1996 Marshall 6100LM Presence Pot Not Working / Mode Volume Levels

[MacFangus]

This is my first post ever, so I would first like to say hello to everyone here. Hello!

I have a Marshall 6100LM here. Channel 1 sounds great, but channel 2 may have some issues. I will list them…
1. JTM45 mode seems very low in volume.
2. Plexi mode seems low in volume.
3. JCM900 mode comes on like gangbusters!
4. Excessive bass on channel 2.

I'm also hearing crossover distortion coming through on channel 2 in all modes. It's bias properly with =C= 6L6GC tubes, and the preamp tubes all check good. I don't know if these are typical issues, or maybe this is normal for the 6100LM?

[Mike_LA]

Greetings and welcome.

I'm sure some of the other guys will chime in but I would recommend a good spraying of contact cleaner in the switches would be a good place to start.

Best of luck,
M

6100 Tech Thread

[MacFangus]

Thank you!

I have the chassis out now. There is a resistor soldered to PIN 5 (green wire) on one of the power tube sockets... Is this normal? It's on the socket closest to the transformers. I was running it last night, and began to get very noisy. I looked in the back of the amp, and noticed that the tubes were red plating, so I shut it down.

[MacFangus]

I just got through cleaning off a ton of oxidation from the chassis surface (for max cooling), cleaned the tube sockets w/Deoxit, and then tightened the contacts in the power tube sockets. I ran through some tests, which all checked-out... No red plating, so I think that PIN 5 may have not been making contact with the tube pin.

I then switched to Pentode/Triode mode which worked fine, but when I switched it into Low Power mode, one of the T500mA power fuses popped! Any ideas what caused this? I will get some new fuses, and power it up with no power tubes installed... I'm thinking that this would indicate a bad tube, maybe? I don't see how, as I've used these tubes in my other amp, with no issues at all.

*I shut the amp down before I made these switch adjustments.

[jeffsmusic4919]

Thank you!

I have the chassis out now. There is a resistor soldered to PIN 5 (green wire) on one of the power tube sockets... Is this normal? It's on the socket closest to the transformers. I was running it last night, and began to get very noisy. I looked in the back of the amp, and noticed that the tubes were red plating, so I shut it down.

If it is the one is this thread, take it out. I had to remove mine as well.
http://marshallroadhouse.com/viewtopic. ... 40#p157840

[jeffsmusic4919]

Crossover distortion is is a sign of a cold biased amp , I think. I would just start with fresh tubes. Once a tube red plates it cooks off all the plating on the plates. It can cause some over heating issues etc. Just make sure you pull the power tubes first and check voltages to the pins first to begin to isolate the issue. Oh, test your switches as well, they can get corroded from non use.

[Anitoli]

Ok, first off that extra resistor on V-11 is there because there is a typo on the schematic. Take it out, it doesn't need to be there. Pin 5 is the bias supply wire, so yes if it was not making contact the tube would definitely red plate.

Generally when the out put fail fuses pop it means a shorted tube. Even if a tube is new it can still go south. With no tubes in what are the voltages on pin 3, 4, and 5?

[MacFangus]

Thank you for the replies. Don't I need to put a new HT fuse in before testing? I have some on the way... They will be here in 2 days.

Damn! I've been using these SED =C= 6L6 tubes for about 3 months in my other amp, with no issues at all. Are they now trashed from the red plating? I put them on my Hickok tester this morning, and they test okay.

Do you think this 5.6k resistor @ V11 could have had anything to do with the fuse popping?

What about the volume differences when switching between modes A (JTM45) B (Plexi) C (JCM900)? There's a substantial difference in volume between the modes, which I suppose may be normal? When I change modes (channel 2) I have to either raise or lower channel 2 volume, depending on the mode I'm in.

Even though I'm having issues, I really dig this amp! I want to get it working properly, and make it a keeper.

[jeffsmusic4919]

Don't I need to put a new HT fuse in before testing?

Yes

Damn! I've been using these SED =C= 6L6 tubes for about 3 months in my other amp, with no issues at all. Are they now trashed from the red plating? I put them on my Hickok tester this morning, and they test okay.

