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Thread: New issues on my JTM45 build while testing tubes.

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    New issues on my JTM45 build while testing tubes.

    I started a new thread since these are issues that showed up after I had selected tubes to use in my JTM45.

    1: As stated in the previous thread ,I had experienced some audible noises when I removed tubes from the amp after it sat unplugged overnight. Never had that happen even after an hour. Was said to be leftover voltage in caps.
    2: After I had settled on tubes to use for awhile I had set the bias ,and left it there for about 30 minutes. It was steady ,so I turned it off ,and left it overnight. The next morning when I powered up to check it again the bias was slowly increasing ,but when it settled it was more than 10mA off. I adjusted again to where I had set it before. As I was increasing it I could hear a little puttering until I got near what it was then it stopped. I left it there for about 30 minutes ,and it was steady.
    3: When I switched from play to standby there was an awful bunch of noise ,and as soon as I switched back to play it was fine. Every time I did this I got the noise. Bad switch, cap, choke? I only tested the switch continuity with a meter. Seems fine. Checked the filter cap attached to the switch with a capacitance meter ,and was within spec. ( I have an ESR meter on the way).
    I have a few more things I want to check ,but I'm leaning toward bad standby switch since I know they aren't designed to handle high DC.

    Had the wrong layout. THIS Triode is what I used.
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    Last edited by gtrplayr1976; 10-10-2019 at 12:37 AM.

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    Quote Originally Posted by gtrplayr1976 View Post
    ......The next morning when I powered up to check it again the bias was slowly increasing ,but when it settled it was more than 10mA off......
    Are you saying the bias supply voltage was slowly increasing or the bias current? The bias was 10mA off, was it too much current or too little?

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    Last edited by The Dude; 10-09-2019 at 11:28 PM.
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    Quote Originally Posted by The Dude View Post
    Are you saying the bias supply voltage was slowly increasing or the bias current? The bias was 10mA off, was it too much current or too little?
    I was using a bias probe. Had it set at 39mA ,and it was 29mA. That seems to be ok for now. I think I need to reroute some wires. I believe when I removed the PPIMV ,and put the control grid wires back I may have them in a bad area. I also twisted them together. Not sure if that's advisable? I may just bypass the standby altogether.

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    I don't see the 0.05µF cap in the layout that connects from the rectifier output to ground (before the standby switch). It should be rated at least for 630V. It helps to reduce the turn-on surge current for the GZ34.

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    Quote Originally Posted by Helmholtz View Post
    I don't see the 0.05µF cap in the layout that connects from the rectifier output to ground (before the standby switch). It should be rated at least for 630V. It helps to reduce the turn-on surge current for the GZ34.
    Yes I noticed that none of the layouts I have from different makers have that cap. The Marshall ,and Fender Bassman schematics both have it. I will either add that cap along with a new switch ,or just bypass the switch.

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    Found AN issue. Three of four points on the wires from my bias pot to the circuit were broken ,or probably just nearly broken. When I moved the wires a little they all snapped. Got that fixed ,and reset the bias ,and the standby switch isn't making noise now? I'm going to play around with it some more ,and see what I can find.
    One thing I saw that confused me, when I was reading voltages on the control grids of the power tubes I noticed on one when I put the lead on it i could hear a rising noise ,and the bias was going up. Could that have been because I had that tube sitting in a bias probe socket ?

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    One thing I saw that confused me, when I was reading voltages on the control grids of the power tubes I noticed on one when I put the lead on it i could hear a rising noise ,and the bias was going up. Could that have been because I had that tube sitting in a bias probe socket ?

    Try to pull the PI tube before you repeat the grids measurements.

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    Quote Originally Posted by Helmholtz View Post
    Try to pull the PI tube before you repeat the grids measurements.

    Speaking of the PI ,what voltage can you typically expect here where the two resistors meet?
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    Speaking of the PI ,what voltage can you typically expect here where the two resistors meet?
    Around 350V. What are your PS voltages?

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    Quote Originally Posted by Helmholtz View Post
    Around 350V. What are your PS voltages?
    That's what I thought. It looks very low ,but I think I'm just getting a bad reading because the plates are 240-250.

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    Quote Originally Posted by gtrplayr1976 View Post
    That's what I thought. It looks very low ,but I think I'm just getting a bad reading because the plates are 240-250.
    In your other thread you mentioned a plate voltage of 450V. What happened?

