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Custom Amplifiers

                                                     Custom Amplifiers        Semi-Custom Amplifiers

Custom Amplifier Build Service 

I can build virtually any custom amplifier you desire. I have experience with many types of tubes and circuits, and can discuss specifics and offer advice based on your needs. I can use transformers you supply, or attempt to procure vintage transformers as needed.

Please spend a little time looking over the pictures and reading the descriptions of a few of the amplifiers I have built.

1.     “Pencil Tube” Mono-blocks- these tubes were originally designed for an article to be published.  These tubes were the last gasp of vacuum tubes and were developed almost solely for military applications.  They are used in missile guidance systems and portable radios. They are used in combination with transistors in many commercial radios.  They are also used in a variety of other commercial applications, such as hearing aids. Early versions of these tubes operated with plate voltages of about 175 volts, but later versions used as little as 22 volts for the plates. Some recent applications for these tubes have been for preamplifiers for condenser microphones, headphone amps and effects boxes for guitars.

There are several examples of amplifiers I have built;

 

                  Mono Blocks (SE) with separate high voltage power supply


 

                 

 Stereo Amplifier (SE) with outboard high voltage power supply

 

 

 
















                             Stereo Amplifier (SE) with 12 volt power

 








2.     6AQ5 Push Pull Stereo Amplifier-

This amplifier was built using transformers from a Harmon Kardon A300 Integrated Amplifier. The chassis and front panel were in very bad shape and the original output tubes used are obsolete and very difficult to find. The 6AQ5 tubes are the miniature version of the 6V6. They are relatively inexpensive and easy to find.  This amplifier puts out about 10 watts per channel and is perfect for an office or a bedroom.  It featured two input selections for use with a tuner and MP3 player.

 

3.     6BQ5 Push Pull Class A Amplifier.  

This was built on a stainless steel chassis (very difficult to machine) and utilizes a pair of Dynaco SCA 35 outputs and an Edcor power transformer. This amplifier was designed by Bruce Heron and operates in pure Class A mode. It uses constant current devices on the output cathodes that keep the outputs in Class A mode. This amplifier also uses an SPRP driver circuit. It puts out about 8 watts/ channel (but really sounds like a lot more) It sounds wonderful, but is relatively hard on output tubes and their life is limited. It works great with Russian tubes. They have heavier glass and a larger envelope and are very rugged. Since I put in a quad of these I have not replaced tubes.

 

4.     6BN8 Stereo Amplifier. This was an experiment with an interesting tube. This tube contains a pentode and a triode in the same envelope. These were used in many integrated amplifiers imported from Japan in the 60’s. Many of these amplifiers were actually quite good with quality transformers.  Radio Shack sold some under the Realistic Brand and even Montgomery Ward’s sold them under the TrueTone Brand. My build was based on the schematic from an integrated amp I restored for a customer. The biggest issue with this tube is NOS are only available from Russia. (You can sometimes find NOS and used ones on eBay in small numbers) The Russian versions do not have as much gain in the triode section and that makes them hard to drive to full output. This amplifier will put out about 4 watts RMS/channel and sounds very "Tubey"

 

5.     6BQ5 Dynaco ST 35 clone

      This amplifier was built on the request of a customer. The original ST 35's were not nearly as attractive as ST 70's, they didn't have a nickel plated chassis, instead they were on a galvanized chassis. This partial kit was purchased from Dynakitparts.com and used some early SCA-35 output transformers and a very unique power transformer (look closely at the cooling fins) The chassis is brushed stainless steel. The boards are all new with a 6CG7 preamp/phase inverter tube. The power supply has about 4 times the capacitance of the original. This amplifier performs as good as it looks and actually put out 20 watts RMS/channel with a 400hz sine wave.

 

6.     6L6 Amplifiers

 Some of the finest hifi amplifiers in the world use 6L6 tubes. These include the Heathkit W-4’s, W-5A, the McIntosh MC 30, MC 40, and MC240’s. I have built more amplifiers using 6L6’s than any other tube. There are many NOS tubes available at very reasonable prices; the Russian 6L6’s are very solid and the even the Chinese ones are passable. There are many very good transformers on the market that work well with 6L6’s. They work in SE, triode, pentode, and ultra linear modes and will generate 18- 24 watts per channel RMS. (SE about 5 watts) Also check out my restoration section, as well as my semi-custom section of this site.         

