Check out the VHT Bias tool.... sold at music stores. It has two sockets and dual mA meters.
Bias adjustment
I need to set the bias on my vacuum tube amplifier. Does anyone know where I can BUY an amp meter that reads milliamps? Most I have seen are for large AMP applications.
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@ richfrank I assume you still have the Cary SLI-80 See page 12. It appears all you need is a plug and of course a multimeter. https://caryaudio.com/wp-content/uploads/2014/06/SLI-80_manual.pdf . |
I got this cheap Woods multimeter at Lowes a few years back for my Cary SLI-80...now use it on my SLI-80HS. Very simple and about $20. https://mobileimages.lowes.com/product/converted/032886/032886958441.jpg?size=xl |
Thanks. I have a simple VOM also, but the CARY techs claimed that my bias setting was wrong and it burned out the KT-88. I know what I read on the VOM and it was reading 80 mA as it should be, but CARY kept insisting I had to send the amp back to them. I wouldn't buy a CARY PRODUCT again if my life depended on it. The techs are rude, sarcastic and think they know better than the client. Screw them! And that's a FACT and you can post it if you want. |
Sad but true. One of my first audio buds was a tech. Sharp as a whip, one glance inside anything and he could see what was going on and what to check and how to fix it or make it better. But all he did all day long was fix one simple thing after another (and to him they were almost always simple) and talk to one idiot incomprehensible and uncomprehending customer after another. It is apparently very hard to deal with morons all day long and then somehow flip around and assume this one here isn't just like all the other morons. Not saying he's right or wrong. Not saying you're a moron. Just saying it is what it is. If you want something really well made that works beautifully and where you will be talking with an intelligent and thoughtful human being in every interaction then you want to ditch Cary and talk to Keith Herron. Or for tube amps Raven Audio. In your situation I would totally go with Raven. |
DMM or Analog A good DMM Digital MultiMeter will cost 30-75 dollars. Self calibrate, there is no analog issues as with stick meters. Analog meters are not accurate, they have to be calibrated with a DMM, so why waste your money, your time, or you breath? If your lucky, 2-3 years. In oil for vibration control, still 2-3 years, but normally can be calibrated, with a DMM. Analog looks good, but really don't trust them... Digital SELF Calibrates. or battery change time, one of the features. There is no arguing with the numbers then. Normally when you burn out a valve, burn it up, use it up prematurally, it normally points to over bias. You, need to know, supply and demand go hand in hand, with any amp, valve amps, more so. Maintain 120 vac. Get good valves, well matched signal and power valves. Use a DMM and you'll be happy. You can tell the tec you DID bias the tubes correctly. But you'll find out they were probably correct... Regards |
@ richfrank I know what I read on the VOM and it was reading 80 mA as it should be,Shouldn’t the bias be set at 75mA as the manual says? Bias Adjustment As oldhvymec mentioned in his post is the meter a DMM, digital multi meter? If the meter is a really cheap one its accuracy may not be all that good. Can you provide a Web Link for the meter you have? Just curious, how many hours are on the KT 88 tubes. General rule for power tubes life is around 1500 to 2000 hours. . |
@richfrank Just to confirm what I think you said, the Cary tech insisted your bias reading was incorrect even though they didn’t have the amp in their possession to dispute your reading? Weird. Yes, the manual says 75mA but honestly 80mA shouldn’t be an issue. How old was your amp? Were they offering to perform warranty work and that’s why they said to ship it to them? If in fact "bias neglect" wasn’t the cause of your power tubes burning out why wouldn’t you want them to look at it and figure out what the REAL problem is? When I bought my SLI-80HS from them last fall it arrived with a buzz that seemed to be related to a wonky triode/UL toggle that I suspect was damaged during shipping. They paid for shipment both ways and fixed the issue 100% at no cost to me. The amp is a sweet little unit that I now run with KT-120s and some 60s NOS 6922 Mullards. I feel like they handled my situation just fine and would recommend them. Just another data point for what it’s worth. |
jea483,321 posts04-16-2020 5:27pm@ richfrank Just curious, how many hours are on the KT 88 tubes. General rule for power tubes life is around 1500 to 2000 hours. I have KT-88 valves that are well over 5000 hours, with 40% left in the tank. VTLs El KT-90s AB amps 15 year old, 20 hours a week for 15 years, still sound very good and hold a bias. The KEY is do they hold a BIAS. Good indicator of a healthy machine and valves. Who makes the valve? MATCH the valves. 1 and 2, need to be close, 3 and 4 need to be close, not 1 and 3, 2 and 4 ok makes a difference, when matching. and then the bias will be closer per side...noise reduction and extend valve life.. If the valve is holding the bias, and you going through valves, it's usually to hot. Go down to 70ma for 2 valves, if it sounds good not muddy, leave it and listen...35ma per valve should be plenty, and extend valve live A LOT.. EL34 6L6, KT66, KT77, and 90 if you can. all at 70-75 per pair Second if you bias to high it WILL distort quicker at higher volumes. Not playing in the band, trying to listen to the band. AY?? Parts Express pt #390-501 under 8.00 usd get 30 dollars worth.. LOL The ones I got there they may not have. LOOK 8-99.00 dollars. Several to choose from and great tec support..Even if you need to make up a pigtail for you meter, and amp, they'll help. Next day delivery if you want. Regards |
So many advises. I thank you all. Let me try to answer each one: 1- I did send the amp back to Cary a few days ago even tho I had no longer a warranty, because there was a clause that when I bot it about 1-1/2 years ago they were only giving me 1 yr warranty and not 3 years as the manual stated. The excuse? A special offering price on the SLI-80. I suspect they were ready to introduce the SLI-80HS and were getting rid of inventory, but I did not know that.... Well; that's neither here nor there. I was raising the "flag" on this issue since way back within the warranty period but they always said "check bias" or "change the rectifier tubes". 2- After the techs received the amp, they claimed the bias was set at 130 mA and the KT-88's were burning up. But why didn't the tube fuses blow? Because they found that the fuses were 8A ..... Yes, you read it correctly: 8A instead of 0.5A and that I didn't know what I was doing and it was my fault. 3- Since I bought the amp in May 2018, I never once changed the tube fuses. I was changing the AC fuse 3A slo-blo constantly, and was reporting the "damage" to Cary to no avail. 4- According to the SLI-80 manual, the bias should be set at 100 mA. When I replaced the rectifier tubes 5U4 for 5AR4's by Cary recommendations, they recommended the bias around 75 mA. I reset the bias at 80 mA [with an analog VOM it can be difficult to differentiate between 75 and 80 mA], well within the acceptable "range". The 5AR4's were burning up and popping the AC fuse give or take every 2-3 months. 5- I now am getting the amp back from CARY in the next week or so having paid for the maintenance, setting up, and testing it, BUT ..... I did not want to have them provide tubes or fuses as I don't trust what they say. If it works after I install the tubes and fuses, so be it. Once it blows up again I will proceed to throw it out into the ocean..... good riddance! It's not worth the stress and headaches and being told I am an idiot! Did I mention that I am a retired electrical engineer with a minor in communications and an MBA in Finances. Oh well .... what more can I say. Thanks for lending me your ear and advise. Have a nice weekend and stay well, wherever you are. Richard |
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Typo Field Flukes Typo They "did" hold a good Bias, smoked um... Cable drop... VTL are broke, not working, capoot, crashed, burned, killed um'. You feel bad.. Yours is easy to fix.. Mine is a friggin' nightmare, TRADE YA!!!!!! 340 WPC, POOF!!!! 16 valves, setting there looking like 16 valves setting there... Hope you can see the humor... Regards |
In reference to adjusting the bias on vacuum tubes, I have a question that no one, no manual, no tech has brought up and I am really interested to know. It is simple: When someone tests or adjusts the bias using a VOM or mA meter and the amp is ON with all vacuum tubes in place....... 1- Should the measurements and adjustments be made with NO load on the amp? In other words no speaker leads connected; no sub-woofer connected? 2- With everything connected and functioning? Why? Is it possible one gets a different reading and how much difference if there is a load or not? Any thoughts on this? |
Biasing can be performed, with or without input connections, but- with a load on the outputs. Otherwise (no output load); one can lose a transformer. Nothing (no signal/music) playing, while being done (amp at idle/you’re setting it’s, "idle current"). Amp should be warmed up (maybe 30 minutes), before checking/adjusting). |
Oh my goodness. If what you are stating is FACT (and I do believe you) then in both cases on my different amps I have been measuring the bias INCORRECTLY. 1- On my SLA-70 Signature amp - which weighs about 50 lbs and sits on a top shelving unit - to test the bias I need to disconnect the speakers so I can take down the amp and place it upside down to unscrew the bottom plate and then proceed to check the bias [as it is inside the amp]. 2- On my integrated amp SLI-80 the jacks are on the top so all I need to do is to insert the plug and read and adjust the bias. And I have been doing it with music playing. In either case, neither the techs from Cary nor the service manual mention anything regarding "how to" with the exception of where the bias should be. Thanks for your input. I appreciate it. Richard |
So it appears that the Cary tech was correct. Your readings were wrong. I have to say, due to poor communication with the manufacturer you end up proclaiming that you'd never buy a Cary product again if your life depended on it despite the issue was user error as Cary correctly pointed out. Ugh. I didn't even think that anyone would consider setting bias with music playing. Yes, you want your speakers connected when you check bias but DO NOT play music while measuring. The whole point is that you're checking the bias while the amp is at idle, and running a signal through the amp is not idle. Depending on how you had the volume knob set you could have easily been running the amp 50mA or more ABOVE idle when you were measuring and hence frying your power tubes. |
I accept your logic. But wouldn't the VOM have read then 130 mA instead of what it was reading? 80 mA? I would never buy a Cary product again. I should explain: It's not the product which is bad! It is that after 1-1/2 years they claim there is no more warranty. It's also that the techs do not give the customer fair and honest information nor ask the "right" questions to try to solve the issue instead of the need to ship it back to Cary. Without warranty, the cost of shipping round trip [+/- $200.00] and repair (?) and new tubes was going to come out at close to $1000.00. If the amp cost me $3000 and after 1-1/2 years I have to put in another $1000.00 that is 30% of the value, I might as well buy me a new amp ..... and from someone more honest and transparent, totally regardless of whether I was "wrong or right". |
I’ve measured my Cary with, and without, a signal. It was the same. I alway check without. I wanted to see if it could cause the reading you had. Mine was fine, so was your.. You do need to recheck at least a few times, back and forth. WHY?. In a stereo one side will change the other, until they are both within 5% or so. Go back and forth a few times. I also checked a pair of older VTLs same, and a Citation II. No change in the bias, I set a high, and low capture on my meter. Never more than 5-7ma from zero volume to 50% volume. All six amps, same. How do you bias? UNIT OFF, turn the bias down CCW. Insert your 1/4" jack. Power up the unit, warm 10 plus min or so. You can leave the input signal off if you like. Bring up the bias slowly CW. Power down, move your 1/4 inch jack, to the other rail, power up and bias. TWO or more times. New tubes, recheck see if the new valves are holding a bias... I just had a bit of a tiff also with Cary. I ask for a schematic, hand written, or partial for the hexfred upgrade. Didn’t ask where it goes, or to hold my hand while someone else does it. I asked TWICE. Just ignored the request. I’ll buy a plane ticket and knock on a door, no problem for me..I’ll get my schematic. If I have to draw my own, I’ll post it for ALL to see. There will be no secrets then. EVERYONE should be able to get technical information on equipment owned by them..Especially a schematic.. That schematic should reflect the safety or lack there of built into unit. It should reflect upgrades also... Regards... |
Hello oldhvymec. Yours has been probably the best and clearest explanation of all the "advices" I've had. I REALLY thank you for taking the time to explain the bias process. I've had a similar issue with Cary when my old SLA-70 Signature ran into trouble some years back. They would not supply me with a schematic at all saying it was "obsolete" and they do not keep old schematics. None of this should have happened if they had a bit of honesty and transparency. I'm sure that Dennis Had (?) in his days never had issues like that. It's a shame the company has changed owners. Such is life. Thanks again for your help. Sincerely, Richard |
I just had a bit of a tiff also with Cary. I ask for a schematic, hand written, or partial for the hexfred upgrade. Didn’t ask where it goes, or to hold my hand while someone else does it. I asked TWICE. Just ignored the request. I bought a case of Coca-Cola and called up there head office and requested the formulae and was also ignored. |
+1 @testpilot at what point does a consumer have a legal or ethical right to a firm's intellectual property just because they purchased a product? When he wants to fix it.. Like I've done for over 50 years. What a dumb thing to say. Is there a repair manual for my 40,000.00 intellectual piece of property setting in my driveway? YES it's required by LAW.. Even a friggin RR provides a complete manuel front to rear on their intellectual property.. Bower, corp. 1-20 million dollar machines, EVERY nut and bolt location, and where it was bought. Think before you speak, I own the damn thing, I have the right to fix it, especially, if I can do a BETTER job...I am qualified to fix what I know, not what you don't... Regards Op, rude person no regards |
three_easy_payments929 posts04-24-2020 2:42pmHow on earth are you guys measuring bias while running a musical signal? The bias moves up and down continuously while the music playing. It must be idle to measure it accurately. I just checked as I read. hee hee, I'm running 4 valves 2 per side. I'm biased at 75 ma with 0 reading on the LED, or the blue stick meters. I cranked it quickly to 80% 76 ma and back to 75, on the button. Mine didn't change a bit... The point is it should not effect his Cary, my Cary, VTL, HK Citation II to the point of BURNING up tubes. Even if he did do it that way. The guy said 130ma or something, suppose to be 75ma.. Broken analog meter, not plug in right, DUNO,, a mistry.. Second, some folks pull the signal tube to bias.. then there is NO signal at all, A real tube tec, red sock, blue sock guy, had me do that a time or two..running down a problem on a pair of old western electrics.. Regards |
rodman999994,287 posts04-24-2020 3:06pm@oldhvymec- If you’re interested in doing a HEXFRED update, on a Cary product, PM me. I can talk you through it. Did my SLM-100s, without their help. Virtually, every other component I own, as well. I also have pics, if we can figure out a way to communicate them. Your a heaven sent sent. A man of integrity indeed. did you read the above?? man, asking about smart rights, excuse me Intellectual tights, oh excuse me again, his panties were just right...LOL Regards, we'll figure it out...Thanks.. |
When he wants to fix it.. Like I've done for over 50 years. What a dumb thing to say. Is there a repair manual for my 40,000.00 intellectual piece of property setting in my driveway? YES it's required by LAW.. Even a friggin RR provides a complete manuel front to rear on their intellectual property.. Bower, corp. 1-20 million dollar machines, EVERY nut and bolt Your auto analogy is a poor one. The reason you have a service manual for your car is that in the automotive world, federal legislation requires auto manufacturers to provide manuals! A lot of electronic manufactures do not release service manuals to unauthorized service centres/individuals due to liability issues. If they send you a service manual or write down on a piece of paper a circuit mod and you go ahead and electrocute yourself, who’s liable? What about intellectual theft and copyright infringement that is prevalent in the electronics industry? Don’t manufactures have the right to protect said property. Electronic manufactures are under no obligation to support the DIY crowd. |
