AR Classic 60 Bias Confusion

You're certain your not thinking of the mention of setting the bias point at 65mv so that the tubes are driven at 32 watts of a potential 48?

It has been a long time since biasing this beautiful amp back in mid 90’s. As i remember it should read an average 65mv, leave the unit at idle for half an hour with no signal before attempting bias. Line voltage should be around 120v.

32.5mv x 2 =65mv. Logic suggests to me that each tube is biased for 32.5 mv but perhaps with this amp you must bias two tubes at the same time, i.e. the setting would be for 2 tubes, 65mv.. I don’t know about your amp specifically but, for example I had an amp which had this set up - you had two tubes (on each side) and the bias was set with one circuit on each side. They even had an amp with 6 tubes on each side and only one bias circuit for each side. Made for getting matched tubes essential.

@perryg reading over the manual a bit, appears to be written more for a tech who's familiar with the basic procedure. Check this too - here is an older post on Audiogon worth browsing. There are varying references to different measurement values of 65ma, 65mv, and .65vdc used interchangeably - pay close attention to that, use of alligator clips, and a few other comments about how to do this. Some of the replies down on the first page are a bit more informative and helpful fwiw. Something else for reference, be sure to note the 2-3hr waiting time to let the amp reach full operation on your very last bias check before closing up.

Adjust the "bias" for an average reading of 65 mVDC (0.0650 volt DC) between TP1 - TP2 and between TP3 - TP4.

Do not make any other adjustments of pots in the amp.

For power tube bias adjustments go to pages 7 and 8.

https://www.arcdb.ws/Database/CL60/ARC_CL60_manual.pdf

The tech most likely got it right with a bias of 32.5ma.

Based on the manual, there is a plate voltage of 420V and a bias of -46 V. If the output transformer is between 4 and 6k ohms, then 65 ma is the correct bias for the 6550 tube as it operates at 65% dissipation in Class A/B (or "enriched A/B" as ARC calls it). But that is a 42 watt tube.

If using the 6550C, which is rated at 35 watts max dissipation, a bias of 65ma will take you into Class A operation (80% dissipation) territory and definitely not a recommended thing to do unless the amplifier is designed from the ground up to operate in Class A/A1. By using a bias of 32.5ma at 420V plate and -46V bias, that is "cold" Class A/B operation instead of "enriched" Class A/B the CL-60 was designed for.

Don’t crank up the bias until you hear from ARC

Many thanks for all of the input.

I will look at the old forum post. I’m comfortable and careful working around high voltages but sadly my knowledge of amplifier/tube function is essentially nil. I know enough to know where to test and how, but beyond that I’m a bit lost. I have enjoyed listening to it which I can do:)

It doesn’t seem as though there should be rocket science here but the bias in my manual is clearly different than the one online.

The amp has two pairs of matched quads. I can’t recall if the Classic 30 has two matched pairs. Maybe my adjustment page should be in a Classic 30 manual?

I just know that the Classic 60 manual in the ARDB says to set the bias to 65mv.

I’m curious to see what the reply from AR service will say.

In the mean time I’ll leave the bias as it is and read back in the forum.

Thanks again for all of the info.

@perryg,

I found this video for the ARC Classic 60.

For the bias adjustment go to time marker 12:20.

Note, when the tech first starts taking about where he has the meter probes attached on the left side of the amp he say points 2 & 4. I pretty sure he meant to say 3 & 4. When he moves to the right side of the amp he says points 1 & 2.

Hi jea48,

I know that same video and have been in touch with Mitch who was the tech who made that youtube post.

He does say 65mv in that clip. I also noticed the labeling of the test points but the locations of the points for each side were really what I wanted to see and he illustrated that well. I figured out the TP numbers on my own.

I suppose I could have checked the Classic 30 manual on the ARCDB website earlier. It lists a bias setting of 32.5mv. 

My guess is the amp was biased to 65mv when it came from the factory. Sounded great for many years. When I changed the tubes the bias was set to 32.5 as per page 8 in my manual but I think my page 8 actually was from the Classic 30 manual. The amp hadn’t sounded very good since the retube.

I have to leave town for 5 days but will set the bias to 65 when I get home. Looking forward to hearing how it sounds.

Will keep you guys posted.

@perryg

I hope you have at least a pair of something like these.

You’ll need something insulated to connect the DMM two probe leads to the two bias points on the circuit board for channel of the amp. You’ll need the meter around the other side so you will be able the read the display as you adjust the blue trim pot.

I thought my ARC VT50 amps where a PITA to bias the power tubes. The Classic 60 looks worse. Older ARC amps were not user friendly when it came to biasing the power tubes.

At least you don’t have to bias the input and driver tubes for each channel like I do on the VT50. That can hours to do...

