Vintage Tube Amp

For sale is an Audio Research Dual 75A factory upgraded from a Dual 51 Chassis. This upgrade must have been done back in the 70s as the tubes are all US made, not USSR/Russia like in most ARC gear. The cost of these tubes alone would be likely half the total cost of the amplifier if you were to replace them.

Since the late 1950′s, engineers at Audio Research have been aware of these problems. To achieve low distortion under the dynamics of music, Audio Research has investigated many approaches to amplifier design, including the conventional bipolar transistor, FET, V-FET, Class D (switching), tube and various combinations of each.

For sale is one of the best, most musical vintage Audio Research amps. All electrolytic capacitors, all Zener diodes, all 715p orange drops coupling caps replaced (with 716p). GE 6cg7, Amperex 12AX7, German (GDR) 12AX7, Vintage, strong SED 6550 set. Adjusted to factory specs and sounds hot in every meaning of this word. Excellent condition and comes with original manual. I will pack it well and send world wide.

For sale is an Audio Research Dual 75A factory upgraded from a Dual 51 Chassis. Essentially, this unit has the transformers from the Dual 51 and the boards from a Dual 75A or possibly a D76. This upgrade must have been done back in the 70s as the tubes are all US made. There are four RCA 6550 power tubes and a single GE 6550 that is part of the circuit as well. These and the rest of the driver/preamp tubes are the originals. I don’t know for sure, but if I were to guess, I believe the owner had this unit upgraded by Audio Research and used it very little when he received it back. The boards do not have a burn spot or any amount of discoloration anywhere. When I received this amplifier, it had been sitting in a closet since it had last been used decades ago. Upon examination, the unit was indeed very dirty. So dirty that I had to completely disassemble it to clean it. The end result is a very clean amplifier. The only real issues are oxidation on the transformers, the screws, and the pots on the amplifier but the chassis is nearly perfect. The cover is in nice shape as well. The faceplate does have some minor nicks and scuffs, but is nice as well. This amplifier not perfect cosmetically, but it is nearly impossible to find an amplifier from this era with all of the original USA made tubes. Moving from the cosmetics, I didn’t simply disassemble the amplifier, clean it, and then put it back together. Many parts were replaced including all of the capacitors (with the exception of the main filter caps), numerous out of spec resistors, and diodes. Additionally, I also replaced the terminal strips (that are nearly always broken) with NOS terminal strips identical to the originals. These took many hours to find. All of the parts used were top quality including CDE capacitors, Vishay CMF resistors, and UF4007 low noise diodes. All of the hard work is done. All sockets, pots, and controls were cleaned. When I got to the point I thought the amplifier was ready to use, I went through the manual and followed the steps to set up the amplifier. Everything was reasonably within spec, thankfully and this amplifier sounds excellent. It is truly something you need to hear. I did not think I’d sell this, but it is time to move it on to someone who will use and enjoy this amplifier more than I have the time to. Price is not cheap I know, but I have 20-30 hours at least into this restoration and ARC would charge a fortune for the amount of work I did. Everything I did was as clean or cleaner than the original factory work. These amplifiers are not easy to work on. Terms (Please read these). Please message me if you have any gear for sale but don’t want to deal with listing. I live in Central California and travel to LA as well as the bay area often! You do not need to live near me. This rules comes from dealing with an individual who offered an attractive trade if I delivered the speakers to him. This ended up being a tremendous waste of time. I sell mostly vintage electronics. As such, these items are old and typically out of spec from when they were new. I do my best to test these items and make sure they work before they’re sent out, but I do not guarantee or represent that they will meet the original specifications. I may not even be able to test them, in which case assume that they don’t work. However, there are times where I may miss something. If that happens, I hope that you would agree that the most efficient way to take care of an issue is if you simply message me before opening a case or doing anything else. I need an outlet for these items online, so I have no reason to send you something that doesn’t work unless I’ve noted that it doesn’t. I pack items correctly. I know what it takes to get an item to you safely. You will NOT have to ask for it. I will do my best to get items out within a few days, however there are instances where that is not possible especially around holidays. The policies here are difficult in this regard because these vintage items in particular need to be moved and packed carefully. I do not want to damage anything by rushing. With this all being said, if there is any reason that you don’t feel that you can leave FIVE stars in every category, please reach out to me before leaving any feedback other than positive with all five stars in every category. Please message me to work this o. This item is in the category “Consumer Electronics\Vintage Electronics\Vintage Audio & Video\Vintage Amplifiers & Tube Amps”. The seller is “doublewalledbox” and is located in this country: US. This item can be shipped to United States, Canada, United Kingdom, Australia, Japan, South Korea, Indonesia, South Africa, Thailand, Bahamas, Israel, New Zealand, Singapore, Switzerland, Norway, Saudi Arabia, United Arab Emirates, Qatar, Bahrain, Malaysia, Brazil, Chile, Colombia, Panama, Jamaica, Barbados, Bangladesh, Bermuda, Brunei Darussalam, Bolivia, Ecuador, Egypt, French Guiana, Guernsey, Gibraltar, Guadeloupe, Iceland, Jersey, Jordan, Cambodia, Cayman Islands, Liechtenstein, Sri Lanka, Monaco, Macau, Martinique, Maldives, Nicaragua, Oman, Peru, Pakistan, Paraguay, Reunion, Vietnam, Kuwait, Denmark, Romania, Slovakia, Bulgaria, Czech Republic, Finland, Hungary, Latvia, Lithuania, Malta, Estonia, Greece, Portugal, Cyprus, Slovenia, Sweden, Belgium, France, Ireland, Netherlands, Poland, Spain, Italy, Germany, Austria, Republic of Croatia, Luxembourg.

