the home of the turntable

Upgrading and servicing my Audio Research Classic 60

amplifiers, receivers and loudspeakers

Upgrading and servicing my Audio Research Classic 60

Postby abelb » 01 Apr 2012 10:35

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.

21116

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.

21117
21118
21119

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.

21120
21121

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.

21122

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.

21123
21124
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:

21125

Safety goggles on before flicking the power switch...

21126
Victory!

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.
Cheers,
User avatar
abelb
member
member
contributor
 
Posts: 163
Images: 21
Joined: 14 May 2009 04:22
Location: Melbourne

Re: Upgrading and servicing my Audio Research Classic 60

Postby Hanuman » 01 Apr 2012 15:52

The original tubes will certainly have been 6550s. Audio Research would have been using Svetlana in those days so all of the tubes were "ring-ins" when you bought it. I've got a couple of points to raise and this is timely because I recently started to look at the options available for my 2-year-old-plus KT-88s (x8 in each of two Antique Sound Lab Hurricanes) and while doing so the KT-120 had come up in the searches. I'm not contemplating '120s for myself but I think you should be aware that the KT-120 is not a drop-in replacement even for the KT-88 in all amplifiers and yours was built for the 6550. The other thing that concerns me a bit is that you didn't mention that you'd re-biased the tubes - if John Atkinson is right in the following quote the Classic 60 is not self-biasing:

"When the output tubes are replaced, the new tubes have to have their bias adjusted. As supplied, the 6550s carry a standing current of 65mA, resulting in "enriched" class-AB1 operation. This bias level is adjusted by an automatic circuit and is therefore independent of AC line conditions. A second DC servo circuit, again based on a high-quality op-amp chip, keeps the output tubes in DC balance."

(from http://www.stereophile.com/tubepoweramps/586/index.html)

I reckon you owe it to yourself to give Audio Research a call and ask one of the techs whether you need to do anything special to prepare the circuit for the KT-120.

By-the-way; back in the '90s "Audiophile" in North Fitzroy did some work for me on some M300s and, also, an SP9 pre-amp. I was surprised at their level of knowledge of a product they didn't sell.
“I don’t hold with furniture that talks.” - Titus Moody
Hanuman
senior member
senior member
 
Posts: 943
Images: 82
Joined: 12 Jun 2007 07:59
Location: Bangkok

Re: Upgrading and servicing my Audio Research Classic 60

Postby abelb » 04 Apr 2012 11:11

They're all good points, thanks. I originally wanted the JJ KT88's however the first repairer suggested the KT120 as better and longer lasting option. I agreed with his logic that basically if parts are over rated for the application they will last longer and perform better as they're not pushed to the limit. This is my reasoning for using the KT120's. When I get a chance I will call Audio Research anyway as I'm interested in their thoughts about this configuration and I'll post the results back here. I've re-biased the tubes to 62.9mv on the left channel and 65.7mv on the right. Having used the amp for about a week now since the service I'm happy to report that there are no issues thus far and it's still sounding great.
User avatar
abelb
member
member
contributor
 
Posts: 163
Images: 21
Joined: 14 May 2009 04:22
Location: Melbourne

Re: Upgrading and servicing my Audio Research Classic 60

Postby timbloke » 15 Apr 2012 18:01

Nice amp. I have an ARC D130 I hope to end up with one of their tube amplifiers someday. Hope you're got her back to spec and sounding as good as I'd suspect it does : )
timbloke
member
member
 
Posts: 73
Joined: 15 Nov 2011 21:42

Re: Upgrading and servicing my Audio Research Classic 60

Postby abelb » 17 Apr 2012 05:59

Thanks timbloke.
Getting her back to spec was the whole point of the exercise. There's always a niggling doubt that maybe Audio Research or a professional would have done something differently but I guess you get that with any older amp which required servicing, whoever serviced it. Anyway, it's sounding great and it's rock solid, it hasn't done any of the funny things it did before the service such as one channel cutting out and the fuse going.
The D130 is supposed to be a great amp from what I've read on the net. The Classic 60 is fantastic as well. I don't think you can go wrong with Audio Research.
Cheers,
User avatar
abelb
member
member
contributor
 
Posts: 163
Images: 21
Joined: 14 May 2009 04:22
Location: Melbourne

Re: Upgrading and servicing my Audio Research Classic 60

Postby jedinite24 » 25 Apr 2012 01:09

Hi Abelb

Great job with the restoration of the ARC 60. Very beautiful on the inside as well as the outside. I'd love to give it a listen but alas I'm quite a distance away from you. I'm curious to know what you find out from Audio Research regarding the Tung Sol KT-120s in your Classic 60 amp.