·        6L6 Pilot Stereo build

This amplifier was featured in an article in AudioXpress . The purpose of the article was to help hobbyists utilize a wide variety of audio and power transformers to build a true hifidelity amplifier. This one uses a pair of Heathkit W4 transformers and a Fisher 400 power transformer. It uses a very nice stainless steel chassis with a Cherry wood base. It also features “eye” tubes to give a visual indication of audio activity.  The filtering on this amplifier was a little bit obsessive. There are separate filter chokes for each channel of the preamp/drive/phase inverter tubes and the pentode power supply also has it's own choke. This is the quietest tube amp I have ever heard.

                                                                                              

 

The same basic Pilot Circuit with Vintage parts and transformers

·        This is another built for the article with the same circuit but with all vintage parts and octal tubes.

 

 

 

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      This is an amplifier built on a Heathkit W 5 chassis. It also features an eye tube to indicate audio activity. This chassis was powder coated in a color very close to the original Heathkit metal flake. These use some Heath W-9 output transformers and the original W5 choke, with a new production Hammond power transformer.

 

 

 


7.     EL34 Amplifiers

The EL34 tube was used in the most common amplifier sold in the US: the Dynaco ST 70. Estimates are that about a million of these amplifiers were sold both in kit and factory wired versions. This amp was rated at 35 watts RMS per channel (optimistic in most cases), but these amps got many people into tubes and they are still pretty common today. They can be rebuilt. (I have rebuilt dozens). The Mark IV is a mono version of the ST70 and represents a significant improvement over the ST70.  Many very fine amplifiers from the 50’s and 60’s used EL34’s including Marantz tube power amps and some very fine Fairchild’s. The EL34 is also extensively used in guitar amplifiers. It is a tough tube and has a sound all its own.

 

Pentode Petite EL34- I have built a number of custom EL34 amplifiers, and one of my favorite is the Pentode Petite. This was an amplifier featured in AudioXpress. It was originally designed with adjustable bias, but I modified it to an automatic bias system. EL34 amplifiers are capable of 30-35 watts per pair of tubes and will drive most speaker systems to painful levels. .






Marantz 8C (clone)

 

     Another EL34 amplifier I built is a clone of the Marantz 8. This one used a circuit board designed by a gentleman from Hong Kong. The transformers are some 1953 vintage UTC A-35 ultralinear transformers. I left them in their original color and paint for nostalgia reasons. The chassis is heavy aluminum plate fastened to a rack mount front with dark oak side panels. This amplifier has individual bias adjustments for each output tube, a small digital LCD meter switches between tubes and the adjustment pot is next to each tube. There is also an adjustment for balance between the driver/phase inverter. This requires a second meter and switching system. This amplifier is big, heavy and sound so good, that everyone who hears it wants to buy it. It how ever is one of my personal amplifiers.


8.     KT88 and 6550 based custom amplifiers.

If high power is your wish you will be interested in my big boys. Dynaco Mark 3’s

and Heathkit Mark 6’s use KT88 or 6550’s and are capable of 60-70 watts output in push pull. There are a number of decent kits for rebuilds of Dynaco Mark 3’s; I particularly like the Welborne Labs kit and Dynakits parts.  Here are some custom 6550’sI built:


Dynaco Outputs Stereo Mark III

  This monster uses Dynaco Mark 3 outputs and a massive torroid power transformer for a stereo 60 watt per channel amp. The top plate is made of .250 aluminum plate that was machine jeweled before the components were mounted. This amplifier is pretty impressive and will drive just about anything above 4 ohms to painful levels.


This is one of set of monoblocks that use McIntosh MA230 output transformers.

The MA230 was a 30 watt per channel integrated amp using 7591’s. The output transformers are large and heavy and capable of 70 watts RMS without saturation. This amp was based on an article by Bruce Rozenbilt that appeared in Glass Audio. These are heavy and complicated and would cost over $1,200 each to duplicate today.

