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Amplifier Restoration

Restoration/Restification

When approaching restorations or restifications, some general principles apply.  All electrolytic power supply capacitors usually need to be replaced. The risk of failure and damage to transformers, and/or homes is too great.  I do analyze all can caps and in most cases replace them with new reproductions or rebuild the originals.  Generally mica or ceramic capacitors do not need replacement, but critical ones will have their value verified, and replaced as necessary. Paper/wax coupling capacitors are problematic. Most are off value and leaky. This leads to poor performance and mushy sound. Many of the McIntosh “bumble bee” caps are fine, as they were very well sealed units, but if they need replacement I recommend new mylar/polyester audio caps. Using these new caps will significantly open up your sound field and liven the sound. If you prefer I do have a small number of NOS bumble bee and black cat coupling caps.

 I measure values of all resistors and replace any out of spec with NOS, or new metal film ones. Where needed, I hand match precision sets. If you want complete originality, I have a stock of carbon comp resistors. If you want precision and low noise the carbon film are the way to go. For restorations, it is carbon comps and restifications it is carbon film. Selenium rectifies must be replaced with silicon diodes. This is for your equipments safety as well as performance. Up to the mid 60’s many bias supplies used selenium rectifiers. As these age they get less efficient. I have seen bias supplies that should have provided -35 volts provide as little as -15.  This leads to output tube over-heating and plates glowing. I many cases I can hide the new silicon diodes and leave the selenium rectifier in place for cosmetic reasons.  Tubes are another huge issue. I prefer to have you supply the tubes you wish to use in your completed amplifier. There are many sources of used, NOS and new production tubes today. Start with eBay or your favorite tube supplier. I can procure tubes for you, but this adds time and expense to your amp. I can make recommendations concerning tubes. My general ranking of tubes is as follows;

NOS→ Used Good→ Some Russian Military→ Some New Production Russian→ Chinese

You also have to take into consideration the use of the tube. Preamp tubes need to be quiet and have well matched sections. Output and rectifier tubes need to have good plate structures and high quality materials.  Burn-in and matching are a plus with ALL new production tubes. I have found some of the new production Russian made tubes to rival British and US made NOS tubes. Some of the military surplus Russian tubes are also excellent. My limited experience with Chinese tubes is not very good. I have seen too many catastrophic failures. If one of these takes out an output transformer the money you saved (and a lot more) just went out the window. I do not recommend you spend hundreds of dollars on “Black plate, Grey plate, Smooth plate or Telefunken tubes advertised to be special. They usually aren’t. I have found a number of great suppliers for restoration items for tube  and guitar amplifiers and want to share them with you.

www.dynakitparts.com -great source of Dynaco restoration parts and new kits

www.tubesandmore.com- great source for guitar parts, tubes and other restoration items

 

Restorations- Some examples of recent work

McIntosh C 20 preamplifier- This unit had a number of issues. I was lucky enough to find a reproduction front glass for this unit. The volume control was open in one channel and this necessitated a long search for a NOS part. Several resistors were noisy and a number of power supply caps were leaky. I have a large stock of carbon comp resistors and was able find some quieter ones. After several reforming operations the cap caps responded very well, with the exception of a single can. That one was replaced with an new production FP.
 All the tubes were still within specs, even after 40 years. After cleaning all the controls and switches this unit functioned as good as new and met all factory specs. This was a relatively easy restoration.

  









McIntosh MC 30 power amplifiers
- (set 1-after and before) These amplifiers were in extremely sad shape. The chassis were rusty, all ,most tubes were missing, the circuit boards were intact but a number of parts were missing and the can caps were shot. New stainless steel chassis were procured and a ground up restoration was begun. A completely original stock unit was used as guide.  All missing bumble bee caps were replaced with NOS units, the can caps were replaces with new production units. The transformers were sanded, the decals masked off and painted with auto-body catalyzed gloss black. When the new chassis’s arrived, the NOS sockets were installed and wired. The wiring was duplicated for the power supply and boards. NOS tubes were selected and the amplifiers were power up and checked. The high voltage supply was fine tuned to meet factory specs. After the restoration the amplifiers were able to put out 34 and 35 watts respectfully. The cost for this restoration was around $1500.  The customer was thrilled and loved them. He thinks they sound better than new.

