Fender AB763 Super Reverb, 1966

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

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

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

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

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)

seems an

and

chassis

the 2

not 0.

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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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)

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

Arfa

mostpart.

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

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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

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)

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

Ron(UK)

--
Lune Valley Audio
Public Address Systems
Hire Sales Maintenance
www.lunevalleyaudio.com

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

Arfa

closure

the

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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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)

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

?

his

better

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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

YES do it now!

Ron(UK)

YES do it now!

Arfa