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    Quote Originally Posted by Helmholtz View Post
    In your other thread you mentioned a plate voltage of 450V. What happened?
    Power tube plates yes. This is the PI plates.

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    I see, but what's your problem?

    but I think I'm just getting a bad reading because the plates are 240-250.
    I don't understand this. What is a bad reading?

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    Quote Originally Posted by Helmholtz View Post
    I see, but what's your problem?



    I don't understand this. What is a bad reading?
    I was getting a bad reading on the meter at the point where the two PI plate resistors meet. Plates are good ,so it was just a funky connection between my meter ,and that point.

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    Quote Originally Posted by gtrplayr1976 View Post
    I was getting a bad reading on the meter at the point where the two PI plate resistors meet. Plates are good ,so it was just a funky connection between my meter ,and that point.
    Now what's the good reading? Do you need further advice?

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    Quote Originally Posted by Helmholtz View Post
    Now what's the good reading? Do you need further advice?
    It's at 390 VDC after initial startup. It runs over 500VDC on startup. The thing I'm trying to find now is that when I take it off standby I can hear a faint puttering until the bias mA reading on the probe/meter reaches about 27-29mA. I just tried jumping the standby switch to see if that was a cause ,but it did the same thing.

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    Quote Originally Posted by gtrplayr1976 View Post
    It's at 390 VDC after initial startup. It runs over 500VDC on startup. The thing I'm trying to find now is that when I take it off standby I can hear a faint puttering until the bias mA reading on the probe/meter reaches about 27-29mA. I just tried jumping the standby switch to see if that was a cause ,but it did the same thing.
    390V PI supply voltage seems very high. What is your heater voltage? (sorry if you already posted it somewhere). Some voltage overshoot before all heaters are hot is normal, but 500V is extreme. A tube rectifier should reduce the overshoot as its own heater delays voltage build-up.

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    Quote Originally Posted by Helmholtz View Post
    390V PI supply voltage seems very high. What is your heater voltage? (sorry if you already posted it somewhere). Some voltage overshoot before all heaters are hot is normal, but 500V is extreme. A tube rectifier should reduce the overshoot as its own heater delays voltage build-up.
    The PI supply voltage on a Bassman is 385 ,so I figured this would be within spec since it is very similar?

    Heater voltage is 6.5VAC. I am using a copper top from Weber for the rectifier ,but I am going to order an actual GZ34/5AR4 to see if there's a difference.

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    The PI supply voltage on a Bassman is 385 ,so I figured this would be within spec since it is very similar?
    True, but the 5F6-A has a lower value dropper resistor (4.7k instead of 8.2k in the JTM45). Nothing wrong with 390V but on the high side for a JTM45. Does your PT have a 240V mains input tap? If available I often use the 240V tap when voltages are high.

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    Quote Originally Posted by Helmholtz View Post
    True, but the 5F6-A has a lower value dropper resistor (4.7k instead of 8.2k in the JTM45). Nothing wrong with 390V but on the high side for a JTM45. Does your PT have a 240V mains input tap? If available I often use the 240V tap when voltages are high.
    It does have a 240 tap ,but is that safe to use with 125v supply ? I've never heard of this being done?

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    Quote Originally Posted by gtrplayr1976 View Post
    It does have a 240 tap ,but is that safe to use with 125v supply ? I've never heard of this being done?
    Sorry, I goofed. Thought you were in Europe. Our nominal voltage is 230V and import PTs or amps often are designed for 220V. So at 230V internal voltages are too high and I switch to the 240V tap if available. So you would want something like a 130V tap.

    Using the 240V tap with 125V input would be safe but give very low amp voltages.


    BTW, what does WV mean?

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    Last edited by Helmholtz; 10-13-2019 at 02:37 PM.
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    Quote Originally Posted by Helmholtz View Post
    Sorry, I goofed. Thought you were in Europe. Our nominal voltage is 230V and import PTs or amps often are designed for 220V. So at 230V internal voltages are too high and I switch to the 240V tap if available. So you would want something like a 130V tap.

    Yeah there's no tap for that.

    BTW, what does WV mean?
    LOL...West Virginia United States.

