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LeGrace

Bob Latino

peterh

CletusB

8 posters

Serious Red Plate event

CletusB: Posts : 234
Join date : 2018-02-11
Age : 69
Location : Trinidad & Tobago

Post n°1

Serious Red Plate event

by CletusB 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: Posts : 234
Join date : 2018-02-11
Age : 69
Location : Trinidad & Tobago

Post n°2

Re: Serious Red Plate event

by CletusB 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: Posts : 1869
Join date : 2012-12-25
Location : gothenburg, sweden

Post n°3

Re: Serious Red Plate event

by peterh 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: Posts : 234
Join date : 2018-02-11
Age : 69
Location : Trinidad & Tobago

Post n°4

Re: Serious Red Plate event

by CletusB 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: Posts : 1869
Join date : 2012-12-25
Location : gothenburg, sweden

Post n°5

Re: Serious Red Plate event

by peterh 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: Posts : 389
Join date : 2016-08-07
Location : Ontario, Canada

Post n°6

Re: Serious Red Plate event

by LeGrace 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: Admin; Posts : 3277
Join date : 2008-11-26
Location : Massachusetts

Post n°7

Re: Serious Red Plate event

by Bob Latino 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: Posts : 389
Join date : 2016-08-07
Location : Ontario, Canada

Post n°8

Re: Serious Red Plate event

by LeGrace 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: Posts : 234
Join date : 2018-02-11
Age : 69
Location : Trinidad & Tobago

Post n°9

Re: Serious Red Plate event

by CletusB 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

cheers

bounce

Tubes4ever: Posts : 167
Join date : 2015-07-14
Location : Star, Idaho

Post n°10

Re: Serious Red Plate event

by Tubes4ever 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.

Roy Mottram: Admin; Posts : 1838
Join date : 2008-11-30

Post n°11

Re: Serious Red Plate event

by Roy Mottram 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: Posts : 234
Join date : 2018-02-11
Age : 69
Location : Trinidad & Tobago

Post n°12

Re: Serious Red Plate event

by CletusB Tue Jan 22, 2019 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: Admin; Posts : 3277
Join date : 2008-11-26
Location : Massachusetts

Post n°13

Re: Serious Red Plate event

by Bob Latino Tue Jan 22, 2019 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: Posts : 234
Join date : 2018-02-11
Age : 69
Location : Trinidad & Tobago

Post n°14

Re: Serious Red Plate event

by CletusB Tue Jan 22, 2019 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

Very Happy

CletusB: Posts : 234
Join date : 2018-02-11
Age : 69
Location : Trinidad & Tobago

Post n°15

Re: Serious Red Plate event

by CletusB Wed Jan 23, 2019 5:27 pm

Watching it like a hawk and so far so good! Laughing

Laughing

bbqjoe: Posts : 98
Join date : 2018-12-03

Post n°16

Re: Serious Red Plate event

by bbqjoe Thu Jan 24, 2019 12:25 am

Glad things are back to normal.
Whenever I have a downed anything; amp, motorcycle, car etc, it's like having a deathly ill kid to me.

I also don't trust those Chinese resistors much.
And who decided blue was a good shade to put other colors on?
I find it awfully hard reading purple violet and black on them most times.

I measure each one as I inventory a kit, then for piece of mind, I measure them again as I install.

CletusB: Posts : 234
Join date : 2018-02-11
Age : 69
Location : Trinidad & Tobago

Post n°17

Re: Serious Red Plate event

by CletusB Mon Mar 30, 2020 11:45 am

Had another 10 ohm cathode resistor open today and red-plated a tube very distressing since were in lock-down and this is when I could really get into my music. Well, I've fallen back to my trusty el-cheapo MosFet amp in the interim! Evil or Very Mad

Evil or Very Mad

LeGrace: Posts : 389
Join date : 2016-08-07
Location : Ontario, Canada

Post n°18

Re: Serious Red Plate event

by LeGrace Mon Mar 30, 2020 12:17 pm

IME its usually the other way around, red plate takes out resistor.

MechEngVic: Posts : 107
Join date : 2019-01-16

Post n°19

Re: Serious Red Plate event

by MechEngVic Tue Mar 31, 2020 1:04 am

CletusB wrote:Had another 10 ohm cathode resistor open today and red-plated a tube very distressing since were in lock-down and this is when I could really get into my music. Well, I've fallen back to my trusty el-cheapo MosFet amp in the interim!

Was the red-plating tube in the same position as the one you reported previously? You said it was the left rear the first time.

I have an Dynaco st-70 series II and my left rear tube position keeps arcing during warm-up and keeps blowing tubes. A new tube lasts a couple of months before this happens. I have grid protection resistors, and I think this is the reason the tube arcs but does not red-plate like it used to before I installed the screen stoppers. My coupling caps are new and this has been happening to the same position before and after a re-build, only thing not replaced was the output transformer.

Definitely go back over Peterh's 2018 recommendations. I think I might be doing the same. I think I will go back over the whole bias circuit, not just the bias resistor.

CletusB: Posts : 234
Join date : 2018-02-11
Age : 69
Location : Trinidad & Tobago

Post n°20

Re: Serious Red Plate event

by CletusB Tue Mar 31, 2020 8:30 am

This time it is the left front. Amp started up and was playing just fine for about four minutes, then I noticed the distortion, looked over and saw the red plate. Quickly powered off and glanced the voltage monitor which was at 117vac. I could smell the resistor burning and confirming with the ohmmeter (pin 8 to ground) found it to be open. We're in lock-down so I guess I will tear into it today.

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Post n°21

Re: Serious Red Plate event

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