Marshall 6100LM... 5 x 5.6k "swamp" resistors?

[67Mopar]

I've found an anomaly in my Marshall 6100LM head. There are 4 x 5.6k swamp resistors mounted on the PCB, each one going to pin 5 on the power tube sockets. The last socket (V11) has an additional 5.6k resistor soldered directly to pin 5. I've read that this is was a typo on the schematic, and so all of the 6100 series amps received this additional part.

Why would Marshall leave this resistor on the socket? What effect would 2 x 5.6k resistors connected in-series (on one socket) have on the amp? Would this not effect the biasing?

[67Mopar]

Here is an archive photo of the redundant resistor...

[IMG:586:395]http://i1250.photobucket.com/albums/hh536/soldanofan/Screen%20Shot%202016-11-09%20at%207.00.28%20PM_zpsqmozzqz6.png[/img]

Here is a photo with the 4 x 5.6k PCB mounted resistors, and an arrow pointing to the redundant resistor.

[IMG:960:716]http://i1250.photobucket.com/albums/hh536/soldanofan/Screen%20Shot%202016-11-10%20at%206.29.25%20PM_zpssfpdhiwo.png[/img]

[xtian]

I don't know the term 'swamp' in this context. Are these the grid stoppers? It would be odd to have one tube out of four have twice the resistance than the others.

[martin manning]

Why would Marshall leave this resistor on the socket? What effect would 2 x 5.6k resistors connected in-series (on one socket) have on the amp? Would this not effect the biasing?

Bias won't be affected by the extra resistor since there is essentially no current flowing through it. The most effective place for a stopper resistor is directly on the socket pin. Marshall may have added this one to control an oscillation, and saved the expense of removing the one on the board.

[Cameron]

I've found an anomaly in my Marshall 6100LM head. There are 4 x 5.6k swamp resistors mounted on the PCB, each one going to pin 5 on the power tube sockets. The last socket (V11) has an additional 5.6k resistor soldered directly to pin 5. I've read that this is was a typo on the schematic, and so all of the 6100 series amps received this additional part.

Why would Marshall leave this resistor on the socket? What effect would 2 x 5.6k resistors connected in-series (on one socket) have on the amp? Would this not effect the biasing?

These resistors should be on the socket for them to work properly and do the job ...that extra one may be for stability...or just an assembly mistake.

Edit to add ...Martin beat me to ..haha

[67Mopar]

Maybe it would be better to removed the grid-stoppers form the pcb, link the solder points, and then solder 5.6k resistors directly to the sockets?

Anyway... I've got the amp up and running. It sounds great, but it doesn't seem like the presence control is doing anything? If memory serves, one of the Marshall JCM models uses a similar DI. Its (front panel) presence control is dedicated to its DI, with an internal trim pot that sets the general presence. I haven't noticed an additional internal trim pot in the 6100, but I will look.

[martin manning]

The 6100 scheme I found has a pseudo presence on the PI, controlled by the "Damping" switch through a relay. There is no traditional presence.

You could move all the screen stoppers to the power tube sockets if you're so inclined. It couldn't hurt, and might clean up some small glitches.

[67Mopar]

Thanks! I tried the low/high damping settings... There is a slight difference, though my ears a bit fatigued from fiddling with it for so long.

I don't know if just it's just me, but channel 3 sounds like is has some blocking distortion happening. Sort-of a non-musical type of distortion. My Cornford RK100 the same thing happening. I ended-up replacing the (RK100) V1A/B cathode-bypass caps to 1/2 the stock value... This solved the issue.

[67Mopar]

The presence is definitely not working.

If it's relay controlled, maybe the relay has failed?

[martin manning]

If the "Damping" switch doesn't activate it (it's hardwired) then the relay could have failed, but there are other possibilities. It could be that there is no power getting to it, the diode across the coil is shorted, or the MOSFET that can also activate it is either always turned on (by some fault) or shorted.

[67Mopar]

Edit.

[67Mopar]

If the "Damping" switch doesn't activate it (it's hardwired) then the relay could have failed, but there are other possibilities. It could be that there is no power getting to it, the diode across the coil is shorted, or the MOSFET that can also activate it is either always turned on (by some fault) or shorted.

Thank you! I centered my efforts on the relay itself... One, possibly more solder points were compromised, so I re-soldered every terminal. Not only is it operating as it should, this is one hell of an amplifier!

[martin manning]

Great, glad you got it sorted out!

[67Mopar]

My RK100 used 10k grid-stoppers... In leu of removing the 5.6k stoppers from the board, could I just add an additional 5.6k stopper at each socket (pin 5) to achieve the same stability? If yes, should I use metal-film resistors as grid-stoppers?

[martin manning]

My RK100 used 10k grid-stoppers... In leu of removing the 5.6k stoppers from the board, could I just add an additional 5.6k stopper at each socket (pin 5) to achieve the same stability? If yes, should I use metal-film resistors as grid-stoppers?

Yes and yes. If you do notice some change in the sound that you don't like you can just short the ones on the board from the top side.