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    Thanks. We also abbreviate our federal states, like BY, BW, NRW, RLP etc. Of course I don't expect foreigners to know.

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    Quote Originally Posted by Helmholtz View Post
    Thanks. We also abbreviate our federal states, like BY, BW, NRW, RLP etc. Of course I don't expect foreigners to know.
    Germany is one of the places I would love to visit especially the old places.

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    Quote Originally Posted by Helmholtz View Post
    Sorry, I goofed. Thought you were in Europe. Our nominal voltage is 230V and import PTs or amps often are designed for 220V. So at 230V internal voltages are too high and I switch to the 240V tap if available. So you would want something like a 130V tap.

    Using the 240V tap with 125V input would be safe but give very low amp voltages.

    What does this sound like to you? It is there at startup when it reaches a certain voltage, and/or bias mA reading. Cap? Well crap, for some reason the mp4 file won't upload. Have to try something different. Hopefully this will work.

    https://www.youtube.com/watch?v=iBYq...ature=youtu.be

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    Quote Originally Posted by gtrplayr1976 View Post
    What does this sound like to you? It is there at startup when it reaches a certain voltage, and/or bias mA reading. Cap? Well crap, for some reason the mp4 file won't upload. Have to try something different. Hopefully this will work.

    https://www.youtube.com/watch?v=iBYq...ature=youtu.be
    Never heard something like that with an amp. Sounds like a motorcycle. Is this with a GZ34? Are your filter caps rated at 500V or higher?

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    Quote Originally Posted by Helmholtz View Post
    Never heard something like that with an amp. Sounds like a motorcycle. Is this with a GZ34? Are your filter caps rated at 500V or higher?
    Cans are 500v. I don't have a GZ34 tube yet. I have ordered some. Using the Weber coppertop right now. Well "motorboating" is a sign of bad filter caps ,but it usually continues ,and doesn't stop like this does ,so I'm not sure either.

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    Well "motorboating" is a sign of bad filter caps ,but it usually continues ,and doesn't stop like this does ,so I'm not sure either.
    True and motorboating typically sounds different. Let's see if its still there with the GZ34. You might even use a 5U4GB for a little lower voltages if your PT supports the higher heater current.

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    Quote Originally Posted by Helmholtz View Post
    True and motorboating typically sounds different. Let's see if its still there with the GZ34. You might even use a 5U4GB for a little lower voltages if your PT supports the higher heater current.
    Should get the GZ early in the week. 5U4GB won't work. PT only handles 2A. I was going to do that a long time ago.

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    I seem to remember having a 12AX7 that made that quiet motorboat noise, try subbing out each in turn with a good one.

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    Quote Originally Posted by pdf64 View Post
    I seem to remember having a 12AX7 that made that quiet motorboat noise, try subbing out each in turn with a good one.
    Did it go away after warmup? This starts at a point during initial startup ,then fads out when it settles? I will give that a go though. Can't hurt.

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    It was a long time ago, sorry, can’t recall the detail, but it got worse and provoked me to track down the culprit. I was surprised it was a tube fault.
    It may have been one of GTs ‘Mullard’ replicas from the early 2000s, I read they had a lot of problems and got withdrawn.

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    Quote Originally Posted by pdf64 View Post
    It was a long time ago, sorry, can’t recall the detail, but it got worse and provoked me to track down the culprit. I was surprised it was a tube fault.
    It may have been one of GTs ‘Mullard’ replicas from the early 2000s, I read they had a lot of problems and got withdrawn.
    I gave it a shot. Still have the noise. I'll keep looking.

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    Quote Originally Posted by Helmholtz View Post
    True and motorboating typically sounds different. Let's see if its still there with the GZ34. You might even use a 5U4GB for a little lower voltages if your PT supports the higher heater current.
    I just tried another set of power tubes ,and the puttering is gone on the first startup. I will try again shortly to verify that. I'm kind of shocked. I've never had an issue with a tube I have gotten from Eurotubes.

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    Quote Originally Posted by pdf64 View Post
    It was a long time ago, sorry, can’t recall the detail, but it got worse and provoked me to track down the culprit. I was surprised it was a tube fault.
    It may have been one of GTs ‘Mullard’ replicas from the early 2000s, I read they had a lot of problems and got withdrawn.
    I just tried another set of power tubes ,and first startup the puttering is gone.

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