I bought this amp about three months ago, it was reported to have been recently re-tubed and serviced. The service notes say only that a 100ohm resistor was replaced and that the 6CG7 driver tubes were “tired”. The amp worked perfectly and sounded sweet, although a look under the hood revealed that two electrolytic's were bulging, and that only two of the 8 power tubes were replaced.
The tubes were also replaced with a different type leaving a configuration of 4 6550 tubes on one channel and two 6550's plus two KT88's on the other. It appears as though the 6550's may have been the original tubes as they're branded Phillips ECG which aren't available anymore, although it's possible they were actually replaced with NOS, or other used tubes.
I made an appointment with a specialist amp repairer here in Melbourne for a complete service and re-tube but he missed the appointment and didn't call me back. I decided to buy the parts myself and take the amp to a friend's dad who is an electrical engineer but as luck would have it he was checked in to hospital on the day I was supposed to take the amp down (he's fine). With little options left at the time and a desire to do this myself I decided to dive in.Purchases:
*8 new TungSol KT120 power tubes to replace the old 6550's and KT88's.
*4 new Electro Harmonix 6CG7 Gold driver tubes
*4 new Evox Rifa 1000uF 450V filter caps to replace the original 800uF caps
*1 new Evox Rifa 470uF 100V 105c to replace the bulging Nichicon 470uF 100v cap.
*1 new Evox Rifa 680uF 60V 105c to replace the bulging Marcon 680uF 35v 105c cap.
In the mean time the amp had also blown a fuse. A check of the schematic revealed that fortunately, it was only due to the wrong fuses in use. A 2AMP fuse where a 4AMP should have been and another 2AMP where a 250milliamp should have been. Both fuses were replaced with new units of the correct values.
The next step was replacing the faulty electrolytics. I removed all the tubes and the four large filter caps (drained caps but wore gloves out of paranoia anyway). Caps desoldered, and then the area was cleaned with a solder pump, solder braid, flux and ethanol.
I put a Rifa 470uF 100V 105c rated cap in where the old Nichicon 470uF 100V cap had been. The Nichicon wasn't labeled as 105c but given the position of the cap right next to the power tubes I figured a bit more head room couldn't hurt. A new Rifa 680uF 60V 105c cap replaced the Marcon 680uF 35V 105c cap on the base of the unit. The new caps are both low ESR.Replacing the large filter caps.
One thing that struck me about the Audio Research amp is the quality and rating of all the components. The build quality and component selection in these amps is really amazing, I would love to own a new unit one day. The old 800uF filter caps all measured at between 760uF and 761uF on my capacitance meter. The new 1000uF caps measured between 860uF and 916uF, all within the 20% tolerance but obviously not precisely matched like the old units were. Does anyone have any thoughts on the effect of matching the caps here? All the caps appeared to be wired to common posts.
The new caps have the exact same dimensions as the old ones so installation was straight forward.
The new 470uF Rifa cap is the silver capacitor on the lower right.New tubes:
The Tungsol power tubes come matched from the factory. I have two matched quads; four 98 / 9100 and four 98 / 8900
I put two + two on each side.
With the new driver tubes:
Safety goggles on before flicking the power switch...
The top cover fits over the KT120's with a couple of mm to spare, but it is a little cramped.Impressions.
First impressions were a bit of a worry. The left channel was extremely quiet and fuzzy sounding so I took the record off and let the amp warm up for another 15 minutes. Bingo, problem gone.
The first thing I noticed is that the amp sounded a bit clinical, perhaps I had grown accustomed to the distortion produced by the 20 yo tubes? Things improved the longer I used the unit.. What really struck me was the depth this amp could now reach to pull out clear well defined bass notes. I put this down to the new KT120's which are apparently known for that.
The mid range and top end are sweet and full. I'm very happy with the results overall and basking in the warm glow of having done it myself.