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AR Amplifier bias pot replacements


soundminded

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I think the author's recommendation of 100ma quiescent current is too high for the AR amp. If you try this current value, make sure that the idling heatsink temperature is only moderately warm. The AR amp does not have a very sophisticated bias vs temperature control system and the amp could run away thermally, especially if played loudly into low impedance loads. 50ma seems more reasonable. What complicates the biasing procedure is the interaction of the upper and lower halves of the push-pull output section idling current with the output dc offset voltage. Slight tweaking of one or the other half will be necessary to maintain minimum offset while not excessively increasing the idle current.

Unfortunately, AR did not specify a nominal idle current in their manual. Their specification of 12 or 13 ohm fixed resistors which are then trimmed puts the idle current into the right range. I still think that this is the safest approach to take.

Bob

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How about this. Turn the Amplifier off. Mark the pot settings with a pencil. Dowse each pot with cleaner such as Deoxit (I, hahaha still have some freon) working them back and forth, set the pots back where they were, let dry for a day or two, turn the amplifier on with the volume control at zero, and adjust the pots until the DC voltage from each speaker hot to ground is as close to zero volts as you can get it. If there is a range of adjustment, leave it midway between the points where the DC offset begins on either side. Any comments?

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I'm back;

AR bypassed the bias pots because they either were, faulty from manufacturing, wondered off specs, poorly designed, unsuitable for this particular application, not intednded for the chassis mounting, or other reason.

If they wanted to change to a different supplier, no big deal.

They could have easily re-routed wiring or parts layout, if a pot was the best long term option for the bias pots.

I believe they investigated the most solidly available, long term option.

Remembering the length of their warantee period, freight, parts and labour included.

Because of the lack of AR supplied technical data, at this time, I would suggest this topic go no further than the search for the missing AR bias pot bypass sheets.

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I don't disagree, Vern but I've had no luck finding this info and no one else seems to have copies - I've offered to pay for them but if anyone has/had them, apparently they don't anymore or prefer not to share them So I don't know what else to do but start from scratch.

The article I referenced specifically described putting in good quality pots to replace original pots which, it seems to me is a good functional option BUT it would make the amp completely non original which is something I was trying to avoid.

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Can't argue with that either! But there is no COMPLETE data for the mod so it seems to me that if you want to correct the problem with a "potted" ;) AR amp, there's no choice but to figure it out yourself. The service manual - and I have all the versions available, including free and "paid-for" versions, are exactly the same and do not, as you mentioned, explain how to actually perform the mod. They partially explain it but leave info out that is necessary - AT LEAST as I read and understand it.

Given that the mod data is not available, the only option that I can see is to either get the original pots to work well OR, replace them re the article I posted. Even with that (replacement or oem pots working), there are no specific AR specs for bias/offset (though everything I can find re amps in genera is that offset should be zero or as close to it as possible).

I don't mean to be difficult or to keep beating this to death but since the AR pot mod doesn't seem to be available at all, what other choice is there? As I stated several times, I would be happy to PAY someone for the complete mod data that you mentioned, is "cloaked" on this site. But it doesn't appear that it is becoming "uncloaked."

Couldn't the person that originally posted it just post it again or send it to me via email for a Paypal fee?

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Mike,

What the mod does is to substitute the fixed/trim resistor combination for the pot. The terminal strips are a convenient place to mount the resistors. You could use one terminal of the pot (with its original lead attached) as a mount for one end of the resistor combination and connect the lead that was attached to the other pot terminal to the free end of the resistor combination. This would not be as mechanically secure as a terminal strip, but should be ok if you do not plan on shipping the amp. This has the advantage that you do not need to perform any drilling/removing.

Just a thought.

Bob

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  • 2 weeks later...

After stating that my AR amp runs very hot but hasn't shut itself down, it has made a liar out of me. Yesterday, after running for a couple of hours it did, for the first time, shut down due to overheating.

So I will be opening it up and trying to sort out the bias/offset with the oem pots in place for the moment. I received some very helpful advice via email from several folks here who are interested in helping me sort this out - THANKS! One possibility includes replacing the oem pots with current, better quality pots. Since the missing info on how to perform the AR fixed resistor mod does not appear to be "findable," the AR factory-type mod is off the table for now. It will probably be several weeks before I have the opportunity to mess with the amp but whatever happens, for good or ill, I'll let everyone know.

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After stating that my AR amp runs very hot but hasn't shut itself down, it has made a liar out of me. Yesterday, after running for a couple of hours it did, for the first time, shut down due to overheating.

