Diminished response

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DrewV

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Hi, I have a 60 watt Mark IIC and a friend has a 60 watt Mark IIB. We both bought our amps new directly from MB. Remembering back thru the fog, our new Boogies exibited amazing touch sensitivity and near infinite sustain even at minimum volumn levels. Over the years we have both noticed that the amps no longer display that touch and sustain quality. Both amps have had the filter caps replaced (a couple of times) and I even changed out all the electrolytics a couple of years ago. Tubes have been replaced as needed, but even new tubes don't bring back that original feel. Has anyone else run into this phenomena? Are we just smoking too much or too little? Is it time to blueprint the amps? Thanks DrewV
 
What about the speakers? Perhaps the cones have deteriorated somewhat.

DrewV said:
Hi, I have a 60 watt Mark IIC and a friend has a 60 watt Mark IIB. We both bought our amps new directly from MB. Remembering back thru the fog, our new Boogies exibited amazing touch sensitivity and near infinite sustain even at minimum volumn levels. Over the years we have both noticed that the amps no longer display that touch and sustain quality. Both amps have had the filter caps replaced (a couple of times) and I even changed out all the electrolytics a couple of years ago. Tubes have been replaced as needed, but even new tubes don't bring back that original feel. Has anyone else run into this phenomena? Are we just smoking too much or too little? Is it time to blueprint the amps? Thanks DrewV
Click to expand...
 
I've got a fairly new set of MB STR454's in the amp right now. It just seems like the "lead drive" channel is not as hot as it used to be. I thought the bias resistors might have gone out of spec and that the 6L6's were running too cold but the resistors read good. Is there a certain grade of 6L6 that would run hotter with the stock Boogie bias settings? Would this be a good option? Or maybe a specific 12ax7 in V1 that would yield more gain early on. Would it be a good idea to replace some of the Sprague "orange drop" caps in the lead drive circuit.? Thanks DrewV
 
...another thing: what's your presence setting like these days? You might try turning it up a bit.


DrewV said:
I've got a fairly new set of MB STR454's in the amp right now. It just seems like the "lead drive" channel is not as hot as it used to be. I thought the bias resistors might have gone out of spec and that the 6L6's were running too cold but the resistors read good. Is there a certain grade of 6L6 that would run hotter with the stock Boogie bias settings? Would this be a good option? Or maybe a specific 12ax7 in V1 that would yield more gain early on. Would it be a good idea to replace some of the Sprague "orange drop" caps in the lead drive circuit.? Thanks DrewV
Click to expand...
 
Speakers wouldn't be a problem. Besides the original stock Black Shadow I've got Vintage 30's, Greenbacks andf Vox Blue Bulldogs. Presence control is usually set to 31/2 to 4. I've got two sets of "Mesa Engineering" made in USA 6L6's. The original set and the first replacement set that I bought from Mesa in 1985 or 86. I 'll stick 'em in there and see if it makes a difference. When I had the amp opened up I notice that one of the large "orange Drop" caps near the lead drive controls was looking rather brown in color. It's value is .005uf /400D/ 715P. Could this cap be affecting the lead drive channel response? thanks DrewV
 
One other factor is ear fatigue. I know in my case, I simply can't hear as much of the nuance as I could 25 years ago. :)
 
xrisj, My Marshall JTM 60 sounds just fine and is very touch sensitive, as are a number of other amps available to me for comparison. thanks DrewV
 
Here's a link to my MKIIC gutshot
http://www.drewswebsite.com/images/GUTSHOT%201.jpg
Thanks DrewV
 
One end of the cap connects to pin 7 of V4. To the left of the cap is a 1 watt carbon comp resistor valued at 680 ohm. It looks a bit discolored and measures 630 ohm in circuit.
 
It could just be where the power tubes are biased at.Even tho you buy tubes from Mesa that are supposedly matched to the last pair you had,it aint necessarily so.I've seen tubes of the same "grade" differ by as much as 20ma's from set to set.Makes a big difference in tone.They will certainly be "safe" but not ideal.Pre-amp tubes make a big difference in the amps response.That is a matter of experimentation,to find what sounds and feels best to you.What kind of caps did you use for the filters?
 