McIntosh MC 30 Power Amps (set 2)

These were a very special set of amplifiers. They were consecutive serial numbers and the chassis’s had no rust, and they were completely original. The owner had stored them in his closet for over 20 years (dry and temp stable) and someone told him about my service and he contacted me.  The filter caps were shot and the coupling caps to the outputs were leaky, the selenium rectifiers were very weak (only -10 volts of bias) and last, but not least, the input tube electrolytic’s and bias cap electrolytic’s were leaky and off value. The customer ‘s goal was to have a set of reliable amplifiers with the stock look. New production FP can caps were used with an extra section and slightly higher capacity. Wired correctly this substantially improves the transient and bass response of the amplifier. (while maintaining the stock look) The paper/ wax coupling caps were replaced with a matched pair of Sprague Orange drop capacitors. (These were extensively used by the famous MC Clinics when bringing MC equipment back to specs) The selenium bias diodes were very weak and were replaced with silicon diode, the bias cap was replaced with a substantially higher value, to stabilize the bias voltage at high volume levels. Another weak capacitor was located in the input tube cathode, this was replaced. Good used and NOS American made tubes were selected for the 9 pin tubes and the rectifier, while Russian made 6L6GC Tungsol re-issues were selected  for the output tubes. The power transformer voltage tap was switched over to the 125 tap and the amplifiers were powered up. As typical with higher line voltage today and the silicon bias cap, the power supply voltages were about 15% high with the bias voltage at -57 volts where spec was -45 volts. A NOS 5R4 was substituted for the 5U4 rectifier tube. This brought down the power supply voltage about 7%, but the high bias voltage still persisted. By adjusting the resistor value that feeds the bias circuit and matching the needed value with fixed resistors I was able to achieve a bias voltage of -45.6 volts. This also resulted in the plate voltage of the output tubes to fall from 479 volts to 444 volts. (spec is 445 volts) The amplifiers were burned in for 5 days and voltages all rechecked. Everything was still in spec and they sounded wonderful. Evaluation of them demonstrated an RMS power without clipping at 35 watts, and a perfectly shaped sine and square wave response. The chassis was polished with a fine diamond based polish, then it and the transfomers were waxed with a high quality paste wax. The total cost of this restoration was $265 per amp.

 


Restification- can best be described as a rebuild utilizing the most current, up to date components and technology. These may include ultra-fast rectifiers, low noise resistors, high quality capacitors and high value electrolytic capacitors. Some examples of resitifications I have done are:

Heathkit W-5 Amplifier- Many of the original coupling capacitors in this amplifier were off value and leaky. They were replaced with high quality IC mylar/polyester capacitors. The AC surge delay was a mechanical unit and was replaced with a new replacement surge suppressor. Many of the carbon resistors were off value and replaced with high quality metal film resistors. The multi-section electrolytic can caps were leaky and one section was shorted, and they were replaced with new replacement can caps. I used new tesla caps with about 3 times the total capacitance.  The input jack was corroded and loose and was replaced with gold plated RCA’s. Cosmetically these amps are far from perfect, but they reside in an audio closet and produce beautiful music. They are built like tanks, sound great and have been functioning flawlessly for 5 years.



Heathkit W-7- This is a very interesting amplifier, similar to the Dynaco Mark IV. They both use EL34’s as outputs  and a single dual pentode/triode as the input and phase inverter. The output transformer is half again bigger than the Dynaco. The power supply with the W-7 is the primary problem. Heath stacked 4 insulated cans caps on the chassis. These capacitors were not very good and almost always need replacing.  New capacitors are much smaller, so I was able dramatically increase the values, thus improving the amplifier transient response. In order to hide the new capacitors I built some small cages. The paper coupling caps were replaced with some high quality mylar/polyesters. This amplifier was a real surprise, it was much better than the Dynaco. This is one of the more powerful monoblock amplifiers from the "old days" It is probably one of the better ones out there. Unfortunately they are a lot rarer than the W5M's or the Dynaco Mark IV's. It had substantial bass and very clear treble. This customer wanted reliable power and was not concerned about originality or cosmetics.
















Heathkit W-6AThis was Heath’s largest amplifier. It used as 12AU7, a 12AX7, a 12BH7 and a pair of 6550’s as outputs. It was rated at 70 watts RMS.  These amplifiers are some of the best ever built, by anyone. The power and output transformers are huge and vastly underrated.As you can see this one arrived to me pretty homely. Beauty is in the eye of the beholder and this  one had a lot of potential. Properly built these amplifiers are virtually flat from 20 HZ to 20 kHz. They too have power supply issues, but have 5 large can caps and a half dozen axial lead caps stacked. I removed all these and built a new circuit board and almost doubled the power supply capacitance and cleaned up the under chassis.  The paper coupling capacitors were awful and were replaced with high quality paper in oil caps.  This amplifier turned out to be one of the best amplifiers I have ever heard. The first picture shows how the amp arrived.  Customer wanted cosmetic external restoration and reliable “guts”. I was able to remove most of the splattered black pint off the chassis without much loss of the gold color. The completed amp looks a lot different, and preforms extremely well.


