JCM 900 SLX 5881 conversion to EL34 question

Clockworkmike said:
So would the r30 being smoked, likely destroying the phase inverter tube in the process, play a role in the screwed up cathode resistance?
Click to expand...
R30 22K 1 watt is probably blown because of a shorted phase inverter tube.
The plate resistor of the PI also overheated.
and so you need to replace 2 resistors and a PI tube.
Also check the rest of the resistors in the PI circuit.
Looks like the tube shorted out.
Is the PI tube black / burned on the insides?
Look at the silver inside the tube. Is it discolored?
 
Amp Mad Scientist said:
R30 22K 1 watt is probably blown because of a shorted phase inverter tube.
The plate resistor of the PI also overheated.
and so you need to replace 2 resistors and a PI tube.
Also check the rest of the resistors in the PI circuit.
Looks like the tube shorted out.
Is the PI tube black / burned on the insides?
Look at the silver inside the tube. Is it discolored?
Click to expand...
Yeah compared to the other 3, it looks discolored, like the silver is dissipated badly, i can see a faint line across it and it looks milkier in the glass below, inside the components looks darker . Looks off to me

Tube from v1 20210214_200109.jpg
PI tube 20210214_200354.jpg
 
Clockworkmike said:
Yeah compared to the other 3, it looks discolored, like the silver is dissipated badly, i can see a faint line across it and it looks milkier in the glass below, inside the components looks darker . Looks off to me

Tube from v1 View attachment 59003
PI tube View attachment 59001
Click to expand...
Replace the PI tube and don't use that tube again.
It may cause the resistors to blow again...I wouldn't take a chance by using it unless you are prepared to replace resistors again.

When the resistors blow, there is a cause.
They don't just blow by themselves.
A shorted PI tube or the PI coupling caps are breaking down.

There is one thing:
The PI coupling caps. Sometimes (in 900 amps) these 2 caps are rated 250 volt, and they should be 500 volt or higher.
And so we might need to pick out 2 PI cap replacements. The lead spacing of these caps is 10mm I do believe.
In our earlier bias test the PI coupling caps passed, but if caps are under rated 250V then those should be changed out to 500V.
I think it's Mouser #: 80-R71VF24704030K , .047 , 520 VDC, 10mm lead space X 4 mm wide

I usually use 630 V caps for the PI coupling, but people call that overkill. I don't think so.

The difficult part is soldering the PC board, you need to be careful because the circuit tracks can be damaged easily. I tried to warn you.
Even an experienced person will be challenged by these extremely fragile PC pads and tracks.
If you guys don't have the experience to work on PC boards, then you need to get some professional help with the soldering.

When you talk about soldering make sure eSGee doesn't find out, because it upsets him so.
I mean, don't push the traumatized individual over a cliff with signs already there to start with, be more sensitive.
 

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Clockworkmike said:
So would the r30 being smoked, likely destroying the phase inverter tube in the process, play a role in the screwed up cathode resistance?
Click to expand...
No.
The cathode track pin 1 and 8 would fry from an output tube drawing too much current.
The 500 ma fuse is supposed to blow...
but maybe somebody put an oversize fuse in there at some point in the distant past...and that would fry the cathode wire path leading to ground.
So if the wire is bad whatever you need to fix that.
 
Amp Mad Scientist said:
No.
The cathode track pin 1 and 8 would fry from an output tube drawing too much current.
The 500 ma fuse is supposed to blow...
but maybe somebody put an oversize fuse in there at some point in the distant past...and that would fry the cathode wire path leading to ground.
So if the wire is bad whatever you need to fix that.
Click to expand...
Man, i cant thank you enough for your time and input on all of this. Sincerely mean that. I will change the messed up resistors,replace the PI tube and check further on this cathode wiring before actually putting everything back together.

As far as the PI capacitors and the delicate soldering: Yeah i learned a lesson years ago on the JCM2000 about fragility, replacing its BR and damaging the traces during soldering, so ill tread cautiously there and if replacing becomes necessary, take that one to the tech. I can solder fairly decently but i know my limits when it comes to precision soldering now lol
 
Amp Mad Scientist said:
The PI coupling caps. Sometimes (in 900 amps) these 2 caps are rated 250 volt, and they should be 500 volt or higher.
Click to expand...
Clockworkmike said:
I will...
Click to expand...

The white PI coupling caps on the amp's board are rated for 400v.

I have already have suggested to not use the current V4/PI tube. Also, replace the dead R30. Replace R28 & R29 with stock schematic values. BTW, R28 looks brutalized!

Definitely check the value of the HT fuse and make sure it's no more than .5A.

Then check voltages with no tubes installed.

I really hope that the amp's problems aren't and deeper than this... :fingersx:
 
syscokid said:
The white PI coupling caps on the amp's board are rated for 400v.

I have already have suggested to not use the current V4/PI tube. Also, replace the dead R30. Replace R28 & R29 with stock schematic values. BTW, R28 looks brutalized!

Definitely check the value of the HT fuse and make sure it's no more than .5A.

Then check voltages with no tubes installed.

I really hope that the amp's problems aren't and deeper than this... :fingersx:
Click to expand...
Definitely good advice! Ill replace all the messed up and inaccurate pieces and give it a good walk thru before hopefully, putting the tubes back in and fingers crossed: it working lol
 
Clockworkmike said:
Man, i cant thank you enough for your time and input on all of this. Sincerely mean that. I will change the messed up resistors,replace the PI tube and check further on this cathode wiring before actually putting everything back together.

As far as the PI capacitors and the delicate soldering: Yeah i learned a lesson years ago on the JCM2000 about fragility, replacing its BR and damaging the traces during soldering, so ill tread cautiously there and if replacing becomes necessary, take that one to the tech. I can solder fairly decently but i know my limits when it comes to precision soldering now lol
Click to expand...
Did you look at the voltage rating of PI coupling caps?
These caps will be connected to pins 1 and 6 of the PI.

Don't forget adjust the bias to -48VDC on pin 5 of the output tube sockets..
 
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