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    Wacky bias readings

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    Dingojazz

    Posts : 68
    Join date : 2010-09-03

    Wacky bias readings

    Post by Dingojazz on Wed Mar 07, 2018 3:15 pm

    Hello All,

    My VTA ST 70 is my back up amp, and I have been using it more lately. The issue is that I'm getting strange bias readings, i.e. (new) tubes will bias up to 400, then later drop to around 350 (or less) and refuse to increase, even at the top of the bias pot. Sometimes (new) tubes won't bias up beyond 250, or even less despite the bias pot setting.

    There seems to be a 'central' issue. I've been getting more mechanical noise from the (original) power tranny. Could this be a PT failure, or perhaps rectifier? Any help appreciated.

    Thanks,

    K.
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    tubes4hifi
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    Join date : 2008-11-30

    Re: Wacky bias readings

    Post by tubes4hifi on Wed Mar 07, 2018 4:40 pm

    you don't say what kind of tubes. If you're getting a mechanical hum from the transformer, likely these tubes are pulling more current (are you using KT88 instead of EL34?)

    Dingojazz

    Posts : 68
    Join date : 2010-09-03

    Amp won't Bias

    Post by Dingojazz on Thu Mar 08, 2018 8:17 pm

    To be clearer.. this is a vintage amp with the VTA updates. Basically everything is recent except the iron and tube sockets; quad cap, boards, caps, etc. The drivers are 12BH7/12AU7, and the power tubes are either (new) 6L6 or (new)EL34, not KT88.
    Up to now, the amp has worked fine since I did the VTA updates about 5 years ago. The tranny doesn't run hot, but the mechanical noise has been a feature for quite a while, and I am told this is often the case with older amps. The inability to bring the bias level up above 300 - 350 is new. I haven't had a chance to do voltage checks yet, but likely will this w/e.
    Just wondered if there might be an obvious culprit to direct my troubleshooting.
    If not, I'll check the voltages and report back.

    Thanks in advance,

    K.

    PS: Rectifier swap results in no change to bias readings.
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    tubes4hifi
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    Re: Wacky bias readings

    Post by tubes4hifi on Thu Mar 08, 2018 8:41 pm

    check the actual bias voltage at pin 6 of an output tube.
    You should be able to vary it from around -30vdc to -45vdc.
    If you can't get the bias reading (which is across the 10 ohm cathode resistor, pins 1 & Cool above 0.30vdc
    then the actual bias is too negative, which means R39 is too small. Usually 6.8K on a VTA70 (7.5K on some older ones) and usually 1K on a VTA120 amp.
    Or maybe your tubes are just worn out. Never hurts to have a couple of new spares on hand.
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    Peter W.

    Posts : 1036
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Wacky bias readings

    Post by Peter W. on Fri Mar 09, 2018 8:50 am

    http://www.leedsradio.com/images-sockets/Testsocket-octal.JPG

    Guys and gals - I have a set of these in various sizes.

    They allow me to test voltages in-situ, without opening the chassis or probing amongst all sorts of other live wires.

    And are perfect for the sorts of tests described above.
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    pichacker

    Posts : 65
    Join date : 2016-08-01
    Age : 54
    Location : Near to London - UK

    Re: Wacky bias readings

    Post by pichacker on Fri Mar 09, 2018 9:16 am

    What rectifier are you using SS or 5AR4? If tube then a low HT will cause lower than normal bias voltages.

    If you pull one set of output tubes (or wind the bias pot full -ve so they conduct less) does the bias on the other pair alter significantly?

    Dingojazz

    Posts : 68
    Join date : 2010-09-03

    Low Voltages

    Post by Dingojazz on Sun Mar 11, 2018 7:14 pm

    Hello,

    Voltage check reveals low voltages across the board; average 100v. low at the tube pins, rectifier and quad cap.
    I was able insert power tubes on one side (plus rectifier) and bias, but when adding the other power tubes, they wouldn't go above .30v.
    The bias circuit appears ok, as it will swing bias from -20 to -40 v.

    Thanks K.