Bogen MA 100 Rebuild- 
The Bogen MA 100 was a 100 watt mono amp designed for sound reinforcement and commercial sound installations. They used four 8417 output tubes running at 500 volts and a single 7247 preamp/phase inverter tube. The had outputs from 4 ohms to 125 volts. They also allowed the use of balance or unbalanced inputs and remote control. The issue today is 8417 tubes haven't been made in 40 years and they are getting scarce and very expensive, plus you needed a match quad for this amp, since it didn't have individual bias controls. These are about impossible to find. I undertook a project to find a way to still use these amps. KT88's or 6550's made the most sense, but because of the socket spacing you can't get even the 6550's in without them touching each other. Instead I used EL34's and set up a new bias supply. All filter cap cans were replaced as well as coupling caps. Added a bias adjustment in addition to the factory bias balance control. You can see from the original picture it needed some cosmetic work too. Oak surround covered extra holes drilled throughout the years and a little engine turned aluminun to cover front ones. The amp produced 87 watts RMS into an 8 ohm  load, also has 4 ohm output.
 


Dynaco Mark VI- (no pictures at this time) This was Dynaco’s larges tube amplifier, producing 120 watts RMS from four 8417 output tubes. Only about 1000 pairs of these ever made and they are relatively rare today. The several issues with these amplifiers;  the output tubes have not been in production for almost 20 years and these amps required matched quads, so it is pretty much impossible to use the amp as is. The bias was set for all for tubes with one control, hence the need for matched quads.  There are two ways I have restified these amps.

1. A mount plate was fabricated to hold 4 bias pots and a control to switch the meter to each individual 8417, allowing the use of unmatched tubes in the output circuit. While this doesn’t solve the problem of tubes made of unobtainium, it does allow relatively stock use of the amplifier.

2. Another solution was to replace the 8417 tubes with EL34’s and modify the bias system to bias a matched quad of EL34’s. This still requires a matched quad of EL34’s, but since these are being manufactured by a number of OUS vendors, the availability of tubes is very good. The power output with El 34’s is a little over 100 watts. While this is 20 watts less than the original amp, 100 watts of tube power is very impressive

3. My own personal pair of these amps were rescued after being modified by someone with little or no metal working and electronics skill. I am not a fan of adjustable fixed bias, so I set up each EL34 with an auto bias system. This allows the use of unmatched tubes and each tube adjusts it’s own bias based on its needs.



Vintage Solid State Amplifer Restification

Dynaco SCA 80-
This was one of Dynaco’s most successful integrated transistor amplifiers.  They were a good value n the 70’s and 80’s and make a very nice amplifier with some updates. All the electrolytic caps were replaced with new units and in some cases tantalum caps, the driver and output transistors were replaced with new matched TIP’s and large die TO3 transistors. Frank Van Alstine recommended a modification to the Dynaco PAT 4(back in the late 70's) that I applied to the preamp/driver boards on this amplifier also. This involves a considerable investment in large (read expensive) film caps and some large tantalum caps. What this mod does though, has to be heard to be believed.  The internal chassis was painted, and rebuilt boards reinstalled. This is a sweet little amplifier that has classic good looks. With a little tweaking this amplifier puts out a solid 40 watts of power and sounds very good.

 



Dynaco SCA 80
with 200 watt Class D rebuild- A repeat customer wanted me to build him an amp for his college bound daughter, that looked classic but “would really kick butt”. I started with an old SCA 80 chassis(as you can see it was pretty much of a junker, purchased reasonable at a yard sale)  and rebuilt the preamp circuit boards with a Frank Van Alstine Modification (discussed briefly in the previous SCA 80 mention) and then installed a 200 watt per channel Class D amp and 24 volt 15 amp switching power supply.These are a relatively new item to be available for experimenters. While the power supplies are quite common, compete class D amps have not been easy to find.  This thing really was nasty starting point and a very nice finish. The chassis was completely stripped of parts, boards and transformer and it was refinished with an antique silver vein paint. The rebuilt boards and controls were reinstalled, and then the ClassD amp and power supply was added. One of the issues that I discovered with these imported modules was that they had a pretty substantial turn on thump, even thoug the mute on turn on function was activated. To solve this problem, I built a small timer delay that prevented the amps delay circuit from turning on for about 10 seconds after the power was applied. This solved the problem with turn on thump. It does exactly what he wanted. Looks good sound good and kicks.