• Model: Dual 75A
• Country/Region of Manufacture: United States
• Type: Tube Amplifier
• Brand: Audio Research
• Featured Refinements: Vintage Stereo Amplifier

This is a MK1 SP8. Contrary to popular belief is better than the MKII. Thoroughly Bench checked works perfect. Best phono section ever. Notice not too many MK1 units for sale. The Mk1 was more expensive to produce. More parts and labor. More power supply storage on the main rectifier.. This item is in the category “Consumer Electronics\TV, Video & Home Audio\Home Audio\Home Audio Components\Other Home Stereo Components”. The seller is “ctech-electronics-store” and is located in this country: US. This item can be shipped to United States, Canada, United Kingdom, Australia, Japan, Korea, South, Indonesia, South Africa, Hong Kong, Bahamas, Israel, New Zealand, Singapore, Switzerland, Norway, Saudi Arabia, United Arab Emirates, Qatar, Bahrain, Malaysia, Barbados, Bangladesh, Bermuda, Brunei Darussalam, Bolivia, Egypt, French Guiana, Guernsey, Gibraltar, Guadeloupe, Iceland, Jersey, Jordan, Cambodia, Cayman Islands, Liechtenstein, Sri Lanka, Monaco, Macau, Martinique, Maldives, Nicaragua, Oman, Pakistan, Paraguay, Reunion, Uruguay.

• Compatible Brand: Audio Research
• Type: Tube Pre-Amplifier
• Brand: Audio Research

