A Friends DIY For Me

[brad225]

This description and pictures will take me a while so I apologize for it dragging on for the whole story.

I have owned a pair of Audio Research 610t amps for 7 years. They were 10 years old when I purchased them from a dealer and were shipped directly from ARC after tube change and through testing.

They use 8 matched pairs of 6550 power tubes, 1 6550 regulator, 16H30 regulator amplifier, 2 6L6GC driver, 1 6N1P input and 1 6H30 follower. I shake my head every time I look at it written out.

During their manufacture life ARC offered an upgrade to KT120 tubes that were more stable as they ran at lower a voltage and increased the watts per amp to 840, not that I possibly need that. The upgrade for that is actually rather simple, assuming you know what you are doing.
In this case my EE friend Phil that worked in Bell Labs for years.

He has built and rebuilt every piece of equipment he has ever owned. Built numerous crossovers for speakers he has created. He is extremely particular about the quality of parts he will use and tests and discards parts that don't match with in very small tolerances.

This description is from notes I have made and I claim no knowledge of electronics. I will do my best to describe it properly.

Last summer I talked to him about doing the upgrade to KT120 tubes as 6550 are less stable in this amp and are getting harder to find. ARC could no longer provide enough matching pair that would meet their standards last time I wanted to re-tube one amp.
We have traded woodwork for electrical work and he said he would do it.

He had a speaker build project last summer and fall and I had spine surgery so that postponed moving the 2 180lbs beasts from my second floor listening room to his home. They have now been there for a couple weeks and he is making great progress.
The upgrade has been pushed to the end of the line as one repair and the decision to replace many old out of spec resistors, capacitors and relays along with some parts I will describe as I describe the process.

 

[Michaels HiFi]

This description and pictures will take me a while so I apologize for it dragging on for the whole story.

I have owned a pair of Audio Research 610t amps for 7 years. They were 10 years old when I purchased them from a dealer and were shipped directly from ARC after tube change and through testing.

They use 8 matched pairs of 6550 power tubes, 1 6550 regulator, 16H30 regulator amplifier, 2 6L6GC driver, 1 6N1P input and 1 6H30 follower. I shake my head every time I look at it written out.

During their manufacture life ARC offered an upgrade to KT120 tubes that were more stable as they ran at lower a voltage and increased the watts per amp to 840, not that I possibly need that. The upgrade for that is actually rather simple, assuming you know what you are doing.
In this case my EE friend Phil that worked in Bell Labs for years.

He has built and rebuilt every piece of equipment he has ever owned. Built numerous crossovers for speakers he has created. He is extremely particular about the quality of parts he will use and tests and discards parts that don't match with in very small tolerances.

This description is from notes I have made and I claim no knowledge of electronics. I will do my best to describe it properly.

Last summer I talked to him about doing the upgrade to KT120 tubes as 6550 are less stable in this amp and are getting harder to find. ARC could no longer provide enough matching pair that would meet their standards last time I wanted to re-tube one amp.
We have traded woodwork for electrical work and he said he would do it.

He had a speaker build project last summer and fall and I had spine surgery so that postponed moving the 2 180lbs beasts from my second floor listening room to his home. They have now been there for a couple weeks and he is making great progress.
The upgrade has been pushed to the end of the line as one repair and the decision to replace many old out of spec resistors, capacitors and relays along with some parts I will describe as I describe the process.

Sounds like a great project!

 

[jadedavid]

Phil rebuilt a pair of Clayton amps for me some years back.
He is a great guy and does class "A" work.
I'm sure you will be very pleased when they are completed.

 

[Mikado463]

Brad, it's good to have friends like that !! I wonder if he knows my friend Mark Obrien (Rogue Audio) ? For Mark worked at Bell Labs years ago.

 

[brad225]

Brad, it's good to have friends like that !! I wonder if he knows my friend Mark Obrien (Rogue Audio) ? For Mark worked at Bell Labs years ago.

Phil said he did know a Mark that was a programer. He looked at his picture and said he was sure Mark O was to young to be the same one.

 

[brad225]

So, the next thing was while waiting to move the amps I turned them on to warm up and more than likely went to the kitchen for a beverage to enjoy with the music.
When I returned to the room it smelled really hot. It is obvious as the room is well air conditioned when all the tubes are on.
I walked up to the amps and I could see that the power regulator tube on the middle shelf was runaway and extremely bright. I immediately turned the amp off and when I did the tube made the huge pop sound they make when they go bad. I shut off the other amp and shook my head thinking I guess Phil will have something else to look at.

A few weeks went by and I was off restriction from surgery and the amps were moved to his home for diagnosis and hopeful move forward with repairs and upgrades.

This is what the amps look like with the sides and top removed. There are parts on the bottom of both boards but I don't have pics of them at this time.

