JCM 900 SLX 5881 conversion to EL34 question

Clockworkmike

Clockworkmike

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Alright well my old trusty SLX went poof today and smoked the Mains fuse and one of the EL34s, as mentioned on another post about a Marshall Power Brake. The Attenuator wasnt the culprit here but rather some amp internal issues

One thing ive came to realize and noticed only recently, is this amp was initially a 1993 2500 SLX 5881 model, sold to me from a store brand new in 1996 as an EL34. It appears to be both used obviously as well as someone attempting the convert it over to EL34 but possibly not making all the proper adjustments. Below are both schematics and values. What im wondering is, if this thing wasnt properly converted, there's definitely got to be issues arising from this correct? I have no problem going over the amp and checking each value of the components but i guess im just wondering if this has been its issue all along?

2500-60-34.gif2500-60-58.gif
 
Checking down the line i see obvious differences from the EL34 to 5881 at resistors
R30
R31
R34
R28
R23

The 5881 has a tap from the 4ohm impedance selector to VL1 with a 33k resistor and a 4n7 capacitor that is not present on the EL34.

Im gonna look inside and verify these values and im growing suspicious this wasnt properly addressed when someone attempted to mod this over to EL34 before i got it
 
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The amps actual Resistors 23, 30, 31, 34 are the correct EL34 schematic values. The R28 installed is not. It is an actual 68k resistor, should be 22k. This is the bias adjustment resistor if im reading this correctly. Thats all i can see different than what should be in this amplifier. So that tells me one of two possibilities:

1) This is a real EL34 from the factory and just put into a chassis with the 5881 stamp.
2) Someone adjusted the correct values to mod from 5881 to EL34.
 
Clockworkmike said:
The amps actual Resistors 23, 30, 31, 34 are the correct EL34 schematic values. The R28 installed is not. It is an actual 68k resistor, should be 22k. This is the bias adjustment resistor if im reading this correctly. Thats all i can see different than what should be in this amplifier. So that tells me one of two possibilities:

1) This is a real EL34 from the factory and just put into a chassis with the 5881 stamp.
2) Someone adjusted the correct values to mod from 5881 to EL34.
Click to expand...
Power tubes removed.
Power on.
Standby switch set to warm-up.
What is the DC voltage on pin 5 of each output socket?

EL 34 will be about -42 to -47 volts DC.
5881 will be about - 57 VDC.
 
Amp Mad Scientist said:
Power tubes removed.
Power on.
Standby switch set to warm-up.
What is the DC voltage on pin 5 of each output socket?

EL 34 will be about -42 to -47 volts DC.
5881 will be about - 57 VDC.
Click to expand...
Unfortunately i couldnt land a T3 or T4 fuse within 100 miles of here today. I got a pack coming in the mail but im at the mercy of that now. So i cant even perform a basic checkdown until then im afraid. As soon as i get them i will respond back asap my friend. But this great info: will let me know for sure what exactly im dealing with. I have lots of questions now, years after i bought it. Hell, i didnt know anything about serial numbers or there really even being differences in the tube models back then. I bought it "new" on good faith that i was getting an honest deal off the showroom floor

Just another reason to hate living rurally
 
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Clockworkmike said:
Unfortunately i couldnt land a T3 or T4 fuse within 100 miles of here today. I got a pack coming in the mail but im at the mercy of that now. So i cant even perform a basic checkdown until then im afraid. As soon as i get them i will respond back asap my friend. But this great info: will let me know for sure what exactly im dealing with. I have lots of questions now, years after i bought it. Hell, i didnt know anything about serial numbers or there really even being differences in the tube models back then. I bought it "new" on good faith that i was getting an honest deal off the showroom floor

Just another reason to hate living rurally
Click to expand...
How many amps is a gum wrapper?

1612679422944.png
 
Clockworkmike said:
Appreciate it bud! Just another hiccup i guess lol luckily i dont use this amp as often, so it at least wasnt my main amp i guess
Click to expand...
I just did one this morning (4100),
and it stays crunchy, even in milk.
I changed the output to EL34s.
I rebuilt the bias supply, I changed the screen resistors to 2200 ohm 4 watt, I rebuilt the 15 volt supplies, and I changed the PI coupling caps. Then I changed some of the under rated coupling caps in the preamp.
They had a habit of using 250 volt coupling caps and I changed those to 630 volt.
But overall, they tended to use under rated caps so I beefed all the ratings up.
I use 105C 10,000 hour caps whenever I can get them.

Then it got a new set of JJ output tubes....
and it's about ready to hit the road again. See ya.
 
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Amp Mad Scientist said:
I just did one this morning (4100),
and it stays crunchy, even in milk.
I changed the output to EL34s.
I rebuilt the bias supply, I changed the screen resistors to 2200 ohm 4 watt, I rebuilt the 15 volt supplies, and I changed the PI coupling caps. Then I changed some of the under rated coupling caps in the preamp.
They had a habit of using 250 volt coupling caps and I changed those to 630 volt.
But overall, they tended to use under rated caps so I beefed all the ratings up.
I use 105C 10,000 hour caps whenever I can get them.

Then it got a new set of JJ output tubes....
and it's about ready to hit the road again. See ya.
Click to expand...
I got some belton octal tube sockets ordered, should be here this weekend with the fuses, as well as 2 Svetlana EL34s i had left over. I gotta test the cabs speaker inputs still yet, hoping to do that by thursday. Its been chaotic as hell here when i get home from work all week long
 
Clockworkmike said:
I got some belton octal tube sockets ordered, should be here this weekend with the fuses, as well as 2 Svetlana EL34s i had left over. I gotta test the cabs speaker inputs still yet, hoping to do that by thursday. Its been chaotic as hell here when i get home from work all week long
Click to expand...
Always the way when you have a project in the plans.

Good luck, guess I know what your plans are next weekend. :unsure:
 
Amp Mad Scientist said:
I just did one this morning (4100),
and it stays crunchy, even in milk.
I changed the output to EL34s.
I rebuilt the bias supply, I changed the screen resistors to 2200 ohm 4 watt, I rebuilt the 15 volt supplies, and I changed the PI coupling caps. Then I changed some of the under rated coupling caps in the preamp.
They had a habit of using 250 volt coupling caps and I changed those to 630 volt.
But overall, they tended to use under rated caps so I beefed all the ratings up.
I use 105C 10,000 hour caps whenever I can get them.

Then it got a new set of JJ output tubes....
and it's about ready to hit the road again. See ya.
Click to expand...
So heres what i began checking down:

The amp turns on with just power switch on and tubes removed. It turns on with standby on. Put the tubes back in and it blew another fuse, so stupid move there.

But i began checking the schematic and each resistor and capacitor. The biggest stand out i see is R30 Resistor here: 22k 1w Metal Oxide is dead. The other two: R28 Bias pot resistor is way oversized ( should be 22k, its 68k)and the R29 in the middle of the two is oversized as well ( should be 8.2k , its 22k)
20210214_132140~4.jpg
2500-60-34 (2).gif
I found incorrect values in the preamp section as well:
R16
R19
R23
R25
R27
 

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