SkyFi Audio – Curators of Vintage Audio Gear. No mid-fi, no gimmicks, no junk. Each piece of equipment is cosmetically restored and fully tested in our well-equipped lab by an engineer with 30+ years of experience in the HiFi world. Through our sister company, which is an authorized dealer for McIntosh, Audio Research, Sonus Faber, ClearAudio, Project, and Sony ES, we have access to OEM parts, service, and support. We are big fans of the VT100 variation which we have a hard time keeping in stock. This is its larger brother VT200 MKI at twice the power, but with the same design and build quality. This is a very powerful tube amp capable of driving difficult loads with the same neutral and clear sound that you would expect from Audio Research. It has facilities for balanced and single ended operation and offers speaker connections from 1 ohm to 16 ohms. Serviced in house and working beautifully. Little to no signs of use cosmetically. Includes a custom dust cover (for when the amp is not in use obviously), and the manual. SkyFi Service Notes. All driver tubes replaced. Front end adjustments and bias set to service manual specification. Tube Notes. All driver tubes replaced with matched balanced JJ E88CC. All output tubes are pre-owned matched quads of Svetlana 6550 ribbed plate (Not Winged C). The SkyFi Testing Process for Tube Amplifiers. We start with a visual inspection of all internal components to make sure that there are no signs of heat stress or damage. Capacitors are checked for telltale signs of predictive failure including bulging, shrunken wrappers, or physical leakage. We also inspect resistors and other passive components for signs of overheating. If tube arcing has occurred in the past we can usually spot discoloration on the output tube sockets. On vintage units we often spot check select capacitors for value and ESR. If the amplifier passes visual inspection, we move on to a full test of all of the tubes. We use an Amplitrex AT-1000 Tube Tester which is capable of testing both emission and Gm with a high degree of accuracy. We document the results of each tube and replace any weak or suspect tubes before proceeding. When we power on tube amplifiers for the first time we usually use a variac and current limited AC supply and slowly raise the voltage up to nominal mains level while monitoring plate, screen, filament, and negative bias supply voltages where applicable. If everything is in order we feed a low level test signal into the amplifier’s input and monitor its output on an oscilloscope across an 8 ohm dummy load. At this point we are just looking to verify basic function and confirm that the output transformers are not damaged. Once we have verified that the amplifier is safe to operate, we connected it to full mains power. For cathode biased amps we monitor the plate to cathode voltage to determine if the output tubes are operating in a safe range. Once the output section is verified we move onto bench evaluation. We start by feeding the input of the amplifier with a low level 1KHz test signal, slowly increase its amplitude while monitoring the amplifier’s output on an oscilloscope for signs of noise, clipping, distortion, or improper channel balance. We continue increasing the signal level until the amplifier reaches clipping. At this point we take an output power measurement and compare it to the spec sheet of the amplifier to verify proper performance. We finish off the bench evaluation with a 1KHz square wave check and a 20Hz to 20KHz sine sweep to assess the amplifier’s frequency response characteristics. This battery of tests will usually reveal if the amplifier has any issues that need further attention. Before the device leaves the bench, we perform a listening test with actual music using a variety of preferred test tracks. Our benches are outfitted with familiar monitor speakers which help us identify inconsistencies that will not always show up on our test gear. The main things that we are listening for are hum or noise with no signal present, proper center image, clicks, pops, or any other obvious undesirable audio characteristics. If the unit passes all of these tests it is moved to our long term testing rig where we simulate real word operating conditions for 6-8 hours. For