My first Tube Halfstack!
First of all, I would like to thank my grandfather for giving me a non-functioning all tube radio amplifier to experiment with. A brief research on the Internet showed that “WEGA Radios” built the “Bambino L” radio in Germany in 1951. The company became a part of SONY in the 70’s.
Only by asking responsible people that knew all-tube audio products did I manage to repair this 5Watts amplifier. The maintenance of parts such as wires, tubes, potentiometers and input/output jacks was the key to the restoration of this classic piece of equipment. Repairing such a device was not easy but it didn't cost me more than ten euros as no valves or transformers had to be changed.
Furthermore, extra features such as a guitar isolation circuit, a power-tube cooling fan and a temperature indicator were added. More specifically, the isolation circuit protects the guitarist, as it is known that voltage values as great as 400Vd.c. are apparent inside tube/valve audio amplifiers. The circuit consists of 1/4Watt 100Kohm resistors and 10uF electrolytic capacitors that reduce D.C. current and voltage peaks.
After experimenting a lot with the configuration of the speaker cabinets and the preamplifier section, I noticed that the amplifier produced the nicest sound when driven by a slightly distorted pedal at high volume. The cabs produced lower frequencies than the built-in speaker of the amplifier and worked fine with the output transformer and the EF41 power tube. The sound produced by this tiny 5watt amplifier varied from crystal clear at low volumes to earth shuttering distortion at maximum volumes!
Extra features:
-Speaker cabinet 1:
1x1” tweeter
1x3” midrange
1x6,5”woofer
impedance: 8Ω
-Speaker cabinet 2:
1x8”woofer
impedance: 8Ω
-Guitar isolation box
Warning: tubes do get hot! Therefore, cooling mechanisms should be used.
(Don’t ever touch them when lit.):
-3” Fan for maintaining power tube’s temperature within reasonable values
Athanasios Sotiropoulos, 1/2/2008
