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    failed rectifier in VTA-70

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    sendwaves

    Posts : 16
    Join date : 2015-08-12

    failed rectifier in VTA-70

    Post by sendwaves on Tue Nov 06, 2018 9:40 am

    Hello all, I've been enjoying the VTA-70 that I build a few years ago.

    After discovering that my mains varies from 118 VAC to over 122 VAC depending upon the season and time of day, I've been powering the VTA-70 with a variac so that the supply to the amp is under 118 VAC.

    In the past two weeks, however, I've blown three 5A/250V fast acting fuses on the variac. After the most recent failure, the VTA-70 stopped producing audio. A faint crackling/sparking sound could be heard at the amp itself with the tubes powered up.

    Bias measured 0V at each of the test points, so I replaced the Genalex U77/GZ34 rectifier with a spare Sovtek 5
    AR4, verified that the bias was correct, and the amp is once again producing the great sound I've come to enjoy.

    There were a couple of incidents in the past few weeks when the power company was working on the lines and interrupted power while the amp was powered on. I've been wondering: could have this power cycling damaged the rectifier?

    In mulling this over, I recalled that the meter on the variac goes momentarily to high scale on power up before settling at the indication that corresponds to the target 117.5 VAC that I measure with my multi-meter. Out of curiosity, I connected my multimeter to the variac output and powered it on. I saw a momentary peak of 154 VAC.

    This leaves me with a few questions for which I would appreciate your thoughts and suggestions:



    1. Are there any other tests that I should perform on the VTA-70 to ascertain if the suspected power cycling and voltage spikes has caused any other damage?

    2. Would a manual reset GFI or some other device between the variac and the VTA-70 protect against the kind of short cycle that could occur as a result of a momentary mains interruption?

    3. Is it advisable to do the tube rectifier diode mod on the VTA-70?

    4. The fuses that came with variac were fast-acting; would it be a better to continue to use fast-acting or switch to a a time-delay fuse on the variac?


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    Peter W.

    Posts : 1003
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: failed rectifier in VTA-70

    Post by Peter W. on Tue Nov 06, 2018 9:53 am

    A couple of things right away (WARNING - Mini Fuse Rant!)

    a)  A 5A fast-acting fuse will have a very short life in a VTA-70, more-so if the unit is blinked a few times.
    b) Bias voltage is developed via the bias supply, not via the 5AR4 and should be independent of it.
    c) But, if you had no B+, that would be the rectifier tube.
    d) The Yellow-Sheet mod is, generally, a good idea, yes.
    e) If you can use a manual-reset GFCI device to trip-off at a blink, yes do it.

    On fuses:  DO NOT us a 5A fuse of any nature in a VTA70. As I remember, the thing is rated around 190 watts, or so, at 120V, or so. This is something you want to verify by actual test in any case.

    That is 1.58 amps.

    If the fuse survives the turn-on surge, it will allow 3.16 the rated current through the device before blowing.  That fuse is designed to protect real-estate, not the device-in-service.

    Slo-Blo (Fuses wound on ceramic inserts) are equally pernicious at any rating.

    Measure actual current use of the amp at the desired operating voltage. Add, perhaps, 10%.

    Obtain a DUAL ELEMENT fuse at that calculated rating. And use it. It will handle the surge, and protect the equipment. Yes, these fuses are not cheap. But, consider the alternative.

    sendwaves

    Posts : 16
    Join date : 2015-08-12

    Re: failed rectifier in VTA-70

    Post by sendwaves on Tue Nov 06, 2018 6:03 pm

    Peter,

    Your point about the differences between the slo-blow fuses wound on ceramic inserts and the dual element fuses is well-taken. I was not aware of the difference in rating between them until reading another post here in the last day or two.

    Just to be clear, the 5A fast-acting fuse I mentioned is what the variac was originally equipped with.

