STINGER – Owner’s Manual – Ver: 2.31
13
Elmwood Amps
TUBES - MAINTENANCE
Tubes – Microphonics
Microphonics is a term used for high-pitched clanging noises that can occur from bad tubes.
You can test for microphonics by gently tapping the tubes with a wooden pen, or similar,
when amp is turned on. It’s normal to hear some noise when tapping the tubes, especially
the pre-amp tube located closest to the input jacket is sensitive.
Tube chart
V1 - closest to the input jack socket, is the first two gain stages in the amp
V2 - the middle tube is distortion and EQ gain stages.
V3 - closest to the back of the amp, is the phase inverter.
V4 - EL 84 output tube
V5 - EL 84 output tube
V6 - EL 84 output tube
V7 - EL 84 output tube
How to replace tubes
To change the tubes in the combo version:
•
On each side, near the upper front of the amp, there are 2 screws.
Remove these totally by turning them counter clockwise.
•
Remove the speaker plug on the back of the amp.
The amplifier chassis can now be removed by pushing it gently backwards.
•
Ones out of the cabinet - remove the 7 screws on the outer sides on top of the
amplifier chassis so the top piece can be removed.
•
Preamp tubes - On the far left, with the front of the amplifier facing towards you, there
are 3 preamp tubes. The shields covering the tubes are removed by turning them
counter clockwise a few millimeters and then pulling them straight up. The tubes can
now be removed by pulling them gently straight up.
•
Power amp tubes - In the near middle of the amplifier are the power amp tubes,
mounted horizontal. These tubes can be replaced by removing the retainer and then
gently pulling the tubes towards the fan on the back of the amp.
•
Be sure to re-assemble all parts and to connect the speaker plug before connecting
the power cord.
The power amp is operating in a cathode biased circuit.
No biasing is necessary when replacing power amp tubes.
WARNING:
This amplifier contains electrical parts. Voltages inside
chassis can be lethal. All service should be performed by qualified
personnel. Always unplug AC power before removing chassis.
Note:
Make sure to replace power amp tubes with a matched quartet of EL84’s.