tube amps we like to run this test at least twice. This allows us to monitor the unit for signs of thermal runaway or intermittent issues that only crop up when it has fully come up to temperature. We find this step to be essential, especially for vintage units. More from Audio Research: The VT200 vacuum tube stereo power amplifier is the most powerful of the three VT-series models. Rated at 200 watts per channel into a wide range of speaker loads, the VT200 encompasses a “dual mono” circuit layout reaching back to the massive single power transformer. Eight matched pair of 6650C output tubes and 674 joules of distributed energy storage give the VT200 unwavering control over the most difficult speaker loads, without sacrificing dynamic freedom or musical nuance. Separate left and right input and output circuit boards, with distributed power supplies, keep signal path lengths at a minimum for quicker transient attack and explosive musical dynamics. As with other VT models, constant-current sources in the input stage help maintain stable drive characteristics and differential balance of the output stage under the rapidly changing demands of actual music reproduction. Ultralinear output transformers with partial cathode coupling to the output terminals give the VT200 higher efficiency and lower output impedance than conventional vacuum-tube amplifiers. This results in better control of the loudspeaker throughout its frequency and impedance spectrum. Both single-ended (100K ohms) and balanced (200K ohms) inputs are included to allow use with any high-quality stereo preamplifier. Custom-machined, gold-over-brass five-way output terminals allow a secure, high-contact connection to a variety of speaker impedances: 1, 2, 4, 8 or 16 ohms. Internally, the VT200 presents a very open architecture within a spacious, rugged chassis. A single cooling fan operable at three different speeds provides unobstrusive cooling of interior components, including the vacuum tubes mounted along the chassis sidewalls. Biasing of the output tubes is easily accomplished via two trim pots for each channel, accessible through holes located along each sidewall of the chassis. Complete retubing kits of tested, selected and matched tubes for the VT200 are available through authorized Audio Research dealers, or from the factory. DETAILS and SPECIFICATIONS – MKI Tube stereo amplifier. 200 watts per channel continuous from 20Hz to 20kHz. 1kHz total harmonic distortion typically 1% at 200 watts, below. 01% at 1 watt. Approximate actual power available at “clipping”:210 watts (1kHz) Note that the actual RMS power output is dependent upon both line voltage and “condition” ie: if power line has high distortion, maximum power will be affected adversely, although from a listening standpoint this is not very critical. (-3dB points) 12Hz to 120kHz. (-3dB points at 1 watt) 0.5Hz to 200 kHz. 2.5V RMS (Bal or SE) for rated output. (27 dB gain into 16 ohms). 200K ohms Balanced, 100K ohms Single-ended. 1, 2, 4, 8, 16 ohms. Approximately 0.8dB 16 ohm load to open circuit. Less than 0.2mV RMS – 109dB below rated output (IHF weighted, input shorted, 16 ohm output). POWER SUPPLY ENERGY STORAGE. 105-125VAC 60Hz (210-250VAC 50Hz) 990 watts at rated output, 1200 watts maximum, 680 watts at “idle”. VT200: (8 matched pairs) 6550C power output, (6) 6922 driver, (4) 6922 input. 19″ (48.3 cm) W x 10½” (26.7 cm) H x 23.6 (60.0 cm) D. Handles extend 1½ (3.8 cm) forward. (46.5 kg) Net; 148 lbs. Yes – Power Only. We have too many items for sale and it is nearly impossible to keep track of offers outside of the official system. We are currently operating from our shop inside our dedicated building in Glen Rock, NJ. We’re not a store, so please don’t make an appointment if you’re just curious as to how a speaker or piece of equipment sounds. In essence – no tire kickers please. This is super useful when purchasing large speakers or heavy amplifiers. It’s also great for ensuring your new turntable is setup correctly in its final resting place. This item is in the category “Consumer Electronics\Vintage Electronics\Vintage Audio & Video\Vintage Amplifiers & Tube Amps”. The seller is “skyfiaudio” and is located in this country: US. This item can be shipped to United States.