 

 

 McIntosh MA230 Transformer Stereo

This amplifier is a stereo version using MA230 output transformers and power transformer. With the exception of the power supply the amplifier is pretty much a copy of the McIntosh output section of an MA 230.  The power supply is upgraded to supply over 500 volts to the plates and provide an increase in the available bias voltage for the 6550’s.

 

9.     Transmitter Tube Amplifiers

 I have built custom amplifiers using a variety of World War II transmitter tubes. The 632 based amplifier uses the dual sections of each tube in a push pull configuration. These tubes were used in radio transmitters in a wide variety of Navy ships up to the early 60’s. A gentleman contacted me after seeing this amplifier and told me he was a radioman during the Viet Nam war. He said the transmitters used 10-20 of these tubes in push pull parallel, so when one failed, they just pulled the bad one and replaced it HOT. Not my idea of a good time, but he said it was a common practice.

632 Transmitter Tube Push Pull Amplifier

This amplifier puts out about 10 watts per channel, and because it doesn’t run these tubes anywhere near their ratings, they run cool and last a long time.  It used a set of decent guitar amplifier outputs. While these don't frequency response much below 30 Hz, it sounds very nice. Mello, tubey and very interesting to look at. Solid Cherry base with 0.25 aluminum plate chassis.

 Another transmitter tube amp is based on the 329 (3B29) tube. This is a larger, heavier tube than the 632, but from the same era. This amplifier is a Single Ended (SE) design paralleling the elements in each tube.  Not quite as much power, but again is used significantly under its rated values. Please also note the 50’s vintage ITC choke, used as part of the power supply. A torroid in housed underneath the chassis for high voltage and filament power. A solid hard maple base finished in a poly-urethane and the top chassis is 0.25 tempered aluminum.The driver tubes are NOS 6SJ7 pentodes  and the RCA's are 1944 vintage NOS RCA's

329 (B29) Transmitter Tube Amplifier


Neither of these amplifiers is suitable for use in homes where children or animals roam. The terminals at the top of each tube are live with 300-400 volts DC. If you want a truly one of a kind amplifier, that will not only sound great, but will impress your audiophile friends one of these types may be for you.

 

10.    Headphone Amplifiers

            Over the years I have used a number of dedicated headphone amplifiers and most leave a lot to be desired.  I built a very complicated one several years ago and it worked very well, but the cost of building it was over $200 and it required 120 volts for operation. I recently built a couple of headphone amps, designed by Rogers Gomez, published on DIY Audio. They use a 12AU7 tube as the voltage amplifier and then a IRF510 mosfet as a current amplifier. These are relatively straight forward. I made a couple of boards and constructed these two.The major issue with them was turn on transients. I designed a delay circuit to allow the amp to stabilize for about 15 seconds before connecting the headphones.The first one used a 1.3 Amp/hr sealed lead acid battery. This gave a battery life of approximately 3 hours on a charge. The second one used a 3.6 Amp/hr rechargeable sealed lead acid battery. This one give about 6 hours of uninterrupted use between charges. If you have never listened to an ipod or MP3 player with an external headphone amplifier, you have never actually heard much music.  The bass is more natural, the treble in non-stressful. 

 





Semi-Custom Amplifiers  


Dynaco/Pilot 6L6 Amplifier (Ruth)

I have built a number of custom amplifiers on Dynaco ST 70 chassis. These chassis are plentiful, and have provisions for a tube rectifier and four output tubes. In the past, I have cut an aluminum plate to replace the stock Dynaco circuit board and then punched appropriate holes for tube sockets for preamp, drivers, and phase inverters. When I had a request for four identical amplifiers, I decided to design a new circuit board for the ST 70 chassis. This took many hours to work out and test, but the resultant performance was excellent. The new board uses three 12AU7’s in a very reliable circuit. The first tube is used as a triode preamp for each channel; the second and third tubes are used as a triode drivers and a phase inverter. This board is designed for a pentode output stage, but will function well in ultra linear or triode mode. (can  be selected when your order) The circuit board cuts down hand wiring time by 3-4 hours and can be utilized with a number of different output tubes and transformers. The front panel receives a custom front trim made out a variety of exotic woods (such as rosewood, American black walnut, coco bolo, black palm, or cherry). I prefer to use “vintage iron”, the 6L6 family of output tubes.

