What I've Seen in the Vintage Amp Repair Business

It has been about a year now since I’ve opened the doors of my shop to the general public. In that year, I have seen an amazing range of different amplification devices. While about 70% of my work involves guitar amplifiers, I have also seen my share of audiophile equipment. Reel to reel decks, linear tracking turntables, amplifiers, preamplifiers, tuners, and integrated subwoofers.

Some of the so called highly respected vacuum tube amplifiers have been rather lackluster in performance. While the outside of the amplifier looks impressive the internals tell another story: sloppy wiring techniques, crosstalk, poor build quality. And you know the funny part? After I’m done fixing these amplifiers, the customers call me up and rave about how wonderful they sound! It doesn’t matter that the amplifier has about 0.7% harmonic distortion. Perhaps it’s some of these distortion products that add something indescribable to the music that is considered pleasing to some ears.

More tricky, is the art of getting the right guitar amplifier tone. In this arena, there are as many unique listening tastes as there are guitar playing styles and guitar players. Every player is looking for a different sound. And some amplifiers have a beautiful harmonic profile when driven at moderate levels, but become downright ugly when driven almost to the brink of clipping. Some are ugly all the way through the range, and usually this can be improved with changes in bias voltage on the output tubes.

I recently had a Bang and Olufsen BEOGram 8000 turntable in here with a customer stating speed problems. Indeed the speed was fluctuating every revolution of the turntable. I quickly discovered that the mechanism for speed control was a transparent tachometer disc attached to the motor spindle and through which an optical sensor counts the apertures in the disc. The problem is that the printing on the disc was dissolving and disintegrating in a few places around the circumference. Fortunately I found a gentleman in Denmark who was manufacturing high quality machined stainless steel discs to replace the plastic ones. I replaced it and that cured the speed problem. Of course, as I tested the turntable further, other problems became apparent: when the turntable was called and the play command was issued, the tone arm would not set down onto the record. After a 10 minute warmup, the tone arm worked as it should. There were also problems with consistency of locating the lead in groove on the record. These problems were traced to faulty solder connections on the main control printed circuit board. After having reflowed solder on about 50 or 60 connections, I reassembled the turntable and it works perfectly now.

Another interesting project that came across my bench was an antique German radio. This one was completely dead when it arrived. The only thing that worked was the tube filaments. There were about nine resisters which were either open or grossly out of tolerance, plus two leaky capacitors. I did a full alignment on the tuner section and got the audio working very nicely with under 1% harmonic distortion from antenna to loudspeaker. Using an RF spectrum analyzer attached to the output of the IF strip and the tracking generator driving the input of the IF strip, I was able to see a real time representation of the IF skirts, enabling me to stagger tune and the IF stages so that any linear audio response could be achieved from the ratio detector. There were many challenges including the fact that the schematic was in German and was not quite the right diagram for this particular radio. However, I drew upon my considerable knowledge of antique radios to troubleshoot various circuits which were not working. The end result was stunning: a radio with rather fine tone and multi band reception. Here it is being demonstrated in my shop:

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Recently, I had a high and vacuum tube amplifier in here which suffered open bias resisters, causing the KT88 output tubes on one side of the amplifier to overheat badly. The manufacturer was several hours late supplying this schematic, so I trouble shot it and repaired it without this schematic. When the manufacturer sent it, it turned out to be the wrong drawing anyway. But the customer was happy just the same.

I’m having a great deal of fun as I repair a diverse collection of audio gear for an even more diverse customer base which is growing every day. I still have a long way to go before I am fully booked, but the shop has made excellent progress in the past two months toward reaching that goal.

Yeah, that M7 radio from Germany was quite a hoot. EL84 pair in the output stage, decent sized output transformer and 5 speakers make for an amazing sound for a 1950's radio. The oddity is that the FM band only covers 88-99Mhz during that era, so this radio has its limitations.

I've invested in somewhat overkill tools to do the jobs that I do, but that's my nature. In the prior 40 some-odd years of building, testing, troubleshooting and repairing audio and broadcast gear, I've relied upon just an oscilloscope and frequency counter and a 60+ year old Precision audio generator for my work. When some audiophile work came in last January and when a broadcaster friend cleaned out his cellar last February, I refurbished and sold a lot of broadcast gear and sunk the profits back into shop gear.

I consider it almost mandatory to analyze audio gear with both a distortion analyzer AND a spectrum analyzer. The SA tells me so much more about the character and sonic signature of an amplifier than just a %THD readout. And I made several important discoveries about how to tweak that sonic signature to reduce harshness and increase warmth of the vacuum tube amplifier's sound.