• Brand: Audio Research
• Type: Tube Amplifier
• MPN: VT200 MKI
• Model: VT200 MKI
• Country/Region of Manufacture: United States

No mid-fi, no gimmicks, no junk. Each piece of equipment is cosmetically restored and fully tested in our well-equipped lab by an engineer with 30+ years of experience in the HiFi world. Through our sister company, which is an authorized dealer for McIntosh, Audio Research, Sonus Faber, ClearAudio, Project, and Sony ES, we have access to OEM parts, service, and support. The ARC 76A is a favorite among vintage audio collectors and tube rollers. This one is in what we call “survivor” condition, original and unmolested. We fitted a new set of tubes and did a light service to get it working nicely. Cosmetic condition is good with only a few visible signs of use, please look over our photos for more info. SkyFi Service Notes: This unit appeared to be all original on initial inspection. After testing all of the tubes it became clear that the output tubes needed replacement. We swapped out all of the 6550′s and biased them up to service manual spec. The input attenuators and jacks were also cleaned. After service, this amplifier tested clean above up to its rated 75W with ease. SkyFi Cosmetic Notes. The terminal strips exhibit the typical cracked barrier guards that we normally see on units of this age. Using period correct small spade connectors or spade to banana adapters is encouraged. Light marking under left handle of face plate. Light black paint marks on face plate. All 6CG7s are vintage GE, all testing well on our Modern and Professional Amplitrex AT1000 Tube Tester. 12AX7′s are a mix of GE and RCA, also tested well on our Amplitrex. The original GE 6550′s were at the end of their lifespan and were replaced with two matched sets of known good Svetlana Gold Label 6550 from our stock. All tubes biased to service manual spec. The 6550 in the power supply section was also replaced with a Svetlana tube. More from Audio Research. POWER OUTPUT 75 Watts per channel minimum RMS (both channels operating) into a 4, 8 or 16 ohm load from 20 Hz to 15 kHz with less than 1% total harmonic distortion. 1.5 volts for rated output. 80K ohms at 20 Hz; 30K ohms at 15 kHz. 4, 8, 16 ohms. 14 at 8 ohms (1 kHz). Intermodulation distortion: less than. 5% at rated power. More than 80 dB below rated output. 100, 120, 200, 220, 240 VAC; 50-60 Hz, 550 watts at rated power output. (4)12AX7, (8) 6FQ7, (2 matched pairs) 6550, (1) 6550. 19″ W x 7″ H x 12-1/4 D. The SkyFi Testing Process for Tube Amplifiers: We start with a visual inspection of all internal components to make sure there are no signs of stress or aging. Capacitors are checked for telltale sings of bulging or leaking, resistors are checked for signs of overheating or cracking, and tubes are inspected for signs of stress overheating or cracking. We then power up the unit carefully and run a simple 1k sine wave while monitoring the output on an oscilloscope for signs of distortion or noise. Finally we run the amplifier to its full rated power into the appropriate resistive load provided by our Sencore amplifier tester. If there is an issue, it will usually be noticeable at this point. If it all looks clean we will often test for THD (Total Harmonic Distortion) measured by our Keithley distortion meter, and compare it against the original manufacturers specifications. The amplifier the remains powered on to “burn in” on our test bench for a few hours to make sure there are no heat related issues. If there are balanced inputs available they will also be tested. We also test all tubes for output gain and leakage on our calibrated B&K 747 tester against the original manufacturers specifications. If needed, the tubes will be replaced from our vast NOS (New Old Stock) vintage or modern inventory. Final testing involves pushing actual music thorough the amplifier. We’ve learned over time that some issues are only noticeable to a trained ear while listening to familiar source material. Our test bench has reference vintage KEF speakers that we’re very familiar with which quickly reveal any discrepancies. Some amplifiers will then move into our listening room where they are tested with our in-house reference speakers of choice. We have too many items for sale and it’s nearly impossible to keep track of offers outside of the official system. We accept trade-ins of equal or lesser value under certain conditions. Please visit our website to submit your gear for full consideration. We will only consider high-end equipment from mainstream manufacturers such as McIntosh, Mark Levinson, Audio Research, Krell, etc. And will typically pass on displays, projectors, receivers, dated surround processors, and mid-fi equipment of any type. We will offer a fair value for your equipment, but keep in mind we’re running a business and will need to be able to resell the equipment at a profit after investing a great deal of time and resources in testing, certifying, repairing, cleaning, and packaging the incoming equipment. SELL US YOUR GEAR (Working or Broken). Simply visit our website to submit your gear for full consideration. We are currently operating from our shop inside our dedicated building in Glen Rock, NJ. We have several well-equipped and dedicated listening areas, plus a dedicated listening studio with tons of equipment of all makes and models. We’re not a store, so please don’t make an appointment if you’re just curious as to how a speaker or piece of equipment sounds. In essence – no tire kickers please. This is super useful when purchasing large speakers or heavy amplifiers. It’s also great for ensuring your new turntable is setup correctly in its final resting place. This item is in the category “Consumer Electronics\Vintage Electronics\Vintage Audio & Video\Vintage Amplifiers & Tube Amps”. The seller is “skyfiaudio” and is located in this country: US. This item can be shipped to United States, Canada.