So I will be opening it up and trying to sort out the bias/offset with the oem pots in place for the moment. I received some very helpful advice via email from several folks here who are interested in helping me sort this out - THANKS! One possibility includes replacing the oem pots with current, better quality pots. Since the missing info on how to perform the AR fixed resistor mod does not appear to be "findable," the AR factory-type mod is off the table for no

w. It will probably be several weeks before I have the opportunity to mess with the amp but whatever happens, for good or ill, I'll let everyone know.

At least 2 or more pages have been deleted or just lost from this website of the bias mod, vapourware.

I lost track of the number of my write-ups, it appears that some are missing.

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At least 2 or more pages have been deleted or just lost from this website of the bias mod, vapourware.

I lost track of the number of my write-ups, it appears that some are missing.

Hi Vern,

There shouldn't be any forum posts missing. I just found my first post back in 2000. If you (or others reading this thread) can give me more information about a particular post (or entire thread) you think is missing I'll look for it first here in the current forum and then in my backup.

Is it possible the bias mod was in the Library and not in the discussion forums? If so, all Library material is on the way back, I just can't do it overnight. ;)

Mark

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  • 3 weeks later...

Well, the amp is now operating well and quite cooly (if that's a word) with it's original pots. I used all the knowledge exhibited by folks here, as well as a few email suggestions I received to sort out the bias/offset settings with the oem pots. I also had to do some refresher math so I could figure current based on voltage readings/resistors.

Here's what I did:

Turned amp on

measured the existing offset voltage at the speaker terminals and the voltage across the emitter resistors. The results were:

L speaker 190.3 mv R speaker 124.4 mv

L emitters showed 85MV for a current of 257 MA (.33 Ohm resistor) R emitters showed 51 MV for a current of 154 MA All voltages were constantly varying by 10-25 MV

Turned the amp off

Obvious first step was to flush the 4 pots with LOTS of cleaner, accompanied by copious turning of the controls back and forth and back and forth and back and forth etc, etc ,etc and set them to their approximate middle position.

Turned amp on

I then discovered that the two pots on each channel interact so that it was difficult to get a decent bias current AND a good offset at the same time just by twiddling the pots. I could get a decent emmiter voltage but a terrible offset or visa versa. Remember, I am NOT a circuit guy - I was doing this based on what I read and (what I perceived to be) common sense! :rolleyes: These interacting pots were unlike the adjustments I made on my Moscode 600 which has a completely separate and independent Bias Pot and an Offset pot for each channel; You set the bias to the factory setting, then adjust the offset to as close to zero as you can. Adjusting the offset pot does NOT affect the bias. I was assuming that the AR amp would be the same - one pot in each channel was for bias and the other for offset. BUT It APPEARS to me that the AR amp does not have an offset pot at all - the two bias pots in each channel interact and the bias setting also determines the offset setting.

Turned amp off

So, in thinking about this, I decided to measure the actual resistance range of the pots which, after cleaning was essentially their rated 50 Ohms. Based on reading in the AR manual that PART of the mod included replacing the pots with 12 or 13 Ohm resistors, I set all 4 pots at 12 Ohms, turned on the amp and measured the results.

VOILA! After warm up the emitter voltages were 20mv L and 18mv R Offset was 16.5MV L and 15 MV R. Not too shabby! Based on a suggestion in an earlier post here, I thought about dropping the emitter voltage to around 12mv but I decided to leave it as it was and check how warm the heat sinks became. There is no variation in the readings now, unlike before pot cleaning.

After running for two hours at mild listening levels (AR3As) the heat sinks are not noticeably above room temperature. Previously, they were too hot to touch in 30 minutes with NO signal.

AGAIN, I want to emphasize that I don't know anything about amplifier circuits. What I have done seems to work fine and the amp sounds very good to me for what it is. I'm especially pleased that it is now working in it's ORIGINAL configuration, with no mod required. Of course the audio experts here can probably point out that the amp may not sound as good as it should based on what may be my faulty procedures, misunderstandings, and assumptions.

But in any case, I'm a Happy Guy!

Thanks again for all the input on the site and the educational emails I received on this subject.

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In the middle of the night I woke up with a (possible) revelation that the bias mod was actually staring me in the face (though I'm not going to do it since the pot cleaning seems to have worked so well).

The incomplete instructions in the AR service manual (all versions that I have been able to find) mention removing the pots, installing terminal strips to accomodate a 12 or 13 Ohm resistor and the "...shunt resistors which will be attached later." But there is no further mention of the "shunt resistors" or connecting them. So how could one perform the mod without knowing what these resistors are and where they attach. There is no further info on the settings or what readings should be obtained so I don't see how to calculate any of the resistance needed to obtain settings that aren't specified! :rolleyes:

AGAIN remember - I don't understand circuitry - those of you that do probably know or can figure out what it means/where it goes.