I found this while I was looking for info about the KT-120 in another forum.

Credit for this goes to A'gon member Hifigeek1. He posted this on another thread.

04-19-12: Hifigeek1

The following is a list of ARC amps that are authorized to use the KT-120:

VS55, VSi55, VS60, VSi60, VS110, VS115, REF110, VM220. I hope people find this useful as there seems to be a lot of confusion about it.
User avatar
jedinite24
member
member
 
Posts: 36
Joined: 27 May 2011 16:41
Location: Plainview NY

United States of America

Re: Upgrading and servicing my Audio Research Classic 60

Postby abelb » 26 Apr 2012 16:04

Hi Jedinite,
Well I finally got on to Audio Research and it seems that the KT120 is not recommended for my amp. Apparently the KT120 requires an additional 0.6AMPs of heater current per tube which "exceeds the rating of the power transformer" :oops: .
Apparently I've received misguided advice about the KT120, but at least I've got now got authoritative information on the use of KT120's in the Classic 60.
The power transformer does not get hot during use but I'd really be kicking myself if the transformer went. I may have to send my new KT120's off to eBay in search of new home.
Thanks for the compliments on the amp. It is a great looking unit and I may post another pic when I find a set of 6550's or KT88's for the thing.

Cheers,
User avatar
abelb
member
member
contributor
 
Posts: 163
Images: 21
Joined: 14 May 2009 04:22
Location: Melbourne

Re: Upgrading and servicing my Audio Research Classic 60

Postby Ampex53 » 28 Apr 2012 00:45

Nice detailed article ... Just remember with recommendations about which tubes suit which amps people, particularly techs working for the coy that makes the amp, will always err on the side of caution. Sure they draw extra current but if the Tx in the amp isn't getting hot & you aren't pushing it or likely to, then I wouldn't worry too much. Just keep an eye on it.
I've just scored an old ARC D-115 ... not sure if its original or MK 2 ... its at least 20-25yo so I'm looking to replace the PS HV caps & I like the look of the rifas? you have there ... 1,000uf @ 450vdc ... where did you get them & how much?

cheers .... cb
Ampex53
junior member
junior member
 
Posts: 8
Joined: 09 Aug 2011 09:20
Location: Tropical Nth QLD

Re: Upgrading and servicing my Audio Research Classic 60

Postby abelb » 28 Apr 2012 14:39

Hi cb,
Thanks for the advice. I've been thinking along the same lines, it's good to hear a parallel opinion. Very nice amp score by the way. The Rifa 1000uF 450vdc capacitors came from RS Components Australia - ordered from their website. They were on special a couple of months ago for $40 each and they may still be.
Cheers,
User avatar
abelb
member
member
contributor
 
Posts: 163
Images: 21
Joined: 14 May 2009 04:22
Location: Melbourne

Re: Upgrading and servicing my Audio Research Classic 60

Postby Ampex53 » 29 Apr 2012 08:55

OK thanks mate ... the D-115 gets posted/freighted from WA this week hopefully. I'm up in Cape York so it'll be at least a week & a half by Greyhound.
The guy's not really too cluey on id'ing tubes & he's taken all the pics from above!! so I'll have to wait till it gets here to figure if its a Mk1 or 2 ... for $500 I figure its gotta be a bit of a bargain.... he did say "....3 of the small tubes are definitely a bit taller than the 4 x 6922 ... " which is puzzling because if its a MK2 it'll have 3 x 6DJ8 types + 4 x 6FQ7?? so I'm wondering if someone's been running a 6DJ8 where a 6FQ7 should be?? I'm not sure on the pinout of the 6cg7 - there's 4 of them in my Stax energiser unit but I've never checked.
So are you happy with the Classic refurb?
I'm looking for an SP-8 preamp if you come across one ... just to keep the classic setup in the family so to speak!
cheers .... cb
$40 is a good price for RS .. I'll phone them 2 moro
Ampex53
junior member
junior member
 
Posts: 8
Joined: 09 Aug 2011 09:20
Location: Tropical Nth QLD

Re: Upgrading and servicing my Audio Research Classic 60

Postby Ampex53 » 02 May 2012 10:11

Well as luck would have it I found those Rifas on ebay for $8 each + postage from the UK ... not a bad score!

