A Tube Amp Restoration Walk Through

[AEA Audio]

Hello! We thought it would be fun to share with you guys and gals a restoration of a Fisher X-100B that we recently did. This write up can apply to amps other than Fishers and hopefully you can get some ideas for your own vintage tube amp restoration. Any comments and/or questions are very welcome! Now on with the show:

Inspecting the Amp as Received:
- It is important to take a minute and look over what you are working with. Some amps have rust that will need special attention to remove, missing components that need to be replaced, or blemishes that might be removable. I usually just mentally mark these areas and needed items. When I get to cleaning, I pay more attention to the spots I marked as a measure of how effective the cleaning process is going in general.

With this Fisher X-100B, there is only the face plate jewel and a knob missing. The rest of the amplifier, while very dirty and tarnished, is rust free and in nice shape. This type of unit should come out nicely without too much dirty work. Lucky me!

Prepping the Amp for Cleaning:
- Here we remove the knobs, face plate, chassis bottom cover, and the transformers. Why the transformers? It is an easy thing to do, keeps them safe from our cleaning chemicals, and allows us to fully clean the chassis and the transformers separately if needed. We can also test them on the bench if we have any doubts as to their condition. If you are going to clean your amp, pulling the transformers is the best way to go. Oh, it also makes handling the chassis so much easier without the bulk of the weight attached! Pro Tip: Be sure to take lots of photos of where each lead off of each transformer goes!

Take special care when removing the knobs, as they are sometimes a little brittle. The best way for push-on type of knobs is to pry them off from the back of the knob while working it forward with your hand, however there is the concern of scratching the faceplate if you use a screw driver or similar metal tool. Pro tip: buy a cheapo car interior removal plastic tool kit. The plastic pry tools are perfect for taking off your knobs while protecting your face plate. Something I brought over from my vintage car restoration hobby. Some knobs have tiny little retaining screws, so be sure to look which type you have, as you don't want to be pulling on your knobs without first loosening a retaining screw.

Also, keep all the nuts, bolts, washers, screws and knobs together. Simple way is to use a plastic sandwich bag, as you can zip it closed to prevent anything from getting lost.

From here we concentrate on the bare chassis.

 

[AEA Audio]

Ultrasonic Cleaning:

What?! Yes! Ultrasonic cleaning! We found out that ultrasonic cleaning works wonders for your amp. It removes tarnish and corrosion from all surfaces of your amplifier, most importantly all the wiring, solder joints, switches, tube socket pins, input jacks, and controls. It cleans everywhere!

Tarnish and corrosion are a big problem with vintage amps, and this really takes care of the problems they can create. And when we go in later with our solder iron, solder flows like the amp was just soldered yesterday. The effect is almost like rewiring your amp, replacing your tube sockets, pots, switches and jacks. Crazy I know, but it really is amazing.

So what is ultrasonic cleaning? To be really brief, ultrasonic cleaning uses a tank of liquid cleaner through which high frequency waves are passed through to create microscopic cavitation in the liquid. Cavitation is basically when microscopic air pockets form that expand and collapse with a lot of heat and energy. This is the action that provides the miracle scrubbing power of ultrasonic cleaning. The cleaning agent depends on what you are cleaning, and helps remove corrosion / dirt / oil. We use a blend of cleaners in our ultrasonic tank that we have found works really well.

If you don’t have an ultrasonic cleaner handy, you can simply skip that step and do our pre-ultrasonic cleaning treatment, which is scrubbing the amp down with soapy water and a soft brush. Soapy water will remove the caked up dirt and sometimes ash that is covering your amp. You want to use a soft brush so that you don’t scratch the chassis surface of your amp or remove any lettering. I use car soap, which is actually really weak soap specifically made to not remove wax/sealant that might be protecting your car’s paint. Hand soap would probably work better if you are not going to dunk your amp into an ultrasonic cleaner, as it will remove more than car soap will. Alternatively, if you are feeling brave, a mix of Simple Green and water would work even better than soap, but you have to be careful not to overdo the Simple Green, as it can remove the factory protective finish on the amp. Simple Green is also conductive, so be sure to seriously hose down the amp after you scrub it. At this point, liquid/water will not hurt anything, so go ahead and give it a real bath. Note: if you left your transformers in, you will want to avoid getting any cleaner into the bells, as it can lead to shorts in your transformer.

