68 Plexi refurbish

[Strat78]

This amp seems to have a red plating issue that comes and goes. Today the amp was idling for about twenty minuets when a faint humming began. I checked the tubes and the plates were beginning to glow red so I turned it off. Again, the amp was idling and only had a wall voltage of 100v with the plates at about 360v. Fired it back up and checked the bias, which had dropped from 2.3v where I had initially set the bias to a now cold 1.3v.

I hate to start swapping out the original Dubilier PI couplers and RS bias capacitors, but do I have a choice anymore. I've swapped out the TCC screens for Rifas so far but have not fired it up again. Now I am wondering if I should stop playing with fire (expensive Syvania's and original tranny's), and just be safe with all new couplers? Are there also resistors that are among the usual suspects when red plating occurs?

[echoplexi1974]

Are all 4 tubes red plating?

[Strat78]

V4 and V5 were definitely red plating, but I think they all had a red glow to some extent. Once the Rifas are done forming I'll fire it up again and see what happens.

[Star*Guitar]

Everything but the trannys are replaceable my friend. Dont stress. Swap out what you need to make it it stable. We will find good NOS parts. Ive waited my whole life to own a 68. Im in no hurry. I got the componant cost covered providing we can find the era correct ones. This amp is a piece of art that needs to be restored. I know it is in the right hands. btw, I could care less about the red dye on the soldier joints. Just do what you need to make this baby happy.

[neikeel]

Everything but the trannys are replaceable my friend. Dont stress. Swap out what you need to make it it stable. We will find good NOS parts. Ive waited my whole life to own a 68. Im in no hurry. I got the componant cost covered providing we can find the era correct ones. This amp is a piece of art that needs to be restored. I know it is in the right hands. btw, I could care less about the red dye on the soldier joints. Just do what you need to make this baby happy.

Surely you test them for dc leakage before pulling them?

I would actually replace the bias caps first with 200v RIFAs as they are similar size to the ones you have and work well, make sure the slider contacts are clean too.

[vh junkie]

I would definitely follow neikeel's advice regarding the bias caps/trimmers... otherwise I don't think you should just start replacing caps.

See this redplating thread that describes changes to the the PI Plate and tail resistors:
http://forum.metroamp.com/viewtopic.php ... &start=124

On the last page Roe references this link that handles it a different way:
http://www.vintageamps.com/plexiboard/v ... 9&start=30

[echoplexi1974]

Also make sure all of your ground connections are solid too. A loose ground anywhere in the circuit could cause intermittent weird issues to happen. The amp heats up and starts to effect those loose connections. I had a mojave peacemaker that red plated on all 4 tubes. Turns out the PPIMV pot on the back of the amp was loose. Just thinking out loud

[danman]

I recently had a redplating problem with a new build. It would play fine for a few minutes and then suddenly redplate from a loss of negative bias voltage. Turned out to be a bad solder joint at the 220k bias splitters. It looked great but needed reheating and fresh solder to fix the issue.

[neikeel]

I recently had a redplating problem ................. from a loss of negative bias voltage.

So check your bias caps

[Strat78]

Thanks for the suggestion, I've gone over all the grounds, reflowed some suspect screen resistors joints, replaced the bias resistors with some nice looking philips caps and after catching a little shock from just flipping on the amp, I finally replaced the original two prong plug with a three prong. Made a chart with all the voltages and everything checked out good. I'll check the sockets to make sure none of the contacts are broken out of sight. I almost hope it is something with the amp and not the Sylvania 6ca7's, because those are so scares these days. Will fire it up tomorrow.

[Star*Guitar]

Lol.. I got one more Sylvania here I can send if you need it. I kinda doubt its them. I only buy then one at a time usually because you get better deals that way. I got one question though, how can you change bias resistors with caps?

[Star*Guitar]

Seriously though cant wait to see how things go. I love listening to the changes.

[Strat78]

Replaced the bias resistors with some NOS philips. Put in the Rifas because something was not right about the old TCC's even though they tested pretty good. Played the amp for about twenty minuets and the tone kept getting better unlike last week when the amp seemed to loose steam after a while. No red plating so far. I'm pretty sure the bias is more stable now, I'll run it tomorrow a little longer. The last two pics are how it looks now:
https://www.flickr.com/photos/120665708 ... 278347414/
Those little Rifas look pitiful compared to the TCC's but they do sound really good. We need to track down some NOS TCC 32uf or 32/32's to beef up that power board again.

[neikeel]

Replaced the bias resistors with some NOS philips. Put in the Rifas because something was not right about the old TCC's even though they tested pretty good. Played the amp for about twenty minuets and the tone kept getting better unlike last week when the amp seemed to loose steam after a while. No red plating so far. I'm pretty sure the bias is more stable now, I'll run it tomorrow a little longer. The last two pics are how it looks now:
https://www.flickr.com/photos/120665708 ... 278347414/
Those little Rifas look pitiful compared to the TCC's but they do sound really good. We need to track down some NOS TCC 32uf or 32/32's to beef up that power board again.

NOS 32uF axials for the screens.......

[Strat78]

NOS 32uF axials for the screens.......

Here are the caps that were leaky, now we need to find some NOS replacements!
https://flic.kr/p/q37hM8
Edit: I'm not leaving the V2A cap and resistors the way they are now, that is just temporary.. Oh, and I ran the amp for a good hour yesterday, no red plating and the bias is stable. Amp is sounding really good!