But maybe it was literally right in front of me. Below are (I hope) two pics. One of the amp innards and the other a close-up of the pots in the left channel. All the pots have a fixed resistor across the pot's posts. It occurs to me that since the pots are now set at 12 ohms - the value of the the resistor that is supposed to replace them, that existing resistor on the pot IS the shunt resistor that is referred to in the AR manual.

IF my ASSUMPTION (yes, I know what they say about assumptions - I used to say it myself) is correct, the bias mod simply consists of replacing the pot with the 12-13 Ohm resistor and connecting the EXISTING resistor (or new one of the same value) in parallel. This is essentially what I did by setting the pots to 12 Ohms.

You circuit experts can, of course, explain why I have this all wrong but hopefully what I have done may help others by, at a minimum, setting an example of how NOT to do this! :)

Oh, and a quick question - the four .33 ohm resistors shown just south of the Heat Sinks each has a diode on top that is supposed to be attached to the resistor. The left channel diodes are attached firmly with a dab of the blue material that looks/feels like very hard glue. The right ones have popped loose from the "glue." I have pushed them so they are touching the resistors but obviously they need to be firmly attached. What is that blue material or what would be suitable to re-adhere those diodes so they will remain in firm contact with the resistor? Thanks again!

DSC_34761.jpg

DSC_34751.jpg

DSC_1687.jpg

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OK, I know you guys are probably tired of me on this thread but now, after another few hours of messing around and further adjustment, I have formulated what I believe (as a total non-knowledgeable person) to be a good way to set up the bias on the POT-equipped AR amp assuming the pots are now clean.

Start with either channel, set the voltage at the outboard Emitter resistor (the pot closest to the front of the chassis) to 12.5mv. Then measure the offset at the same channel's speaker terminal. Adjust the offset to as close to zero as possible using the OTHER pot on the same channel (the one toward the rear of the chassis. Do the same with the other channel. That's it.

Trying to set the voltage at each emitter resistor takes a lot longer because they interact. Adjusting the offset AFTER setting the voltage at ONE of the channel's emitter resistors is much faster and provides the same result. Since they interact, the voltage at each emitter will be about the same but will be DIFFERENT than the 12.5MV originally set. I found that (at least on my amp) the voltage to the emittters drops, ending up around 10 - 10.5mv which is the range that Bob suggested in an earlier post.

One thing I have found is that, as has been often stated, the oem pots are pretty bad. Even after a LOT of cleaning they still tend to jump around a lot as you turn them- a tiny movement can produce a LARGE reaction. Perhaps even MORE cleaning will improve their action. But newer high quality pots would be a better deal if you want pots. I have to admit that I believe that good pots will allow you to get better and more equal channel balancing than fixed resistors. I'm not sure that the oem pots will necesarily do that no matter how clean they are.

I have been running the amp through the AR3as for several hours now pretty LOUD (currently playing Warren Zevon's "Transverse City." The heat sinks are quite warm but NOT hot to the touch. As I said in my first post in this series, the heat sinks were formerly to hot to touch after 30 minutes with NO signal.

OK, I won't write any more about this.......probably! :)

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Hi Mike,

I've never been involved with an AR amplifier, so I personally cannot offer insight, but I did speak to Larry Lagace ("Vintage AR") on your behalf recently. He seems quite familiar with the fixed resistor mod, and has performed it a number of times.

Larry said that the over-heating problem is simply caused by oxidation/corrosion of the pots (as in the speakers) and that the fixed resistor mod was designed to eliminate that flaw in the design. If you keep the original pots you will continually be dealing with the overheating issue. He said he would be happy to assist you if you contact him.

Roy

PS How did the midrange driver work out?

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Hi Roy,

The midrange driver was PERFECT! Both speakers now sound identical to my ears - and as measured with a few test tones and an SPL meter.

My "mostly AR" vintage system is now totally operational!

AR amp (first verstion w/pots),

AR XA TT,

AR3a (Sep 1968 per date stamp on woofers)

and the non AR member of the team - an Ampex F44 Reel to Reel tape deck.

I owned the above AR components back when they were new and being produced though I had sold them all by the early 70's. I have only acquired their "replacements" fairly recently - the speakers and TT a little over a year ago, the amp about 6 months ago. I have owned the F44 since it was new in 1965. It had new oem heads and a "tune up" about 5 years ago.

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  • 6 months later...
Has anyone got the link for the replacement of the bias control pots in an AR amplifier with fixed resistors? I just can't seem to find it. I think there were at least two versions depending on serial number. Thanks in advance.

See this post for the requested documentation:

http://www.classicspeakerpages.net/IP.Boar...?showtopic=4554

Robert_S

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