Mystery SL-X bias issue?

AndyK11

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My '93 EL34 equipped SL-X 2500 has had plenty of the needed upgrades. The BR3 and surrounding caps have been upgraded/replaced, the filter caps, the FX loop jacks too.

I am getting zero sound with the EL34's installed. When I hook up my bias probe to measure MA, I get a super low reading. Strange thing is, when I plug in my TAC tube adapters that are self-biasing, the amp works fine.

I figure it has to be something in the bias circuit?
 
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Tatzmann

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Measure the negative controlgridvoltage.
Should be somewhere between -35 and
-55volts.

What is it?

Put those adapters away for now, it just
gets confusing.
 

AndyK11

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I’m seeing -39 to -42 on pin 5 for both sockets with the amp in standby.
 
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rocker68

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To eliminate variables, verify fuses and battery in meter. Check ALL fuses in the amp with meter. Then measure ohms across screen resisters. Bias pot wonky ?
 

Tatzmann

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Dont put the tubes back in until you have
sufficient negative voltage there or they
might get damaged.

Bias circuit needs to be checked.

C15 is reportedly problematic in jcm900
amps.
 

Pete Farrington

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Make sure pin 4 … on your sockets go to ground!
Pin4 is the screen grid, surely it needs to be up >400V DC for the amp to work as intended?
I’m only getting 3 mv at pin 5 of both sockets, with no power tubes installed?
You’ve got your meter set to DC voltage, right?
 

Gunner64

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The sl-x has a weird 22ohms above chassis ground grounding scheme iirc.
 

AndyK11

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So it seems I get -42 on pin 5s in standby, and when the amp is turned on. When I put the power tubes in, using my bias probe, it makes static sounds, and I get 12 ma reading. Turning the trim pot makes no difference, and the bias is still way low at 13 ma.

It did the same thing with the tubes the amp came with (old Sovteks). I thought because they were old, it must be the tubes. But it's doing it with relatively new JJ's too.

What would cause the bias to be that low when the neg grid current is relatively fine?
 

AndyK11

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Pins 1 &8 are grounded - the tube sockets are original and factory wired.
 

AndyK11

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I changed the bias caps for good measure. Still doing the same low bias, no sound. Not sure what to check next?
 

Pete Farrington

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Turning the trim pot makes no difference, and the bias is still way low at 13 ma
I guess that there's a fault around R28 22k VR2 47k

But nonetheless, as the output valves seem to be passing some anode or cathode current, that wouldn't stop the amp from working.

Does twiddling the bias trimmer alter the VDC on pin5 of the output valve sockets?

Are you sure the 13mA reading isn't actually 0.13, and the amp's cathode fuses are actually blown?

Smicz TAC tube adapters https://www.smiczamps.com/tacs/



The schemnatics seem to be incomplete, whatever they're difficult to make sense of. Aren't these amps supposed to have reverb?
There's supposed to be an FX loop.
If the FX send is conencted to the input (ideally FX return) of another amp, do you get signal? ie plug into the 2500 and listen via the other amp.
If you plug into the FX return of the 2500, do you get any signal out of the 2500?

The M5201 switching opamp in this era of amps is notorious for failing. It's IC2 here I think https://www.drtube.com/schematics/marshall/slxa-61-02.gif
 
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AndyK11

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When I connect to pin 5, and turn the bias control, it goes from 39 to 43 or so.

The 2 fuses on the board look fine to me. If they were blown, or the op amp was failing, wouldn’t that kill the sound, even with the Smicz adapters installed?
 

Pete Farrington

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The 2 fuses on the board look fine to me
What's the board reference for those fuses?
The output valve cathode fuse is F2. Dunno where in the amp it's mounted.
Whatever, looking ok ain't enough, they need to actually be proven to conduct, using a meter. Breaks in the fuse element may not be visually apparant.

If they were blown, or the op amp was failing, wouldn’t that kill the sound, even with the Smicz adapters installed?
Dunno what's in the TAC thingies. A bit of HT current to 0V common can still flow via R9 - LED2.
 

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