How do you adjust the bias of a tube amp?
I own an old Ampeg combo amp that is all tube, just recently changed all of the tubes. How do set the bias? It has 2 test points on the back of the amp and an adjustment screw. I have no schematics for the amp, but I do have a very good knowledge of electrical science. I live to far away from civilization to take it to a tech. Please help!
# 1
[font=trebuchet ms]I'm a bit confused by your description. The only Ampeg I know of that had test points and adjustable bias was the original SVT head. I have a 1976 VT-22 (2x12") combo and a 1977 VT-40 (4x10") combo. Neither of these has any test points, and the only adjustment on the rear panel is a "Hum Balance" trim for the heater filaments. The bias is non-adjustable, other than by changing out one or more resistors to tweak the voltage divider circuit in the power supply. I installed multi-turn trimpots in both of my amps while I had them apart to replace the old electrolytic filter caps. Your amp may have been modded by a previous owner.
Have you had the chassis out of the cabinet? Most old Ampegs have a schematic pasted to the chassis cover, where you can't see it until you take the guts of the amp out of the box. On my VT's, the drawings are pasted to the cover which has the reverb tank bolted to it.
I don't have schematics for them that I can post, but Aspen Pitman's "The Tube Amp Book" has some for several different years and models.
You can also try the Discussion Board at the >Used Ampegs< website.
In my VT-22, I calculate the plate current by measuring the voltage drop across the 5 Ohm plate resistors. The VT-40 doesn't have any plate resistors, so I installed precision 1 Ohm resistors between the Cathode and Ground of each 7027A power tube.
Be very careful in there! Ampegs have very high (like 600 Volts!) plate voltages on the the power tubes. Even after turning the amp off, there can be enough stored charge on the caps to KILL you![/font] :eek:
Have you had the chassis out of the cabinet? Most old Ampegs have a schematic pasted to the chassis cover, where you can't see it until you take the guts of the amp out of the box. On my VT's, the drawings are pasted to the cover which has the reverb tank bolted to it.
I don't have schematics for them that I can post, but Aspen Pitman's "The Tube Amp Book" has some for several different years and models.
You can also try the Discussion Board at the >Used Ampegs< website.
In my VT-22, I calculate the plate current by measuring the voltage drop across the 5 Ohm plate resistors. The VT-40 doesn't have any plate resistors, so I installed precision 1 Ohm resistors between the Cathode and Ground of each 7027A power tube.
Be very careful in there! Ampegs have very high (like 600 Volts!) plate voltages on the the power tubes. Even after turning the amp off, there can be enough stored charge on the caps to KILL you![/font] :eek:
# 2
The amp is a VT-60 Tri-Axe 1X12 combo. You can run it in Triode or Pentode mode. Its the loudest cleanest amp I've ever heard.
# 3
Thanks for the web site. Found some info in their forum, went home and adjusted the bias pretty easy to do, did'nt even kill myself, amp is loud and clean once again.
# 4
Originally Posted by: bsnake27Thanks for the web site. Found some info in their forum, went home and adjusted the bias pretty easy to do, did'nt even kill myself, amp is loud and clean once again.
:cool: 7890
# 5