Marshall JCM800 2203 reissue / One power tube failed / Burnt Hum balance pot

[spunko]

Hello fellas,

The owner says one power tube went red plating and burnt, the amp started to sound awful and then no sound. The owner took it to his friend and he found the hum balance trimpot burnt, along with R31 and R32. His friend replaced those parts, put new power tubes and took it home.

Now the owner says the amp has two problems:

1) With the master volume at 0, the amp still makes a little sound, you can hear the guitar really low, but is there.
2) It seems it lacks of power, the amp sounds loud, but not that loud like he is used to. The owner says that he used to play at 1.5 master volume, now he has to play it on 3.5 master volume to get the same output level.

When I opened it, I noticed his friend replaced the burnt 100 ohm trimpot with a 200 ohm pot and a 200 ohm resistor in parallel. R31 and R32 have the correct values. I replaced the trimpot with the correct one, 100 ohm as shown in the schematic.

I don't have another JCM800 to test, but I do have an ENGL fireball 100, and I know these are two different amps, but the ENGL is louder than the JCM800 at low volume (1, 1.5 on the master). I have to set the JCM800 master higher to get the same output level of the ENGL.

I have checked for common voltages on the output section and all seems fine. I have to check voltages on the pre-amp section yet, but is 23:45 in here, I'll do that tomorrow morning.
I also shorted to ground the wiper of the master volumen to check if that pot is bad, but the guitar still sounds. I have had amps that behave like that, but I don't know if JCM800 does.

What would you suggest I should check?

https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-2203-Reissue-Schematic.pdf
 

[pdf64]

Start with the basics, a full voltage survey, are all the screen grid resistors good, what’s the idle cathode or anode current of each output valve, is it able to put out a reasonably clean sine wave of 100W into a resistive load.

[spunko]

Sorry for the late reply.
I don't have a scope to check the sine wave.

Here is what I've checked so far:
- All screen resistors test 1k - good
- Voltages on all power tubes are the same V4, V5, V6 ,V7:
PIN3: 445 VDC
PIN4: 442 VDC
PIN5: -38 VDC
PIN6: 443 VDC
PIN8 V4: 48mV
PIN8 V7: 44mV
EDIT: On V5 and V6 I connected my tube probes and read:
V5: 24 mA
V6: 12 mA

- Voltages on pre-amp tubes and phase inverter:
V1
PIN6: 213 VDC
PIN8: 1.6 VDC
PIN1: 250 VDC
PIN3: 2.5 VDC

V2
PIN6: 167 VDC
PIN8: 0.74 VDC
PIN1: 286 VDC
PIN3: 170 VDC

V3 PI
PIN6: 207 VDC
PIN8: 34 VDC
PIN1: 221 VDC
PIN3: 34 VDC

[spunko]

So I bumped up the Bias pot to its limit (full) and got 36mA per tube EL34. There is one colder tube and gets to 22mA while the remaining three stay between 33-36mA.

The thing is, I played the amp with the bias at 36mA and about 2 minutes later, out of the sudden, the amp bumped up in volume a litte bit, but it was very noticeable, and now it has stayed that way. Turned off the amp, came back after 1 hour, and it hasn't changed    
So is it fixed now? 
What could possibly cause these symptoms?

[pdf64]

… out of the sudden, the amp bumped up in volume a litte bit, but it was very noticeable, and now it has stayed that way.
 …
What could possibly cause these symptoms?

Try repeating all the measurements to see if that reveals something that’s changed.
When checking stuff inside an amp, it’s good practice always to note the mains voltage being fed to it.

[spunko]

I biased the amp at 36mA per tube, the cold tube reached 24mA. All voltages are pretty much the same, plate voltage lowered a bit because of the higher bias, but that's it.

The problem happenned again, and now I understand what the owner is feeling with the amp. Is like it loose a bit of punch or a bit of distortion. I'm sorry, I don't know how to put it in words, besides that I don't speak english very well 

I re-checked all the voltages, I taped all the PCB components with a plastic stick, all the cables, all the conections on the sockets, I tightened all the tube socktes, none had any affect on the problem. It just happens randomly, I'm playing the amp and out of the sudden it bumps up in punch/gain/distortion and then it goes down again, or viceversa. It is just a bit punch, the amp sounds good, not a flat or dull sound at all, and super loud.

[spunko]

Could a pre amp tube cause this symptom? Loss of punch intermittently

Because the problem persist.
Sometimes it manifests after 1 minute of playing.
Sometimes I can play for 20 minutes and then it happens

[trobbins]

I'd initially worry about the set of output stage valves.  Do you have another set to tube roll, or even to tube roll one new one around. Maybe also clean the terminal pins and check around the original failed valve, as that appears to have failed causing overcurrent.

PS. I'd also suggest cleaning around and between pins 2 and 3 on each output stage valve base, as the original fault may have been arc-over from pins 2 to 3 on the base.

[nandrewjackson]

Have you cleaned the "effects card" connections?


JS1 , JS2 , W3 , W4.

[spunko]

Hello, thanks for the replay
I have cleaned all the sockets, FX loop, buttons, etc. and the drop/bump still happened. The socket where the tube died is ok. All power tubes have the same negative BIAS, and the resistors are the same value on all of them, I can't seem to find anything wrong where the tube died, besides R31,R32 and VR2 that burnt and were replaced.

I've been playing the amp with 3 new 12ax7, putting back the originals one by one starting from PI/V3, and now V2. I have yet to try with the original V1.
I also changed the colder EL34 because it has a tiny little crack in the glass I didn't notice, and it has 2 tiny white dots (like it lots vacuum maybe). It really is so tiny that you can even tell is there.

So far I haven't encountered the drop/bump problem. I have to try V1, but I did change that tube with the owner in his home before doing anything, and it didn't make any difference there, so I doubt that is the problem. Maybe that colder power tube with a tiny crack?

I also tried with a complete set of new tubes, power and pre, and it didn't bump/drop that period of time, but it is intermittent, so I can be sure that one tube is the problem. That is why I'm trying tubes one by one.

The only voltage I noticed it changed a little is in V1 PIN3, went from 2.5VDC to 3.0VDC, not a dramatic change, could this be related to the hotter bias?

I'm going to put back the colder/cracked EL34 to check if the drop/bump problem returns, which is also tiny. This drop/bump problem is like if I'm playing with my gain on 8, and it bumps to 10, or vice versa.

[tubeswell]

The sudden changes in volume could be related to a cracked/cold  solder joint (although you might also notice intermittent pops if that was the case?). Or a loose pin clamp in a tube socket.


Red plating output tubes can also wreck (or mostly wreck) the OT, so it could be that