• Brand: Audio Research
• Type: Tube Amplifier
• Model: D-76A
• MPN: D-76A

No mid-fi, no gimmicks, no junk. Each piece of equipment is cosmetically restored and fully tested in our well-equipped lab by an engineer with 30+ years of experience in the HiFi world. Through our sister company, which is an authorized dealer for McIntosh, Audio Research, Sonus Faber, ClearAudio, Project, and Sony ES, we have access to OEM parts, service, and support. Super cool and rare upgraded Dynakit ST-70 tube amplifier modified by Audio Research. What could be better than a well regarded ST-70 with a bit of love and magic from our favorite tube amp maker Audio Research. They made 3 versions of this unit over the years and this is the latest and greatest 3C version. Best of all it’s fitted with vintage GE tubes that tested really well on our modern and professional tube tester. Not sure how many pieces were actually modified, but I have not found many examples of this amp online. So it’s relatively rare. This one looks great and sounds amazing. It was tested in our lab and listening room and is working perfectly. Test Notes: Uses (3) 12AX7, (4) 6CG7, and (4) 6L6GC. 40W into 8 ohms with both channels driven. Set Bias to. 410V across 7.5ohm resistors as suggested in schematic. We have too many items for sale and it’s nearly impossible to keep track of offers outside of the official system. We accept trade-ins of equal or lesser value under certain conditions. Please visit our website to submit your gear for full consideration. We will only consider high-end equipment from mainstream manufacturers such as McIntosh, Mark Levinson, Audio Research, Krell, etc. And will typically pass on displays, projectors, receivers, dated surround processors, and mid-fi equipment of any type. We will offer a fair value for your equipment, but keep in mind we’re running a business and will need to be able to resell the equipment at a profit after investing a great deal of time and resources in testing, certifying, repairing, cleaning, and packaging the incoming equipment. SELL US YOUR GEAR (Working or Broken). Simply visit our website to submit your gear for full consideration. We are currently operating from our shop inside our dedicated building in Glen Rock, NJ. We have several well-equipped and dedicated listening areas, plus a dedicated listening studio with tons of equipment of all makes and models. We’re not a store, so please don’t make an appointment if you’re just curious as to how a speaker or piece of equipment sounds. In essence – no tire kickers please. This is super useful when purchasing large speakers or heavy amplifiers. It’s also great for ensuring your new turntable is setup correctly in its final resting place. Yes – Power Only. This item is in the category “Consumer Electronics\Vintage Electronics\Vintage Audio & Video\Vintage Amplifiers & Tube Amps”. The seller is “skyfiaudio” and is located in this country: US. This item can be shipped to United States, Canada.

• Brand: Audio Research
• Type: Tube Amplifier
• MPN: ST-70
• Model: ST-70
• Country/Region of Manufacture: United States

The front has some minor marks, mainly on the left handle, a little in the center, and some on the bottom right corner. The sides have a couple marks as well. Overall still looks very good, and no dents or damage. Please use zoom to examine all areas closely and let us know if you have any questions. The amp has been thoroughly inspected and tested by our vintage audio technician and everything works well. Sound is clear, powerful, and evenly balanced. We listened for hours and did not detect any hums, buzz, or crackles. All tubes and buttons function properly. Thank you for viewing! This item is in the category “Consumer Electronics\Vintage Electronics\Vintage Audio & Video\Vintage Amplifiers & Tube Amps”. The seller is “quyenvintage” and is located in this country: US. This item can be shipped to United States.

• Model: Classic 30
• Country/Region of Manufacture: United States
• Featured Refinements: Vintage Stereo Amplifier
• Type: Tube Amplifier
• Brand: Audio Research

This Amp is Model Dual 51 and it is a Tube Amp. We acquired this beauty from a friend of ours Estate Sale who had some very nice stuff. We took it to a professional and had it tested out and there was one tube that needed to be replaced, so it was replaced. It is in WORKING order and works just PERFECT. There is minimal wear on this amp, just a couple light scuffs, but very minimal. It looks as though this item was hardly used… Overall it is in EXCELLENT condition! I’m not sure how this will be boxed yet, but I will talk to the professional again and find out what he suggests. The item “VINTAGE AUDIO RESEARCH MODEL DUAL 51 HIGH DEF AMP AMPLIFIER TESTED WORKS” is in sale since Sunday, December 5, 2021. This item is in the category “Consumer Electronics\Vintage Electronics\Vintage Audio & Video\Vintage Amplifiers & Tube Amps”. The seller is “virgilsgeneralstore” and is located in Valparaiso, Nebraska. This item can be shipped worldwide.

• Brand: Audio Research
• Type: Tube Amplifier
• Model: DUAL 51