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Post by crimsonqueen on Nov 9, 2020 9:35:41 GMT -7
Hi,
I rented a rehearsal space yesterday. I can't access our regular space because of Covid. All their cabs were 4 ohms. I had never used that output before on the amp.
We played a couple of tunes and the amp quited down suddently and a made "pffffff" sound until it stopped completely. Everything remained lit and the fuse is fine. I switched to the house amp.
Just looked inside, nothing looks burned. I have changed the screan resistors a couple of years ago, but there are fine now.
I turned the amp back on and only plugged a guitar. It's working now. It's humming more than usual, though. I also had been hearing some popping sounds here and there lately, but the caps look good. It's a 15th anniversary Z28.
What could have happened? (Loose tubes?)
Do dying power tubes do this? (stop working, but then work again?)
And is this related to my other thing about the sound being brighter?
Any help would be appreciated. I can effectuate repairs myself, but cannot diagnose problems, unless they are super obvious.
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Post by Jaguarguy (Mike) on Nov 9, 2020 9:55:49 GMT -7
Here's Doc's troubleshooting guide - works pretty good. When was the last time tubes were changed? That may be your problem. Also, make sure all tubes are seated properly. I'm sure others will chime in. drzamps.com/info/faq-troubleshoot/
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Post by crimsonqueen on Nov 9, 2020 10:41:16 GMT -7
I have been using this set of power tubes since April 2017 (JJ's). I was primarily using a Quilter head for practicing and gigs until late 2018, when I decided to only use the Z28 for everything (jamming, practicing and gigging). My band practices for about 5 hours every week, and I play at home during the week. I use an Airbrake on the amp at home.
How often should I change power tubes and how can I tell they need changing? I was more obvious with other brands of tubes, like the Tung Sol's I had before, or the tubes that were in the amp when I bought it (used).
I don't want to buy power tubes if I don't need too, but then again, I have changed all the other tubes recently.
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Post by premiumplus (Dave) on Nov 9, 2020 12:33:46 GMT -7
I agree with Ridgeback, and you'll be happy with the improvement. Get a matched pair and bias at 28 mA per Z's website. JJ is making good 6V6 tubes, they're rugged and sound great.
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Post by crimsonqueen on Nov 9, 2020 13:41:00 GMT -7
I will order some new JJs and rebias the amp. I will report on the situation after. Maybe all the weird stuff I have been noticed are related to the tubes. My amp is "on" a lot.
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Post by zpilot on Nov 10, 2020 9:05:44 GMT -7
Just a point to note. Some players like attenuators a lot but they fail to realize that while they are controlling their volume they are also actually running the amp a lot harder than is apparent. That cuts short the life of the tubes more than if you are using an OD pedal. I'm not saying it is bad, just that you need to allow for that when figuring what to expect for the life of those tubes. It can reduce that by as much as half. It is like driving your car around in 2nd gear. You will get there and acceleration will be quick but you are going to be filling the tank and replacing parts more often.
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Post by wraparound (Steve) on Nov 10, 2020 20:13:23 GMT -7
In answer to one of your questions, yes tubes can exhibit weird things as they die like making noise then not and so on.
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Post by Chilly Gibbons (Todd T.) on Nov 11, 2020 7:41:50 GMT -7
My money is on worn out output tubes. Especially if you are experiencing a hum now. One of the tubes has died and while still partially functioning, is now severely mismatched with the other and is producing the hum. A new set and bias should have you back to great performance.
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Post by crimsonqueen on Nov 11, 2020 8:22:45 GMT -7
I ordered a new set of JJ 6V6 tubes and bought a Sovtek 5AR4. I had to change the rectifier to a JJ a while back, and never liked the amp as much afterwards. I bought the amp with a 5Y3 and changed it to a Sovtek 5AR4 right away, but that tube died about a year ago. I should have the tubes this Friday.
For the attenuator thing, I use an Air Brake at home. I can't get amp distortion at any reasonnable volume even with the attenuator. I only use it to be able to get a little volume. I have never used with a band or on a gig. My drive comes from the pedals.
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Post by crimsonqueen on Nov 11, 2020 10:53:58 GMT -7
You seem knowledgeable about biasing so I assume that you reset the bias when you swapped rectifiers. Different rectifiers can have a big impact on the DC voltage going to the power tubes. Personally, I have found that a rectifier change does not impact tone much if the idle current is set similarly before and after the change. Feel\sag yes if changing types, tone not so much. I suspect that the new tubes will fix the issues you are having. Good luck. Yes, I rebiased when going from the 5Y3 to the Sovtek 5AR4, and again when putting in the JJ 5AR4. The amp sounded and felt better with the Sovtek. I will keep the JJ as a spare. But, I think the power tubes are indead the real problem here. Thanks.
