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Should I re-bias Cary SLI-80 when replacing rectifier tubes and/or

97.94.130.80
Posted on July 24, 2015 at 14:49:57
Terry P
 the pre-driver (6SN7) tubes? The Cary manual and website instructions say to adjust bias when replacing power tubes. An on-line search showed that a few other manufacturers recommend to also re-bias their amplifiers when replacing rectifier tubes. What do my fellow inmates recommend?

I'm also interested in your opinions regarding the Cary instructions to unplug all speaker and interconnect cables from the amp prior to adjusting the bias.

A very experienced dealer recommends in his on-line instructions for setting tube bias that speaker cables remain connected. He has carried Cary equipment and other manufacturers. I don't want or mean to start a conflict thing here, I only want to ensure that I don't shorten the life of my equipment and get the most accurate bias setting for my amp.

I've only had the Cary for a year, so I'm just starting to do some tube rolling. Thanks!

 

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RE: Should I re-bias Cary SLI-80 when replacing rectifier tubes and/or, posted on July 24, 2015 at 16:33:32
airtime
Audiophile

Posts: 11287
Location: Arizona
Joined: February 4, 2003
 Generally I check the bias every so often even if I don't change the tubes. Heck, can't hurt to check.

 

RE: Should I re-bias Cary SLI-80 when replacing rectifier tubes and/or, posted on July 24, 2015 at 17:34:25
Blackdog
Manufacturer

Posts: 1505
Location: Ontario
Joined: March 20, 2006
 Check the bias when you change either the power tubes or the rectifier tubes. A new rectifier may make the high voltage change (higher) especially if you originally biased with weak rectifiers.
Unplugging everything including speakers is not necessary. Make sure there is no signal entering the amp by turning the volume down. Unplugging the speakers is recommended to prevent a loud pop when you plug or unplug the bias test cable. Otherwise it doesn't matter.


Dan Santoni

 

Something is screwy here, posted on July 25, 2015 at 00:43:00
Frihed89
Audiophile

Posts: 15703
Location: Copenhagen
Joined: March 21, 2005
 Every fixed bias amplifier I have owned or seen is adjusted with everything powered up, playing music. Whenever i switch power tubes, i hook up my DMM and test each tube periodically until the bias i set is stable.

I'm not doubting the Cary procedure. I just don't understand it.

 

RE: Something is screwy here, posted on July 25, 2015 at 04:53:52
Blackdog
Manufacturer

Posts: 1505
Location: Ontario
Joined: March 20, 2006
 Powered up yes, but not playing music. This is Idle Current you are adjusting, so the amp must be idle.
Wherever you got that information from is wrong.
Dan Santoni

 

Humm. Thanks, posted on July 25, 2015 at 08:43:32
Frihed89
Audiophile

Posts: 15703
Location: Copenhagen
Joined: March 21, 2005
 At idle, they both read far too low and can't be biased higher. I wonder what's going on?

 

Agreed, posted on July 25, 2015 at 09:34:34
mbnx01
Audiophile

Posts: 7956
Location: Eagle, Idaho
Joined: October 22, 2004
 Dan is spot on.



'A lie is halfway around the world before the truth gets its boots on'. -Mark Twain

 

RE: Should I re-bias Cary SLI-80 when replacing rectifier tubes and/or, posted on July 25, 2015 at 12:20:51
Terry P
 Thanks Dan! I always like to know the reason why something is the way it is. Also, should I unplug the power cable for the tube replacement itself? I would of course have the unit "dead cold" (off overnight) prior to the tube replacement.

The reason I'm asking about this is because it's a real pain to get behind my equipment rack to do all the cable unplugging.

 

RE: Should I re-bias Cary SLI-80 when replacing rectifier tubes and/or, posted on July 25, 2015 at 12:28:36
Terry P
 Yes, I should probably do that. It's a real pain to get behind the equipment rack and unplug everything. Between that and following the Cary bias instructions perfectly, setting the bias the first time took over an hour. Thus, my question. Now, thanks to everyone's replies, it will take considerably less time because I don't need to unplug everything prior to setting the bias.

 

Just turn it off and change the tubes, posted on July 25, 2015 at 14:46:57
Jack G
Audiophile

Posts: 9740
Joined: September 24, 1999
 It's not a big deal.
Jack

 

RE: Humm. Thanks, posted on July 25, 2015 at 16:39:56
Blackdog
Manufacturer

Posts: 1505
Location: Ontario
Joined: March 20, 2006
 What amp? There a number of reasons for low bias current.


Dan Santoni

 

RE: Humm. Thanks, posted on July 25, 2015 at 17:30:51
Frihed89
Audiophile

Posts: 15703
Location: Copenhagen
Joined: March 21, 2005
 Two custom made DIY PP amps, one by Jeff Larson and the other by Peter McAlister.Both sound very good.

 

RE: Humm. Thanks, posted on July 25, 2015 at 18:54:23
Blackdog
Manufacturer

Posts: 1505
Location: Ontario
Joined: March 20, 2006
 I don't know the Jeff Larson, but the McAlister's lack of bias could be anything from changes he made, variations in tubes or his poor build quality.
Typically when you can't get bias current high enough it's too low a plate voltage, variation in output tubes or bias voltage too high overall.


Dan Santoni

 

RE: Humm. Thanks, posted on July 26, 2015 at 04:36:51
Frihed89
Audiophile

Posts: 15703
Location: Copenhagen
Joined: March 21, 2005
 I had the amp rebuilt by a tech. It sounds lovely.

 

I sold a jillion of those amps - here's a couple tips, posted on July 26, 2015 at 11:48:54
Kevin Deal
Dealer

Posts: 369
Joined: May 29, 2000
 The reason that some manufacturers want speakers left hooked up is because without a load the tubes may run away. That is fairly rare. I can only think about a couple amps that do that. The Cary doesn't.

As long as the volume is turned all the way down or the amp is in mute you don't need to unhook anything.

That amp is not 100% tube rectified, FWIW. It is partially tube rectified. Tube rectification is not the best way to do it in a power amp. In a preamp, yes. I much prefer tube rectification. In a power amp there can be voltage sag.

I would suggest to you to change the 6922 tubes to something that is really a color that you like. That's where the rubber meets the road.

I would also suggest you look at changing the selector switch to something higher quality. We did that in the F1 versions we made years ago. There is a lot of bleed over and crap that comes in through the other inputs. If the other inputs aren't used, make sure you have shorting caps on them.

 

RE: Just turn it off and change the tubes, posted on July 26, 2015 at 11:50:46
Terry P
 Yep, thanks. I assumed that was the case but I wanted to check with people more knowledgeable than me if there could be a standby current in the unit. There's lots of equipment on the market that has a small current draw even when completely off.

 

Thanks Kevin!, posted on July 26, 2015 at 13:57:48
Terry P
 I really appreciate the information and tips!

 

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