Mark II C+ plate voltage question

[Mr. Blues]

I regularly gig with my C+ and then right in the middle of a set it blows a fuse and smokes....
smells like burning wires. Finished the gig with a back up baby Marshall from a friend, and
the boogie is now fixed with two new 470 ohm 5W resistors, new 2.5 slo-blo fuse, also a couple of 12AX7 tubes blew,
...so considering these tubes were three years old, I decided to play it safe and retubed with JJs, because I have a gig this weekend
(I happen to like them, the price is right for me... so please don't flame me), and want to make sure there are no new surprises.
ANYWAY ... I asked the repair guy for plate voltage figures, and he said they were around 390?
Last time I recorded these numbers (3 years ago,) they were at around 435.
So now I got a head trip about LOWER numbers. The amp sounds GREAT.... everything works
but these #s seems lower than three years ago, (around 435) and lower than what I've read a new 60 watt C+ used to be.
The repair guy is not always available for further inquiry and his supervisor says he's never seen a boogie
with numbers above 400? I point out the numbers from the boogie users manual and he says those are max numbers, that should not be exceeded? Now I'm losing confidence in this guy and looking for a second opinion.
I don't have much "hands on" experience regarding repairs/specs.... but I READ.... and try to at least be able to converse with the techs and learn from them...and now I'm even more confused.
Bottom line.... is my C+ on it's last leg?
Are the lower plate voltages normal for a working aged C+?
Does it depend on where (i.e. which pin) these numbers are measured.?
In which case it may be a false alarm, and no need for concern?
There are SO many so called amp gurus out there and it seems it's like some like fords and some like chevys and everyone has an opinion....
...so...give it to me straight forum doctors...i can take it... Is this the beginning of the end of my C+.
If so...worse case scenario...eventual tranny will die.... and given all the magic mojo around C+ trannies,
can a replacement be obtained/cloned/purchased...or once it goes it will never sound the same?
Sorry for the drama.... but I dearly LOVE my C+ and get lots of compliments about the amp/tone. It loves P90s.
Thanks for all replies

 

[JOEY B.]

The repair guy is not always available for further inquiry and his supervisor says he's never seen a boogie
with numbers above 400?

The spec for a 60 watt IIC+ is 448V at pin #3 of the power tube sockets. That is not a max number. It should be close to the spec as soon as the amp is powered up. I have seen Mark series Coliseum amps with 525V at the plates. Sounds like the supervisor is either confused or talking out of his ***. :lol:

 

[Mr. Blues]

Thanks for your reply...
Went to an amp buddy (who builds amps) and he measures plate voltage from each power tube socket
and sure enough both were right at 447. Whew... what a relief...although now
if ones turns the master volume past 3 the digital readout gets lower, then SOARS to 500+, and I quickly
turn it down.... this behavior exists only on one of the power tube sockets....so...another thing to
screw with my head....perhaps it might be a diode thingy on the tube socket is having a bad day?
Swapped out tubes in different sockets and behavior exists only on the left 6L6 socket. I don't think this major fluctuation
is "normal"... or is it?
I played a gig with the amp last night and everything was just fine.
But then I didn't play much past 1.75 on master volume.

Any suggestions?
Thanks again for your reply

 

[Mr. Blues]

FWIW...got still another opinion....amp guy says not to worry...couldn't find anything wrong...
says the weird numbers when turning master volume are from some kinda oscillation (?)
that effects the meter?
Oh well....twang on

 

[>Photi G<]

That can't be good for the OT. There's definitely something wrong if the plate voltage exceeds 500V. The PT can only supply 450ish to the plates, so I don't know. Maybe it is oscillation. Maybe it's not.

BTWay, Power tube plates never have resistors on them. You probably blew the screen grid resistors earlier. A common problem with older amps.

 

[stokes]

Are you saying you are taking the plate voltage while playing and "turning the master past 3"?Or is the spike with no guitar plugged in?If an oscillation were occurring,the plate voltage would drop,an oscillation is just an unwanted signal passing thru the circuit,if a signal passes thru the circuit the plate volts should drop as the current rises.If all else seems fine with the amp,I would suspect a faulty meter.If this voltage spike is happening with no guitar plugged in,I would think something is happening with the bias voltage to that tube.Have your friend check pin five of the tube that is spiking to see what is happening with the bias voltage.If the negative voltage goes up,the current draw is decreasing and would cause the plate volts to rise.

 

[Mr. Blues]

the second amp guy i took it to got the weird spike without guitar plugged in....said there was something wrong and referred me to the third guy....third guy doesn't like people looking over his shoulder when he works (understandable) and says he couldn't find anything wrong...the following is from the work invoice....
"B+ is 450V, output is 60W clean
6L6 tube slightly unbalanced but ok (brn 43 mA, blu 37mA)
output transformer ok (ratios are 4140:8, 4160:4)
all tube test ok"
I don't know exactly what the above translates to but I played a gig with it ... so far so good
Thanks for all replies