I had a pair red plate while a friend was repairing my Vibrolux and he just replaced them. Maybe there isn't much to it though, I was just repeating what he told me.

Do you think this 5.6k resistor @ V11 could have had anything to do with the fuse popping?

It didn't effect mine in that way. However, Anitoli is the grand master of repairing these so, wait for him to answer.

What about the volume differences when switching between modes A (JTM45) B (Plexi) C (JCM900)? There's a substantial difference in volume between the modes, which I suppose may be normal? When I change modes (channel 2) I have to either raise or lower channel 2 volume, depending on the mode I'm in.

I think mine is the same, since they aren't foot switchable, I am not sure the engineers really cared much. Although I would have to say, mine could just be broken in the same way yours is. Did I mention Anitoli is the grand master of repairing these amps.

Even though I'm having issues, I really dig this amp! I want to get it working properly, and make it a keeper.

If the volume thing is permanent, these are excellent pedal platforms. Pick the three channels you want and add a pedal or two for the fourth and or fifth.

[MacFangus]

Thanks! I will wait for Antitoli.

Question... Can a power tube test AOK on my Hickok 6000A, yet fail under actual use?

I think the red-plating may have been the result of the power tube pin sockets being stretched open by the JJ 6L6GC tubes that were in it. Would explain a no continuity issue in regards to pin 5, if that's what in-fact happened. Those JJ pins are huge!

[Anitoli]

Thank you for the replies. Don't I need to put a new HT fuse in before testing? I have some on the way... They will be here in 2 days.

Damn! I've been using these SED =C= 6L6 tubes for about 3 months in my other amp, with no issues at all. Are they now trashed from the red plating? I put them on my Hickok tester this morning, and they test okay.

Do you think this 5.6k resistor @ V11 could have had anything to do with the fuse popping?

What about the volume differences when switching between modes A (JTM45) B (Plexi) C (JCM900)? There's a substantial difference in volume between the modes, which I suppose may be normal? When I change modes (channel 2) I have to either raise or lower channel 2 volume, depending on the mode I'm in.

Even though I'm having issues, I really dig this amp! I want to get it working properly, and make it a keeper.

If the HT fuse didn't blow, it doesn't need to be changed.

If the tubes test ok they should still be good. The extra 5.6k on V11 wouldn't cause that it just doesn't need to be there. The voltage on that pin is the negative bias voltage and it's not really flowing much current so there won't be much of a voltage drop if any. Take it out.

All the 6100's i have seen have a volume difference between modes A-B-C, with C being the loudest. This is due in part because the gain structures are much lower in A-B as compared to C. If you look at the schematic when in mode A the triode in V2b is eliminated from the circuit which leaves mode A using only two triodes. Mode B and C use this triode and C specifically uses lower cathode resistances to push up the gain. So yes, it's normal to have to adjust the volume when switching modes on channel two as long as it is not too drastic it's ok.

[MacFangus]

Understood. Thank you.

I replaced the blown T500mA fuse today. Out of curiosity, I bell tested the contacts of the impedance selector switch. All 6 terminal legs (regardless of the switch setting) read continuity between the next... Is this normal?

I've got the fuses in, checked in both high and low power modes... Fuses are holding! But, when I switch the amp to low power mode, the bias climbs 5mV?

[MacFangus]

I have removed the redundant resistor and re-soldered the wire to pin 5, checked all fuses, it is biased properly...but now I get no sound out of the amp. All three channels are dead. The channel buttons all work. Rather, the lights are changing on the front panel... I can hear an occasional pop from my bench speaker when I switch between the dampening modes, but that's it. :(

Update: I flipped the amp upright, started messing with the buttons and knobs, and it started working! It wasn't that the DI button was pushed-in, because I checked that several times. Weird! I biased the SED =C= 6L6 tubes at 38 @ 100 watts (42 @ 50 watts).

I'm thinking about spraying the slide switches (high/low power - impedance - pentode/triode) with Deoxit Silver and then some Deoxit Gold spray lube.

[MacFangus]

Update... The amps sounds great, except my presence control doesn't seem to do anything?

[jeffsmusic4919]

Check the dampening switch, in auto is should work. In high, I think the circuit is by passed. Flip the switches while the amp is off by the way.