Another Marshall SLX issue - no heater voltage on one socket?

AndyK said:
I read on the old Lord Valve bias FAQ document (the Bible in the early days of the web) that “some Marshalls don’t like having extra wire between the tube and probe, and may oscillate”. Not sure if the SLX is doing that.
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Our TTR brother, Ivan H, encouraged me to build bias socket adapters that break the circuit at the sockets anode/plate pin 3. He advised to keep the adapter's leads very short for the reason that you have described.
 
I ordered some 1-ohm resistors, will see if I can get a reading when I install one.

On another note, seems the presence control does absolutely nothing. What would that be caused by?
 
AndyK said:
I have a Eurotubes bias probe socket adapter that measures MA, and the amp doesn't like it. All I get is buzz and no readings. I'm thinking the 1-ohm resistor method may work better, measuring MV.

Also, the wire joining pins 1 and 8 goes to ground somewhere, but there is no actual ground lug like in most JCM800's. So I would attach the resistor to the socket, cut the black wire and splice it to the other end of the resistor, if that makes sense.
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Maybe the ma fuse in your meter is blown?
Weak battery?
 
How did your under board pcb inspection go ?? any broken solder joints on the under side of the control board.
 
AndyK said:
I’ll check the meter, thanks. What about the non working Presence ?
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The presence control only affects the super high frequencies.

If you plug a CD / music player into the guitar input...
adjust volume for clean sound...
you should be able to verify that the tone controls and presence are working.
The presence only controls the brightness of the very highest frequencies.

If you plug the amp into a stereo speaker:
Play CD music as above.
the stereo speaker has a tweeter which allows hearing the highest frequencies.
You should be able to hear the presence working.

Guitar speaker:
does not reach high frequencies. Only goes to about 6Khz, barely any highs.
This might make you think the presence isn't working. Because the speaker won't reproduce highs.
 
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I’ve had many Marshalls and in every case, when I lowered the presence, then raised it, there was a noticeable difference in brightness. I have Greenback speakers. They pick up presence changes in other amps. So I’m guessing my presence isn’t functional.
 
AndyK said:
I’ve had many Marshalls and in every case, when I lowered the presence, then raised it, there was a noticeable difference in brightness. I have Greenback speakers. They pick up presence changes in other amps. So I’m guessing my presence isn’t functional.
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Can you post pictures of the wiring?
 
AndyK said:
I’ve had many Marshalls and in every case, when I lowered the presence, then raised it, there was a noticeable difference in brightness. I have Greenback speakers. They pick up presence changes in other amps. So I’m guessing my presence isn’t functional.
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FWIW... My two 50 watt JCM800's presence control effect is very subtle. Amps are modded but the presence circuit is of stock 2204 values and layout. The louder the amp is, the more I'll hear the effect of the presence control.

Another method to hear if your amp's presence control is doing anything, is to turn on the amp, with nothing plugged into the input jack. Turn up the master and preamp volumes till you can hear the tranny-hum and preamp-hiss in the background. Now rotate the presence control... You should here a good distinction of tone change (brighter and louder) in the last half of travel.
 
Update: I installed my 1-ohm resistor, and discovered the bias was set to 17ma, and that's with the screen current. So, maybe that 13 I was getting with the bias probe was accurate? Doesn't explain the noises though.

Of course that means Fuchs didn't check the bias at all. Or, should I say the apprentice that actually did the work didn't bother to check the bias. With the 1-ohm resistor, I was able to use the trim pot and set the bias to 35ma (around 65% since the plate voltage is 455). I actually set it to 39, allowing for the screen current.

I put the master on 2, and fired up the amp, and almost peeled the paint off my walls! Damn! This thing screams, and is plenty bright and punchy now. Also, my presence works fine now that the bias is set.

I guess severely under-biasing the tubes doesn't sound good?
 
AndyK said:
Update: I installed my 1-ohm resistor, and discovered the bias was set to 17ma, and that's with the screen current. So, maybe that 13 I was getting with the bias probe was accurate? Doesn't explain the noises though.

Of course that means Fuchs didn't check the bias at all. Or, should I say the apprentice that actually did the work didn't bother to check the bias. With the 1-ohm resistor, I was able to use the trim pot and set the bias to 35ma (around 65% since the plate voltage is 455). I actually set it to 39, allowing for the screen current.

I put the master on 2, and fired up the amp, and almost peeled the paint off my walls! Damn! This thing screams, and is plenty bright and punchy now. Also, my presence works fine now that the bias is set.

I guess severely under-biasing the tubes doesn't sound good?
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If your bias amperage is too low, it's called "cold bias" and while it won't really hurt the tubes, it makes them about as anemic as you can get. Too much restraint and never reach their potential.

Inversely, going too much leads to red-platting and kills your tubes, often some other things. There IS a sweet spot where you can go a lil hotter and get a better punch but it will shorten the life a bit
 
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