(written October 2019; overhaul Aug 2021)

(referencing a 1989 Ampeg SS-140C with a fluctuating volume)
Learn all about the VH series and how it differs from the SS series here.

Notice: Skip down to the bottom for other causes of this issue. The addition of a fan did not solve my volume problem, though it seemed to reduce the occurrence for a short time. This page explains one thing I tried after reading about a few owners with overheating chorus boards, and other possibilities. In any case, this fan mod should theoretically extend the life of your SS or VH circuits. If you have any wisdom to share to help keep these amps alive, please shoot me an email.

VOLUME LOSS: I had bought a used 1989 SS-140C in 2019, and was 45mins into playing it when the volume began to repeatedly dip and return like a water swell (similar to a modulation pedal that is underpowered). Forum searches had me blaming the cliff jacks (plug a single patch cable into the loop to diagnose/remedy this; clean the jacks) or requiring a touch-up on all the solder joints. Ugh! Then I came across two actual SS-140C owners mentioning overheating chorus boards.* This appears to be something Ampeg may have corrected in the VH series? A few days later, I pulled the electronics chassis out to do a quick exam of the circuit boards and then played it again with no problems. It seemed the open air was apparently enough for the chorus board to radiate excess heat away. However, I had noticed this area of that particular board had a subtle discoloration to it and now the two ceramic resistors there were very hot to the touch - Ampeg did cut holes in this circuit board directly under these large, white resistors to allow heat to radiate off, but maybe it just wasn’t enough? There are also two heatsinks on the underside of the chorus board here, though they are very small. Take note that these particular resistors produce substantial heat regardless of the chorus being engaged or disengaged, so it does not appear to be as simple as just leaving the chorus effect off.

* this cooling mod was unsuccessful, but I hope it will help to keep the remaining circuits in good health longer.

COOLING OPTION: I like my vintage amps to look original (even if modified), so I wanted to install an internal power source operated by the power switch along with a discreetly placed computer fan. I made sure the amplifier was unplugged, marked the reverb RCA cables to their respective jacks (the one closer to the transformer is the signal return) and disconnected them from the chassis’s underside, removed the entire chassis out of the cabinet (see below), and remained careful not to poke around where it was not needed because capacitors can store electricity.

I settled on a very generic 12V power supply, referred to as "the adapter" from now on, to maximize the fan's airflow.* This adapter is rated to step 100-240V AC down to 12V AC/DC (whichever the item it is powering needs). It can deliver a maximum of 1A, not that that is needed. It automatically switches polarity, so assembly will be fool-proof. I opened this adapter using a Dremel to cut a notch along the seams, then inserted and twisted a flat head screwdriver to break the housing open. The circuit board layout of this supply was not as compact as the previous one I had chosen, so things were going to be a little tight when mounting it inside the chassis.

* I originally started with one of those crumby BluCoil 9V 670mA pedal supplies. It will work, but the nine volts didn't spin the fan fast enough for my own liking. Also, I ran across a similar modification done to a homemade amp that instead used a 9V battery, but the math says that might last 3-5 hours before needing a battery change. I would certainly not waste any of your time with that solution.

I ordered the Noctua NF-A6x25 FLX computer fan, which is rated for DC12V at 0.12A (120mA). This very quiet fan will only draw the small amperage that it requires. Converting to the DC current will also reduce any unwanted cycle hum from the new circuit. A 60mm fan fits perfectly on the back of the chassis (80mm might fit snugly, but it would certainly encroach on the serial number), however I moved where the power cord exits the chassis because of this fan’s 25mm thickness.

After taking reference pictures of the circuit boards for reassembly, I removed the Ampeg’s power cord by disconnecting it from the power supply board and the common ground screw. The plastic strain relief grommet requires a special tool that I didn’t have, so I used pliers to squeeze it together and pull it all out the bottom. Since my power cord was showing excessive wear, I chose to replace it. The factory installed an 18AWG cord that is about 9-feet long, but I decided to order a 10-foot 16AWG cord along with an assortment of "cable gland grommets." I almost regret not getting the 15-foot cord for added convenience, but that’s a fairly easy swap if I ever change my mind. Why the upgrade to 16AWG? Mainly because I couldn’t find an 18AWG cord over 6-feet that appeared to be the same quality as the one I bought. Thinking in my normal overkill fashion, the thicker copper wire more than guarantees delivery of all the power this amplifier could need.*

* the SS-140C manual specifies that it draws a max of 6A (18AWG can carry 10A; 16AWG can carry 13A) and the transformer wires are 18AWG. I later noted that the wires to the courtesy jack appear larger than 18AWG, so a 16AWG power cord seems like a fairly practical upgrade to me. Plus, a good rule of thumb is to choose electrical supplies rated to handle twice the need (6A vs 10A vs 13A).