 

After I finish the amplifier, I fine tune the voltages for each of the tubes. This allows the tubes to function at their peak of efficiency and sound. After this fine tuning, I burn in the amplifier for a week, and re-check all the voltages. I then run a series of tests while monitoring the output on a dual-trace oscilloscope and a Sencore amplifier test system. .I name each amplifier based on a number of factors. The amplifier shown was Ruth.


Another one I recently sold was Racine. Her new owner is an amateur audiophile and sent me a follow-up to his purchase and has allowed me to share it with you.

Racine arrived this past weekend, you packed her very well, double boxed method provided excellent protection for her journey to Mooresville, NC.  First thing I did was put her on my shop bench to run some square wave signals through her.   I used a WaveTech generator, a Heath 25, True RMS AC/Volt meter, an HP Freq. counter along with a 30 MHz dual trace scope.  Racine’s outputs were loaded with a pair of 25 watt, 8 ohm resisters.   I started the test after Racine had warmed up for about 10 min.  Test were run with an input level of .9 volts.  The initial test ranged from 18hz to 12Khz all was found to be flat and with no  measurable output level change.

 I then ran the input voltage up to 1.5 Volts and the observed output wave form remained relatively flat measuring over 55 volts at the output making about 22 to 25 solid watts per channel.   

 I returned  to the .9 volt input level, and ran through the higher freq. to find an excellent response from 13khz to 22Khz observing a flat response showing a small rise time overshot (less than 5% of the rise time amplitude) for about three (decreasing in level) cycles,  As the freq increased this overshot remained relatively small and had no observed sonic consequence.   I went on to 30 khz input signal for grins, observing a surprisingly good pattern and relatively little signal level drop off, less than .3 volts at the outputs.

 I did not make any THD measurements due to my THD instrument being out for calibration at the time of  this test. If what I hear is any indication, I don’t expect there will be any issue. 

Played through both a set of JBL (refoamed) 4311bs and pair of Fisher XB7b, Simply put, your amp is warm and clean from its musical lows to its pristine highs.   Yet in each of those extremes, she is not muddy, nor harsh, the middle freq. were also a joy, reproducing remarkable clarity, while again remaining beautifully musical.

 I finally have a spectacular tube amp….

 Thank You!  Chris M

Another recent customer Steve B sent the following email,


The amp is powerful in ultralinear mode with a crystalline high end,  a throaty midrange and a thunderous base.  I have not begun to play out all the alternatives: two amplifier circuits, preamp treble and bass controls, loudness control and the active bass woofer crossover.  But I did run the amp with Triode switches tripped.  I adjusted the woofer crossover only-controls at flat.  The Triode sound is smaller but clear and tight with lots of air-bass is very tight, treble is presented up front  and the midrange is remarkable-clear and multifaceted.   I can't imagine what this will sound like after the amp gets broken-in and I dial get the settings dialed in. 

 Hammond Organ Amplifiers

 

A few years ago I purchased a pair of amps from a 1950s series Hammond organ. These amplifiers were very interesting and led to a development project. They used four 6V6 tubes as outputs, with either a single 6SN7 or a pair of 6J5 tubes as the preamp tubes. The problem with these amplifiers was that they were originally used with field coil speakers as part of the power supply and they used an interstage transformer since they were designed as part of a tone cabinet, and have no phase inversion as part of the amplifier. They built like tanks. The power transformer/supply was designed to power the tone generator tubes, which are not part of the amplifier, so there is plenty of current capability. 

The power supply also featured two chokes and a couple of big oil caps. The output transformer is quite heavy and even has cathode feedback windings (similar to a McIntosh). These amplifiers have a huge bandwidth and are able to reproduce very deep pedal notes on an organ. They were conservatively designed, for long life, so many times I have found all the tubes to test like new, even after 50 years of use. I designed a preamp/driver/phase inverter section for this amplifier and rebuilt the power supply.

 

Before/After Hammond/Jensen amp

These amplifiers came in a variety of configurations, most are visually “plain” at best. As you can see from the photos, a great deal of time and effort goes into preparing these amplifiers for home use. The will produce about 20-25 watts RMS; are smooth, and easy on tubes..