ampage
Tube Amps / Music Electronics
For current discussions, please visit Music Electronics Forum. New: view Recent Searches.
New: visit Schematic Hell!
The sunn still shines online!

 
Listen to great tunes streaming live right now!

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  

I'm perplexed , JTM45 Problem ( I think)


 
11/2/2003 1:46 AM
db
email
I'm perplexed , JTM45 Problem ( I think)
I've been doing some final tweaks to my JTM45 clone which sounds great but the two strats I've being using had what I thought to be scratchy volume pots, actually they are somewhat but much more so with the JTM45 and much less with a JTM50 clone and a November. I cleaned out the pots, even took one apart and cleaned it out good. I will have to change to pots but I find it strange that the scratchiness is more predominant with the JTM45 clone.  
Just tried my kids guitar and me thinks itís the amp! His guitar is fine through my other amps but scratchy though the JTM!  
I went through the amp and no leaky caps , the amps controls work fine with no scratchiness .I swapped out the 250pf in the tone stack, tried different tubes, nuthin. Double checked the wiring and all looks good, each set of jacks has a separate ground wire like my other amps, is it possible that the 1meg resistor it's self is the culprit?  
I'm not sure what else to look at. Any ideas?  
 
Thanks  
 
David
 
11/2/2003 5:13 AM
Richie
email

Try useing a different tube in the first 12AX7,sometimes the tube itself can cause this..and some make it worse than others..maybe for a test..try swapping the preamp tubes from the other amp you built and compare them..See if the problem goes to to the other amp with those tubes..  
 
Richie
 
11/2/2003 3:36 PM
db
email
still perplexed
Hi Ritchie,  
 
I did try the tube swap unfortunately to no avail. It's the same sound ya get with the presence pot. What really gets me is that everything looks good, the amp is quite, the pots on the amp are noiseless. I wiggled the tube sockets and they are tight with no apparent noise.  
Either I did something really stupid that is staring me in the face or some part is bad.  
Thanks  
David
 
11/2/2003 11:10 PM
db
email
Re: still perplexed but a step closer
I found the reason for the scratchiness, there is voltage on the input jack! Something is amiss somewhere for sure but no luck yet finding the root of the problem.It's right under my nose, just can't see around it:D  
 
 
 
my readings  
 
JTM45 clone  
 
-1.1 ( standby)  
-.3 ( ON )  
 
in contrast my JTM50 clone reads the following  
 
 
-.002 (standby)  
-.002 (ON)
 
11/2/2003 11:22 PM
Dai Hirokawa
email

besides a leaky coupling cap, and a bad tube (with grid emission??--as Richie already mentioned), too much length using a "hot shield" lead on the grid, and a local fdbk. lp. (ala Dumblar) seemed to lead to scratchiness on the gtr. pot, to my recollection.
 
11/3/2003 2:15 AM
db
email

Hi Dai,  
 
I think something else is going on, tried a bunch of different tubes. I pulled all the tubes except the recto and the first 12AX7 ,with the amp in standby there is -2 volts on the plates and -.5 grid of the AX7! I also pulled the HT fuse and the results are the same. Can't figure out where this voltage is coming from, the only thing active on the board is the bias. Looked at the wiring again and found nothing wrong but my brain hurts now. haha  
 
What is Dumblar?  
 
 
 
David
 
11/3/2003 4:23 AM
Matt H
email

dumblar= dumble  
 
 
now we're getting soemwhere!!!  
 
 
make sure that your input jacks are isolated from the chassis.
 
   Page 1 of 4 Next> Last Page>>