JCM 800 lead series problem

[andresound]

Hi guys,
I have a friends early eighties JCM 800 lead series 100w amp. It has reduced output. Have changed the tubes and no change.
I noticed when I try play loud, the tubes start to red plate. When I stop the tubes go back to normal.
basic voltages are a bit lower than usual (10% across the board).

Any ideas?

[andresound]

Here are the voltages

V1
P1=264.3
P3=2.64
P6=229
P8=1.7
V2
P1=173.3
P3=1.02
P6=303.4
P8=175
V3
P1=225.7
P3=39.4
P6=215
P8=39.4

V4
P3=476
P4=-45
P5=472
V5
P3=470
P4=-45.3
P5=470
V6
P3=467
P4=-45.4
P5=470
V7
P3=467
P4=-45.3
P5=471

V 6&7.... Isn't P3 supposed to be greater than P5 ???

[andresound]

Could the OT be suspect??

[Stevem]

unless you are reading the output tube socket wrong that negative voltage should be on pin 5 and high plus voltage coming in on pin 4 off of a resistor!

I don't say this to be mean, but the fact that you do not see this mistake would show to me that you should not have your nose in this amp until you are better armed with trouble shooting skills to deal with the high voltage section!

[andresound]

OUCH!!! Yes Stevem, I did read the pins incorrectly (see below).
I am the first to admit, that I am a complete idiot when it comes to understanding a lot of this hobby. However, that IS WHY I AM HERE!!! TO LEARN!!!.
I am sure the forum rules do NOT stipulate that a electronics degree was a necessity to join. But no offence taken, I am here to LEARN. How do I achieve trouble shooting skills, without starting somewhere! This will be my first attempt at "repairing" and amp.

V4
P3 =474
P4 =471
P5 =-45.5
P6 = 471
V5
P3 =474
P4 =473
P5 = -45.7
P6 =471
V6
P3 =472
P4 =469
P5 =-45.5
P6 =470
V7
P3 =473
P4 =471
P5 =-45.6
P6 =470

Thank you for the sobering input anyway Stevem. I'm getting there.

[Stevem]

Its cool, it just that in the high voltage high current section of any amp you can have a mistake happen and kill a output transformer really fast, and if so then thats a big buck item in a 100 watt amp that I guess you would have to pay for should a issue take place!

The first rule of working on a tube amp that has its output tubes in place is to always have a speaker hooked up or atleast a 4 ohm load resistor even if it's only a 10 watt resistor.

Getting back to your amp the first thing I would do is to remove one output tube from each side of the OT, this way if things are screwed, or you screw up something atleast you only kill just 2 tubes and not 4!

With the voltages you have reposted now I am sorry to say that your buddy's amp would seem to need a new output transformer, and in replacing it I would try not to have to get a Original Marshall replacement, but a better made one with meater core.
You do not have to spring for a Murcury magnetics type unless that's what your friend wants to pay for , but do get something better then OEM!

Once repaired if you want to add insurance to your friends new OT then wire a 250 ohm 10 watt resistor inside the amp across the 16 ohm output leads of the OT.

[Stevem]

For future reference note that pin 6 is just used as a terminal ,as pin 6 if the tube does have one is not connected to anything internal within the tube.

[andresound]

Thank you Stevem,
Thank you for the advice. BTW, I always connect a speaker load when firing up an amp with tubes inserted.
I suspected the OT as previously stated. My friend has another JCM800 of similar vintage, which he says, does not have the same "MOJO" as the one on my work bench before it started to sound sick. I will suggest we use that OT as a donor.
Your thoughts please. Also, how do I test an OT for faults?

[andresound]

Stevem (Steve?),
As a humble scholar, please enlighten me. How can you tell from the voltages, that the OT is toast?
Regards Andre

[rp]

http://www.geofex.com/ampdbug/outtrans.htm

http://www.radioremembered.org/outimp.htm

at the very least check ohms ct to taps both sides, and test for shorts across the sections.

If the amp looks 100% and you have an other OT it's usually faster to swap or jumper one in than to tear your hair out. Kind of bass-ackwards trouble shooting but it can be fast.

BTW It's usually not the OT.

[andresound]

Thanks for the link rp.
I did measure every component in the amp. One was out of spec (think it was the cathode v2b resistor), which I replaced. Will measure the the OT as suggested and report.

[Stevem]

From prevous experience this is what is going on with the amps output stage in all likelyhood!
So let's list the facts.
1) The output tubes ( atleast one on each side of the OT are good ) are red plating.

2) The tubes are red plating only when driven and not at idle.

3) The tubes being driven means the PI section and the preamp section are working.

4) The tubes are red plating due to working like hell to try and drive the shorted output transformer primary which is like having a near short placed across each output tube.
Also not that any output tubes that have been red plating for more than some 20 seconds will likely have become very weak and likely to short out at some point.

[andresound]

Thanks again Stevem,
NOW I have LEARN'T something!! All of the above makes perfect sense. You are a better teacher than you think . All of the symptoms will be forged in memory for future recall.
Once I have installed the "donor" OT, I will report back. Hopefully the "old" OT was not the major contributing factor to its MOJO. The amp, even in "sick" form, had a wonderful blooming type character and would take off on almost any note.

[Stevem]

I can't say, but many times cranked amps do sound best just before they go south!

[andresound]

Replaced the OT and all is good. Thanks all for the help. TAG rocks as usual