Gregory Apollo 800

Submitted by imre on Sat, 02/26/2011 - 21:47.
in

Tube guitar amp from the 60's. Tube lineup: 6CA4, 2x 12AX7 and 2x 6BQ5, 12" speaker combo. The speaker is a 12 inch Quam-Nichols ceramic speaker. All the Gregory amps came with this make of speaker. Actually sounds pretty good with a nice breakup when pushed. Some of this is probably due to the cathode biased class A output stage. Gregory Apollo 800 frontGregory Apollo 800 front Decent output at 10.5 Watts at 5% distortion. 12" speaker gives you good low end, but it's not very efficient, so it won't get super loud. You can crank this think and it won't blow your ear drums out. Plenty of gain, the 12AX7's are configured with 1 Meg resistors. Good quality transformers round out this nice budget amp from the glory days. Not much repair needed from me, I cleaned the pots, added a 1/4" speaker jack so you can hook it up to a cabinet and upgraded to a 3-prong power cord.

Inputs and controls from left to right are as follows:

  • "Hi Gain" - supposedly the high gain input, but there's actually more voltage division going on here than the "Lo Gain" input.
  • "Lo Gain" - in any case, Hi and Lo are basically the same. See the schematic.
  • "Bright" - adds a second high pass input before the preamp.
  • "Volume" - controls the output voltage of the tone stack to the second preamp tube.
  • "Tone" - simple one knob tone control
  • "Tremolo Speed" - self explanatory
  • "Tremolo Strength" - modulates the bias voltage of the power amp stage to generate tremolo effect.
  • "Ft Sw" - foot switch to control tremolo. You don't need the footswitch to control the tremolo, it is also completely controllable from the knobs.
  • "OFF" - on/off switch
  • Fuse - 1 amp fuse
  • Pilot Light

    Transformer details, voltage readings, etc. are all on the schematic I marked up. I started with the Mark XV / Mark X schematic since it was similar and I documented the differences. The schematic can be found here: http://knausz.com/content/gregory-apollo-800-schematic

    Shrine dedicated to this tube amp company: http://www.diyguitarist.com/GuitarAmps/GregoryAmps.htm

    Check out the rest of the gallery here: http://knausz.com/category/image-galleries/vacuum-tubes/amp-repair/gregory-apollo-800

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