Mark IVa R1 volume swells issue grows worse

[aaronlyon]

Read some old posts, but couldn't answer my questions. So:

I let my Mark IV warm up in standby for a minute before turning it on. Still, during the first couple of minutes of playing, Rhythm 1 channel (only--doesn't seem to happen in other channels) fluctuates volume up and down every couple of seconds. This problem goes away after a while. Which tube is most likely causing this? (Manual says there's not one tube component specifically for R1.)

I want to buy a couple spare 12AX7 tubes anyway, and I understand the tube in V1 (input stage) can impart a bit of character. True? Is it worth shopping other brands of 12AX7 for this purpose? What about Mesa's SPAX7:

MESA SPAX7 preamp tubes are a super-premium grade substitute for any 12AX7 positions - They are a "Special Requirement Preamp Tube" that possess an even higher standard of overall performance, due to their incredibly low sensitivity to microphonic noise - This is the ultimate preamp tube to use in all critical gain stage positions - It is the absolute best of the best!

 

[aaronlyon]

I let my Mark IV warm up in standby for a minute before turning it on. Still, during the first couple of minutes of playing, Rhythm 1 channel (only--doesn't seem to happen in other channels) fluctuates volume up and down every couple of seconds. This problem goes away after a while. Which tube is most likely causing this

This problem is getting worse. Quite noticeable during first few minutes of operation. What is likely causing this volume fluctuation? 12AX7s?

 

[Boogiebabies]

Clean the pots for Ch. 1, specifically the Gain pot and the R1 Volume. One may be damaged, so take a look.
Sometimes any front impact with the pots knocks the back and wafer off.

Also, if you have a dead spot in one of the pots, try lower settings until you figure out
what pot may need to be replaced.

 

[aaronlyon]

I already replaced two bad pots in R1 channel (R1 Treble and R1+R2 Bass). The other pots are smooth and noiseless when R1 is operating properly.

Next best guess?

 

[Boogiebabies]

Still my first best non "Take it to a Tech" guess. The gain and volume pots.

The R2 and Lead would not work either if the V1 was bad. The only other components in the way are LDR's 5 and 6 that may be acting like an old Fender Tremolo
effect and oscillating the volume up and down from the photo resistor opening and closing. LDR 5 would open and close the tone
stack and LDR 6 would open and close the shunt to ground. Either one oscillating could cause volume swells on R1.

 

[aaronlyon]

Still my first best non "Take it to a Tech" guess. The gain and volume pots.

The R2 and Lead would not work either if the V1 was bad. The only other components in the way are LDR's 5 and 6 that may be acting like an old Fender Tremolo
effect and oscillating the volume up and down from the photo resistor opening and closing. LDR 5 would open and close the tone
stack and LDR 6 would open and close the shunt to ground. Either one oscillating could cause volume swells on R1.

Time to revisit this issue, because it's pretty bad. I'm confident it's not a bad pot, nor tubes. That leaves your suggestion of LDRs. Could I possibly test this by using an alligator jumper to short (bypass) an LDR to see if this makes the volume swells stop?

 

[aaronlyon]

Here's an interior shot of my Mark IVa. I replaced the pots for R1 Treble and R1+R2 Bass.

It looks like a major undertaking to access the bottom of the main circuit board. Looks like you'd have to disconnect every darn front and back panel pots, switches and jacks. True? This is probably a job for Mesa.

 

[aaronlyon]

Why do all amp makers put hot glue on the caps?

 

[aaronlyon]

Something new and confusing: You know the Mode Select knob on the back? Five positions: Ft Sw, R1, R2, Lead, Ex Sw. I don't have anything plugged into the External Switching Jacks. But when I turn the Mode Select knob to Ex Sw, I hear some COMBINATION of channels. I mean, even when I turn R1 Gain, R2 Gain, and Lead Gain all to zero, I still hear some amplified guitar, which level I can adjust with the overall Output Level knob.

In this "mystery mode", the sound responds only to the Output Level knob and the EQ. None of the other knobs, for any of the channels, affects the tone at all. AND, there is a significant 60 Hz hum that's audible only when the Mode Select is set to Ex Sw.

Is this how the Mark IVa is supposed to work, or is this further evidence that I have LDR problems?

-a

 

[guitarzan1143]

take it to a tech. mines sitting in a techs bench right now getting the resistors changed and general maintenace. it needs it, its about 20 years old now. stop beating around the bush before something seriously wrong happens in the amp!

 

[aaronlyon]

Mesa just called. Indeed, they found bad relays on R1 and replaced them. Should have it back in a week.

In the mean time I built a 1-watt Firefly tube amp which is super cool: http://ampgarage.com/forum/viewtopic.php?t=11248

 

[Tuna141]

aaronlyon
Post subject: Re: Mark IV preamp tube questions

Why do all amp makers put hot glue on the caps?

The hot glue is a good thing - it's for improved reliability. Its purpose is to keep the longer lead and heavy components from moving when the amp is moved/transported/played. Since the glue restricts the component's movement, it reduces the chance of a component lead breaking over time due to continual flexing.

 

[guitarzan1143]

I got the filter caps replaced on mine since it is 20 years old now, fixed the problem entirely.