I've got a little more info from the seller but he's incognito for a few days so it'll be at least a fortnite before I see the amp. In the meantime I'm fairly certain its a MK 1 so I'm after a schematic & service manual. any clues on where to find one? I'm thinking seriously of ordering a quad x 2 of 6550 & leaning towards the Tung-Sol reissues. I used original Tung Sols 30 years ago in a Burnett-Elliot 100w/ch Australian power amp which began life with GEC & MO Valve KT88 ... I always preferred the 6550's. Later I subbed KT90's in the early 90's & I preferred them. Not sure if they are still available.
Any suggestions from users??

cb

cb
Ampex53
junior member
junior member
 
Posts: 8
Joined: 09 Aug 2011 09:20
Location: Tropical Nth QLD

Re: Upgrading and servicing my Audio Research Classic 60

Postby abelb » 02 May 2012 12:58

Hi cb,
You can get the schematic over on ARCDB http://www.arcdb.ws/D115/D115.html
You may be able to get the service manual from ARC if you're up for a 1:00AM phone call. Also, I have defintely seen new KT90 tubes around. I think they're under the Electro Harmonix brand.
Cheers,
User avatar
abelb
member
member
contributor
 
Posts: 163
Images: 21
Joined: 14 May 2009 04:22
Location: Melbourne

Re: Upgrading and servicing my Audio Research Classic 60

Postby Ampex53 » 02 May 2012 13:19

Hi Abelb .... yes I've seen it .... but its a MK 2 ... I'd like to know if the PS is different for the MK 1 ... I suppose it'll mean a phone call.

thanx ... cb
Ampex53
junior member
junior member
 
Posts: 8
Joined: 09 Aug 2011 09:20
Location: Tropical Nth QLD

Re: Upgrading and servicing my Audio Research Classic 60

Postby Ampex53 » 31 May 2012 10:09

G'day again .... well the ARC D-115 arrived from WA today - took about a month I think!! - and its definitely a MK 2 ... but badly packed so the rack mount handles are actually bent!! amazing that its in tact ... there doesn't appear to be any serious damage
... I've just ordered 8 x Mazda 6CG7/6FQ7 from Ned Carlson for the front end - these are short bulb jobs made by Brimar & raved about as the best 6FQ7 around ... I got an extra 4 for the Stax unit I have. I've used clear-tops etc & not really found that much difference between this tube type ... maybe the hype about these Mazdas will turn out to be true ... who knows.
So what I'm really trying to find out with this post is how the KT120's are working out for you . Is your filament winding still intact? I'm thinking seriously about them for the 115 ... it came with 8 x GEC KT88 which I haven't tested but they appear to be in good shape. I'm not a KT-88 fan & have always replaced them in favor of 6550 or KT90 ... but I'll try them after I clean everything up & test them on the tube tester.
This amp appears to have been in a cupboard for years ..... the topside of the PCB has a grimey dusty gunk over a lot of it ... the 0.01uf bypass wondercaps are a greyish colour from the dust .. so it all needs a good clean with rubbing alcohol or ssimilar.
Any opinions on the KT-120 let me know

cheers .... Ampex53
Ampex53
junior member
junior member
 
Posts: 8
Joined: 09 Aug 2011 09:20
Location: Tropical Nth QLD

Re: Upgrading and servicing my Audio Research Classic 60

Postby abelb » 02 Jun 2012 05:35

G'day Ampex,
I've been meaning to post back here to see if you'd received your amp yet. The Mazda tubes look like a pretty good deal, let me know what you think of them in your STAX preamp. Now that I've got about 30 hours on my tubes I'm planning to put the thing back on the bench to re-check the bias so I'll have a look at the tubes. I'm no tube expert so you'll have to tell me what to look for and where the "filament windings" are. I'll take a macro pic and put it up here. I'm enjoying the amp, no issues at all but that niggling doubt planted by the ARC guy is still knawing at me so I might try calling ARC back to see if I can chat to someone else who isn't so keen on getting me off the phone with a photocopy answer. Else I might still put the tubes on eBay for 50% of what I paid before they get too old and just get a set of =C= 6550's as originally used by ARC in this amp.
User avatar
abelb
member
member
contributor
 
Posts: 163
Images: 21
Joined: 14 May 2009 04:22
Location: Melbourne

Next

Return to Amplifiers and Loudspeakers