Click here to download the free modification manual
The components are available in any electronics shop
I didn't expect this to happen, I thought I already had brought the MCML05 valve pre amplifier to the highest level, no need for any modifications. Reality is different, because I recently discovered an impressive improvement, which I would like to share with you.
The standard valve in the line stage is the ECC82. I replaced this valve by the ECC99 (at a larger anode current) and noticed impressive improvements. The distortion weakened by a factor of 5, the output impedance went from 4 kOhm to a nice 1 kOhm and the maximum output voltage in 10 kOhm load improved from 6 to 14 Vrms.
The sound character did not change, but the reward is much more details and depth. Many of my Tube Society students heard these improvements and advised me to share it with my customers and to introduce a new modification parcel, containing the necessary components.
The ECC99 is taller than the ECC82. So, you can see its top above the upper plate, as the picture shows. I consider this as an advantage. Now you even better see that the MCML05 is a true valve pre amp.
Foto-1: MCML05 with ECC99 line stage.
The reason why the ECC99 performs better becomes clear when you study the characteristics plus schematics as shown below.
Figure 1: ECC82 characteristics (click to enlarge)
Figure 2: ECC99 characteristics (click to enlarge)
Figure 3: schematics of the line stage (click to enlarge)
The high voltage supply equals 200V. So, every half tube gets 100 V approx between anode and cathode. Then the maximum current for the ECC82 equals about 5 mA, while the ECC99 easily can handle 15 mA. At these larger currents the linearity of the ECC99 is far more better than the ECC82. This explains the impressive reduction in distortion. Also the characteristic lines of the ECC99 are much steeper, explaining the lower output impedance. This mod is so simple: replace the valves and replace the cathode resistors for larger anode currents. As normal, simple is dangerous. There is more going on. The filament current demand of the ECC99 is twice the filament current of the ECC82. The power supply of the MCML05 must be adapted for this larger current, else the regulation and stabilization transistor T2 will burn. I therefore replaced this transistor by a new type, having twice the power capability compared to the earlier one.
Foto 2: New power transistor T2 for the filament supply.
Also the anode currents are larger now, but the supply with the mosfet stabilization can handle this demand without any further adaptation.
Which components are in the MCM-MOD-1 modification parcel?
2 x JJ ECC99 gold pin selected
4 new cathode resistors R33 and R34
New high power stabilization transistor for the filaments
New heat exchange waver plus wires and heat shrink tube and screw
Detailed (pictured) manual for this very simple but truly effective modification.
Foto 3: content MCM-MOD-1 modification parcel.
The modification parcel is on stock, its price is Euro 63,- without VAT and shipment. You can order the parcel at my website, select "order" to find how.
Zwolle: 2-4-09