Fender AB763 Super Reverb, 1966

[N Cook]

Cured the main problem but the owner seems to live with what to me seems an
excessive amount of stray mains interference, without any signal leads and
input contacts shorted to ground.
This amp is 110V running off a 240/110 V autotransformer with the chassis
having an earth to mains earth (UK)
Other than probably something to do with the awkward cross-linking of the 2
sections of 2 7025, of normal and vibrato channels . It is possible to
minimise the noise using one "normal" signal input and nothing in the
"vibrato" inputs but the vibrato volume set on about 3 of the scale, not 0.
Is there a recognised fix/amelioration of this effect. ?

 

[Eeyore]

Cured the main problem but the owner seems to live with what to me seems an
excessive amount of stray mains interference, without any signal leads and
input contacts shorted to ground.
This amp is 110V running off a 240/110 V autotransformer with the chassis
having an earth to mains earth (UK)

Probably illegal and should almost certainly be linked out. You might want to
consult your liability insurance

Other than probably something to do with the awkward cross-linking of the 2
sections of 2 7025, of normal and vibrato channels . It is possible to
minimise the noise using one "normal" signal input and nothing in the
"vibrato" inputs but the vibrato volume set on about 3 of the scale, not 0.

You're just cancelling various sources of 'hum'. Some are in opposite phase so
you'll find various nulls like that.

Is there a recognised fix/amelioration of this effect. ?

First off, have you got a 'balanced' source of 110V ? Balanced mains tends to
have a useful effect on a lot of old kit.

Another thing .... the PSU reservoir caps may need replacement. If they're old,
their capacity has almost certainly degraded and as a result there'll be more HT
ripple than there should be. Also, imbalance of the bias current in the output
stage will cause ripple to appear in the output. In the absence of individual
bias pots, a new set of output valves might help. This hum will be there even
with the preamp pots down.

Graham

 

[Meat Plow]

Cured the main problem but the owner seems to live with what to me seems an
excessive amount of stray mains interference, without any signal leads and
input contacts shorted to ground.
This amp is 110V running off a 240/110 V autotransformer with the chassis
having an earth to mains earth (UK)
Other than probably something to do with the awkward cross-linking of the 2
sections of 2 7025, of normal and vibrato channels . It is possible to
minimise the noise using one "normal" signal input and nothing in the
"vibrato" inputs but the vibrato volume set on about 3 of the scale, not 0.
Is there a recognised fix/amelioration of this effect. ?

did you pass this by alt.guitar.amps? There are still a couple people
reading there that know these amps well.

 

[Ron(UK)]

What do you consider excessive mains interference? They are quite noisy
amps at the best of times with the volume up.
When you say interference, I presume you dont mean mains hum. If it IS
mains hum, check the smoothing caps and make sure that the bias is
correct and both output valves are ok. Some Fenders have both a bias and
a hum bucking adjustment.

If it`s more of a 'fizzle' it might be fairly typical of am amp of that
age. Are the screening cans still present on the preamp and reverb drive
tubes? Are the shorting jacks actually shorting the input when there`s
no plug in the socket? You'll probably have to trace it back with an
earpiece type probe.

They do tend to be noisier out of the cabinet, if there`s a foil screen
above the chassis, make sure that it`s making contact with chassis earth
or that will make it worse.

You might want to change all the 100k resistors in the preamp section,
that gets rid of a lot of rustling/frying noises.

hth

Ron(UK)

 

[N Cook]

What do you consider excessive mains interference? They are quite noisy
amps at the best of times with the volume up.
When you say interference, I presume you dont mean mains hum. If it IS
mains hum, check the smoothing caps and make sure that the bias is
correct and both output valves are ok. Some Fenders have both a bias and
a hum bucking adjustment.

If it`s more of a 'fizzle' it might be fairly typical of am amp of that
age. Are the screening cans still present on the preamp and reverb drive
tubes? Are the shorting jacks actually shorting the input when there`s
no plug in the socket? You'll probably have to trace it back with an
earpiece type probe.

They do tend to be noisier out of the cabinet, if there`s a foil screen
above the chassis, make sure that it`s making contact with chassis earth
or that will make it worse.

You might want to change all the 100k resistors in the preamp section,
that gets rid of a lot of rustling/frying noises.

hth

Ron(UK)

Its external mains bourne dimmer type noise but not hum in the mostpart.
Whether inside cabinet with mesh closure or outside with improvised closure
shield. Not internal crackles.
All input null shorts are fine.
I may try the suggestion of an isolating and balancing feed of 110V to the
amp just as a try out as simple to arrange temporarily.

 

[Ron(UK)]

Its external mains bourne dimmer type noise but not hum in the mostpart.
Whether inside cabinet with mesh closure or outside with improvised closure
shield. Not internal crackles.
All input null shorts are fine.
I may try the suggestion of an isolating and balancing feed of 110V to the
amp just as a try out as simple to arrange temporarily.

I generally find that I get more hum and noise using the isolating
transformers on my bench than I do when an amplifier is connected
directly to the mains.

Ron(UK)

 

[Ron(UK)]

I generally find that I get more hum and noise using the isolating
transformers on my bench than I do when an amplifier is connected
directly to the mains.

Ron(UK)

Appols for replying to my own post but, make sure that the connections
to the reverb tank are the right way round and that there`s a good
ground to the tanks case.

