The third member of the LoWatt family is based on a British original - the AC30. But - we didn't model the entire AC30 in minature; only the EF86 channel.
As can be seen from the schematic, the Molly is clearly derived from her big brother; only a few changes were necessary to adjust the proper signal level. This means that Molly's sound comes very close to that of the AC30, but with lower output. But - be careful! Even with only 4 watts, Molly can scream loudly when she is really driven.
The preamp, as stated above, is based on the EF86 channel of an AC30. That means the input tube - the heart of the amp - is a small signal pentode. Two inputs are available, with high and low sensitivity. The EF86 directly drives a long-tailed-pair phase inverter, implemented with a 12AX7 dual triode. This is the difference between Molly and her siblings, the LummerlandExpress and Emma, which use a cathodyne inverter.
The ECC99 is again used for the output tube. A 12AU7 can also be used, but the headroom is greatly reduced, and then this amp has little in common with a real AC30. The input signal to the ECC99 must be reduced to avoid overdriving it too quickly (voltage dividers R13/R20 and R14/R21). The tone can be adjusted with the 'Cut' pot.
The output transformer is the often-used Hammond 125C, wired with a primary impedance of 12.8kOhm. The layout shows an 8-ohm load. If a 4- or 16-ohm load is used, the appropriate connections must be taken from the 125C data sheet.
Our small standard power supply from the shop is used which provides more than enough power while requiring very little space. The connections B2 and B3 in the schematic refer to the respective connections on the power supply unit. The 6.3V filament supply is taken directly from the power transformer.
We were able to compare Molly with an original AC30 from the 60s, and we were pleased to find Molly's voice is very, very close to that of her big brother. In all fairness, we must admit that the low-power version doesn't develop quite the punch of the AC30, especially during the attack. But the AC30 character and charm is clearly there.
A further advantage of this amp is its effect-friendliness - for example, the Pod; which multiplies Molly's potential usefulness. However, it always remains, at heart, a small AC30.
Caution is necessary. Even though this isn't a particularly powerful amp, it nevertheless works with voltages which are potentially lethal. This amp is NOT appropriate for beginners !
Reading and understanding schematics, use of measurement devices, and good experience with voltages over 60V are prerequisites for the successful completion of this project.