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    Serious Red Plate event

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    CletusB
    CletusB

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    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.
    CletusB
    CletusB

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    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
    peterh
    peterh

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    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.
    CletusB
    CletusB

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    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.
    peterh
    peterh

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    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
    LeGrace
    LeGrace

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    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.
    Bob Latino
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    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
    LeGrace
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    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!
    CletusB
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    Re: Serious Red Plate event

    Post by CletusB on Sun Jan 20, 2019 1:11 pm

    Been working my butt off on some home renovations, and finally got to working on the amp yesterday.
    Problem solved:
    Turns out the 10 ohm cathode resistor associated with that particular valve ended up at 7 ohms. So when I was happily adjusting bias for that KT-88, and setting up on my DMM for 0.550v (55mA) I was in fact, unknowingly, biasing the tube at 78.5mA, because of the lowered resistance value.
    But, I supposedly caught the red-plate in time and powered-down the amp, as the tube is working just fine in the amp now that I've changed out to a good 10 ohm resistor, which I got from Bob.
    I have no idea how that resistor ended up at  ohms.
    One other thing cropped up yesterday, is that there was quite a bit of hum when the tubes heated-up even through the bias pots were at their minimum at the time.  I at first figured the quad cap had gone south, but later discovered my driver tube (12BH7AEH) had died. Swapped-in a 5963 and all is well again!

    BTW My line voltage is usually around 113V peaking at times to 117V
    Very happy ending! cheers bounce
    Tubes4ever
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    Re: Serious Red Plate event

    Post by Tubes4ever on Sun Jan 20, 2019 10:13 pm

    CletusB wrote:
    Turns out the 10 ohm cathode resistor associated with that particular valve ended up at 7 ohms. So when I was happily adjusting bias for that KT-88, and setting up on my DMM for 0.550v (55mA) I was in fact, unknowingly, biasing the tube at 78.5mA, because of the lowered resistance value.

    Setting the bias higher on your KT88 isn't the reason that the tube red plated.  78mA isn't enough to cause the plate to glow.
    I would keep a close eye on that tube for awhile.  I have an ST70 and one of my EL34 tubes red plated on turn on one day.  I moved it to another socket and it red plated again. Proving that it was the tube at fault.  After allowing the tube to cool.  It began working just fine again and tests just fine on my tube tester. I replaced it and no longer use it because I can't trust it. I'd move the KT88 to the other socket on the same channel on your system.  If it happens again, you know for sure it is the tube at fault.
    tubes4hifi
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    Re: Serious Red Plate event

    Post by tubes4hifi on Mon Jan 21, 2019 7:26 pm

    if you're using a VTA driver board, also make sure that R29-32 are 100K, NOT 150K
    CletusB
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    Re: Serious Red Plate event

    Post by CletusB Yesterday at 8:51 am

    tubes4hifi wrote:if you're using a VTA driver board, also make sure that R29-32 are 100K, NOT 150K
    Hmmmm, They are 150K resistors, as supplied with the kit from Bob.
    What are you saying, the resistors sent with the kit is incorrect?
    Bob Latino
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    Re: Serious Red Plate event

    Post by Bob Latino Yesterday at 8:56 am

    It may say 150K on the parts list but you were supplied with 100K resistors for R29 - R32. On recommendations from Roy, R29- R32 were changed from 150K to 100K about 3 years ago.

    Bob
    CletusB
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    Re: Serious Red Plate event

    Post by CletusB Yesterday at 9:00 am

    Bob Latino wrote:It may say 150K on the parts list but you were supplied with 100K resistors for R29 - R32. On recommendations from Roy, R29- R32 were changed from 150K to 100K about 3 years ago.

    Bob
    Ah!   .....ok, my bad!   Just physically checked, they are in fact 100K on the board. Will make a note in the literature.
    Thanks Bob! Very Happy

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