VTA 120 output tube red plated -- what do you check if you're using the AB board?

Hi,

One of my KT88 output tubes red plated this morning. I caught it pretty quickly, but I want to query the board to make sure I check what I'm supposed to.

I know in the normal scheme you should check the 10 ohm resistor connected from pin 8 to ground and the 1K resistors connected on pins 5&6.

However I have the AB board installed and the 10 ohm resistor is taken out of the circuit (or at least moved to the AB board). I'm pretty sure it's replaced by the 10ohm resistor that sits on top of the AB board (4 of them of course--I've sent a note to Pavik to check).

I checked each of those and they measured 10ohms back to pin 8 of the socket. I also measured all 4 of the 1000 ohm resistors on pins 5&5 just to be sure.

Lastly, I moved the output tube that red plated to a different socket to see if it followed the tube or the socket (after cleaning the sockets and tube pins)....but since reinserting the tubes the amp has been running fine.

Kind of unsettling to see that thing red plate and then not-any advice would be appreciated.

Jay

Have you read the note from Bob in Sticky #6, bottom of the first post, about Auto Bias boards?

I know this isn't directly helpful with regards to your issue, but it may sort of point you to look for problems with the Auto Bias board itself.

ttocs wrote:Have you read the note from Bob in Sticky #6, bottom of the first post, about Auto Bias boards?

I know this isn't directly helpful with regards to your issue, but it may sort of point you to look for problems with the Auto Bias board itself.

Hi,

If you mean that they are no longer recommended? Yes I'm aware of this, it's installed and would be very difficult to uninstall to say the least. Besides, I have been using it for over two years now without issue. As I understand the situation with AB board reliability, it is rooted in the auto bias setting pot (or whatever kind of variable resistor it is), being played with and ruining the resistive coating. Since I've never played with it, I'm assuming this is not an issue on my unit.

Of course this is analog electronics, so you never know. I've been running the amp at pretty high volume for a while since the initial red plating hoping that it would do it again (thinking it was maybe heat related?) But so far can't reproduce it.

Jay

If you are in the U.S., Erhard Audio is the distributor for the bias board and can be counted on to provide reliable service.

I suggest you contact Holger at the Erhard Audio website.

jsl1234 wrote:Hi,

One of my KT88 output tubes red plated this morning. I caught it pretty quickly, but I want to query the board to make sure I check what I'm supposed to.

I know in the normal scheme you should check the 10 ohm resistor connected from pin 8 to ground and the 1K resistors connected on pins 5&6.

However I have the AB board installed and the 10 ohm resistor is taken out of the circuit (or at least moved to the AB board). I'm pretty sure it's replaced by the 10ohm resistor that sits on top of the AB board (4 of them of course--I've sent a note to Pavik to check).

I checked each of those and they measured 10ohms back to pin 8 of the socket. I also measured all 4 of the 1000 ohm resistors on pins 5&5 just to be sure.

Lastly, I moved the output tube that red plated to a different socket to see if it followed the tube or the socket (after cleaning the sockets and tube pins)....but since reinserting the tubes the amp has been running fine.

Kind of unsettling to see that thing red plate and then not-any advice would be appreciated.  

Jay

A tube that has redplated is not to be trusted. It should be replaced !

peterh wrote:

jsl1234 wrote:Hi,

One of my KT88 output tubes red plated this morning. I caught it pretty quickly, but I want to query the board to make sure I check what I'm supposed to.

I know in the normal scheme you should check the 10 ohm resistor connected from pin 8 to ground and the 1K resistors connected on pins 5&6.

However I have the AB board installed and the 10 ohm resistor is taken out of the circuit (or at least moved to the AB board). I'm pretty sure it's replaced by the 10ohm resistor that sits on top of the AB board (4 of them of course--I've sent a note to Pavik to check).

I checked each of those and they measured 10ohms back to pin 8 of the socket. I also measured all 4 of the 1000 ohm resistors on pins 5&5 just to be sure.

Lastly, I moved the output tube that red plated to a different socket to see if it followed the tube or the socket (after cleaning the sockets and tube pins)....but since reinserting the tubes the amp has been running fine.

Kind of unsettling to see that thing red plate and then not-any advice would be appreciated.  

Jay

A tube that has redplated is not to be trusted. It should be replaced !

Of course I ran it after it red plated!!

Fortunately it seemed to run fine. I take your point completely though and won't repeat my foolishness--I have pulled all four KT88 OTs and will be replacing them with the original set of 6550s that came with the amp (which have very low hours on them). The KT88s are over two years old and have been run pretty regularly for 4-6 hours at a time (I'm retired and this amp is in my personal space where I spend 90% of my awake time in the house re: so as not to get in the way of my more productive wife...)

