Red plating

This has happened again. After the first incident about 3 years ago, I upgraded all the caps as well as the coupling caps. I have been running the Genalex KT-88 reissues from Jim McShane. This amp has been very stable and the bias has never drifted. Recently, the 5AR4, arced and had to be replaced. I readjusted the bias and it held steady.

All good but the tube that redplated, right rear, was the same position that redplated before.

Questions are: where to begin, what to look for and, perhaps, it may have been just a bad tube?

First thing I'd check is the bias resistor to ground on that socket. Maybe a weak solder connection that occasionally goes fubar ... I had one that looked perfectly fine but had gone open when I wasn't looking. Replaced it, and no problems since.

sKiZo wrote:First thing I'd check is the bias resistor to ground on that socket. Maybe a weak solder connection that occasionally goes fubar ... I had one that looked perfectly fine but had gone open when I wasn't looking. Replaced it, and no problems since.

Also check the Pin 5 hole contacts on the socket, and tweak the contacts it in a bit tighter towards center (turn the juice off, first!). If that baby's loose and the tube pine loses contact you've lost your grid control and the covers fly off the tube. If it's happening in the same hole. It's a common problem for tube-rollers. Take a teensy screwdriver and force the three contacts in the socket from the topside back towards center.
Any time I change output tubes I do the same step with every contact on every tube socket, then a little de-oxit on tube pins and the holes as well.
BTW, just because you've red-plated a tube doesn't mean you've murdered it, just shortened its lifespan a bit.

This is great advice!!!!

audiobill wrote:This is great advice!!!!

It's what Bob L told me to do after I fried a coupla tubes in the same hole.
Also a distillation of what Andy Bowman said, which to paraphrase, "You guys spend thousands of dollars on cabling but you don't clean your tube pins and tube sockets."
And that is where the action is.
I learned this lesson the hard way, having used Russian tubes, then switched to antique Tung Sols. The Tung Sol tube pins are just a micron's width difference smaller than the Russians and went red-plate. So the Russians blow out the socket-holes just enough that the old 6550s don't quite make full contact on all six of their active pins. A micron ain't squat to us big people but it's a helluva jump for an electron.

Or you could just replace the socket. Belton sockets are really tight.

corndog71 wrote:Or you could just replace the socket.  Belton sockets are really tight.

If starting from scratch that's what I'd do, but if you've already wired the kit, that's about like re-doing it.

deepee99 wrote:Andy Bowman said, which to paraphrase, "You guys spend thousands of dollars on cabling but you don't clean your tube pins and tube sockets."

This is a good point. I use DeoxIT D to clean my tube sockets. The dissimilar metals between the tube pins and sockets can cause corrosion over time. I recently restored a roached out 1962 RCA SE amp. There was so much corrosion that I was unable to remove the tubes, until a good soak of DeoxIT. Also works well on cleaning bias pots, etc. My local Guitar Center carries it, but it is kept back where the techs work. It is not displayed out front, with the other snake oils.

BTW, the NOS, octal Cinch(tm) sockets that Dynakits sells are original equipment for Dynaco, Scott,Fisher and others. They used the black version of the sockets. I used them in all builds except when the buyer requested the ceramic explicitly. The Cinch(tm) hold the pins snugly even after many years. They're Mil-Spec.

Good check lists for trouble shooting the red plating problem.

Bob Latino wrote:Red plating on an output tube ..

Most often caused by a bad tube but can be the fault of bad contact (as others have said) or a problem in the bias system of the amp that causes the bias for that socket not to be present.

1. If that same tube red plates IN ANOTHER socket socket while another tube does not red plate in that socket, nearly always the tube is bad. Replace the tube .. Another clue is that this tube may bias properly and the amp may play OK

2. If ALL tubes red plate in that socket then ..
   A. There may be a contact issue on one of the pins as others have said > Clean or retension the socket pins a little. (bend in a little the sides of the pins with needle nose pliers)
   B. Check the value of the 10 ohm resistor (on VTA amps) on that tube socket that goes from pins 1 and 8 to chassis ground.
   C. Check the value of the 1000 ohm 1 watt resistor that is between pins 5 and 6 on that tube socket
   D. Check the 270K (or 100K in the VTA ST-120) resistor on the driver board that feeds bias to that tube (R29 - 32 depending on the tube socket)
   E. Check the Russian PIO cap that feeds that tube socket (it should have no continuity across the cap)

Under #2 above > A is the most likely problem. Also check on B, C, D and E above that the part mentioned is soldered properly on to the driver board..

