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Post by joedan on May 7, 2021 6:12:56 GMT -7
Hi everyone! I have a Jetta combo that appears to have a 7591 that has gone microphonic. What do I need to know about getting new tube? I don’t know much about the whole bias thing, I just know it’s a term that is important switch some amp designs. Do I have to worry about that with this amp?
Also was hoping to get some explanation on a issue that I did have, but seems to be passed now. I noticed yesterday a really bad low frequency hum that would go away when I tilted the amp forward of backwards. I scoured the forums but didn’t see anything about it, but I eventually just decided to take off the tube buffers and it appears to have gone away. When I got the amp it had three tube buffers on each 7591 and I realized the other day they the middle one on both tubes broke and was no longer tight on the tube. Not entirely sure if that caused it the hum as I only started paying close attention because I was trying to figure out the following issue.
The other issue that came up recently was that as I played the amp for a while, there would be some random crackling coming through the speaker. I’ll have to wait and see till I play a long gig again to see if it still makes that noise, but was curious if anyone had any other idea what it could be.
Sorry for the long post, I’m very mechanical by nature so I’m really trying my best to troubleshoot the issue and see what’s going on. Thanks everyone!
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Post by KeithA on May 7, 2021 8:13:59 GMT -7
Hi everyone! I have a Jetta combo that appears to have a 7591 that has gone microphonic. What do I need to know about getting new tube? I don’t know much about the whole bias thing, I just know it’s a term that is important switch some amp designs. Do I have to worry about that with this amp? Also was hoping to get some explanation on a issue that I did have, but seems to be passed now. I noticed yesterday a really bad low frequency hum that would go away when I tilted the amp forward of backwards. I scoured the forums but didn’t see anything about it, but I eventually just decided to take off the tube buffers and it appears to have gone away. When I got the amp it had three tube buffers on each 7591 and I realized the other day they the middle one on both tubes broke and was no longer tight on the tube. Not entirely sure if that caused it the hum as I only started paying close attention because I was trying to figure out the following issue. The other issue that came up recently was that as I played the amp for a while, there would be some random crackling coming through the speaker. I’ll have to wait and see till I play a long gig again to see if it still makes that noise, but was curious if anyone had any other idea what it could be. Sorry for the long post, I’m very mechanical by nature so I’m really trying my best to troubleshoot the issue and see what’s going on. Thanks everyone! Welcome to the forum. The Jetta is 'cathode-biased'. So, that means it's pretty much 'plug & play' with a new set of tubes. When you buy them just ask for a set that is matched and equivalent to a 'medium' rating, as far as tubes go. The other the frequency hum, as you note, may have been a buzzing. Sometime the tube internals can be a little microphonic as well. I suspect it this happens consistently over time a tube replacement may fix it. Crackling, in my experience, is usually preamp tube-related. Sometime it occurs and then goes away once the amp is heated up. Like above, if this persists it's likely just a tube replacement required. Most times quirks like those above are tube-related. It's the just the nature of tubes these days
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Post by "Z" Steve on May 7, 2021 8:20:58 GMT -7
Welcome to the forum joedan! You will be able to find answers to most of your questions here on the forum or on the Z website ( drzamps.com/support/tube-info/ ). I do not own a Jetta but have had a number of Z amps to know how to trouble shoot some issues. Re your thought on a microphonic tube, have you tried tapping the suspected tube with a pencil or pen and see if it make popping or crackling sounds? Tap each tube and if one is microphonic you will hear a huge difference in noise as tapped. If in fact it is a tube issue, I saw on the Z website that there is no bias necessary re the Jetta - plug and play, so makes it easier to fix a tube issue. On that note it is always good to have extra backup tubes for times like this (same with fuses). Hang tight here and I am sure someone else will join in and help you more with this issue! Again, welcome to the forum and congrats on a great amp! Well DANG, Keith beat me to it! Your amp will have Red tolex in no time
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Post by Jaguarguy (Mike) on May 7, 2021 8:42:31 GMT -7
Hi everyone! I have a Jetta combo that appears to have a 7591 that has gone microphonic. What do I need to know about getting new tube? I don’t know much about the whole bias thing, I just know it’s a term that is important switch some amp designs. Do I have to worry about that with this amp? Also was hoping to get some explanation on a issue that I did have, but seems to be passed now. I noticed yesterday a really bad low frequency hum that would go away when I tilted the amp forward of backwards. I scoured the forums but didn’t see anything about it, but I eventually just decided to take off the tube buffers and it appears to have gone away. When I got the amp it had three tube buffers on each 7591 and I realized the other day they the middle one on both tubes broke and was no longer tight on the tube. Not entirely sure if that caused it the hum as I only started paying close attention because I was trying to figure out the following issue. The other issue that came up recently was that as I played the amp for a while, there would be some random crackling coming through the speaker. I’ll have to wait and see till I play a long gig again to see if it still makes that noise, but was curious if anyone had any other idea what it could be. Sorry for the long post, I’m very mechanical by nature so I’m really trying my best to troubleshoot the issue and see what’s going on. Thanks everyone! Welcome to the Forum - a lot of great people and information here. Here is Doc's AMA (Ask Me Anything) on troubleshooting an amp.
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Post by doctorice on May 7, 2021 9:34:12 GMT -7
Welcome to Z Talk.
One of my Jetta tubes went microphonic recently. For a while, the "bad sound" disappeared when I tilted the cab, but ultimately the tube was no good.
I purchased a new, matched pair from The TubeStore and installed. One of those tubes went microphonic almost immediately. (All tubes were Tung Sols.) I just received a replacement but have not had time to install it and test.
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Post by headshrinker (Marc) on May 7, 2021 9:39:41 GMT -7
Another vote for a tube issue. My guess is all your problems may be caused by the 7591. Crackling can be caused by having a cell phone in the vicinity of the amp.
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Post by LT on May 7, 2021 14:12:57 GMT -7
Welcome to the forum joedan! Good folks and lotsa knowledge here.
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