Ok folks,

Here is the response straight from the hoARse’s mouth. (Sorry for the bad pun).

 audio research HIGH DEFINITION®
5740 GREEN CIRCLE DRIVE / MINNETONKA, MINNESOTA55343-4424 / PHONE 612/939-0600 FAX612/939-0604
CL-60 /V-70 BIASING INSTRUCTIONS
NOTE: This supplement is not intended to replace the owner's manual instructions included with these models. Please familiarize yourself with that information before attempting the following.
1 . Ensure unit is turned off and unplugged.
2. Remove tube cage top on CL60 or V70. Unplug fan cord to remove cage.
3. If replacing tubes, remove old tubes. Note any tube that looks different from the others. Forexample, a tube with a whitish crust inside the glass envelope (which should appear clear with silver spots) may have lost vacuum. Or a tube that hasa brownish cast inside the envelope may have overheated. These clues should be noted in case the amplifier does not function properly when retubed, as component damage may have occured due to a tube failure.
4.
If the amplifier had a tube failure evidenced by an arcing (flashing) tube
or has blown either fuse, you should inspect the 100 ohm screenresistors
f o r damage.
These act as a "fuse" in the event of severe output tube failure and may be burned, cracked apart or low in value. The 100 ohm resistor is
a brown 9/16" long resistor with brown, black, brown, gold identifying bands, These will be mounted on the top of the circuit board near each output tube socket on the CL60, or on the underside of the circuit board beneatheach output tube socket on the V70. Use ARC #43100200 (2W 5% wire wound) replacements. Refer to illustration A.
5.
Should yoube installing Russian 6550si n t o a CL60 which previously used
either Philipso rChinese 6550so rKT88's, no change is necessary. See Step. 6. I fyou are replacing KT90 or KT91 tubes ina V70 withRussian 6550's, you will likely need to change bias trim resistor R97 (49.9K ohm) to a higher value
such as 76.8K ohm or 100K ohm in order to properly bias the tubes within
the range allowed by the bias pot. Refer to illustration B for part location.
6. Install tubes. Note that matched pairs should be installed as follows: V12 and V8; V10 and V6; V7 and V11; V5 and V9.
7. Locate black .5 ohm resistors at front edge of circuit board. Using a digital volt meter capable of accuracy to .ImVDC, connect negative probe to TP-4
and positive probe to TP-3. Note that t e s t points are the "legs" of the resistor.
VERY IMPORTANT: You m u s t u s e i n s u l a t e d t e s t l e a d s t h a t do n o t c o n t a c t t h e chassis. Do not attempt this adjustment if usingtypical pin-type probes
as you could s l i p and touch the chassis which will damage the unit or your meter and probably necessitate a factory repair! A fully insulated probe with a retractable hook that may be securely attached to the test points will

 Page 2
CL-60 / V-70 Biasing Instructions
prevent mishaps. Most electronic supply stores carry such a probe.
Ensure correct fuses installed: MDX 6-1/4A line fuse; AGC 1/4A high voltage fuse.
With your DVM set to mV DC scale 200mVDC or less (if not auto ranging), turn on amp.
After a few seconds, bias reading should start to rise. Check across other channel's test points TP-2 and TP-1 to ensure proper operation also. Initially
s e t RV-1 bias pot to about 55mVDC reading. This will r i s e as tubes warm up.
8. After full warmup of approximately 20 minutes, set RV-1 to final reading of 65mVDC across TP-1 and T-2, TP-3 and TP-4. Bias readings from channelt o channel may vary up to 4m due to minor tube variations. If greater than
AmV difference, you may need to swap tubes from channelt o channel t o reduce the variation.
9. After a t least 30 minutes warmup, recheck bias. If correct, proceed to check servo balance.
10. Move positive probe from TP-1 to TP-5. Leave negative probe connected to TP-2. Reading should be plus or minus 2mVDC. If out of calibraion, insert plastic probe into hole in side rail of that channel to engage trimmer. With probes connected as above, turnprobeclockwise to make reading positive, counter-clockwise to make negative. Adjust trimmer no more than 1/4turn
a ta timet oprevent over-adjustment. Allow 5 minutes for servo t ostabilize Observe result. Readiust as necessary.
11. Repeat step 10 with other channel. Connect negative probe to TP-4, positive t o TP-6.
1 2 . Should you experience difficulty, Audio Research Field ServiceDepartment. may be reached at (763) 577-9700 8:00 am - 4:00 pm Monday through Friday

So there we have it. 65mv. I’m not sure that I like the the disclaimer about this supplement not supposed to replace the supplied manual because that’s exactly what I’ll be doing. They didn’t officially admit to two iterations of the Classic 60 or to the error in my manual but this was the response.

Thanks for everyone’s help.

I finally got back home and biased the Classic 60 up to 65mv.

I thought that the amp sounded so lean and lifeless initially because the new tubes needed to break in.

All I can say is the amp is back. The wide soundstage, detail, beautiful mids, solid bottom end and floating highs with all instruments all back where there they shoild be.

This has been a long time coming. Will have to listen to my music again.

As said before, thanks to all. The gon community is like no other.

Perry