PCB Tube Amp saga : ’89 Ampeg VT-60

PCB amp, component side flipped upside down in the chassis = impossible to work on “visually” while plugged in … almost all resistors are 1/4w – pretty chancy stuff … with ridiculously narrow traces in some areas on the PCB … and that’s just for starters // the rat’s nest around the PT is quite a sight … perfectly accurate schematics for this beast are not available, let alone a layout diagram (C11 as drawn in the one I have is not present in the chassis)

for starters, the input pin (5) on one power tube socket was lying at +325v instead of -v bias voltage … the pcb short was eliminated by wiring in grid bias resistors, signal caps from PI circuit, and grid blockers “flying” over the pcb board in pt-pt fashion

At first the main B+ voltage was lying at +515vdc, filaments at 6.7vac and Bias feed tap was too low to provide appropriate voltage to bias tubes below 60mA (each) … way too high for 6L6’s / not to mention providing very short filament life

obviously, the primary winding on PT had shorted windings – so voltages had to be brought down to sane levels by introducing a power resistor in series with Prim winding … B+ thus brought down to 483vdc, filaments to 6.1vac, new bias circuit introduced wider range voltage source // set to -49v (with room to spare), power tubes were biased at 27mA/30mA …

then, a leaky signal cap in the preamp section was replaced to removed DC which made vol/tone controls inoperative … channels C & B then worked ok / high gain channel A was still dead – and would require much more work to suss out … luv that switching system, a total circus !

tone of first two channels is good and worth the work … as much to expect from such an unfriendly design … if you own such an amp and it has worked flawlessly consider yourself fortunate

6 Comments on “PCB Tube Amp saga : ’89 Ampeg VT-60

  1. I really need to see how those wire hook up. I have an Ampeg VT 60. PLEASE help. I just need to a different angle of that picture


  3. Fixing a VT-60 for a friend, a good cleaning of all sockets and jacks (especially the Line In jack) cured the typical volume fluctuation and noise issues. Only remaining problem is the Reverb control acts like a master volume (for all Channels) whether the Reverb is footswitched on or off. Can’t imagine that is normal, but without a user manual or decent schematic really don’t want to poke around inside. Any suggestions welcome.

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