We first use soap to clean off as much surface dirt as possible, then dunk it in the ultrasonic cleaner, then soap it again and thoroughly wash it off with water. We also dunk the bottom cover at this time as well.

Drying the Amp:
This will take several hours at the very least. You can actually use your home oven for this step. The important thing is to make sure the temp stays below 140 to be safe - we don’t want anything melting. Also watch out for your power cord, it can’t touch the sliding tray of your oven, or it will melt. Just make sure it is on top of you amp. Place the amp on a baking sheet, double thick preferred, and let it cook for several hours. If you left the transformers in, you can flip the amp upside down, so that moisture can more easily evaporate out of the holes in the bottom of the transformer bells. Best to time this to not interfere with dinner.

Cleaning the Other Parts:
- While the amp is drying off, we clean and prep the other pieces that we took off in preparation for dunking the amp. This would include the faceplate, knobs, and transformers.

Faceplate:
We actually dunk the faceplate briefly in the ultrasonic cleaner for this type of Fishers. It really cleans it up well. If I’m afraid of the lettering being weak on the faceplate, I use car detail spray and a microfiber towel to gently rub the faceplate, horizontally in the direction of the groves, trying to keep the rubbing action off the lettering as much as possible. Some people like to wax the faceplate, but carnuba wax has a very short lifespan. For instance with cars, we always put down a layer of synthetic sealant before waxing, because wax, while it evens out the surface and provides a great reflection, doesn’t last very long. Pro Tip: Therefore, rather than waxing or using sealant on the face plate we use….silicone oil! Yes, we use a silicone impregnated cloth to rub on a very thin layer of oil. This not only provides an amazing sheen to the faceplate, it lasts a lot longer than wax, and has the benefit of protecting against tarnish. Using oil to preserve metal parts in storage is another vintage car trick.

On this faceplate we also needed to replace the jewel. We like to use green jewels instead of red, as we feel it lends to a softer classier look Fisher amplifiers. Although you are free to disagree.

 

[AEA Audio]

Knobs:
Here we use a smaller ultrasonic cleaner to clean the knobs with. Using an ultrasonic cleaner easily dislodges nasty built up grime in the knobs’ ridges, and removes the tarnish on the brass rings and caps. If you don’t have access to an ultrasonic cleaner, we recommend using a brass brush to get the grime out of the ridges and a hand dremel to polish out the brass. If you don't have a small dremel, you can use hard cotton swabs, but please don't give yourself carpal tunnel syndrome. If we have a stubborn knob that doesn’t want to clean up completely in the ultrasonic cleaner, we break out the brass brush and hand dremel as well. Hard cotton swabs with some polishing compound also work great for a final polishing after the knobs are cleaned. Another Pro Tip: We like to use “black trim restorer” on any faded plastic. While it doesn’t completely return the color of the plastic in a way that is permanent, it will permanently darken the plastic a shade if properly allowed to absorb into the clean plastic. For this particular Fisher X-100B, we also needed to source an additional knob from the parts bin and an aftermarket brass cap, which I super glued on.

​

Transformers:
- Now we take a look at the transformers and see if they need to be repainted, or if they can simply be compounded and polished. This really depends on the amount of scratches and level of rust on the transformers. If there are rust bubbles forming under the paint, or large areas where the paint has been replaced with rust, it is best to simply repaint them. If there are only a few minor scratches, then a simple compound and polish will do the trick.

Painting the transformers:
- Now you can be happy you already have the transformers removed. You did remove them right? If not, you can simply unbolt them, pull them up a little and use painting masking paper (or newspaper) to mask off the entire chassis under the transformers. Since we always pull the transformers, I simply mask off the leads with painters tape and get on to prepping them for paint.

