markbass tube 800 scratchy pots

[agedhorse]

Except that this is not true. I use JFET op-amps in most of my designs (over more than 30 years) without this problem. Other designers have as well.

 

[ijump206]

Apologies for re-awakening a twice dead thread, but I have a Little Mark Tube 500, long out of waranty, that I had shelved because the master volume pot is so amazingly scratchy as to be useless. The last couple of times I have used the amp, I took to adujsting the volume by using the gain pot, which is suboptimal.

Reading this and a few other threads, I tried turning the knobs a bajillion times, but I also tried a few various knob positions in the gain and eq chain to see if this has any effect.

Interesting to me:

• If I turn the Gain Pots (either channel) to the minimum level needed to get any appreicable signal through the amp chain, I can then adjust the "Master Volume" Pot with much less (almost none!) scratching.
• If I dial in any amount of Gain Pot to the first hashmark or greater, the "Master Volume" Pot becomes amazingly scratchy between 0 and 3 or 4 of the 12 displayed hashmarks printed on the amp head casing.
• If I turn down the "Low" EQ Pot to the minimum setting, I can then adjust the "Master Volume" Pot with much less scratching.
• If I dial any amount of "Low" EQ Pot (say, to 9 o'clock or greater), the "Master Volume" Pot becomes amazingly scratchy between 0 and 3 or 4 of the 12 displayed hashmarks printed on the amp head casing.
• Similarly, If I turn the "Line Out" Pot up to Max and then adjust the Gain Pots for either channel 1 or 2, turning the Gain pots produces scratching at higher gain input levels, say from 12 o'clock onwards.

It feels like something is not right in the totality of this particular signal chain, or set of signal chains, as the settings of upstream pots appears to have an impact on the behavior of the downstream pots.

I am no electrical engineer, and yet I am having a devil of a time finding anybody locally to service this thing - nobody close wants to touch it.

I have opened the housing and extracted the A50K Master Volume Pot module - it would be amazing to be able to replace the entirety of this module as a modular component, but so far I have had no luck in sourcing this as a complete component.

Do we think that replaying the Master Volume pot even has a chance of solving this?

 

[SJan3]

Apologies for re-awakening a twice dead thread, but I have a Little Mark Tube 500, long out of waranty, that I had shelved because the master volume pot is so amazingly scratchy as to be useless. The last couple of times I have used the amp, I took to adujsting the volume by using the gain pot, which is suboptimal.

Reading this and a few other threads, I tried turning the knobs a bajillion times, but I also tried a few various knob positions in the gain and eq chain to see if this has any effect.

Interesting to me:

• If I turn the Gain Pots (either channel) to the minimum level needed to get any appreicable signal through the amp chain, I can then adjust the "Master Volume" Pot with much less (almost none!) scratching.
• If I dial in any amount of Gain Pot to the first hashmark or greater, the "Master Volume" Pot becomes amazingly scratchy between 0 and 3 or 4 of the 12 displayed hashmarks printed on the amp head casing.
• If I turn down the "Low" EQ Pot to the minimum setting, I can then adjust the "Master Volume" Pot with much less scratching.
• If I dial any amount of "Low" EQ Pot (say, to 9 o'clock or greater), the "Master Volume" Pot becomes amazingly scratchy between 0 and 3 or 4 of the 12 displayed hashmarks printed on the amp head casing.
• Similarly, If I turn the "Line Out" Pot up to Max and then adjust the Gain Pots for either channel 1 or 2, turning the Gain pots produces scratching at higher gain input levels, say from 12 o'clock onwards.

It feels like something is not right in the totality of this particular signal chain, or set of signal chains, as the settings of upstream pots appears to have an impact on the behavior of the downstream pots.

I am no electrical engineer, and yet I am having a devil of a time finding anybody locally to service this thing - nobody close wants to touch it.

I have opened the housing and extracted the A50K Master Volume Pot module - it would be amazing to be able to replace the entirety of this module as a modular component, but so far I have had no luck in sourcing this as a complete component.

View attachment 6981458


Do we think that replaying the Master Volume pot even has a chance of solving this?

I had the same amp with the same scratchy Master pot. I didn't wait. I returned it pronto and got a LittleMark 58R. Much better results. I'm no engineer either but if out of warranty see if you can replace the Master pot with one that doesn't incorporate the push/pull mute. It's suspect in my view and you have nothing to lose. Too bad really. I did use the amp on 2 gigs before returning it and really liked it's tone but that scratchy Master was just too much. You might try contacting VST in Ca and see if it's ok to replace the pot without the mute. Maybe they can source you one. Can't hurt to ask..

 

[ijump206]

I had the same amp with the same scratchy Master pot. I didn't wait. I returned it pronto and got a LittleMark 58R. Much better results. I'm no engineer either but if out of warranty see if you can replace the Master pot with one that doesn't incorporate the push/pull mute. It's suspect in my view and you have nothing to lose. Too bad really. I did use the amp on 2 gigs before returning it and really liked it's tone but that scratchy Master was just too much. You might try contacting VST in Ca and see if it's ok to replace the pot without the mute. Maybe they can source you one. Can't hurt to ask..

Thank you for your response. I am checking here in Germany with some of the authorized service centers to see if they can get me replacement parts. We'll see. I have had this amp for over 10 years but not really used it for at least 7 of those, having moved onto my rock solid Mesa Boogie Subway D-800 where a lighter head is the right option for a gig. I have a couple of gigs where I drag out the lead sleads for fun, but the Mesa usually handles everything with aplomb. I am just looking to get this MB head back to functional to serve as a backup, as I am playing more and more gigs, some of which are very well attended, and the need for a backup of everything is becoming quite apparent to me.

 

[agedhorse]

That’s a custom pot, you aren’t going to find one off the shelf.

Since it’s not being used as a PCB mount part, you might check the Mesa Boogie website since we use a similar part in some of the legacy guitar and bass amps. Don’t know if we use the same pot value, and it’s likely that the bushing diameter is a few mm larger and the shaft will be 6.5mm round instead of 6mm splined, but if you can’t get the factory part then it’s an option.

All of the MarkBass pots that I have seen have been 9mm sealed, this must be quite old to use a 16mm part.