I have a number of repeat customers which come in about 1-2 times a month with various pieces of gear. One has a lot of Accuphase gear (I recently repaired a meter movement on a T105 that never worked because the manufacturer forgot to solder the lower spring to the armature, and got the meter working for the first time since he owned it), another brings in a few of his 80 or so guitar amps that he's collected over the years for me to refurbish and restore to full performance. Some of my customers bring in gear that they bought on eBay, for me to verify performance and do detailed cleanup and cosmetic restoration. I have one guitar amp in the shop this week that the customer is about to sell on eBay, so he wants to make sure that it is working properly and for me to clean up the pots.

At some point, I hope to max out at about 4-5 units per day, as I average about 2 hours per repair in most cases, with Japanese reel decks of the consumer level taking the longest and Fender guitar amps being the easiest and fastest to repair. I find the pro grade equipment easier to work on, simply because it is not crammed so tightly together and is not like a Chinese jigsaw puzzle. But that doesn't matter if a customer has a liking for a particular deck and is willing to pay the cost of a full restoration.

Many of my customers are in the medical, middle management and some are self-employed. You'd be amazed at the number of BMW 7 series and Toyota Priuses I've seen parked in my driveway in recent months. Many musicians have good 'day jobs' in well-paid professions. One is in middle management at IBM and is installing high end audio components in his office at work. He's bringing me an Accuphase P300 next week, for me to work my magic on, as I did with his P200 and T105 earlier.

Indeed, having a great time. And no commute, so I save gas and no longer waste 4-5 hours per day of my life sitting in traffic, as was the case with broadcast engineering. The closest radio station I have worked for in recent years was just over 90 miles from home!

Don, LOL! At least my clients have proven to be pretty knowledgeable and sporting a lot more common sense than that farmer!

Aren't guitar players similar to a segment of the audiophile population? Many of the same characteristics.
Tubes are a familiar world that I am completely comfortable with, as I've worked with them for half a century. I always get a relaxed feeling when a nice VT amp comes in. Kind of a warm and fuzzy feeling, if you know what I mean. Even the more oblique VT amp problems are solveable. I have a Marshall DSL401 in the shop now, whose problem was loud popping when operating the volume knob, plus a continuous crunching or static noise that increases as the amp warms up. The pot was also controlling the bias. Marshall volume control advances the bias toward less negative with higher settings of the control. DC flowing through the pot, when encountering any irregularity in resistance change means big audio noises. Changed the pot and fixed that, along with several screen resistors that were burnt, but the other noise remained. Turned out to be one of four EL84s was producing the noise itself. So I have a new set of JJ EL84s on order for it. The kid is gonna love this amp when I'm done with it. He liked the tone when it was working, but the tubes were pretty well worn.

I'm amazed where some of the referrals are coming from. A customer of mine recently told me that a recording studio in NYC recommended me. I never had any contact (that I know of) with that recording studio, but their client list reads like a Who's Who in the Music Biz. Another client told me that Sam Ash Music recommended me to him. It's amazing. One of the big advantages today is there is almost no competition. A major antique radio restoration business in CT closed shop and the owner retired last year. The business is for sale for about half a million dollars. Mostly tube inventory! So I'm about all that's left of the people who know how to restore antique audio here in the northeast. My only advertizing is word of mouth. We are in the "MP3 Generation"--today's kids aren't interested in electronics. They go into other fields, and none of them know what a tube is. Forty years ago, the last time I tried to run an amp repair business, there were scores of repair shops competing with me. I had a few customers, but not enough back then. Today, the odds are very much in my favor.

LOL, Don! Good one!

Pickups, tap points and how you phase them can completely change the sound you get. I remember toward the end of last summer, a musician brought in a guitar for me to rewire the mess that was left by another shop. He couldn't figure out what was wrong, but to my eye it was quite obvious. Within ten minutes I had the whole thing correctly rewired and he tried it into another customer's Fender Twin that was awaiting pickup, and was amazed by the sounds and VERY happy with the result. Even I was somewhat impressed with the variability of tones with just changing taps and flipping phase between two pickups.

I have been asked a number of times if I would consider designing my own line of guitar amps. I have some ideas for a new concept, based on a powerful, clean output stage and doing all the tone shaping on the low level stages. My intent is to design circuits with ample 2nd harmonic output. Just how to maximize that without introducing 3rd, 5th etc is where the engineering comes in.