Prep is the same as prepping most things for paint: Level, key, clean, and spray. Leveling can be done with sand paper, or what I like to use, red 3M scuffing pad from the automotive store. Pro Tip: The red 3M pads have just enough abrasion to remove rust, without causing any scratch marks. It also provides a good enough key on the painted surface for new paint to adhere to. I then either use acetone or POR15 Marine Clean to clean off all the residue from leveling and keying the surface of the transformer. Then it is down to painting.

Wear a proper painting mask and have plenty of ventilation (I usually paint outside, as contaminants aren’t really a big deal with this application). We use Rust-Oleum gloss black universal spray cans with the nicer spray nozzles, as they give a pretty nice even wide spray pattern and have a trigger for ease of use. A couple of nice light coats, and let them dry overnight. You could clear coat over the paint to really make them rugged, but in an amplifier that is taken care of, they shouldn’t be exposed to incidents where they can get heavily scratched.

For the transformers in this Fisher X-100B, I decided to simply compound and polish them. I clean them off with detail spray, a microfiber towel and some cotton swabs. Then I compound them using an automotive paint compound (3M compound). Compounding is to level out the paint, and polishing smoothes it out for better reflection of light (shine). I use an automotive polishing sealant to shine them up. After, I coat them with silicon oil for additional shine and protection against future corrosion.

Pro Tip: The black disk the transformers are resting on is a "lazy susan." They are cheap and make spinning your amp around while you work on it a breeze. Dragging a heavy amp on its side can scratch the transformer bell, the side of the chassis and the faceplate if it wraps around.

Odds and Ends:
I take all the screws, washers, nuts, bolts and tube shields that I removed from the amp when tearing it down for cleaning and place them into the ultrasonic cleaner. This removes all the tarnish and corrosion from them, and they come out looking nice and new! I then hand polish the tube shields, and they really shine up after taking an ultrasonic bath.

 

[AEA Audio]

Polishing the Amp:
Now that the amp is completely dry, we move on to polishing. After ultrasonic cleaning, the fresh surface of the amplifier is completely primed for polishing. We found that polishing is almost twice as easy after ultrasonic cleaning, compared to normal cleaning. This means less coats of polish is needed, and ultimately less of a sore arm and hand. Pro tip: After trying many different polishing compounds, including many of the high-end automotive ones, we have found that Blue Magic is simply the best. It achieves the best results for the least amount of work.

Polishing is sort of an art…not really. It all depends on how clean and soft the surface is and how hard or abrasive the compound and applicator are. The harder the surface, the more abrasive you can get. The absolute must before polishing is to have a completely clean surface and applicator. Otherwise, you will get massive amounts of swirl marks (micro marring of the metal surface), which really detracts from the finished product. I always start with a fresh microfiber towel and a very clean amp.
Controlling the level of polish to achieve an even shine can be difficult. If you rub too much in one spot, you will need to rub that much pretty much everywhere, or it will look splotchy. I tend to polish in light applications, doing multiple runs to achieve the level of shine I want. Be careful around the lettering, as you can polish it off.

Different towels have different pile, and will either be more or less abrasive. The towels I use have a thick and shorter pile side. I use the shorter, more abrasive side for applying the polish, and the thicker more absorptive side for wiping the polish off. Also, Cotton swabs are necessary to get into the tight spaces around the can capacitors, tube sockets, input jacks, and back panel of the amp. Like with towels, cotton swabs come in varying softness. I prefer to use hard cotton extra long swabs to polish with, and soft Q-tip brand cotton swabs to wipe polish off with. Get them in bulk, as you will go through an amazing amount polishing. Finally, please do wear a mask or at least be in a highly ventilated area, as you do not want to be breathing in polishing compound fumes. Pro Tip: Silicon oil works as a great sealer for a polished amp. It provides shine, and protects the amp against future corrosion much better and longer than waxing or using a synthetic sealant.

