Does anyone feel the JP2C is overly compressed?

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GJgo

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There's no doubt that the JP is much more compressed than the original, it's the first thing you notice when you A/B them. I know with my other Boogies as you increase the gain it gets more compressed, and the JP has a TON of gain on tap.

The Mark V guys have had great success with putting a 12AT7 in (?) V6, which they say REALLY opens up the amp. I've heard clips & I'd agree.

Does anyone with a JP & a 12AT7 feel like rolling one through the amp to see if there's a slot where the less gainy tube opens the tone up a little?
 
I have both but not sure if I am willing to do such at this point. If I could see a schematic then perhaps I might (would like to determine if the 12AT7 tube would cause issue with the components used in the circuits. I really do not what to address a warranty claim on this amp. If it is anything like the original Mark IIC+ it lacks the added grid to cathode capacitance on the cascaded pair that generates the gain structure which is why the Mark V benefited from the 12AT7 in V4. Not sure if this helps. I do have another set of NOS Beijing tubes (same tubes used in the Mark III up until 1992 before they switched over to JJ tubes). I loaded up the Mark V and wow what a difference and it even sounds killer with the 12AT7. If you have a Mark IIC+ is it Simul-Class or the Class A/B and what preamp tubes are in it?

The Mark IIC+ has a similar gain stage ladder but somewhat different but it appears that V4 would be the tube position to use.

Mark IIC+ schematic can be found: V1A -> V1B -> high gain drive cascade [ V3B ->V4A (change this one) ]--> V2A -> [ reverb send/return and FXloop] -> V2B - > V5 (PI).
Reverb send is V4B and reverb return is V3A. If I get around to it I may give it a try. I will also see if the tube chart matches the flow.... It does compare to the original IIC+. My concern is the plate voltage on the reverb drive circuit as it may be much higher than the Mark V. At least there may be hope with the JP-2C or even the Mark IIC+. IF the drive cascaded pair shared a common tube it would not be possible to do this trick.

I doubt this would bypass the presets that were done to compensate for the separate drive control or pull switches I also noticed on the IIC+ schematic that the plate voltage on the reverb was marked with 334V but was difficult to actually make out the numbers due to the scanned drawing.
 
Interesting & worthy thoughts, thanks. I have a 100 watt IIC+ and a Simulclass IIC++ so in thinking it through further I'm probably better off leaving well enough alone with the JP since it's the only compressed / modern amp that I currently have. If I want the old sound, play the old amp!

These days I'm using the Mesa / JJ 12AX7 tubes in V1-V4 in my Mark amps & a Mullard long plate in the PI. (To me it opens up up nicely.) I like the old Chinese tubes in my Recto, but they're a bit fizzy to my ear in my Marks.
 
I did not notice any fizz, I just loaded my Mark V with them and I was impressed. Also tried the 12AT7 in V4 and that was a win for me.

I am pondering what the effect will be on the JP if any. I am concerned about the reverb circuit and its plate voltage. I may try it but hopefully I will not regret that. the JJ/Mesa tubes are okay but I have had better tubes. I had to replace all of the preamp tubes in the JP with new ones (had enough of the Mesa tube in my stash to replace them). Makes me wonder if the quality of the JJ tube is declining. If I get to the 12AT in V4 I will let you know if it works or I killed something in the amp. It should work just as good as the JJ tube has the same voltage ratings (perhaps +20 V higher). I have been curious myself. So why not just do it. (fingers crossed and hope no smoke).
 
For starters the old Mesa square getter tubes (Beijing) are awesome in the Mark V. No fizz. However in the JP-2C they do sound fizzy. Interesting. The stock JJ tubes seem to be the best preamp tubes for this amp. The Clean channel sounds thick and sweet, ideal for the blues but with the square getter tubes it was bright and not as nice. Mesa definitely tuned the preamp circuits for the impedance of the JJ tube characteristics. I cannot complain at all as the stock Mesa tubes just work with this amp.

The 12AT7 in V4 did reduce the gain as well as the Master volume. I did not notice much of a change in the temperament of CH2 or CH3. Perhaps a little darker in tone but I felt the JP sounded better with the stock tube. I had assumed as much since the secondary gain stage (V4A which is driven by V3B) in the cascaded drive circuit does not have the positive feedback cap between control grid and cathode (assuming it is the original IIC+ circuit) that is present in the Mark V. The feedback cap adds more gain to the secondary tube but it also is frequency dependent, the higher the frequency the larger the positive feedback which causes the amp to become an ice pick with higher gain settings. The 12AT7 in the Mark V (V4B which is driven by V5A) helps to reduce the high order harmonics and roll off the gain on the higher frequencies resulting in a better sound and distorted signal. This feed back capacitor is not present in the JP-2C as the side effect of the 12AT7 resulted in reduction in the output of the cascaded drive section. The 12AT7 will work in the JP-2C and try it if you like. The JAN/Phillips 12AT7 will work just fine in the V4 position but for how long is uncertain. I do believe the plate voltages are quite different in the JP-2C compared to the Mark V. The trial (as I was interested to find out for myself) with the 12AT7 in V4 took more away from the amp than what I would have hoped for. The reduction in gain also reduced the loudness of the amp. May have made the bottom end a bit tighter but not much in the change of gain character or sense of compression. I do not believe the change in the gain structure to reduce the compression will be relieved with a simple tube change. Perhaps a 5751 would be a better choice than a 12AT7. I would assume having the 5751 in V1 may help as the voltage divider circuit on the tone stack maintains a higher signal level which adds to the drive level. Either that change the V3 tube and leave V4 as is. Some experimentation may be necessary. I like my JP with the stock tubes as is. the Mark V with the 12AT7 brings the amp into the same ball park in tonal character of the JP-2C.
 
V4 on the JP-2C appears to do the same job that V4 on the Mark V does, in both triodes (but reversed). So that would be your best bet.

There is no equivalent to V6 in the JP-2C.
 
Just a thought but haven't some found the AT7 works well in the PI position with iic+'s and iii's? Worth a shot here with the jp2c?
 
Not overly compressed. More compressed than a Mark II or III but less compressed than a Mark IV or V. Much less compressed than most high gain amps I've tried including Marshall JCM 800 amps.

I like the mild compression, it's part of the ease of use that this amp has. If I pull the gain knob on ch.2 or ch.3 there's a definite compression of tone and by turning the shred mode on, I can keep the gain lower, lessening the compressed feel. Keeping the treble low and presence high helps me get a very good heavy metal tone with the gain low enough that any compression is kind of wanted actually to thicken up the sound.
 
I find these amps the least compressed (Mark Series) they are very strident compared to the other High Gain amps which all use the Marshall type topology
 
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