Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on July 17, 2021, 02:22:49 pm
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Hello All! It's been a while being on the forum. Hope all is well with all of you. One of the strangest things i have seen in years. This Dr Z Maz 18 Watt Junior has blown (3) 12AX7 preamp tubes at various times over the past month. V1,V3,V4. Filament voltage is good at 3.45 VAC. Cathode voltages are in spec range. Plate voltages are sitting at 195VDC on V4 the latest one to blow and turn white. I know tubes are a real crap shoot these days, but really,like this often? Something is amuck. Hopefully not the power tranny.
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"Turn white" usually indicates a loss of vacuum, due to rough handling (cracked glass) or manufacturing error. Are all three tubes from the same manufacturer? Try a different brand.
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That's what i am suspecting. Two months ago i replaced a blown white TungSol 12AX7 and the next month it blew a JJ 12AX7. Now it has blown a Mullard 12AX7. I really see nothing wrong with the amps voltages. If there were a massive surge in voltage i could understand that better. For now the obvious thing to do is put in a set of Sovteks this time and see what happens.
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That is strange. Seems like just bad luck. And agree with Sluckey on white look being a bad tube that’s lost it’s vacuum. Ie: Not the amps fault.
I’m on the Z forum as well, as I own two Z’s. You could message DRZ or Don on the forum. They are really great about responding. Can also email repairs @ drzamps .com
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Is it a combo or head? Im
Not sure, but wonder if vibration in a combo could contribute to the issue.
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Jordan86 it is a combo amp with an attenuater mounted near the speaker. I just have not seen this, this often. Really strange/ Oh! and as well every blown tubes base was cracked and a small piece of glass came out as i pulled every tube. I am starting to think it could be miss handling of the amp itself.
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It could be that the tube is not biased properly and is routinely running stupidly outside maximum ratings. Not saying this is your problem. What are the voltages?
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I know nothing of the amp, so..
Is it a PCB amp?
if so
are the sockets on a pcb?
does it have random glichy, crackly, odd noises while jamming, especially after long sessions where the amp is getting worked hard?
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Handwired. No pcb.
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It could be that the tube is not biased properly and is routinely running stupidly outside maximum ratings. Not saying this is your problem. What are the voltages?
Yes Hand wired. Good quality build.
Cathode voltages. V1 Shared 1.185 VDC -V2 .645 - 11.45 VDC - V3 4.96 - 1.958 VDC - V4 Shared 1.905 VDC - V5 PI Plates 190 VDC - Cathode 46.6 VDC. Bias is +12.45 VDC @ 350VDC
It looks like someone paralled a 47uf filter cap across the blue Sprauge Atom to reduce noise.There is some mention online about noisy Z amps
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Gutshot looks a bit low-res for my tired eyes.
Got a schematic handy?
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No schematic online that i have found. Yes my phone camera sucks big time. Going over all the components i have concluded that it is pretty close to a Marshall clone with some other different resistor values. 470pf SM treble peak cap.. Similarities are: 56k Slope r. / 820 cathode r. / 220k Grid / PI split load resistors though both are 100k. And many other values i have yet to document.
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every blown tubes base was cracked and a small piece of glass came out as i pulled every tube.
Did you closely inspect the tube socket pin grippers? Something is cracking the glass after you put the tube in.
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.... running stupidly outside maximum ratings. .....
Sensible; but not likely in typical circuit. Say 400V, heck 450V of raw B+. As confirmed later, the plate resistors are 100k. If the tube were dead-short, the resistor would have 450V^2/100k= 2 Watts. But the resistor is OK. Zero V across the resistor and 450V across the tube is outside voltage rating, but fine for heat; and we know Macs pushed 400V on 12AX7 witout trouble. The worst-case is half-and-half, 225V on tube and 225V on resistor. That is 0.5 Watts (in either part) and is not about to annoy a 12AX7.
1) the roadies are DROPPING the amp every night.
2) we've been on break for the last 15 months. That should not bother tubes, but is there some mechanism where all the expected failures for a year+ are happening all in one month?
Sell him a transistor amp, and a fifth of Jack Daniels to take the edge off.
