Hi RATs,

A friend asked me to look over his 1971 Fender Twin Reverb amp. I'm not
a guitar amp guy, but the electronics are pretty much like hi-fi, so I
took a stab at it. Cleaned up the controls and got rid of the
scratchies. Cleaned the "bright" slide switches. Since I don't own a
guitar, I fed my audio generator into the input and the amp plays just
fine. I can't get any reverb. The tank has an output according to my
scope, and I can get a very small trace from the input side. The tank
was tested disconnected from the amp with scope probes across the RCA
plugs. All the tubes check at an acceptable level, above 80%. Any
suggestions?

Thanx,

Raymond

"Raymond Koonce"
Hi RATs,

A friend asked me to look over his 1971 Fender Twin Reverb amp. I'm not a
guitar amp guy, but the electronics are pretty much like hi-fi, so I took
a stab at it. Cleaned up the controls and got rid of the scratchies.
Cleaned the "bright" slide switches. Since I don't own a guitar, I fed my
audio generator into the input and the amp plays just fine. I can't get
any reverb. The tank has an output according to my scope, and I can get a
very small trace from the input side. The tank was tested disconnected
from the amp with scope probes across the RCA plugs. All the tubes check
at an acceptable level, above 80%. Any suggestions?


** Reverb drive comes via that baby sized transformer and a 12AT7 with its
two halves wired in parallel. With no tank connected a strong signal should
be found on the reverb output RCA socket. The low impedance end of the tank
goes to the transformer - its drive coil should read about 1 ohm on meter.
If wired the right way round a very loud crashing sound should be heard
through the amp's speakers when the tank is bumped.

The reverb drive 12AT7 normally runs noticeably hotter than the other
ubes - check the voltage on its cathode resistor.

Reverb drive transformers occasionally fail.



.............. Phil

Phil Allison wrote:

"Raymond Koonce"

Hi RATs,

A friend asked me to look over his 1971 Fender Twin Reverb amp. I'm not a
guitar amp guy, but the electronics are pretty much like hi-fi, so I took
a stab at it. Cleaned up the controls and got rid of the scratchies.
Cleaned the "bright" slide switches. Since I don't own a guitar, I fed my
audio generator into the input and the amp plays just fine. I can't get
any reverb. The tank has an output according to my scope, and I can get a
very small trace from the input side. The tank was tested disconnected
from the amp with scope probes across the RCA plugs. All the tubes check
at an acceptable level, above 80%. Any suggestions?



** Reverb drive comes via that baby sized transformer and a 12AT7 with its
two halves wired in parallel. With no tank connected a strong signal should
be found on the reverb output RCA socket. The low impedance end of the tank
goes to the transformer - its drive coil should read about 1 ohm on meter.
If wired the right way round a very loud crashing sound should be heard
through the amp's speakers when the tank is bumped.

The reverb drive 12AT7 normally runs noticeably hotter than the other
ubes - check the voltage on its cathode resistor.

Reverb drive transformers occasionally fail.



............. Phil


Thanks Phil. I checked the tubes and they seem OK, but I haven't
checked the transformer. I'll give that a try tomorrow. I'll also
check the cathode R voltage to see where the tube's operating.

Regards,

Raymond

Looking at the schematic the most obvious thing I can think of is,
did you connect up the reverb pedal?

RAY J
On Sat, 14 May 2005 17:04:16 -0500, Raymond Koonce
wrote:

Hi RATs,

A friend asked me to look over his 1971 Fender Twin Reverb amp. I'm not
a guitar amp guy, but the electronics are pretty much like hi-fi, so I
took a stab at it. Cleaned up the controls and got rid of the
scratchies. Cleaned the "bright" slide switches. Since I don't own a
guitar, I fed my audio generator into the input and the amp plays just
fine. I can't get any reverb. The tank has an output according to my
scope, and I can get a very small trace from the input side. The tank
was tested disconnected from the amp with scope probes across the RCA
plugs. All the tubes check at an acceptable level, above 80%. Any
suggestions?

Thanx,

Raymond

RAY J wrote:

Looking at the schematic the most obvious thing I can think of is,
did you connect up the reverb pedal?

The reverb pedal *kills* the reverb, rather than enabling it.

It does this by throwing a short across the output of the
spring tank. Fender reverb will work just fine with no
switch connected. Most likely problem: the small wires
inside the tank which go to the drive/recovery transducers
from the RCA jacks. These are often broken. The input
jack (on the spring tank) should show a DCR of around one
ohm. The output should be in the vicinity of 200 ohms. Also
common: bad cables between the amp chassis and the tank.
Lastly, Fender (for some reason) decided to label
the reverb jacks on the amp backwards. The "OUPUT"
jack is really the input jack, and the "INPUT" jack is really
the output jack. I believe they labeled them with
where they were headed at the other end of the
cable, i.e., the wire in the "INPUT" jack goes to
the INPUT jack down on the spring tank. Audiophools
often wind up hooking them up backwards. Don't
believe everything you read, even if it's printed on
the amp's chassis. ;-)

Lord Valve
Expert

Hi Ray,

The plug jack for the reverb pedal has a shorting plug in it.

Raymond

RAY J wrote:
Looking at the schematic the most obvious thing I can think of is,
did you connect up the reverb pedal?

RAY J
On Sat, 14 May 2005 17:04:16 -0500, Raymond Koonce
wrote:


Hi RATs,

A friend asked me to look over his 1971 Fender Twin Reverb amp. I'm not
a guitar amp guy, but the electronics are pretty much like hi-fi, so I
took a stab at it. Cleaned up the controls and got rid of the
scratchies. Cleaned the "bright" slide switches. Since I don't own a
guitar, I fed my audio generator into the input and the amp plays just
fine. I can't get any reverb. The tank has an output according to my
scope, and I can get a very small trace from the input side. The tank
was tested disconnected from the amp with scope probes across the RCA
plugs. All the tubes check at an acceptable level, above 80%. Any
suggestions?

Thanx,

Raymond

Lord Valve wrote:


RAY J wrote:


Looking at the schematic the most obvious thing I can think of is,
did you connect up the reverb pedal?


The reverb pedal *kills* the reverb, rather than enabling it.

It does this by throwing a short across the output of the
spring tank. Fender reverb will work just fine with no
switch connected. Most likely problem: the small wires
inside the tank which go to the drive/recovery transducers
from the RCA jacks. These are often broken. The input
jack (on the spring tank) should show a DCR of around one
ohm. The output should be in the vicinity of 200 ohms. Also
common: bad cables between the amp chassis and the tank.
Lastly, Fender (for some reason) decided to label
the reverb jacks on the amp backwards. The "OUPUT"
jack is really the input jack, and the "INPUT" jack is really
the output jack. I believe they labeled them with
where they were headed at the other end of the
cable, i.e., the wire in the "INPUT" jack goes to
the INPUT jack down on the spring tank. Audiophools
often wind up hooking them up backwards. Don't
believe everything you read, even if it's printed on
the amp's chassis. ;-)

Lord Valve
Expert


Hi LV,

Thanks for the expert input. I'll go pull the shorting plug and see if
I get reverb.

Regards,

Raymond

Raymond Koonce wrote:

Lord Valve wrote:


The reverb pedal *kills* the reverb, rather than enabling it.

snip
Lord Valve
Expert


Hi LV,

Thanks for the expert input. I'll go pull the shorting plug and see if
I get reverb.

Regards,

Raymond

You da man LV! Works like a charm.

Raymond