Peavey Heritage VTX ..uh oh..

[ohmy!!]

Hey guys, I'm having an issue with my VTX.  It was working fine until the other day.  It started sounding really bad and I had looked at the tubes and one of them was out completely.  So, naturally, I got a new set of the same tubes that were in there and have been in there for a few years (Sovtek 6L6WXT+).  I had plugged them in and started it up, after about 2-3 minutes of it being on it makes a loud pop and then nothing but loud humming.  I check the tubes, 2 of the tubes are glowing insanely bright red.  I flip it off, unplug it and check the fuses later.  One of the fuses had gone, I went ahead and replaced all of them, filter caps look fine.  So I figured maybe those two tubes were bad, I went through and swapped tubes between each plug and the same two plugs cause the tubes to get real hot and lots of loud humming going on.  So I unplug those 2 tubes, turn it on and it's working, no humming.  Any tubes I plug into those 2 spots get red hot and cause it to hum very loud.  I'm not sure if there's a bad resistor anywhere or what's going on. Any help would be appreciated.  I snapped off a picture for you guys to see it.



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[skynyrd]

Sounds like screen grid resistors are shot, the big cement looking resistors on top of the tubes. If you have a multimeter I would check the ohmage on the 2 spots compared to the other 2.

[ohmy!!]

Here's a picture of the circuit board where the two plugs are and the resistors.



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[ohmy!!]

So I need to replace both of those power resistors there, the 100ohm 5W 10% above the sockets?

[bry melvin]

I've had this happen when a driver transistor goes Thats the large transistor on the heat sink. It's happened about every seven years actually.

If the resistors check out...look at the transistors.  I've always changed the pair and the transistor just before it in the schematic.
Careful of the latter Subs with the same pin out are rare.

(q6 q7 q8 q9)

[ohmy!!]

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this pair here?

[bry melvin]

yes if the resistors check the same (good) one of those transistors q7 q9  I always change q6 and q8 along with them...

You'll need the cross reference I uploaded...you'll never find the original peavey #s.

You will need to remove the transistors from the circuit to test them properly.

Make sure you drain the caps before sticking your hands in there.

[ohmy!!]

Now I just need to find my multimeter...off to the garage to look through some boxes (gotta love moving, right?).  I'll have to check it out tomorrow and report back what I figure out. Thanks guys!

[bry melvin]

https://docs.google.com/fileview?id=0B5Lr8KpxtYCuYWQwZDY5M2YtOThjYy00NzE2LWEyMzYtYWI2ZTdkMDAyZDc1&hl=en&authkey=CN3fuK0M

heritage vtx schematics   

[Jack1962]

first thing you have to do is replace the screen resistors , then buy new tubes those did whayt is known as red plating , this means they are now damaged . and then you ave to rebias your amp , anytime you replace power tubes in ANY amp you have to rebias.

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[bry melvin]

VTX is cathode bias  there is no adjustment to change.

If the tubes were red for more than a very brief period of time...particularly if they are recent manufacture tubes they ARE damaged, ( Current tubes have more metal impurities AND are not as deep a vacuum as old US and British made) how badly depends on whose tubes and how long the red.  Red plating causes metal impurities to build as gas in the tube. I would check the tubes before throwing them away but you would need a tube tester for that.

Use a matched pair or better a quad for best results.

This amp has very little tone difference with different ones Ruby Tube set from musicians friend is proabably the cheapest that's not junk.


BTW while you are doing all this ...testing....make sure you have the power on LOW setting it reduces the voltage to the tubes.

Amp usually sounds better there anyway ....I've only used that amp on High in auditoriums.

[Jack1962]

Sorry bro (brv melvin) ANYTIME powertubes a changed any GOOD technician the bias is adjusted because the current flow is never the same in any 2 tubes , weather or it be fixed or cathode bias has nothing to do with it.


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[J M Fahey]

In this particular circuit (which is not like any other tube amp, except the Music Man) , the bias is *always* being adjusted automatically, no matter what the tube, new or old, because it has a SS servo bias circuit.
The tubes will always pass whatever current the cathode-driver transistors feed them, until they die suddenly at the end of their life.
Their grid, in fact, always has +15V (+24v in MM's) and their cathodes will float to whatever voltage is necessary to pass the current fed by the drivers.
Those redplating ones either had some internal short or whatever *or* were fed too much current by a malfunctioning drive circuit.

[bry melvin]

ANYTIME powertubes a changed any GOOD technician the bias is adjusted

well it's not truly an normal cathode bias ...but the point was there is no way to "adjust"

Most cathode bias involve substituting different values of resistors... not "adjusting" a pot

Personally if it's "within limits" as such I never change resistors. If I have to suddenly throw in new tubes in a performance I prefer the resistors be the factory values. I've actually seldom had tubes go performing....transportation is usually the cause of unplanned tube changes for me.

As JM says this one is even different. There simply is no way to adjust it.

The symptoms described are somewhat common and usually caused by a shorted driver transistor q7 or q9 depending on which tube pair is acting up.



I've had to do this twice on one of my VTX once on the other.  that's around  5000 hours of use.

[ohmy!!]

In this particular circuit (which is not like any other tube amp, except the Music Man) , the bias is *always* being adjusted automatically, no matter what the tube, new or old, because it has a SS servo bias circuit.
The tubes will always pass whatever current the cathode-driver transistors feed them, until they die suddenly at the end of their life.
Their grid, in fact, always has +15V (+24v in MM's) and their cathodes will float to whatever voltage is necessary to pass the current fed by the drivers.
Those redplating ones either had some internal short or whatever *or* were fed too much current by a malfunctioning drive circuit.

You're right, with this amp you pretty much just stick whatever 6L6 types of tube in there.  I've never had an issue with any tubes in it.  The only thing besides tubes that's been changed over the years is the filter caps which I changed last year.  The previous owner had it since the early-mid '80s.  He even gave me the original tubes that came with it which were still working.  I haven't been able to test anything because I couldn't find my multimeter and I've got too many other things to pay for to really spend any more money until I get paid at the end of the week.  I'll have an update on what went wrong or if I run into any problems.