The amp has 4 el84 tubes and 4 12ax7 pre amp tubes, the amp lost about half of its volume and the power tubes are red plating, is there any thing other than the tubes that I should be looking out for that could cause the volume loss and the tubes over heating?

  • 3
    \$\begingroup\$ Electrolytic capacitors degrade over time. A bad smoothing cap can cause noise, but a bad cap coupling stages of an amp can attenuate the signal (lower the gain). In an old tube amp, besides checking the tubes, always check the caps. However, you might not see any leakage and checking capacitance in circuit is sometimes difficult. If you have a scope, check signal levels between each stage. 50% volume may be difficult to track down. At your own risk, you might try adding a cap in parallel to each coupling cap, at a fraction of the coupling caps rated capacity (same voltage), and check volume. \$\endgroup\$ Commented Aug 30, 2021 at 0:44
  • \$\begingroup\$ Measure the grid voltages on the EL84s. If there's a significant difference between the two halves of the push-pull, investigate why the higher ones are wrong and fix it. Might be supply failure, AX7 failure, leaky caps, blown resistors. \$\endgroup\$
    – Neil_UK
    Commented Aug 30, 2021 at 4:42
  • \$\begingroup\$ Coupling caps won't usually be electrolytic, usually around 0.1uf, but the old paper in oil "Visconol-X" ones did start to leak after a few decades. As they couple the phase splitter anode (100-250V) to the output stage grids ( Megohm grid leak) they tend to pull the grid positive, increasing output current. As Neil says, measure grid voltage. But this is unreliable given the high impedance : measure the cathode voltage too (that and the cathode resistor gives you the bias current, if this is an automatic bias output stage). TL/DR : replace those caps. \$\endgroup\$
    – user16324
    Commented Aug 30, 2021 at 13:32


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