 

[brad225]

With the Power regulator tube going down the first thing he needed to do was find out why and did it do damage to anything else. He checked parts in the path the power followed and everything except the resistor directly at the tube was fine. It appeared the resistor went bad and allowed the tube to runaway at full power. He temporarily replaced that resistor and checked the value of EVERY resistor, diode and capacitor in the amp.

Phil had been able to acquire a complete set of schematics and parts list. originally used by ARC along with the upgrade schematics and parts list.

After determining that the other parts in the amp were fine to operate he put it in standby for 25 minutes then in operate position for 15. Everything was operating properly so he moved to the second amp and tested all parts as in the first.

This pic is really only exciting to Phil and myself when the repaired amp glowed happily.

That is it for know. 2 and 3 year old grand niece and nephew are coming to spend the night.

 

[brad225]

Once it was determined that the amp was functioning properly it was time to move ahead. After a few conversations about the amps I said that any parts he thought should be replaced and he was willing to undertake I was in favor of. I had know idea how much work I had created for Phil when I said that. There had to be moments he thought, what have I gotten myself into.

The first thing to strip all of the parts to be replaced on the top board relating to the power tubes and driver tube resistors. The top pic.

Middle pic shows parts removed and all holes cleaned to receive new resistors and relays.

Bottom pic shows all the new parts installed on the top board. All of the resistors he uses are noninductive type.

 

[brad225]

For the proper resistor pairing Phil first checks the value of each one of the 220 resistors being replaced. For some to achieve the proper .2 ohm value
2-.1 ohm resistors are soldered together and value checked again. He is able to match the values for each tube pairing for purposes of biasing the banks of tubes.
Virtually all of the pairings are exactly the same with the farthest difference being .001 ohm.

First pic shows testing of 64 of 220 single resistors and their value.

Second shows pairing and again the value so pairing can be completed as perfect as possible.

 

[Mikado463]

Wow Brad, looks like a 10, 000 mile service on a Ferrari !

 

[brad225]

Next is changing out the low voltage power supply capacitors. That is all the blue ones and then the soft start relays.

 

[brad225]

The first set of power supply capacitors on the lower level next to transformers. When he pulled the board to replace them he found a huge ground loop on the bottom of the board. He cut through the trace to eliminate it then put on the new capacitors.

It was interesting and surprising when Phil told me that the capacitors in the amps had a life span of only 3,000 hours before the begin to degrade. He said this is true for most equipment make as longer life are expensive. We tried to get 18,000 hour caps but I would have had to purchase 3,000 of them and so we ended up with an 8,000 hour capacitors for the 48 power supply capacitors needed for the 2 amps.

This was the board and the 8 power capacitors were replaced. These look to dusty to be the new ones.

 

[brad225]

The main power supply board that holds the other 16 capacitors had no ground loop.
Here is the board with old ones removed awaiting new capacitors.

That is where it stands right now. As the week goes on there will be more progress. Still waiting on some parts that hopefully will come early this week.

 

[brad225]

Phil rebuilt a pair of Clayton amps for me some years back.
He is a great guy and does class "A" work.
I'm sure you will be very pleased when they are completed.

I'm sure I will. We have done a few projects together. I'm doing 2 bass speaker towers for him after the first of the year. They will go with the panel speakers and subwoofer we did together.

 

[brad225]

Wow Brad, looks like a 10, 000 mile service on a Ferrari !

I can't imaging what that bill would be Dave. I guess if you can afford the Ferrari you don't have to ask.

 

[JohnJ]

Brad-

One heck of an interesting project.
I was not a double E major but what I find of particular interest is the discovered ground loop. Of late, a lot of aftermarket audiophile dingleberries are appearing for grounding purposes.

Was this ground loop a manufacturing defect or design flaw? Or something that developed over time?

John

 

[brad225]

Hi JohnJ
It was clearly a design and manufactured situation. Phil said cutting through the metal was a substantial endeavor.
He could not come up with any reason that it would have been put there.

 

[brad225]

Today was not real exciting but very satisfying. I went up to Phil's with numerous cleaning items to attack the interior while everything is discharged.

I set about first cleaning all 276 pin sockets in 46 tube sockets and 2 IEC connections with abrasive pipe cleaners and isopropyl alcohol. Though tedious it was clearly noticeable.

Once I was finished with that I used pieces of cloth with limited alcohol on them to wipe off 17 years of dust and crud. Long forceps made it possible to reach tight areas.

Around the tiny diodes and resistors on the mid shelf compressed air will be used to gently remove the dust that has accumulated.

 

[Mikado463]

Brad, you are clearly demonstrating to me why I went back to SS amplification !

 

[brad225]

Trust me I gave it some thought Dave, but, for what the upgrades cost, I don't think a SS exists that would sound as good. Though I have not heard it yet I'm confident it is going to be stunning. I can't wait.