    The 3A fuse in the VTA-70 did not blow. It appears to be the type of construction you are strongly recommending against, and is probably a spare that I purchased a couple of years ago without knowing about the difference in over-current rating compared to a dual element design. I will be ordering some dual-element fuses for the VTA-70.
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    Sfguitarworks

    Posts : 8
    Join date : 2018-10-31

    Re: failed rectifier in VTA-70

    Post by Sfguitarworks on Tue Nov 06, 2018 10:20 pm

    Hi, do you have a link to the proper dual element fuse for the st-70? Just want to make sure I’m getting the right thing. Thanks!

    Sent from Topic'it App
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    Peter W.

    Posts : 1003
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: failed rectifier in VTA-70

    Post by Peter W. on Wed Nov 07, 2018 8:22 am

    Sfguitarworks wrote:Hi, do you have a link to the proper dual element fuse for the st-70? Just want to make sure I’m getting the right thing. Thanks!

    Sent from Topic'it App

    https://www.mouser.com/datasheet/2/87/Bus_Ele_DS_2044_MDQ_MDQ-V-335887.pdf    (Data Sheet)

    https://www.mouser.co.uk/_/?Keyword=MDQ-3&FS=True    (UK/Euro Source)

    http://www.btw-electronics.net/Electro-Mechanical/Fuses/2a-250v-time-delay,-dual-element-fuse,-mdq-2,-bussmann  (Canadian Source)

    https://www.newark.com/c/circuit-protection/fuses-fuse-accessories?ost=MDQ+fuse&searchref=searchlookahead&product-range=mdq-series  (US Source)

    https://www.digikey.com/products/en?keywords=MDQ%20Fuses  (Another US Source)

    How to size a fuse:

    a) Using an accurate watt-meter or ammeter, operate the item for at least 20 minutes. If an audio device, operate it as "fully driven" as possible.
    b) Observe the peak current used during that period.
    c.1) For ultra-safe mode, choose a (dual-element) fuse that is rated closest to the actual peak current used, over/under. Use that one.
    c.2) For devil-may-care mode, choose a (dual element) fuse that is rated no more than 10 - 15% GREATER than the peak current used.

    .1 will protect the equipment best, but suffer from the occasional nuisance-failure. They will also age-out and fail over time - I would guess about every 500 hours or so.
    .2 will also protect the equipment (transformers) well, but not so much the tubes against a slag or cascade failure. These events are exceedingly rare.

    And, at ~$5 per fuse in small quantities, economics do apply. These fuses come in fractional values from 1/10 A to 10 A, and the required precision drives the cost.

    Purchase only from established, reliable sources - counterfeits abound!

    StevieRay

    Posts : 27
    Join date : 2009-01-09
    Age : 56
    Location : Central VA

    Re: failed rectifier in VTA-70

    Post by StevieRay on Wed Nov 07, 2018 10:25 am

    I just purchased some MDQ-3's (3 amp) from Mouser for my new VTA ST-120 build; they were $2.33 each.

    Not cheap but not that bad.  I bought 3 'just in case'.
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    Wotan

    Posts : 3
    Join date : 2018-02-11

    Bias

    Post by Wotan on Wed Nov 28, 2018 9:57 am

    [quote="Peter W."]A couple of things right away (WARNING - Mini Fuse Rant!)


    b) Bias voltage is developed via the bias supply, not via the 5AR4 and should be independent of it.

    In Bob's VTA125 manual he refers to the voltage at the test jacks as "bias"--confusingly in my opinion.  What this looks at is voltage across a cathode resistor, too small to develop a significant cathode to grid voltage on the 6550/KT88 and used exclusively as a cathode current monitor.  He refers to setting the standing current as setting the bias.  While the standing current is technically a "bias" in the sense of a DC superimposed on the desired AC signal, it's not bias in the sense usually understood in tube terminology (monitoring the actual grid DC voltage would  not be very useful).  The manual also correctly suggests if there is no "bias", in this sense, on any of the tubes, to check the rectifier.  I'm guessing the terminology in the 70 is the same. As I've said, not incorrect but confusing.

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