A Note on Rust: Without an ultrasonic cleaner, we like to use Naval Jelly to remove rust. Be sure to wear a proper mask and have lots of ventilation when using it, and apply it evenly, as sometimes the byproduct or residual “goo” can leave streaks in the shape of the application strokes you used to put it on. I apply it, let it sit for a few minutes, then wipe it completely off. Polish and then seal it before the exposed metal can tarnish. Sealing a bare metal spot with silicone oil really protects the spot from tarnishing quickly again.

Lubricating Controls:
- As ultrasonic cleaning gets into all the controls and cleans them out, we need to re-lubricate them. This is a must even if you only used soap or cleaner on the amp, as these old controls can almost always benefit from a clean and lube. We like to use DeoxIT Gold, as it cleans, flushes, and lubricates. Spray it into the top openings of the pots and switches and rock or roll the switch repeatedly until it is nice and smooth to use. No need to use a lot of force or speed. Sometimes a few applications is necessary, and yes, that nozzle is hard to control for all of us.

Guess what? We are done with the cosmetic portion of this restoration!

Putting Back in the Transformers:
- Using photos and the schematic of the amp, we can easily put back in the transformers and wire them back up. Be sure to triple check your work, as misplacing a wire here can lead to serious problems when you power up the amp.

Powering Up:
- Now the amp is on the tech bench. Before installing the tubes, we connect our voltmeter and oscilloscope probes to several key points in the power supply before powering the amp up just slightly on a variac to ensure that basic power supply systems are operating properly. Using a variac to slowly bring up voltages also allows the filter caps to properly re-form, as they have likely been unused for a very long time. Please use extreme caution when poking around under the chassis of an amp when it is powered on and even after you power it off. There are some big voltages going on under there and you do not want to feel them - trust me.

Then we tube the amplifier, hook up a test input signal and speakers. Then we again slowly ramp up the operating voltages using the variac. Meanwhile, we closely monitor the amount of current the amp pulls as the power supply voltages ramp up and the tubes begin to activate. Often we find problems at this stage with amplifiers that are barn pulls. Luckily, this one came to life and started playing music.

Recapping:
- As we had already decided to replace both the signal capacitors and the power supply capacitors, we went ahead and broke out the soldering iron and got to work fitting in the new caps. We chose to leave the original can caps, and simply solder in new caps under the chassis. This is easier, allows us to choose the specific values we want to use, and retains the original classic look of the amplifier. Fitting them under the chassis can be a puzzle though at times. Luckily the Fisher X-100B has a lot of room to work with.

While maintaining factory values for the signal path coupling capacitors is debatable when it comes to preserving the sound of the original circuit or modifying it, we do highly recommend you upgrade the capacitance of the power supply. These vintage amps are often under-capped in the power supply, simply due to the cost of high capacitance caps back when these amps were made. Believe me, if Fisher could have bought caps at what they cost today, there would be much bigger capacitors in these power supplies. We went up 200% in capacitance of the power supply.

We also went ahead and replaced all of the signal path capacitors with high-quality caps of the same value. While most epoxy dipped caps don’t go bad, we decided to make this amp as reliable as possible by replacing them all.

We also install an inrush current limiter in the amp’s power supply. This is a simple, but very effective modification to help increase the longevity of your tubes. If you watch your amp power on, you might see the tubes light up very briefly as they receive a surge of filament voltage from a cold start. This is not good for your tubes, and will shorten their lifespan. The lifespan of your amp’s filter capacitors and on/off switch are also greatly improved with an inrush current limiter, especially in amps with solid state rectifiers like this Fisher X-100B.

The amp is no now ready to begin a series of tests and measurements to verify its operating condition and identify any issues that may need adjustments or repair.

Biasing:
- Now that the amp is recapped, and doesn’t have any problems, we are on to biasing the tubes. The tubes we selected for this amp are all new production, Tung Sol 12AX7’s and Electro Harmonix 7868 output tubes. The output tubes are a “factory matched” quad, however we do check for actual matching using our curve tracer. Surprisingly, factory matched tubes are often not matching within 5% at the voltages they will be running at in the specific amplifier we are using them with.