Before I run the amp again however, I plan on inserting the varistor and fuses that Pavel sent. These apparently are to ensure that the AB board does not suffer any damage in the event of a shorted OT. OF course I DIDN'T insert them right after he sent them to me last month...do you detect a pattern here?

I also plan on contacting Erhard Audio, their US distributor, per Pablo's advice above, just to ensure I know how to check of the AB board is OK before turning back on and to confirm my understanding of how I apply the varistor/fuses.

Lastly, the space is very very tight inside the VTA 120 with the AB board as you know--if anyone on the forum who uses the AB board and has already added the varistor/fuse upgrade, I would greatly appreciate a photo to see how you have inserted these components into the unit.

Jay

Red plates usually stem from an issue arising within the bias regulation circuitry leading to an excess current condition. But in this instance this seems unlikely. IME modern KT88 is less robust then either the 6550 type or my current preferred tube ie KT120. 2 years at 4-6 hours or more per day is really good for a set of 88's. I have never been able to get that much duration out of them.

So more likely tube getting tired versus AB circuit issue. Interesting that you mention cleaning sockets and problem does not reappear afterwards. Solid pin contact is important to proper function, I like to perform annual socket PM. In any event the 88's are long in the tooth and replacing with those 6550 is a good idea and will sound just as good IMO and should last even longer.

As far as the fuse mod I inquired about this with Erhard and they replied they did not feel it was necessary, but if it would make me feel better then go for it. So I never bothered.

LeGrace wrote:Red plates usually stem from an issue arising within the bias regulation circuitry leading to an excess current condition. But in this instance this seems unlikely. IME modern KT88 is less robust then either the 6550 type or my current preferred tube ie KT120. 2 years at 4-6 hours or more per day is really good for a set of 88's. I have never been able to get that much duration out of them.

So more likely tube getting tired versus AB circuit issue. Interesting that you mention cleaning sockets and problem does not reappear afterwards. Solid pin contact is important to proper function, I like to perform annual socket PM. In any event the 88's are long in the tooth and replacing with those 6550 is a good idea and will sound just as good IMO and should last even longer.

As far as the fuse mod I inquired about this with Erhard and they replied they did not feel it was necessary, but if it would make me feel better then go for it. So I never bothered.

Thanks, just posted about that very subject in a new thread (KT88 life).

I just got off the phone with Erhard--super helpful. Pavel also has been very helpful and supplied me with a connection diagram if anyone is interested (I'm going to install the mod plus purchased the 6.3 volt isolation transformer from Erhard since I figure while I've got the unit open might as well bring it up to date as much as possible).

Of course the irony of this situation is I was going to sell my VTA 120 since I don't plan on using it once my Philharmonic BMR monitors come in later this year re: not enough power to drive those speakers. Since the red plate last night, I've pulled the for sale ads off usaudiomart and this board since I don't feel right about passing the unit on to someone else in its current state.

You are the kind of person I would wish to purchase from but seldom encounter. Enjoy the new speakers.

Red plating, if caught very quickly, may not damage a tube.
If the KT88 were purchased as matched pair or quad, the red plate tube should be tested and compared to it's mate[s]. Randomly swapping may cause issues with matched tubes.

When inserting the AutoBIAS transformer, ensure that it is well away from the audio circuitry and it does not increase noise. Personally, I fail to see the need or electrically how it benefits. Ditto on the fuse mod.

Tubes willingly oscillate at tens of MHz. Some designs include snubbers between the transfomer and the plate and screen. Dynaco used them in the Mk VI which had 4 output tubes.
Circuit changes due to capacitor aging can cause a formerly stable amp to fail.

IMO, the pentode / triode was always a bad idea from a minimal, direct wiring perspective. If you never use it, remove it and wire the amp for the desired mode.

ALL connections should be replugged every six months.
Tubes should be replugged at the same interval.

Red plate under a minute is rarely terminal. Any tubes I’ve caught that quickly were still fine afterwards.

With AB I’ve stopped paying attention to matching.

I only use triode mode. However see no gain in rewiring direct.

Replugging (and cleaning at same time) good advice, at least once a year is my rule.

LeGrace wrote:Red plate under a minute is rarely terminal. Any tubes I’ve caught that quickly were still fine afterwards.

With AB I’ve stopped paying attention to matching.

I only use triode mode. However see no gain in rewiring direct.

Replugging (and cleaning at same time) good advice, at least once a year is my rule.

Unfortunately I really don't know how long it went before I caught it--could have been less then a minute or 3-5 minutes before I noticed.

I did replug them and cleaned the sockets. Ran the unit maybe an hour after the incident, without a reoccurrence.

That said, not comfortable running them again since they are 3 1/2 years old and could have as many 2,000 hours on them. Just going to use up my original 6550s that came with the unit once i get it back together.