I originally intended to move the common ground to the other side of the bridge rectifier (the square item with four red wires coming off it), but the ground wire from the amplifier's built-in courtesy outlet wasn’t long enough and I didn't want to extend it. So, I decided the best way to make everything fit was to first move the bridge rectifier closer towards the chorus board. When remounting the bridge rectifier, apply some fresh CPU paste to improve heat transfer to the chassis surface. I used a Rotabroach to make a new hole for the power cord’s cable gland between the common ground hole and the original bridge rectifier hole. I was careful with placement of this gland, as I didn’t want it too far towards the front (where it may rub on the speaker) or too close to the transformer. To do over, I might have moved the common ground closer to the new bridge rectifier position and then placed the new power cord hole between the new common ground and the old power cord hole - this would have alleviated all concerns of the power cord ever rubbing on the speaker. I hot glued a piece of Maxmoral Fan Mesh Dust Filter material onto the chassis floor to cover the original power cord’s pass through hole.

These pics show the before and after layouts. Note how the power cord was originally interfering with this fan.

For the new fan hole, I used a 2.25” Milwaukee Dozer hole saw centered within the rear chassis’s Caution and Warning information. I did my best to vacuum out any debris after every cut or drilled hole. Using a half-round file and some sandpaper, I smoothed out the fan hole and removed the burrs. Then I added some light-colored tape to the chassis and lined up the fan so I could transfer the mounting hole locations to the tape. I double-checked this alignment with the metal fan guard, and drilled the four holes out to 7/32”. I had originally planned to sandwich a piece of Maxmoral Mesh between the chassis and fan guard to help keep dust out (and hide the white print), but decided it would hinder the flow of air from this fan too much. Unfortunately, this will now allow dust into a previously “sealed” chassis. So, I’ll plan to remove the chassis and clean the boards at least once a year, just like my desktop computer - I use a soft brush to kick up the dust and then a shop vac to get rid of it.

To mount the fan's power supply inside the chassis, I made a bracket from some spare plastic pickguard material and 1/16” x 1/2” x 1/2" angle aluminum. Taking the half of the adapter housing that has tiny risers to prevent the solder points from touching plastic, I cut the outlet prong section off then used my belt sander to level the housing down to about half the original thickness. To make the housing shorter, I also cut the ends off that extended beyond the edge of the circuit board. I cut a rectangular piece of pickguard about 6cm x 7cm and drilled a hole in it to make a mounting surface. Placing the housing against this pickguard surface and clamping it, I applied hot glue within the drilled hole and the prong hole to join it to the adapter housing. I re-secured the circuit board to the housing/bracket using two small screws through the board's mounting holes.

To get the 110V from the Ampeg’s power supply board to the salvaged adapter board, I soldered 15cm of 18-guage wire (scavenged from the amp’s original power cord) to the adapter's posts, then added a 0.25” female spade connector to the opposite ends of these wires. The "posts" on my supply were a little odd to utilize, but I managed to solder the wire within the clasps and then reinforce each with large shrink tubing. It doesn’t matter which wire is positive or negative because the adapter’s outlet prongs were the same dimensions - I confirmed that the unit changes polarity on its own. The Noctua fan included a two-wire 4:3-pin adapter that I cut and hardwired to the output wires of the adapter's circuit board - now I can do a quick disconnect of the fan, if ever needed. The adapter’s new spade connectors will get plugged into the two available tabs along the bottom of the amplifier’s power supply board once it is mounted.

I secured the supply and bracket to the angle aluminum with a rivet, then mounted this entire assembly to the bottom of the amplifier chassis with two rivets, tightly situating it between the Ampeg’s power supply board and built-in courtesy outlet.

