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"matched" quad installation tweaking question?
"matched" quad installation tweaking question?
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Old 18th September 2019, 07:24 PM   #1
robertb1958 is offline robertb1958  United States
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Default "matched" quad installation tweaking question?

New to me Audio Research VT130, tubes were old so I just got two matched quads 6550's Sovteks.

The quads have matched plate current and similar gains, on nice labels at the base of each tube.

Just getting back into tubes. From a long time ago, I had a Hickock 800 that never worked, an Eico 667 that I had forgotten about, and a BK 60X. The BK says all tubes are about the same good, but the Eico seems to work well and definitely gives different %cts for different tubes.

Checking the quads that I am sure were analyzed on something better than a 50 year old Eico, I did find different percentages for tubes within a quad. Yes, I know that I don't have a Hickock or something newer or better, and that the Eico likely tests at reduced voltage levels

Should I:

A) Install the quad with the closer matching (by Eico) in "parallel"?

B) Install the closer matching pair on diagonals?

C) Ignore my old cheap Eico and don't worry about it?

Want to fire up the amp tonight, so some quick advice would be appreciated, Thanks!

PS, my Eico did indicate that a new matched quad of 6CA7s for another amp were very close.

Last edited by robertb1958; 18th September 2019 at 08:06 PM. Reason: more info.
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Old 18th September 2019, 08:44 PM   #2
petertub is offline petertub  Sweden
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"matched" quad installation tweaking question?
Meaningful matching of tubes should be done at high voltage and load, something a

typical "tubetester" does not.
If the tubes comes from a reputable dealer he has matched them in a maximatcher or similar device, plug them into you AR.
You will find out when biasadjusting the AR, if they are matched you should have adjustment pots alike. I guess you also have a AC balance, but if you don't have

equip for distortion measurment, just leave it centered.
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Old 18th September 2019, 11:24 PM   #3
robertb1958 is offline robertb1958  United States
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I plugged them in, fired up the amp.

All good!

Bias is supposed to be 65mV across a resistor, left is 66, right at 64. I think that is pretty good for an amp with different (new) tubes.

I think I will let the tubes settle in a bit before messing to get that last mV.
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Old 18th September 2019, 11:59 PM   #4
Steve Morley is offline Steve Morley  Canada
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Those old EICOs are almost comic in their ability to "test" tubes. I have a 666, the devil's own tube tester, and I like to say it won't tell you how good a tube is but it will tell you how bad it is. OK for picking out shorts and gas but as far as putting a value on "goodness", highly suspect.

I think ARC are supplying Sovtek 6550s now that there are no more Winged C 6550s being made.

Cheers, Steve
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Old 19th September 2019, 01:09 AM   #5
PRR is offline PRR  United States
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EICO 667 is an *emission* tester. It ties all loose electrodes together at lowish voltage and reads the total current.

The emission of a tube can be a little or a lot more than required, and it does not matter in a normal circuit.

It is mildly useful if a 10mA tube can only pass 1mA. But I bet all your 6550s are passing >400mA (not that the 667 has real numbers or voltages) which means they are plenty healthy.

A 667 once saved my butt. I had a 7027 which was fine when cold but shorted when heater got hot. This explained the smoke out of the power transformer, and told me which tube to trash.
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