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    Serious Red Plate event

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    CletusB

    Posts : 194
    Join date : 2018-02-11
    Age : 63
    Location : Trinidad & Tobago

    Serious Red Plate event

    Post by CletusB on Wed Oct 31, 2018 4:36 pm

    Was just listening to some music (moderate volume) and suddenly I heard a decrease in volume with a slight hum and when I looked at the amplifier, the left rear tube (KT-88) was red plated and I mean seriously RED.
    Too upset to go troubleshooting right now, will tear into it later.  So, it's back to the MOSFET amp for the time being.
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    CletusB

    Posts : 194
    Join date : 2018-02-11
    Age : 63
    Location : Trinidad & Tobago

    Re: Serious Red Plate event

    Post by CletusB on Wed Oct 31, 2018 5:19 pm

    From Bob's list (have not cracked it open yet, just top-side measurements):
    A. Socket is fine
    B. That 10-ohm resistor from pin 1 & 8 to ground now reads 7-ohm ......I was expecting to see it seriously increase in value or open, not decrease in value.
    c.  1K-ohm between pins 5 & 6 reads correctly
    d.  R29 - 32 (mine are 100K) all read fine
    e.  PIO cap reads open as it should
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    peterh

    Posts : 1054
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Serious Red Plate event

    Post by peterh on Wed Oct 31, 2018 6:37 pm

    CletusB wrote:Was just listening to some music (moderate volume) and suddenly I heard a decrease in volume with a slight hum and when I looked at the amplifier, the left rear tube (KT-88) was red plated and I mean seriously RED.
    Too upset to go troubleshooting right now, will tear into it later.  So, it's back to the MOSFET amp for the time being.
    The most likely causes are :
    - coupling cap leakage
    - bias chain broken, possibly bad solder or cracked resistor
    both these may be diagnosed by power on with powertubes removed, and measuring pin 5
    ( grid) on the powertube socket with oscilloscope ( best) or voltmeter. It should
    measure DC - some tens of volt, stable and not fluctuating. Turning the bias adjustment
    pot should result in a smooth change of voltage. Tapping components witha wooden stick should
    not result in any fluctuations.

    - or it could be the tube itself. The redplated tube is toast anyway, throw it away and
    order a new matched pair. Replace the 10ohm resistor ( and check the
    others to make sure that all are 10ohm. Bias adjustment will be wrong if these resistors
    are of wrong value. They can be check without desoldering or removal of the tube if amp
    is powered off.
    Some folks recommend bending the sockets to increase spring tension, especially if
    "tuberolling" has been done. Normally with good quality sockets it's not needed but
    do examine pin 5 anyway to make sure it cannot make bad contact with the tubes pin.

    There is no need to replace the tubes in the other channel or to do anything with
    the 12au7 tubes or rectifier.
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    CletusB

    Posts : 194
    Join date : 2018-02-11
    Age : 63
    Location : Trinidad & Tobago

    Re: Serious Red Plate event

    Post by CletusB on Wed Oct 31, 2018 6:46 pm

    Thanks Peter, I will pick up a few 10-ohm 2W resistors and go through it tomorrow evening.  Also will order another matched pair of KT88s from Jim McShane.
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    peterh

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    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Serious Red Plate event

    Post by peterh on Thu Nov 01, 2018 4:12 am

    CletusB wrote:Thanks Peter, I will pick up a few 10-ohm 2W resistors and go through it tomorrow evening.  Also will order another matched pair of KT88s from Jim McShane.
    I would use 1/2w resistors. The idea is that if a tubes flashes the resistor will vaporize, a fast fuse to save the transformer .
    Metal film vaporizes best, "flame-proof" might be found.

    One example :
    https://www.ebay.com/itm/5Pcs-600V-15A-4-Positions-Barrier-Strip-Terminal-Block-Wire-connector-TB-1504/152092477511?hash=item23696b0847:g:TfIAAOSwopRYZ23n:rk:7:pf:0


    Last edited by peterh on Thu Nov 01, 2018 6:40 pm; edited 1 time in total
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    LeGrace

    Posts : 260
    Join date : 2016-08-07
    Location : Ontario, Canada

    Re: Serious Red Plate event

    Post by LeGrace on Thu Nov 01, 2018 3:28 pm

    Having lived through same I can commiserate. People advise me to check this and that, you have bad solder somewhere that you need to find, etc. I spend a lot of time looking amp over and over.  But turns out problem is not with amp, it is with local utility. House voltage is spiking high. I insert transformer and no more red plate event since.
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    Bob Latino
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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Serious Red Plate event

    Post by Bob Latino on Thu Nov 01, 2018 9:26 pm

    LeGrace wrote:But turns out problem is not with amp, it is with local utility. House voltage is spiking high.

    There are many places in the USA and Canada where the line voltage is not steady. You "think" that your line voltage is the same day after day but, for many people, this is not true. I have here a Kill-A-Watt meter that monitors the AC line voltage in my workshop 24/7. I check it out every now and then. The line voltage is fairly steady here. It is almost always between 119 and 120.5 VAC. As your line voltages go UP all the AC and DC voltages in your amp goes up. Higher line voltages in the amp will cause shorter tube life - especially the rectifier. You should get a line voltage monitor for your home. Home Depot carries them  ... Link below ..

    Kill-A-Watt at Home Depot

    Bob
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    LeGrace

    Posts : 260
    Join date : 2016-08-07
    Location : Ontario, Canada

    Re: Serious Red Plate event

    Post by LeGrace on Fri Nov 02, 2018 6:58 am

    I monitored mine daily for several weeks and observed a range of 119 to 122.8. Of the various issues the high reading caused the red plates were definitely the nastiest. With a red plate, if not around to catch it, who knows what could be the eventual consequence, the heat is incredible. Esp when it involves more than one tube as it did on occasion! And when it comes to a NOS tube like a GZ37, seeing one of those red plating is cringe worthy.

    The transformer drops my incoming @3.5 volts. The contribution this small change has made towards stable operation is striking. I certainly don’t miss the light shows!

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