For the hidden exhaust ports, I chose to put two 1.75” holes in the bottom of the metal chassis, near the front corners. First, I moved the Caution label (advises what fuse to use) to sit between the polarity switch and courtesy plug with 3M Super 77 Spray Adhesive. I removed the chorus board* to center the one exhaust directly below the resistor on the underside (being mindful of the transformer, again). I took some Stainless Steel Woven Wire Mesh and cut squares a little larger than these holes with tin snips, then hot glued the mesh to the chassis floor. I chose the wire mesh here because it is hidden from view and allows much more airflow than the mesh dust filter. Before reassembling, I took this opportunity to clean all the cliff jacks with DeoxIT D5 and insert a standard cable plug several times. I also cleaned the reverb RCA jacks on the underside - during my volume diagnosis, I was also suddenly not getting any reverb from my pan because of a dirty connection.

* take a pic of the entire board and then carefully remove the four ribbon-style connectors from the chorus board only. Pull the three chorus knobs straight off, unscrew the retaining nuts on these three pots, then remove the two Phillips-head screws holding the board down. Tilt the rear of the board up, then slide it back, up, and out.

Results: I tested it to make sure everything was still functioning with the chassis outside of the cabinet before placing the chassis back in. The fan was noiseless at 9V, but because the fan spins faster at 12V, I was able to hear it if sitting at the back of the amp. The 12V supply I bought has a red LED that I left mounted to the board, which lights up the fan. The fan seemed to have helped, but not completely - I still noticed the volume dip and swell on occasion, but the difference was not as dramatic as it once was.

REVERB NOTES: I had several issues with the reverb. During one of my tests, the reverb was not functioning and I believe the RCA connections were dirty - some DeoxIT D5 fixed that problem. While diagnosing that, I removed the brittle cardboard from the bottom of the reverb tank, but replaced it with Darice foam sheeting (similar to neoprene). Strangely, this caused the reverb tank to produce an unstoppable feedback.




The additional cooling didn't fix the issue, but it was still likely heat-related. Something was on the brink of failure and enough heat build-up would push it there. I dug some other possibilities up after I sold my SS, just in case it ever happens to my beloved VH. Some people have blamed the ICs (Integrated Circuits) in the preamp circuitry and I found a page for trouble-shooting them here. I had eventually discovered that I could eliminate the issue if I bypassed the preamp (just plug the guitar into the effects return), so maybe this was my problem?

One of the more common things I have seen over the years that cause solid state amps to fail is the power amps. For some reason, this is why some claim to avoid them - but from what I can tell, it is repairable. I read a few forum posts that I hadn’t seen before by someone who lived in St Louis and knew a few former-SLM engineers. They told him that the volume swell is commonly caused by the stereo circuitry, where one of the two power amps begins failing. I'm an electronics noob, but I believe the suspect parts ("power amps") would be the two MJ15001 NPN and two MJ15002 PNP transistors soldered to the power amp board. The original ones used by Ampeg are Motorola that were made in Mexico. Here is a video I found showing how to replace them.

(not mine)

Sometimes, like in the above photo, the boards fry (note the four power amp transistors). I have run across a few people that have etched their own replacement boards for these amps! A guy name John Heisz built a VH clone from scratch and posted his efforts on diyAudio here and here. John even shows us how to print our own boards here - John's site, and all of his projects, are incredible!

HOW TO REMOVE THE CHASSIS: Make sure everything is unplugged from the rear of the amplifier and unplug the two reverb RCA cables from the underside of the chassis (mark one jack and it’s corresponding cable with a Sharpie to facilitate reassembly). Still at the rear, remove the upper cross-bracket that essentially adds rigidity to the combo cabinet. Supporting the underside of the chassis with one hand, unscrew the four silver Phillips-head machine screws located at either side of the top of the cabinet - the chassis will slowly lower it’s weight onto your hand. Tilt the rear of the chassis down so the front lip of the control panel clears the top edge of the speaker cover frame, remove the speaker cover, then slide the chassis out the back. I used a couple old effects pedal boxes to level and support the chassis so it wasn’t resting on the transformer and the heat sink while working on it. The speaker cover is velcroed on, so you simply pull it off - I removed both speakers and cleaned the dust off the back of the cones with a soft brush. Reassembly is the exact reverse and a flashlight will make lining up the top-mount screws much easier.

SCHEMATICS: Here are the original schematics for the SS-140C and the VH-140C.
John Heisz discovered a possible misprint on the chorus board schematic. The pic below is the issue, and this is his version of the board. Note that a pro might be able to find the correct circuitry on the SS schematics above.

MANUALS: Here are the original owner’s manuals for the SS-140C and the VH-140C.


CrankyGypsy (established 2001)