For biasing we use a spectrum analyzer. This allows us to watch the spectral content of the output of the amplifier while we tune the bias to null out as much as possible the odd order harmonics. Odd order harmonics, especially those of the 5th, 7th, and 9th order are largely what create an unpleasant listening experience. By tuning the bias to null out these odd order harmonics, we bias the output tubes for the best real world listening performance they can provide. This takes a lot of patience and dedication with the bias pots to determine the best setting that provides the best harmonic content throughout the ideal volume range (up to clipping).

The result of all this can be seen in the performance graphs of this Fisher X-100B, which has limited factory bias adjustment which we tweaked to work better with the EH output tubes:

 

[AEA Audio]

Putting it all Together:
As you may have noticed, we don’t fully reassemble the amp until it is off the bench. This is simply to prevent the risk of the faceplate being scratched while probes, screwdrivers, and soldering irons are flying around. It is just a precaution we like to take, as a lot of work just went into the cosmetic restoration.

Putting the amp together is the reverse of taking it apart. Pro tip: We like to turn all the controls over to one side before putting on the knobs, as it makes locating the key on the shaft easier.
So what does the finished amp look like?

Take a look!

And how does it sound?

Have a listen on YouTube HERE! The video also shows a computerized spectrum analyzer being used to do a live comparison of the input (orange) and output (blue) signals.

We really hope that this was informative, and dare we say, educational for you to read. Please let us know your thoughts and comments. We try to take criticism constructively. Also, feel free to add any restoration tips you may have for others who are looking to do their own restorations.

 

[HVHiFI]

Great write up AEA! The ultrasonic cleaning is incredible. Its hard to believe you can submerge an amp in liquid but the proof is in the beautiful final result. Great work!

 

[catama]

Superb Post. Loved it. Can I send you my Scott 299?

 

[AEA Audio]

Great write up AEA! The ultrasonic cleaning is incredible. Its hard to believe you can submerge an amp in liquid but the proof is in the beautiful final result. Great work!

Thanks! We used to wash the chassis tops in a parts washer bin with soap/water or cleaner/water with the transformers in. It was heavy and awkward to do, and we didn't do the underside for fear of getting cleaner into the transformer housings that would cause problems if it didn't completely dry out.

It took a little trial and error when we first tried submerging amps into an ultrasonic cleaner. We did try it with the transformers in, and even after 12 hour dry time in the dehumidifier, the transformers were completely shot - lesson learned. With the transformers out, there isn't anything that can get hurt with the limited time the amp is submerged and the long length of time we dry them out for.

I highly recommend removing the transformers if you want to clean your amp at home. It makes the whole cleaning process easier. You can hit the underside of the chassis without fear, you get to scrub down the areas hidden by the transformers on top, and you can easily repaint them while they are out. Just be sure to take a lot of photos of where each lead goes for when you go to re-install them.

 

[gadget73]

I'll definitely give you guys props for the cosmetic work. Looks very nice.


Have you had any trouble from full immersion cleaning like this? I've always been scared to death of dunking something entirely, especially the pots and controls. I've heard of people doing it, along with running stuff through the dishwasher and getting good results, but it has always made me nervous.

 

[Overundr1]

Amazing write up, thanks. One query, how do you clean the transformer wires themselves and repair any insulation cracks other than heat shrink?
-Lee

 

[SPL db]

Great write up and thanks for posting that! :yes: :thmbsp:

 

[AEA Audio]

I'll definitely give you guys props for the cosmetic work. Looks very nice.


Have you had any trouble from full immersion cleaning like this? I've always been scared to death of dunking something entirely, especially the pots and controls. I've heard of people doing it, along with running stuff through the dishwasher and getting good results, but it has always made me nervous.

Thanks!

We haven't had any trouble since we decided to take the transformers out before ultrasonically cleaning the amp. The transformers did not like getting dunked. Some turned out okay, others were destroyed, so we changed our policy. Now we always remove the transformers.

The pots and controls really benefit from a good cleaning and lubrication actually. As long as you completely dry the amp off, and re-lube all your controls, they work a lot better after the whole process.