Bob

Great list Bob!

1.  Yup.  Tube biased THEN went to the darkside.  

2 A. Cleaned and retensioned all the octal tube sockets.  As someone else posted, this is the weakest link.  Deoxit is my friend...  You shipped this kit out to me October 25, 2010 and the sockets have not been touched since.  The amount of built up corrosion and mung were astonishing.    

2 B. 10 ohm resistors all within 1% tolerance.  good

2 C. 1000 ohm resistors all within 1% tolerance.  good

2 D. 270K resisters all good.

2 E. Russian PIO caps show no continuity.  good

While I was under the hood I took the opportunity to do the mod on the rectifier using the diodes to protect the power tranny.  Comes under the heading of killing two birds.

Fired the amp up using the same set of tubes and all was well.  Set the bias and for about five minutes all was good then the same tube red plated.  Everything else checks out so it must be the tube!  That sucker got hot!  Replaced that tube with another Genalex KT-88 I had in the wings and it is now behaving.  We shall see.

I love this stuff!  My daughter keeps telling me that I need a dog but dealing with this is so much more enjoyable to me!

Fingers crossed?   

Added Bob's list to my "tech tips" database ... good stuff to have handy!

PS ... it's not like you've wasted your time either. All stuff that should be checked every now and then anyway, especially when the hours start to pile up.

mhardyman wrote:

I love this stuff!  My daughter keeps telling me that I need a dog but dealing with this is so much more enjoyable to me!

And you never have to walk it.   

And don't forget the 10 ohm should not be metal film, which tend to act act as little fuses unless they're flameproof...

Fired up my ST-70 this morning and let it settle in for a few hours. The KT-88 has stayed stable and I'm trying a new EH 5AR4 to see how it holds up. I get about 6 months on the Shuguang rectifiers and hope to do better. My track record with NOS rectifiers is terrible so I gave up.

My current setup for this amp is this:

iMac > via Toslink > Maverick Audio D1 DAC > Transcendent Sound Grounded Grid > ST-70 > DCM Timewindows. Way overkill for a desktop system but I like it!

