Intermittent in 1984 Fender Super Champ

[bluesboy54]

I am the original owner of a 1984 Fender Super Champ.  Recently, the reverb began “cutting out”, especially when strumming my guitar hard, in short bursts.  The reverb also cuts in and out, regardless of what or how I am playing.  I installed each of the two “test good” replacement reverb tubes (type 6C10) I have but the intermittent reverb persisted.   Using a DVMM, I measured the input and output resistance of the reverb tank and found the values to be within “normal” (same as my blackface Deluxe Reverb and Super Reverb amps AND the specifications for an Accutronics replacement tank).  I inspected the reverb tank and found no broken components, obstructions, etc.  Before I pull the chassis and start the “needle in haystack” search for a bad solder joint or other wiring/interconnect fault, I am hoping that someone with experience with and knowledge of this fault will provide recommendations, guidance,  etc. to help me resolve this problem.  I am an electrical engineer with solid skills in electronic assembly and some experience in tube amp and spring reverb repair.  Thanks, in advance.

[HotBluePlates]

Only the reverb cuts out, but the dry signal remains? Or does all sound cut in/out?

Judging from the schematic, 2/3 of the 6C10 is part of the output section (pre-gain and phase inverter stages), and would have to be working or you'd get no sound.

Does the 12AT7 (reverb driver stage) feel tight in its socket? If not, socket to tube pins would be a possible spot for intermittent connection.

Also suspect cables to/from the reverb pan for intermittent or failing connections.

I wouldn't suspect major faults with the tank or amp, unless you know the tank have suffered sever mechanical shock (from a fall).

You might also try injecting a signal from an oscillator into the reverb return jack (no need to pull the chassis) to verify the reverb return path.

Is there any physical movement of the amp/guitar when the cutting out happens?

I was in a guitar shop over the holidays, and someone was trying out a vintage Twin Reverb. They were getting a cutting out of the sound, and because the store workers insisted the amp was fully functional, they thought the problem was a bum cable. I told the onlooking sales guy to press against the cable plug where it entered the guitar's jack; intermittent stopped. There was a slight play in the contact between the 1/4" cable plug sleeve and sleeve contact of the guitar's jack. The fix was to retension the 1/4" jack's hot contact to cause a firm connection between plug and jack.

So be aware of exactly what conditions under which cutting out seems to occur. It is usually not due to a major fault, but some small connection you'd routinely assume to be "good."

[bluesboy54]

Thanks for your reply.  My responses to your post, as follows:

Only the reverb cuts out, but the dry signal remains? Or does all sound cut in/out?
>Correct.  The reverb effect stops but the “main signal” is otherwise unchanged.

Judging from the schematic, 2/3 of the 6C10 is part of the output section (pre-gain and phase inverter stages), and would have to be working or you'd get no sound.
>Agreed.  At the first incidence of a fault I swapped the suspect 6C10 with first and then second replacement 6C10 that I have; all three 6C10s “test good” and the rest of the function of the amp is 100% normal.

Does the 12AT7 (reverb driver stage) feel tight in its socket? If not, socket to tube pins would be a possible spot for intermittent connection.
>Yes.  The 12AT7 tube fits, firmly, in its socket AND I bought and installed a new one, with no improvement.

Also suspect cables to/from the reverb pan for intermittent or failing connections.
>I swapped known good cables, prior to my post.

I wouldn't suspect major faults with the tank or amp, unless you know the tank have suffered sever mechanical shock (from a fall).
>The amp has not left my home or been subject to anything more than the force required to set it in place on the floor when moving it from one room to another … no kids, no pets, etc.

You might also try injecting a signal from an oscillator into the reverb return jack (no need to pull the chassis) to verify the reverb return path.
>I’ll give that a try.  What is the desired output or outcome of that experiment?

Is there any physical movement of the amp/guitar when the cutting out happens?
>No.

I was in a guitar shop over the holidays, and someone was trying out a vintage Twin Reverb. They were getting a cutting out of the sound, and because the store workers insisted the amp was fully functional, they thought the problem was a bum cable. I told the onlooking sales guy to press against the cable plug where it entered the guitar's jack; intermittent stopped. There was a slight play in the contact between the 1/4" cable plug sleeve and sleeve contact of the guitar's jack. The fix was to retension the 1/4" jack's hot contact to cause a firm connection between plug and jack.

So be aware of exactly what conditions under which cutting out seems to occur. It is usually not due to a major fault, but some small connection you'd routinely assume to be "good."
>Good suggestion … I will try some Monster cables with RCA connectors that I know are 100% functional and if that makes no improvement I will “pull/twist/torque" the female connectors a bit to see if I can induce the failure.  If not, I will be sure to thoroughly inspect the female connectors when I pull the chassis.

[HotBluePlates]

It may be worth looking in the chassis to see how those RCA jacks terminate. I wouldn't want you to torque on them if they are PC mount (I don't think they are, but don't remember for the Super Champ).

[bluesboy54]

The 1984 Super Champ is the point-2-point wired, Rivera design so the risk is low but "treading lightly" is my style so risk of damaging the RCA connectors is nearly zero, for me.

If I look at it from the perspective of probable events only, I would argue that the most likely cause is either the 6C10 or the 12AT7.  I have swapped in two "tests good" 6C10s and permanently installed a new JJ equivalent for the 12AT7; none of that corrected the problem.  the easiest and least expensive "repair", although not next in the line of probable causes, are the RCA cables; having tried two pairs of good cables without improvement tells me that its not the cables.  That leaves the reverb tank as the next most likely culprit.  I measured the input and output resistance to 1 ohm and 190 ohm respectively.  Those DC values of resistance are the expected measurements, according to an article I have that includes the impedance and corresponding resistance for Accutronics reverb tanks, like the one in my Super Champ.

SO, I would like to know, what are the measurements and evaluations that must be performed to establish that a spring reverb tank, like mine, is fully functional, or not?  I would like to verify the most likely culprit, from my perspective, before proceeding with the more tedious and unproductive search for a bad solder joint, etc.

[sluckey]

SO, I would like to know, what are the measurements and evaluations that must be performed to establish that a spring reverb tank, like mine, is fully functional, or not?

Substitution.

[punkykatt]

Does the reverb turn off and on with the foot switch?