets to loud to do this for a longer period of time. You would be surprised how much power those few watts deliver. You can turn the master volume up to about three quarters till the pumping effect sets in which show how much reserve is still left. With master set above three quarters the sound gets worse which is the „fault“ of another part, the power tube. The tube works at its limits and distorts rather heavy and not very beautifully. But some of you might exactly like this type of tone.

You should consider that the power valve has a great influence to the overall sound as it starts to distort rather quick. This effect gets bigger with more demand of power which results in different sounds at different volumes. The advantage is a real tubish British sound even at low volumes and the powersoak can be left ont the shelf. It gets really marshally as wanted. But keep in mind that a small 5 watts single ended poweramp will never sound like a big 2 or 4 EL34 push pull stage. It gets quite close; closer than all modelling amps.

As short comment to the combo version. I don´t know it and I don´t know which type of speaker is assembled there, but I don´t think it´s a high quality type. This can again be a big influence on the amp´s sound; those speakers often have harsh treble. I hope those speakers are not sized too small as far as power handling is concerned. I would always recommend the head version of this amp with a separate cabinet with a high grade speaker chassis. A 8“ would be ok, a 10“ even better, it´s worth it.

But back to topic now and some elementary ideas. For a additional stage another tube is be needed. A common candidate would be a 12AX7/ECC83. If you would use both triode systems for a gain stage it would produce a highly unstable unit. Even with a 12AU7 which has just about 20% of the amplification of the 12AX7 it still would be too much. The use of a single triode system is absolutely enough, the second one stays unconnected. This is not the most pleasant solution, because system two stays unused. A solution would be the use of a switch to control to different amounts of gain with one or both systems used. This is possible, but the difference between „more gain“ and „less gain“ is not as big as you might think. A solution would be the use of a 12DW7 which is nothing else than a 12AU7 system and a 12AX7 system in one tube. It would fit perfectly for our use here.

The extended schematic looks like:

As you can see, the signal is coming from the gain wiper to a 2 pole rockerswitch which decides if the signal is sent to the 12AU7 or the 12AX7 half of the 12DW7 and then back to the driver.
The wiring of the 12DW7 is done this way intentionally. Both stages are built the same, other than the addition of a cathode bypass capacitor and differing coupling capacitors, but they sound still different. They got almost the same „volume,“ which was the intention.

To build the stage this way you need to mount a new socket to the chassis which will need drilling again. The heater voltage can be connected directly at the PCB at capacitor C12. You´ll need to disassemble the PCB to get to the bottom side of it to solder the new wires. The anode voltage can be taken from capacitor C8, solder a wire here too. Ground is found at a number of points here, but the best position would be the central ground or somewhere nearby.

When the new wires are soldered the socket can be loaded with its parts. Solder the resistor as near to the socket as possible, especially the grid resistors at pin 2 and 7 to avoid oscillation.

For this reason, the gain potentiometer is redone here. In the first version R16 and R17 were soldered directly to the potentiometer. They´re now soldered directly to pin 2 of the new socket, the rest stay untouched. For the assembly of the switch another hole must be drilled, but if you have completed the socket hole this should be no problem. And the amp is still simple but effective.

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