Good to see you got things worked out. Those Chinese 5AR4's always scared me, with their cherry nuclear glow. Didn't much like the sound of the copper cap, and scored a GZ37. Plenty of reserve. I got an ST120, and not sure how that would work out on the ST70 ... I've also got a nice low mileage Mullard GZ34 that I save for special occasions.

~~~

Love my Maverick TubeMagic DAC ... so much so, I got two!



The D1+ is a major improvement for USB with the Tenor chipset. I prefer that to the toslink side.

** Just realized - I haven't rolled a tube in my D1+ in at least a couple months. Got a Westinghouse 5670 in a grab bag assortment some time back, plugged it in for shitzengiggles, and wouldn't you know, it jumps to the top of the list of favorites. Little known fact - Westinghouse DID built their own tubes back then, and also did a lot of contract work for RCA ... this one is a plain jane yellow label, no military markings or whatever.

+1 on the TS GG. I used one for a couple of years. By the way, Roy offers a few simple mods to improve on the GG. I had him do thise for me, and found it worthwhile.

Skiz', can you actually hear the difference between a solid state rectifier and, say, the NOS Mully?

There's a definite subtle difference to my ear, even with the GZ37, but especially with the 5AR4. Way I understand it, a lot has to do with the quality of the audio signal going in and the speakers you're using as to whether you'll discern any difference. In the case of the 5AR4 and GZ34, I could actually hear the amp starting to sound constricted and get a bit thin when cranking high SPLs, but those are operating at the outer edge of their capabilities anyway, so not that big a surprise. The constriction wasn't an issue with the GZ68 copper cap as that supplies plenty of power, but it sounded a bit sterile and noisy to me. Once again, subtle, but hey, sometimes it's the little things. Not sure if there's a speed factor involved either, and I've never seen a head to head tech comparison for switching noise and such. Maybe just the copper cap I got?

Lots of factors come into play. Probably doesn't help that I'm running KT120's at 60mA. Maybe the bigger chokes I used for my custom build? Something I haven't tried is putting the copper cap back in now that I'm using a bucker to drop the wall voltage from it's typical 124vac to around 117vac. Have to give that a go someday, but it's sounding sweet just the way it is. Like I said, the inter-connectedness of all things. Ohmmmmmmmm ...

Then again, maybe this is all just a way for me to justify spending the big bucks on the big bottle too.   

PS ... file this in the same place people put interconnects, power cords, and cable risers. Those can' possibly make a difference either ... depending on what camp you're in.

Thanks for posting your impressions, Skiz!

Your comment about interconnectedness reminds me of something an audiophile-physicist friend told me a while ago, which was (iirc) that the power supply is in series with the amplifiers output, and the the amplifier must be understood as serving to modulate the power being supplied by the power supply. This is why the quality of the PS has an important impact on the sound.

mhardyman wrote:Fired up my ST-70 this morning and let it settle in for a few hours.  The KT-88 has stayed stable and I'm trying a new EH 5AR4 to see how it holds up.  I get about 6 months on the Shuguang rectifiers and hope to do better.  My track record with NOS rectifiers is terrible so I gave up.

Mark,

There is no such thing as a 5AR4 EH, just so you know. Did you mean Sovtek?

Also, the reissue U77/GZ34 Genalex and the 5AR4/GZ34 Tung-Sol reissue are proving to be VERY fine tubes, and very reliable. If the NOS scares you the new stuff is a great alternative at a fraction of the price.

Jim McShane wrote:

mhardyman wrote:Fired up my ST-70 this morning and let it settle in for a few hours.  The KT-88 has stayed stable and I'm trying a new EH 5AR4 to see how it holds up.  I get about 6 months on the Shuguang rectifiers and hope to do better.  My track record with NOS rectifiers is terrible so I gave up.

Mark,

There is no such thing as a 5AR4 EH, just so you know. Did you mean Sovtek?

Also, the reissue U77/GZ34 Genalex and the 5AR4/GZ34 Tung-Sol reissue are proving to be VERY fine tubes, and very reliable. If the NOS scares you the new stuff is a great alternative at a fraction of the price.

AAARRGGGG! My bad. I had a small set of the Sovtek 5AR4 tubes to try out. So far so good. The Genalex line is of interest to me. The quad of KT-88s I bought from you have been superb!

Michael

mhardyman wrote:AAARRGGGG!  My bad.  I had a small set of the Sovtek 5AR4 tubes to try out.  So far so good.  The Genalex line is of interest to me.  The quad of KT-88s I bought from you have been superb!  

Michael

My bad too MICHAEL, not Mark...    But I'm gonna cop a plea here, I have a customer named Mark Hardyman too!!

My apologies for the name confusion!

Back on the subject of red-plating. I'm having the same issue in one socket. The amp was playing great for weeks and all of a sudden out of the blue a newer KT77 went red.. to the point of shorting out internally, DOA. This happened to the original tubes that came with the amp in the same spot but only after a few years, prior to this it was perfect and I did not roll tubes. When the original tube failed I chalked it up to just a tube gone bad and that is when I bought a set of Genalex Gold Lion reissue KT77 tubes. I put them in and biased the amp conservatively and then checked in after warming up and then a few days later. One of these tubes in the same socket as last time is the one that red-plated. The sockets are still nice and tight. So it's that tube socket. I'll double check and adjust the tube socket. I checked the resistors all is well. The tubes were biased conservatively.
has anyone ever installed snubber resistors on the control grids to prevent any high frequency oscillations? Seeing as how these amps are hand built it is possible that some stray inductance is hanging out in the wrong places isn't it? If it is oscillations, would a 1k Carbon Comp at 1/2 watt from the screen to ground be appropriate?