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Post by crimsonqueen on Nov 11, 2020 10:55:59 GMT -7
There is one thing I forgot to mention. I have always used an 8 ohm cabinet (I have a few), but had to use the 4 ohm tap for the first time. The space only had 4 ohm cabs. Is there any reason to think that contributed to the issue?
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Post by crimsonqueen on Nov 11, 2020 18:54:28 GMT -7
Nope. Even a 4\8 ohm mismatch either direction wouldn't be an issue for a limited time. I'm inclined to think it's a factor of play time on the tubes combined with the use of an attenuator based on your posts. As much as I like the Z-28 (my favorite Z), it seems like you might be happier with one of the Doc's master volume amps or the upcoming MKII version of the Z-28. Let us know how the tubes work out. I don't use the attenuator when playing with my band or on gigs. At home, I don't even raise the volume knob past 2. I am not using the attenuator to achieve power amp distortion. All my distortion comes from pedals, even with the band. I don't need to change amps, nor can I afford to. I have been using these tubes since 2017. It may just be time to change them.
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Post by crimsonqueen on Nov 17, 2020 15:34:32 GMT -7
I received my new tubes and rebiased the amp. My next rehearsal is Sunday, so I won't be using the amp loud for a long period of time until then.
One quick question, though. I could not get the bias exactly at 28.0. Any adjustment was taking it to 28.2 or 29.7, even after a long idling period. Is that a problem? And what is better slightly over 28 or slightly under? Or does it even matter?
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Post by Chilly Gibbons (Todd T.) on Nov 17, 2020 16:38:29 GMT -7
Wont hurt anything to bias a little warmer or cooler. I usually bias my Marshalls a little hotter to add some warmth at the cost of tube longevity. All of my Zs are cathode biased so I do not have a generalized preference like I do for Marshalls. In fact, it would be impossible to get it to a perfect 28ma and have it stay there for any appreciable amount of time.
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Post by premiumplus (Dave) on Nov 17, 2020 17:19:52 GMT -7
Bias will drift a little bit, that's normal.
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Post by zpilot on Nov 18, 2020 4:33:05 GMT -7
I received my new tubes and rebiased the amp. My next rehearsal is Sunday, so I won't be using the amp loud for a long period of time until then. One quick question, though. I could not get the bias exactly at 28.0. Any adjustment was taking it to 28.2 or 29.7, even after a long idling period. Is that a problem? And what is better slightly over 28 or slightly under? Or does it even matter? That is plenty close enough. It is not uncommon for the current draw at idle of a "matched set" of tubes to differ that much between tubes. When they are like that I use the reading of the hottest tube.
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Post by crimsonqueen on Nov 18, 2020 7:38:04 GMT -7
I don't think I was clear enough.
Forget about idling and I am not asking about a biasing method. I use the transformer shunt method.
What I meant is that if the bias reads at 27.8 and I try to get it to 28, it goes to 28.2. If I lower it again, it goes back to 27.8 or something like that. There is no fine enough adjustment to make it stop at 28 exactly.
My question is then, what is preferable? Slightly less than 28 or slightly more?
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Post by premiumplus (Dave) on Nov 18, 2020 7:49:27 GMT -7
If you can't tell any difference in tone, I'd go with 27.8. It's such a small difference that either one would be okay.
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Post by crimsonqueen on Dec 6, 2020 9:38:08 GMT -7
We will have our second rehearsal today since getting the new tubes and rectifier. First one went well. I just checked the bias again. It had shifted up a little after the 4 hours of loud playing at last rehearsal. I adjusted it again. and everything should be fine.
It definitely looks like the whole problem was worn-out power tubes.
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Post by thatstatuemoved on Dec 16, 2020 9:48:39 GMT -7
Back when I was gigging on a regular basis, I changed all my tubes once a year. This was probably overkill, but I did not have to worry as much about an amp going down at a gig. I would keep the old tubes as spares in my gig bag in case I needed them. I would put a piece ot tape on back of amp notating the date of tube change. I never got into the NOS tube scene as that would get pretty expensive. Worked pretty well for me.
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