Gerald Weber harp mods as summarized by Ronnie Schreiber in Harp-L.
  1. Low plate voltage on the preamp tubes. This causes tube break up at almost any volume and shifts the frequency band down. This fattens the notes and reduces high-freq feedback. Weber suggests 80-90 volts.
  2. Larger than normal coupling caps. Allows more lower freqs to pass the circuit. Weber suggests 0.1uf/400-600v.
  3. Simple two-stage design. More gain stages = more feedback problems.
  4. Paraphase-style phase inverter. Weber suggests the use of a self-balancing version of this obsolete design. He says it dirties up the tone.
  5. Cathode-biased, Class A push/pull output stage. Yields more compression, more 'singing' tone and a 'spongier' response.
  6. High Idle plate current in the output stage. Weber doesn't say why but my guess is it keeps the power tubes close to saturation.
  7. Tube rectifier. Weber says it yields more compression than solid-state.
  8. How to stop feedback and fatten the tone: In a self-balancing phase inverter, a resistor common to both circuits goes to ground. Weber found in using a potentiometer to determine the resistor value that when set to a value yielding a non-symmetrical waveform it fattened the tone for harp, and he found that if he wired the speaker reverse polarity to which half of the waveform is bigger that it significantly increased the feedback threshold, even at high volume levels. So he left the pot in as a control to fatten the tone plus reduce feedback when cranking it.