Dynaco ST120- Several million of these transistor amps were built and they still represent a solid amplifier, with some modifications and updating. All the board electrolytic caps are replaced, ceramics replaced with tantalums. Driver transistors are replaced with matched TIP’s and outputs replaced with large die TO3’s. (identical to the transistors Dynaco used in the final version of the 120) I run all your large chassis mounted caps on my reformer/tester to determine their expected life. To non-TIP boards I add a couple of ceramic capacitors that Dyna added for stability. These stop the problem oscillation seen with some types of preamps. The power supply is replaced with a mosfet regulated supply with much larger outboard caps.  (This was designed by Frank Van Alstine and by itself it is one of the best things you can do for a 120)  It also adds individual fuses for each output channel. At you request, I will free of charge, replace your neon power indicator with a Blue (or what ever color you want) LED. I also add a surge suppression varister. If the case is in good shape I polish them with a diamond based polished, but more and more they require refinishing. Some cases are externally pretty good, but the inside is nasty.(As you can see on the left- this was a group of amps I bought on eBay-they were described as "good") I strip the components and paint the inside with an antique silver vein finish.  I can also do this with the outside of case in situations where the nickel is flaked or rusted.

Another option is powder coating, but this adds about $50 to the cost of the project. It is by far the most durable and is available in just about any color you could want. I have had several customers who had me rebuild their 120’s, tell me that the sound was absolutely better after the restification. My personal belief is that the power supply with the mosfet makes the biggest difference in units that are still actually working. I have done a number of units, just adding the power supply and the bass and transient response is great improved. The rest of the rebuilds on these is just rplacing parts that have probably aged  and are contributing to the veiled sound. The performance of these amps has degraded as the capacitors have naturally aged.  (original expected life was 10-15 years) One of my complete rebuilds or ones I sell on my site, are guaranteed for 1 year. This does not include any original large can caps not replaced during the rebuild.I can rebuild your 120 with prices starting at $125, not including large output or power supply large cap replacement.




Dynaco ST 80- These are great power amps and a lot of people think they actually sound better than 120’s. The circuits are very similar, the major difference is the power supply.  The 80 does not have a regulated supply. I think this may the reason they sound different. They are not as common as the 120’s and a little harder to find, but worth it.


This rebuild used the original power supply, updated boards, with new caps and drivers and new TO3 output transistors. This one also got gold plated binding posts and gold plated inputs. (the speaker terminals on 80's are awful) Most 80's power supply caps are still OK, probably because the were not as exposed to as high voltage as 120's. For some unknown reason  the chassis on 80's stand up better to age than 120's. As you can see in the picture, the nickel in very nice shape.


AMPEG G20 Guitar amp- I don’t do a lot of guitar amps, mainly because of their shipping costs, but this one I restored, for my nephew. The G 20 is a rather obscure 6L6 based amp with dual input channels, a spring reverb and built in distortion and pedal effects. The most significant issue was finding a replacement reverb spring. Also the dial was very bad. After a long fruitless search, my wife took a shot at repainting it.  The result was a lot better than the original. The power supply was rebuilt with significantly better caps and the widow maker switch was replaced with a new SPST switch and a 3 prong grounded cord. For those of you who do not know, a widow maker is a DPDT switch that reverses the 2 conductor AC line and grounds either one through a .01 mf 1KV ceramic cap. This was at one time helpful to eliminate ground loops when a number of pieces of equipment were used on stage. The problem is that if the .1 mf cap fails (they do frequently) you can have the live side of the AC line connected to chassis of the amp. Since the input jacks are grounded to the chassis, you end up with your guitar or microphone live. I explain this just to let you know that if I work on your guitar amp it will come back to you with a 3 way grounded cord and elimination of the widow maker.



The case of this amp was very dirty and took several good scrubbing with soap and water to get it clean. It was then waxed with a dark polish and a new replacement top handle and badge were added. (many guitar amp parts are still available or are being reproduced) The power supply caps were all replaced with new equivelent ones and several of the coupling caps were replaced in addition. All of the tubes needed replacement and a mixture of NOS and Russian ones were fitted.


This is only a small overview of some of the amplifiers I have done. I have rebuilt, restored or restified hundreds of amplifiers over the last 20 years.  If you have specific questions, or just want to discuss you options, please fill out the contact form. (Link on Home page)  I will get back to you via email within a day or 2. You may wonder why I do not list my email address  anywhere on this site? Spammers do rountine searches of websites looking for email addresses. These addresses get sold and bombarded with SPAM.

My basic rate is $55/hr. and all work is guaranteed.  I am currently book out about 8-10 weeks. When you amp arrives it goes into the rotation.

Please use the contact form on the Home page for your questions. When you send to me I will respond with my email and phone number. The reason I do not publish my phone or email on this site is that spammer bots look for them on websites, and then you know what happens.

If you do not include an email in the contact form, there is no way I can contact you.