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Sell him a transistor amp, and a fifth of Jack Daniels to take the edge off.
:laugh:
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I wonder if it might be due to a bad batch of valve sockets, ie that somehow put excessive pressure (lateral?) on the valve pins?
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every blown tubes base was cracked and a small piece of glass came out as i pulled every tube.
Did you closely inspect the tube socket pin grippers? Something is cracking the glass after you put the tube in.
All preamp tube socket pin base's are raised where the tubes sit and are clear of touching any metal. This is FUN!!!
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.... running stupidly outside maximum ratings. .....
Sensible; but not likely in typical circuit. Say 400V, heck 450V of raw B+. As confirmed later, the plate resistors are 100k. If the tube were dead-short, the resistor would have 450V^2/100k= 2 Watts. But the resistor is OK. Zero V across the resistor and 450V across the tube is outside voltage rating, but fine for heat; and we know Macs pushed 400V on 12AX7 witout trouble. The worst-case is half-and-half, 225V on tube and 225V on resistor. That is 0.5 Watts (in either part) and is not about to annoy a 12AX7.
1) the roadies are DROPPING the amp every night.
2) we've been on break for the last 15 months. That should not bother tubes, but is there some mechanism where all the expected failures for a year+ are happening all in one month?
Sell him a transistor amp, and a fifth of Jack Daniels to take the edge off.
Ha!!!! That's what i am thinking. Though one thing i forgot to mention because i just remembered. I had another amp a few months ago have the same problem but with only one socket. Whiteout blown 12AX7. That amp has been fine since. Getting this one in and seeing this for a second time kinda tells me we may have a bunch of bad manufactured tubes out there. That's my thinking after all else has been ruled out.
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Gutshot looks a bit low-res for my tired eyes.
Got a schematic handy?
Boy your not kidding about tired eye's. Gona try another phone in a few minutes and see if i can get better pictures.
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Getting this one in and seeing this for a second time kinda tells me we may have a bunch of bad manufactured tubes out there. That's my thinking after all else has been ruled out.
Hmm, you originally said this has happened ***THREE*** times to V1, V3, and V4. First Tungsol, then JJ, then Mullard. Very, very, very unlikely a manufacturing defect.
You ain't gonna find the cause of three broken tubes by looking at pics of the amp. Better chance of discovering the problem by interviewing the owner and tell him that glass is breakable. ***HANDLE WITH CARE***. :l2:
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Getting this one in and seeing this for a second time kinda tells me we may have a bunch of bad manufactured tubes out there. That's my thinking after all else has been ruled out.
Hmm, you originally said this has happened ***THREE*** times to V1, V3, and V4. First Tungsol, then JJ, then Mullard. Very, very, very unlikely a manufacturing defect.
You ain't gonna find the cause of three broken tubes by looking at pics of the amp. Better chance of discovering the problem by interviewing the owner and tell him that glass is breakable. ***HANDLE WITH CARE***. :l2:
The customer did say on a few occasions that he lays the amp down in the trunk. Tubes facing floor board. But the back panel fairly protects the tubes from being hit. Unless there is something else in the truck flying around. :laugh: :w2: :w2: :w2: :w2: :w2:
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Pot holes!
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Tell him to put it in the back seat. Or put padding underneath the amp in the trunk.
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Tell him to put it in the back seat. Or put padding underneath the amp in the trunk.
Or it own flight case.
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Way off center tube shield springs. Enough pressure to make tube unhappy? :think1: Not to mention these tube shields today are so cheap and most wont grab and stay in place with the tiny bump that the shield pushes down on and turns to lock in place.
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a motor vehicle is designed so that the passengers between the four wheels have a comfortable ride. transporting something relatively delicate like a guitar amp in the trunk could certainly be the cause of these issues
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Yes, Operator brutality would be my guess. As other have said, seems very unlikely for this to happen on three different valve sockets. But could just be bad luck with manufacturing. Owner could NOT transport in his trunk for 6 months to see if that fixes it.
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Operator brutality
I like that phrase. :icon_biggrin:
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I usually follow up with a customer to make sure everything is fine. And i will this time as well. Just taking my time. :w2: :laugh: