Bias adjustment

BTW I got a pair of great DMM from PE, 49.00 each, will tell you the numbers through your phone. Just as good as my Field Flutes just not as rugged, or as visible (orange).

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
Set bias for both sides of the amplifier to 75mA

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.

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@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