The dishwasher is a funny one. I guess it would be like putting the amp in a large parts washer. I used to use one at an automotive restoration shop to restore WWII Jeeps, but it only was for getting the dirt and oil off of parts before blasting the corrosion off. I'm not sure how effective a dishwasher would be for corrosion on amps.

 

[audiodon]

Good stuff. Thanks for taking the time. I'm always up for a fresh perspective.

 

[AEA Audio]

Amazing write up, thanks. One query, how do you clean the transformer wires themselves and repair any insulation cracks other than heat shrink?
-Lee

Thank you!

Interesting. I imagine you are referring to the wire leads coming out of the transformer covers/bells. Usually I hand clean them while the transformers are out, but I haven't looked into alternatives for heat shrink to repair insulation cracks. You could take the transformer apart enough to solder on new wires completely, but we never found the need to do that.

 

[gadget73]

maybe I'll try your method next time I have a project to do. I will pull the transformers though, dunking them seems sketchy. The clean and lube on the pots is no big deal, they need it anyway. I was more concerned with water wicking into the phenolic wafer and lifting the carbon track off. Also, excuse to buy a big ultrasonic cleaner. I want one that is record-sized anyway.

I'll skip the dishwasher though. My water here is horrible to the point where I won't wash my car or make coffee with it. Given that its rotted the rack out of the dishwasher in 7 years, I don't want that in my amplifiers.

 

[drew_t]

Very informative write-up; thanks for posting it. The company history section of the AEA website is also a great read.

Question: is there a particular commercial product example of silicone oil (you meant silicone, not silicon, right?) that you can recommend?

 

[SteveinAlaska]

After reading through this post (thanks) I did a quick web search for ultrasonic cleaners. Most were big enough for jewelry or watches. What could you recommend for a complete chassis??

 

[Dandy]

Thanks for a great write up.

 

[Retrovert]

To begin, that was a wonderful writeup.

We haven't had any trouble since we decided to take the transformers out before ultrasonically cleaning the amp. The transformers did not like getting dunked. Some turned out okay, others were destroyed, so we changed our policy. Now we always remove the transformers.

I wonder if this was not the dunking, but microfractures in the winding insulation. Remember, the ultrasonic vibrations affects the wires differently than the (shellac) insulation, and will literally spall the hard surface material (which cannot flex) off the wires.

This is just my theory, as I haven't looked at transformer windings post ultrasonic cleaning.

The dishwasher is a funny one. I guess it would be like putting the amp in a large parts washer. I used to use one at an automotive restoration shop to restore WWII Jeeps, but it only was for getting the dirt and oil off of parts before blasting the corrosion off. I'm not sure how effective a dishwasher would be for corrosion on amps.

Industrial circuit board production puts the boards in a dishwasher to remove flux and other debris.

Funny you should ask about removal of corrosion in the dishwasher. I've tried removing rust in my dishwasher using citric acid. My main goal is to remove calcium deposits from the pump, which the citric acid does. Use the bulk powder instead of LemiShine which costs a lot more. As long as I'm doing this I clean my glassware and silverware. Do not EVER clean plates as the acid will strip off the protective glaze. I've also used this to remove plaster from my plastering tools every once in a while. Works great, except for one problem.

Metal items washed in hot acid will quickly oxidize, corrode, and rust once the surface oxide layer is removed, so one must passivate the surface. I have not had this technique work particularly well. The molasses rust removal trick works in the same way; the sugar is converted to acids which then slowly reduce the iron oxides.

I would instead suggest the electrolytic removal of rust from a chassis. That will work far better. I've used that technique on all sorts of things and each emerged from the bath with no rust at all, and since it won't touch the harmless oxides the surface won't rust like crazy afterwards.

 

[AEA Audio]

Very informative write-up; thanks for posting it. The company history section of the AEA website is also a great read.

Question: is there a particular commercial product example of silicone oil (you meant silicone, not silicon, right?) that you can recommend?

Thanks for pointing out the typo! Haha, yes, I did mean silicone, and not silicon. Silly me.

I actually like to use Hoppe's silicone oil impregnated cloths to apply a light layer of silicone oil. They work great and last for many uses.