Fender 6G4-A Super: simple device with elusive problem

[soapfoot]

I've had a 1961 Fender 6G4-A Super-Amp for over fifteen years. I've always done routine maintenance--electrolytic caps, basic troubeshooting, etc--to keep it in good working order. Nothing about the design of this amp is (or should be) at all mysterious, and yet it's recently developed a bizarre intermittent symptom whose cause I can't seem to trace.

When plugged into the "normal" channel, once in awhile the amp will start playing much louder than it should. In the fault state, the following conditions are true:

• Turning the "normal" channel's volume control to zero doesn't change anything--it keeps playing loudly as long as there is input signal.
• If the tremolo ("vibrato") intensity is turned up on the "vibrato" channel, a tremolo will be audible, even though the instrument is plugged into the "normal" channel only. This is only true when the "abrupt increase in loudness" fault occurs
• Nevertheless, the "vibrato" channel's volume control doesn't seem to have an impact either.

An enthusiastic "whack" on the top of the cabinet usually sets things right... sometimes for weeks or even months(!) This makes it difficult to troubleshoot. I've taken the following steps so far:

• Cleaned and re-tensioned all tube sockets
• Applied pressure to every single solder joint in the amp with a chopstick to try and excite a fault
• Applied chopstick pressure to every single passive component in the amp to try and excite a fault
• Ensured continuity to ground for most ground points, particularly those around/near front panel potentiometers and jacks
• Cleaned and inspected all front and rear panel jacks
• Checked all schematic voltages to verify that they're nominally close to spec

None of the above has revealed anything at all. The only thing I can think of at this point would be to remove the eyelet board and check for anything stuck there that might cause an intermittent short-circuit (but that would be a considerable undertaking with lots of desoldering and rework). Before I try that, I thought I'd check here in case someone has seen something like this before and has more-specific ideas? Schematic here:

 

[Gus]

Check the volume controls have good grounds. If the ground end is not connected you could have full volume.

 

[soapfoot]

Check the volume controls have good grounds. If the ground end is not connected you could have full volume.

Thanks! This is good advice, and indeed was the very first thing I checked.

After verifying grounds with the meter, I also reflowed all connections to the pot and tightened the nut on the front. It didn't have any effect on the symptom, unfortunately.

 

[tony hunt]

Longshot #1:
Conductive fibre eyelet board; playing up in humid weather?
Probe for voltage on the board.

 

[abbey road d enfer]

Thanks! This is good advice, and indeed was the very first thing I checked.

After verifying grounds with the meter, I also reflowed all connections to the pot and tightened the nut on the front. It didn't have any effect on the symptom, unfortunately.

The carbon track may be cracked...

 

[soapfoot]

The carbon track may be cracked...

This is a good lead to chase. The volume pot on the other channel had to be replaced a few years ago, so it would be logical if this one were also on its way out

 

[Disco Volante]

I had the exact same fault on a Quad 33 preamp, until I swapped out the volume pot for a very expensive attenuator, only to have it come back 6 months later. Turns out the threads in the cast body (where the clicking happens) were badly made, which made the tiny screws holding the wafers together come loose. A few fiddly hours and some loctite later, and 33 quit yelling at me;-)

Replace the potmeter, is my suggestion...

 

[soapfoot]

The carbon track may be cracked...

I replaced that volume pot and have been trouble free for a good little while now. The problem was intermittent enough that I'm not ready to declare victory just yet, but it seems promising.

On a side note, the new (CTS) 1M Audio taper pot I installed isn't nearly as nice to use as the 1960 Stackpole part it's replacing. The taper isn't as smooth, particularly at the lower end of its rotation, and mechanically it doesn't feel quite as nice, either.

I suppose I could've sprung for a PEC part (which may or may not have been nicer!), but in the meantime it's at least working okay (it seems).

 

[CJ]

a stepped attn via a grayhill type wafer sw might be nice,

 

[soapfoot]

In theory perhaps, but maybe overkill for a guitar amp volume knob! Also it just kind of "feels wrong" to me to modify this otherwise-original 1961 amp in that way (maybe irrational?)

Also irrational: I can't shake the feeling that the old (carbon) volume pot sounded better, but that seems completely unreasonable. Most likely, the new one is just outputting a lower level for a given position of the knob.

 

[grid_stopper]

are the values on the old pot still readable? maybe try taking a few readings at multiple positions, and find a pot that is close in taper. maybe a lot of effort for minimal returns, but then again, maybe not

 

[Tubetec]

Could you slug the pot wiper with a resistor to get a more pleasing curve ?

Its seems hard to find hi-z pots that give just the right amount of swell nowadays , like in a tube amp or with a magnetic pickup the volume setting often feels lobsided at one end of the travel of the dial.

I might try and dig out a few old style audio taper pots from the tube era and post values for 50% rotation maybe we can compare with modern log pots of equivalent value ,

 

[abbey road d enfer]

Also irrational: I can't shake the feeling that the old (carbon) volume pot sounded better, but that seems completely unreasonable.

Most new pots are also carbon type.

 

[Tubetec]

Ok first pot I measured , 1940's style big body 1 1/2 inch dia ,
449kohms total ,
At the 50% rotation I measure 35 kohms from the wiper to what would be normally ground lug on the pot , 415 kohms to the top of the pot

 

[abbey road d enfer]

Could you slug the pot wiper with a resistor to get a more pleasing curve ?

Slugging the pot may result in loss of tone at higher settings.

 

[abbey road d enfer]

I might try and dig out a few old style audio taper pots from the tube era and post values for 50% rotation maybe we can compare with modern log pots of equivalent value ,

That's an issue. Quite often, old pots had uncertain tapers. Fender, in particular, had custom-taper pots.
Today, most pot manufacturers offer a selection of audio tapers, varying from A5 to A25 (5% to 25% at mid-rotation), but the ones that are commonly available are just marked A.
For guitars, the standard taper is A10, which means the level is 20dB down at mid rotation, which doesn't make for very precise adjustment on stage. That's why many replace them with a custom type that has ony 10dB attenuation at mid rotation (some refer them as "J" taper).

 

[Tubetec]

second one 1950's manufacture 560k ohms total , 44.5k to ground 519 kohms to input end

 

[Tubetec]

another 50's style 290kohm
56kohms to ground at 50% rotation , but even at minimum setting it still hold 16kohms to ground .

 

[abbey road d enfer]

second one 1950's manufacture 560k ohms total , 44.5k to ground 519 kohms to input end

This would qualify as an A10.

 

[abbey road d enfer]

another 50's style 290kohm
56kohms to ground at 50% rotation , but even at minimum setting it still hold 16kohms to ground .

Looks like it's very worn.