    PS: Tubes are new, and switching rectifiers doesn't appear to make a difference.
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    Bob Latino
    Admin

    Posts : 2713
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Wacky bias readings

    Post by Bob Latino on Sun Mar 11, 2018 8:51 pm

    You could have a bad quad cap ? You said the quad cap was original and not replaced. The quad cap is like a container for DC voltage. If your DC voltages are low with TWO different rectifiers, then the quad cap is probably leaking ? Try replacing the quad cap. Original quad caps on Dynaco ST-70's are now at least 42 years old and some of the earlier ones are now approaching 60 years old. When you rebuild/refurbish an original Dynaco ST-70, replacing the quad cap is the first thing you should do. Get a CE 80, 40, 30, 20 quad cap from Dynakitparts. Have the 80 section face the rear of the amp and all the other sections will be in the right place.

    Bob

    Dingojazz

    Posts : 68
    Join date : 2010-09-03

    New Quad Cap

    Post by Dingojazz on Sun Mar 11, 2018 10:49 pm

    Hi Bob,

    Thanks for the reply.

    Hard to be clear on every detail here, but yes the quad is new, along with the addition of the full VTA upgrade 5 yrs ago.

    --The transformers, choke and tube sockets are about the only original parts..
    As noted, everything's been working ok up 'til recently.

    Are there specific diagnostics that will help pin down the cause of the low voltage readings?

    Thanks again for responses.

    K
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    Bob Latino
    Admin

    Posts : 2713
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Wacky bias readings

    Post by Bob Latino on Mon Mar 12, 2018 7:23 am

    If you have a capacitor tester you should test the 4 sections of the quad cap EVEN IF that quad cap was new 5 years ago. Disconnect all the wires temporarily from the quad cap and then test the 4 sections of the quad cap separately with the amp OFF. If you test the quad cap in circuit with the wires attached, you can't get a valid reading for each section.

    It could also be a bad choke? Do a resistance check across the two leads of the choke. You should get about 50 to 65 ohms.

    Bob

    Dingojazz

    Posts : 68
    Join date : 2010-09-03

    I believe

    Post by Dingojazz on Mon Mar 12, 2018 10:36 am

    I have a VOM that will check the quad cap.
    I'll also check the choke!

    Thanks,

    Kevin
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    Peter W.

    Posts : 1036
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Wacky bias readings

    Post by Peter W. on Mon Mar 12, 2018 10:51 am

    Dingojazz wrote:I have a VOM that will check the quad cap.
    I'll also check the choke!

    Thanks,

    Kevin

    I have a Fluke VOM that has a capacitance setting. And is generally useful for simple checks. Some things to know:

    The typical standard-tolerance electrolytic cap may be -20%/+100% and still meet "spec.".
    Electrolytic caps in the earliest stages of failure may read high - giving a false sense of 'goodness'.
    The higher the operating voltage, the more prone such caps are to false readings.

    Only an ESR meter, and/or a cap checker that tests at operating voltage is truly effective. BUT! If you have a known-good cap to test against, you can note any differences in behavior.

    As always, make sure any cap is fully discharged before testing.

    Good luck with it!
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    deepee99

    Posts : 2035
    Join date : 2012-05-23
    Location : Wallace, Idaho

    You might try this . . .

    Post by deepee99 on Tue Mar 13, 2018 9:24 pm

    I kit-built a Hafler DH-200 kit right after they came out a century or two ago. It was noisy as hell, both mechanically and through the speaks. Somebody suggested I might put a brick or concrete block on the cabinet, and QED. Another example of KISS.

    DarthBubba

    Posts : 85
    Join date : 2012-05-05

    Re: Wacky bias readings

    Post by DarthBubba on Wed Mar 14, 2018 3:16 pm

    deepee99 wrote:I kit-built a Hafler DH-200 kit right after they came out a century or two ago. It was noisy as hell, both mechanically and through the speaks. Somebody suggested I might put a brick or concrete block on the cabinet, and QED. Another example of KISS.

    Not to change direction of the thread, but I had the same problem with an early Hafler DH-200 (one of the first kits to arrive in Southern California).  Wrote Hafler about it and the sent me a copper strap Faraday Shield with instructions for installation.  Mechanical buzz/hum fixed!  Very Happy

    -DB

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