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#1 2014-02-20 03:18:29

From: na
Registered: 2010-01-26
Posts: 134

redplating KT88's on V5&V6 --> cracked tubes!

Man it's been a long time since I posted on here!
I'm having an issue with my modded V4B here are some pics of the amp and a few of the mods:

I got this amp in January of 2009 in White Sulpher Springs WV.
It was already correctly modded for 6550's, had a grounded power cord and the ac outlet on the back removed when I got it but, aside from that the amp was fully orignal and operational and in very nice condition for its age.

I have since added:
*a removable IEC power cord
*a 4 inch AC cooling fan
*an adjustable bias via trim pot
*a quad of Golden Lion KT88's and 4 spring retainers
*replacment Screen and Plate resistors
*replacment JJ preamp tubes
*a post phase inverter master volume
*Steve's tone stack mod
*an internal, switched 9v power supply(for pedals)
*replacment CE & sprague filter cans/cap
*replacment rectifier diodes and bias supply
*replacments for various noisy coupling caps
*new switches and neon lamps
and prob. some other stuff I have forgotten.

The issue I am having is that the other day while I was playing the amp it suddenly cut out and turned off.

Everything has been working fine for quite sometime(2-3 years) so It cought me off guard when it just died.

As soon as it cut out I looked behind the amp and 2 of the KT88's (V5&V6) were completely redplating and super HOT.

So hot in-fact that V6 right infront of the fan cracked the glass envelope.

I checked the fuse and it was blown so thats why it lost power but, I still dont know what caused the 2 tubes to draw so much current and redplate.

In the pictures you can see I have the fan installed so it is blowing right on V5&V6. Im pretty sure the tube wasn't cracked before I started playing.

I'm thinking it cracked because the glass was so hot from replating and the air from the fan cooled it too quickly thus causing the glass to crack.
Either way this KT88 is a dead player:

So once again I pulled her apart to see what I could see, expecting to find some flaw in the master volume or bias supply or something.

But nothing, no burnt resistors, no popped caps, no burnt traces...

With the power tubes out I probed around with the meter and all the voltages were good.

So at this point I decided to pull the other tube that had replated(V5) thus leaving me with 2 good KT-88's. Then I moved V7 to the V6 position on the other side if the output trans to balance out.

So I am now running in a Half-Power configuartion to get by for now.

SO thats the back story, I am still wondering what went wrong though...

Is it possible I just had a bad KT88 and when it failed it caused its Pair to redplate with it?

I have since swapped in the un-cracked replat-er while in Half-power configuaration just to see what would happen and it ran just fine...

Is it possible a failing tube could drag its partner down with it??

Thanks and as always any insight is appreciated

Last edited by ROBO (2014-02-20 03:51:03)

Less talk, More rock



#2 2014-02-20 06:03:37

From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: redplating KT88's on V5&V6 --> cracked tubes!

It sounds like one of your kt-88s went bad.   If one of the kt-88s goes bad it can definitely drag its partner with it.   They don't have individual bias feeds,  each side shares to a degree. 
How long did you have the gold lions in there ?



#3 2014-02-20 07:29:31

From: na
Registered: 2010-01-26
Posts: 134

Re: redplating KT88's on V5&V6 --> cracked tubes!

I put the KT88's in sometime in late 2009/early 2010 if I recall correctly.
But honestly for the past 2-3 years the amp hasn't seen much use at all. I wasn't playing much at the time and so I would turn it on and play here and there but no shows or prolonged sessions.

But now that I have a band again I'v been using it more and more and for longer periods of time so its no surprise that the gremlins would start to be awoken.

I have always heard the worst thing you can do for an amp is to not play it, the longer all the components go without being charged and cycled
the more they tend to drift, dryout and go bad.
Letting them sit for a long time un-powered is hard on them.

I still have the quad of 6550's that came out of the V4 when I got it BUT I used them on my SVT to get it back to life, when I got the SVT believe it or not it was missing 4 of its 6 6550's and I just happened to have my old ones laying around!

Less talk, More rock



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