Ron(UK)

 

[Arfa Daily]

Probably illegal and should almost certainly be linked out. You might want
to
consult your liability insurance

I don't think I'm following what you're saying there. Explain ??

Arfa

 

[N Cook]

Appols for replying to my own post but, make sure that the connections
to the reverb tank are the right way round and that there`s a good
ground to the tanks case.

Ron(UK)

Yes that is another odd query
Are USA RCA connectors exactly the same as UK phono connectors or do they
get loose with age in a way that phonos don't.
The outer connections mate ok, but the inner socket ones seem very sloppy,
but not apparently fractured or bent or anything

 

[Michael A. Terrell]

Yes that is another odd query
Are USA RCA connectors exactly the same as UK phono connectors or do they
get loose with age in a way that phonos don't.
The outer connections mate ok, but the inner socket ones seem very sloppy,
but not apparently fractured or bent or anything

The inner contacts float in some RCA connectors, to prevent damage.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Ron(UK)]

Yes that is another odd query
Are USA RCA connectors exactly the same as UK phono connectors or do they
get loose with age in a way that phonos don't.
The outer connections mate ok, but the inner socket ones seem very sloppy,
but not apparently fractured or bent or anything

In my experience, the outer part gets corroded and makes poor contact,
and the inner tube seems to lose it`s grip on the shaft (if you'll
pardon the expression) I guess there are varying quality of connectors
in all parts of the world.

Ron(UK)

 

[Ron(UK)]

I don't think I'm following what you're saying there. Explain ??

Arfa

I was hoping he meant that he was using an autotransformer in addition
to the amplifiers own 110v mains tranny.

Ron(UK)

 

[Arfa Daily]

I was hoping he meant that he was using an autotransformer in addition to
the amplifiers own 110v mains tranny.

Ron(UK)

Yes Ron, that was my thought too, in which case, I don't see any particular
issue ?

Arfa

 

[N Cook]

I generally find that I get more hum and noise using the isolating
transformers on my bench than I do when an amplifier is connected
directly to the mains.

Ron(UK)

With only the main HT and next "B" supply to the 2x 6L6 and the
splitter/pre-driver , remainder disconnected, there was normal amount of
noise.
Similarly reconnecting all the supplies but pulling all the valves except
the first stage amp valve and those last 3, so basic "clean channel" amp
then also acceptable amount of noise.

Trying with an isolating 240V to 55-0-55V transformer with 0 grounded
feeding the 110 V transformer of the amp and full compliment of valves and
function ,then no reduction in noise performance from autotransformer feed
situation.
It seems I have to confirm that the auto-transformer is wired to take 110V
off the neutral end and not the live end of the 240V, to feed the original
USA isolating 110V.

 

[Eeyore]

I don't think I'm following what you're saying there. Explain ??

I misread it as a chassis to mains earth switch. Such things have been known.

What DID he mean by " with the chassis having an earth to mains earth " ?

Either the chassis is earthed (grounded) or it isn't !

Graham

 

[Ron(UK)]

I misread it as a chassis to mains earth switch. Such things have been known.

What DID he mean by " with the chassis having an earth to mains earth " ?

Either the chassis is earthed (grounded) or it isn't !

Graham

As I understand it, he used an autotransformer to step down[1] the UK
mains to 110, then fed the amplifiers own mains tranny from that, adding
an earth wire from the amplifiers chassis to mains earth. His general
complaint has, I believe, nothing to do with the mains supply
arrangements, rather interstage noise generated by the vibrato/reverb
circuits etc.

[1] probably incorrect term I know. I`m sure someone will soon be along
to set me straight. ;^)

Ron(UK)

 

[Arfa Daily]

I misread it as a chassis to mains earth switch. Such things have been
known.

What DID he mean by " with the chassis having an earth to mains earth " ?

Either the chassis is earthed (grounded) or it isn't !

Graham

I have seen the chassis to power-earth switch on some American imports
(which obviously, this is), but not on UK versions of the same amp. If the
switch is fitted at all, it's usually just not connected. I assumed that his
statement just meant that as is the common convention, both sides of the
pond, and when there is not a grounding switch fitted, that the metal
chassis was bonded to power-earth. Could perhaps have been put a bit better
grammatically, but I think that I got the meaning intended.

Arfa

 

[N Cook]

I have seen the chassis to power-earth switch on some American imports
(which obviously, this is), but not on UK versions of the same amp. If the
switch is fitted at all, it's usually just not connected. I assumed that his
statement just meant that as is the common convention, both sides of the
pond, and when there is not a grounding switch fitted, that the metal
chassis was bonded to power-earth. Could perhaps have been put a bit better
grammatically, but I think that I got the meaning intended.

Arfa

The "ground switch" is still present on this chassis.
A capacitor connecting the metalwork to either one side of the mains input
110V line or the other. Would you recommend disconnecting the capacitor ?
Either way setting makes absolutely no difference to noise immunity.

 

[Ron(UK)]

The "ground switch" is still present on this chassis.
A capacitor connecting the metalwork to either one side of the mains input
110V line or the other. Would you recommend disconnecting the capacitor ?

YES do it now!

Ron(UK)

 

[Arfa Daily]

YES do it now!

Ron(UK)

YES do it now!

Arfa