Don’t manufactures have the right to protect said property. Electronic manufactures are under no obligation to support the DIY crowd. No one ask for a repair manual, No one needs a repair manual to repair a Cary amp.. Geez look inside.. simple as could be. I asked for a schematic, You know like on the back of my washer, my dryer, my refrigerator, my microwave, you get my point.. It is required by law for as long as the manufacture builds, or supports the product and for a period of 10 or 11 years after production stops..Unless there are some extreme circumstance. Company buy out and loss of properly trained staffing and such. Been there done that. many many times.. It’s of no consequence anyway, I already drew the circuit out and have a great working schematic with voltages, cap and resistor values.. I figured where to put the 3.00 part X 6, 18.00 usd. Cary charged 25.95 each plus shipping and handling and tax..and, and, and, and. I did a little upgrade to boot. I have a working schematic, I’m good. The new HiPerFreds are on the way. Thanks rodman99999 They are required to provide working schematics with details of their product and how it meet all the electrical standards and requirements of the country they are being sold to.. They are not under obligation to reveal trade secrets, patented or copyrighted material. Cary is not Mcintosh, there is not some secret sauce in there machines. Mac with their different, transformer setup, did actually have patents. I’m not sure Cary even has copyrights. They sure can’t have patents on a valve amp, like Cary. It's just too much like all the rest. Red paint and a fancy logo will get you nothing at a patent office. Only copyrights Cary has is USPS mail, packaging and return a copy of schematics and pictures to themselves. A consumer has the right to repair his or her own stuff. Void Warranty stickers are VOID if there is a fuse holder inside, you know that.. NO you didn't.. If you read most warranty information "only qualified, or authorised repairs will be PAID for".. If the work is done properly, it voids NO warranty, but you may not be reimbursed for YOUR labor.. I always have been.. Normally at a reasonable rate. I may DIY, but usually I’m a lot picker than most bench tecs about quality. I’ve repaired STUFF a long, long time. Regards |
@oldhvymec - Try this, for the HEXFRED pics:
https://systems.audiogon.com/systems/8604 Thus far, I haven't been able to bring them up, any other way. OP: sorry, for the highjacking! |
rodman999994,300 posts04-27-2020 5:52am@oldhvymec - Try this, for the HEXFRED pics: https://systems.audiogon.com/systems/8604 Thus far, I haven't been able to bring them up, any other way. OP: sorry, for the highjacking! Maybe OP need to get hold of rodman99999 for future repairs, sure fixed me up.. Thanks rodman99999, nice job. I'm using turret boards, only because I needed to order a few for a cable build project.. I don't have those little vents either.. As long as OP got his amps up and going, without smokin' valves.. Man oh man... OP make sure you match those valves, in pairs, very close on one side then the other, ok it make a difference.. I read this thread a few times.. Cary, really did mess this guy around.. After what the OP and I went through. Cary not having a DAMN schematic. I wouldn't buy new from Cary. I should post before and after pics, of my rats nest clean up of my Cary.. Really sloppy work if this is factory...Man oh Man, Look at Decware, Carver. Heck the old Macs are a LOT cleaner...HK Citation II, art... Hay OP, my Cary sounds good, very pleased, with the sonics, now. Regards |
Hi here’s some random babbling . I use a meter with a digital readout . Most of the tube amps I’ve biased require that you short the input , I use a rca with a jumper soldered down the center to ground , and then just soldered to the positive shoulder . In the last year I’ve had a Winged “ C “ 6550 flash over cracking the glass, with no damage to the amp . Then a few months later , a New Gold Lion KT-66 arched and blew out the capacitors and a 5 watt resistor in the power circuit . They exploded with enough force that it broke the wiring terminal that was holding them . After two hours with tweezers and a Q-Tip removing paper, foil and oil, I packed the amp and sent it back to Dennis Had . He fixed it for free , after my warranty had expired and didn’t want to charge me . He stated that the me the tube was the cuprite , as I marked the power tubes and sent them along . He also stated if my amp wasn’t repairable , he’d build me a new one . But he no longer has Cary and works from his home shop . He’s a class act and we always talk about God . I’m sorry you’re experience was less rewarding . But having grown up with Dynaco gear , I’ve ruined my share of stuff . And have experienced many variables I didn’t expect . But I’ve always learned from others by asking . Happy listening, Mike . |