I have improved the response and tone by replacing the out of spec 680 ohm resistor. Also, the amp seems louder. Can someone tell me what this resistor does? thanks DrewV
 
Here's what I have done to the amp so far. As noted earlier I replaced the .005/ 400 Sprague 715P with the same type at .0047/400. (Mouser did not have a .005). The original cap reads OL out of circuit. I also replaced the 680 omh resistor. Also in the area of V4 I replaced a couple 470kohm resistors. I replaced a 330Kohm resistor near V2 and a 10M ohm carbon comp connected to the input (I think). I changed out one resistor on the power board, a carbon comp 10k ohm/ 2w connected to the red wire at the binding post. I used a Vishay metal film resistor. Measuring the voltage at the each of the two 2.2k bias resistors I got -48vdc. Is this the proper voltage for this amp? Measuring the two 100ohm carbon comp heater to ground resistors I get 49.5 ohms at each one in circuit. Lastly, I cleaned all tube pins and sockets, retensioning the 6L6 sockets as well as Deoxit on all input and output jacks and plugs.
The tone and feel of the amp has come back really strong. The amp sounds better than it has in years, with very detailed and defined Clean and Lead tones. ANd the reverb is back to its' original lush state. There's just points of contention left. #1 At one time this amp had a very spongy (for lack of a better term) feel to it, like walking on foam rubber. I seem to remember noticing it once after replacing the 6L6's, possibly with Sovtec 5881WXT's (around 1987). #2 When the amp was new I ditinctly remember being able to get near infinite sustain at extremely low volume levels. The amp while getting a nice rich overdriven sound ( a bit similar sounding to a Sovtec Mig 50 and then some) doesn't sustain like that now. I have checked nearly every component finding most within spec. Where should I look to bring back to spec/ improve these two qualities of this amp. Thanks in advance DrewV
 
How do the transformers look? Any visible rust? If the laminations short out due to rust, tone will be effected, including possible perceived loss of power.
I know they are not inexpensive to replace, but it is something to consider if they are visibly rusty. I wish you......

Good luck! :D
 
The 680 Ohm 2W is for the LDR supply. It turns -30V with a diode to +12/13. It's discolored because it's the hottest running resistor
in the whole amp. It runs blazing hot even though it's only dropping 30V. The 10K 2W from the plate's to the terminal strip/red cloth wire
is your reverb supply voltage. That resistor needs to be at least 1 to 2W. All the 1/2 watt droppers like in the MK III eventually cook.
The 10M at the treble cap is only there to prevent a pop when switching the 750pf cap in for the "Pull Shift" on the Treble pot. The ground reference resistors
at 49.5 are fine and the -48V at pin 5 are perfect. Pin 3 should be 448V to the plates. Sadly, the only resistors I would have worried about in these amps
is the plate load resistors, screen grids, the two 5k6 droppers on the power amp board and the 1K dropper on the preamp to the 30uf/500V cap.
Knowing the preamp supply voltage from the buss and looking at the voltage drop across the plate resistors would be the first place to start in the preamp
after cleaning the pots and jacks. The rest of the resistors in the preamp rarely if ever see more than a few volts after decoupling.
You may have resurrected the amp with the cleaning and freshened up the reverb, so in the end it's all good.

For the tonal issues, a cap job may be the cure. The difference in sustain and tightness is remarkable. :D
 
For that spongy feel and the sustain you describe that it once had,I would say the power tubes bias point could be the problem.I have had people bring me Boogies with that complaint and was able to "cure"it with a bias adjustment.
 
"For the tonal issues, a cap job may be the cure. The difference in sustain and tightness is remarkable."
Hey,B.B I remember a time when we strongly disagreed on this fact.I would have said the same thing,but I thought I said it in one of my earlier responses,must have been thinking I was in a different thread.That has been my standard response to any amp over ten years old,I cant believe you beat me to it this time,B.B.
 
stokes said:
"For the tonal issues, a cap job may be the cure. The difference in sustain and tightness is remarkable."
Hey,B.B I remember a time when we strongly disagreed on this fact.I would have said the same thing,but I thought I said it in one of my earlier responses,must have been thinking I was in a different thread.That has been my standard response to any amp over ten years old,I cant believe you beat me to it this time,B.B.
Click to expand...

I agree with you 100%. I took a hard look at ESR as well as age thanks to you. Even though the caps may read perfect after 20 years on
a cap meter seeing how much the ESR is effected is another eye opener. I would love to see the condition of the dielectric after so many years
in use. They must be ready to arc at any time.

Taking advice and listening to the difference is why I totally understand your ten year rule. :D
Good advice !!!!
 

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