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CONFIGURATION JUMPERS – NEGATIVE FEEDBACK SELECTION:
The Model 1 has optional negative feedback connection provided by jumper J161 (J162) – FBK.
By moving the shorting block horizontally across the two lower pins, the negative feedback will be
engaged. The upper two pins are shorting block holders when feedback is not engaged. By using
negative feedback you will reduce the amplifier’s gain, increase bandwidth and reduce distortion.
All the good stuff, but harmonic content will change, and many people do not have much respect
for that kind of sound. Again, option is available, and you need to find an answer for yourself. The
amount of feedback applied can be modified by changing value of R181 and R182. You will notice
variations in the sound, but be careful, poorly compensated feedback will cause the amplifier to
oscillate.
CONFIGURATION JUMPERS – TRIODE PARALLEL CONNECTION
Jumpers J171, J172 are used in the Full pack configuration to parallel cathodes of the input
triodes when the bypass capacitors (C61, C62) are used. If you do not need an extra gain this
capacitor provides, you can safely take it out, and then disengage J171/172 too see if you may
prefer the new sound.
HEATER CONFIGURATION JUMPERS:
Ten zero ohm resistors R290-R380 can be used for different heater configurations. The most
commonly used configurations are:
- Two tubes, Basic configuration (V1 and V2), heaters in series (Vf=32V, If=0.3A): use R290,
R370 and R380
- Four tubes – Full pack (V1-V4), V1 and V2 heaters in series, V3 and V4 heaters in series,
the two groups of heaters connected in parallel (Vf=32V, If=0.6A): use R290, R370 and
R380 (V1 and V2) and R310, R340 and R360 for V3 and V4 heaters.
- ECL82 heaters (6.3V) – connect all heaters in parallel, using R290 to R360.
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