|
Post by jestes70 on Aug 29, 2008 19:21:47 GMT -7
I have researched and found Dr. Z's Trouble shooting help post. The amp has a loud hiss. I pulled v5 as Dr suggested and the hiss disappeared. Moving on to the front end of the amp I tried 2 completely different sets of tubes for all the possitions and the hiss remains. I checked the reverb cables and when you turn the reverb completely off the hiss remains. Then replaced the rec with a mullard I had in a maz jr. Then tried different cabs and still the hiss remains. I haven't tried different power tubes because I don't have them and following docs guide they are likely not the problem. So I have a gig tomorrow and hope I can fix the 38 because the jr can't keep up outdoors. Thanks guys for any help you can give.
jeremiah
|
|
|
Post by benttop (Steve) on Aug 29, 2008 19:47:01 GMT -7
It's not very clear exactly what you did there. You pulled the PI and then started fiddling with the preamp tubes?
Each stage of the preamp will increase the level of any hiss present. If one stage has more than it should, the next stage will increase it. Your rectifier has nothing to do with hiss. Nor, typically, do your power tubes. It's one of those preamp tubes almost certainly. Swap them out, one by one, until you find the one that's causing it.
|
|
|
Post by jestes70 on Aug 29, 2008 20:03:25 GMT -7
I swapped out every one of the 12ax7s and 12at7s in the amp with new tubes. The sound is a hum and not a hiss as I first posted. It only becomes prevalent when one or both of the volumes are up past 12:00( straight up).
|
|
|
Post by benttop (Steve) on Aug 29, 2008 20:35:32 GMT -7
Hum and hiss are a different whole thang. You can get hum from a number of sources, such as loose reverb cables, bad reverb tubes, and even from mismatched power tubes. You can also get hum from an electronics problem in the power supply. If it were my amp, I'd plug in a set of power tubes and if that doesn't do it, and you're sure the reverb cables are all fine, time for a tech.
|
|
|
Post by jestes70 on Aug 29, 2008 20:39:11 GMT -7
Thanks benttop. A tech may be in the near future. It sounds to me like a ground hum. I've tried different circuits throughout my house to no avail. I'll try it at a different house tomorrow. Thanks.
jeremiah
|
|
|
Post by jestes70 on Aug 30, 2008 7:37:08 GMT -7
Here is a link to a soundclick post of the hum noise. Turn your speakers up to hear it. This w3as recorded with the master at 12:00 , the volume at 2:00 and no guitar plugged into the amp. THanks for any help. www.soundclick.com/bands/default.cfm?bandID=869110
|
|
|
Post by dei305 on Aug 30, 2008 8:24:58 GMT -7
jestes70, Turn the amp on to where the hum is audible. Then, take a look at the output tubes..see if any are glowing red. Also, if you can gently tap on each output tube using a chop stick, it's possible a tube may need to be reseated in the socket. It's also possible you may need to replace the entire output set. I agree with bentop in also checking the reverb cables and tubes. The 12AT7 sometimes is a culprit. Good Luck! Rick
|
|
|
Post by jestes70 on Aug 30, 2008 9:20:24 GMT -7
jestes70, Turn the amp on to where the hum is audible. Then, take a look at the output tubes..see if any are glowing red. Also, if you can gently tap on each output tube using a chop stick, it's possible a tube may need to be reseated in the socket. It's also possible you may need to replace the entire output set. I agree with bentop in also checking the reverb cables and tubes. The 12AT7 sometimes is a culprit. Good Luck! Rick Thanks for the suggestion but no luck. The tubes all seem to check out fine. I'll get another set of power tubes to change them out just to be sure before I take it to a tech. Thanks. jeremiah
|
|
janlf
Full Member
Moment's Notice
Posts: 173
|
Post by janlf on Aug 31, 2008 2:49:49 GMT -7
Hi. Sorry to hear about your problem . It might be a grounding problem. Try to tighten the 4 chassie bolts on top of the cab and see if that might help . Jan
|
|
|
Post by jestes70 on Aug 31, 2008 9:25:41 GMT -7
Thanks Janlf but I already tried the screws and they are tight. I took the amp to a gig last night and it performed well all night along except for the hum. The hum was controllable with the volume at 12:00 and master at 2:00. Even with overdrives kicked in the hum wasn't to bad but when I would stop playing it was certainly there. If nobody else has any ideas I guess I'm gonna look for a tech. Thanks.
jeremiah
|
|
|
Post by DRZ on Aug 31, 2008 10:03:57 GMT -7
Boy that is a strong hum sounds like a transformer hum. By pulling the PI you have proved the problem is in the front-end of the amp. Does this old maz have a buffered effects loop ? If so it sounds like a bad cathode resister on V5 should be a 12AU7 E/F driver. To help you try pulling one pre-amp tube at a time from the PI V6 , try V5 ,V4 and so one and see were the hum stops. It's not a tube hum, it is a filter stage or a shorted by-pass cathode cap or resister, or the reverb transformer. Let me know what tube you pulled to eliminate the hum. I doubt you'll able to fix this your self but a good tech with some help should nail it very quickly. I guess also has it always done this, I assume you bought it used and did have this problem from day one. That can be 15 year old amp, and that's 100 years in boutique amp years . DR.Z
|
|
|
Post by jestes70 on Aug 31, 2008 11:13:40 GMT -7
THanks very much for replying Dr.Z . THe amp starts with s/n K so I'm guessing a 2001 model. It has an effects loop but I don't know if its buffered. It has 5 preamp tubes. I did just buy it used from the second owner. The amp has all the original J/J tubes and has seen very little use. Less than 20 hours according to the previous owner. It pulled the tubes one at a time from the PI (V5 it doesn't have V6) as suggested. When I pulled V3 the hum went away. The same is true for V2 and V1. Thanks so much for your reply and I have a local tech lined up who can work on the amp if you point the way. Much appreciated, and I can't believe how good this amp sounded at my gig the other night even with an apparent problem.
jeremiah
|
|
|
Post by DRZ on Aug 31, 2008 11:29:08 GMT -7
That is a 2000 circa
Serial loop not buffered , no tube or Send and Return level pots.
V3 12AT7 reverb drive circuit.
Check tub and cables to reverb tub, and could also be the Reverb Tranny.
Also make sure reverb tub is not sitting on top of your power transformer.
That's were I would start.
Z
|
|
|
Post by jestes70 on Aug 31, 2008 13:02:56 GMT -7
I checked the reverb cables. I can unplug them both and the hum is still present. The reverb tub isn't sitting on the power tranny. Its mounted above the reverb transformer on the left side looking at the back of the amp. I reseated the reverb cables and still the hum. Is this leaving the reverb tranny as the culprit or is there another internal component? Any way to test the reverb tranny? The reverb tranny is labeled Dr z cin tran 3229 if that makes any difference. Thanks.
jeremiah
|
|
|
Post by DRZ on Aug 31, 2008 14:12:33 GMT -7
I think you have checked all things you can, it's time for a tech or send back to factory and we can fix it up.
Z
|
|
|
Post by jestes70 on Aug 31, 2008 14:39:44 GMT -7
OK. I'll try my local tech first to avoid shipping. I'll tell him to start with the reverb circuit. If it needs a reverb tranny can I order one from you? Thanks for your help.
jeremiah
|
|
|
Post by jestes70 on Aug 31, 2008 19:20:25 GMT -7
A little off subject but my studio lead head looks like it has a choke on it. My maz jr reverb doesn't. Do the new maz seniors have the choke or is this just part of the old design? Thanks.
jeremiah
|
|