Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: MF76 on May 26, 2015, 03:35:47 pm
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Wired the Hoffman 5e3 board up and I swear i've double checked everything 10x but i am not getting any voltage to pins 4 on either power tubes. I have 220vdc through the lightbulb limiter to the 1st filter cap and to both pins 3 of the power tubes(6v6) and 0 going to the second fc. any suggestions? fwiw i used classic tone 18016 pt, and 18002 ot. Maybe I'm missing something simple but my layout was nice and clean until I started tryin to troubleshoot. Any help or suggestions are appreciated. This is my first build
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and 0 going to the second fc.
Sounds like your second filter cap isn't connected to B+ supply. If that cap is supposed to supply pins 4 with voltage, that explains that. :)
Your bulb limiter is not glowing brightly, is it?
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Is your 5K "intranode" power resistor open, in other words, does it read 5K end-to-end? That's almost gotta be the issue. And then of course the wiring to and from same.
Do you have any volts on your preamp tubes? (If the 5K was open, no "lower B+" nodes would be getting any power)
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With power off, measure resistance from each pin 4 to ground. What do you get? Does that 5K/5W resistor get hot?
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The lightbulb is glowing halfway or less. No voltage past the first filter cap. The 5k5w reads 5k across it. I'll read pin 4 to ground when I go back out to the shop. If any of this tells u guys anything the.n please carry on with the advice. Thanks!
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I don't know if you've seen this but it is an excellent way to double check an amp build with it's schematic.
We've seen a good number of guys who couldn't find what was wrong with their build and using this method they found it.
Doug did a great job on this example;
http://el34world.com/Forum/index.php?topic=17701.0 (http://el34world.com/Forum/index.php?topic=17701.0)
Brad (http://el34world.com/Forum/Smileys/default/icon_biggrin.gif)
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I saw that in a message from Doug earlier. Been at sons baseball game this afternoon. I had to put the amp and aggravation on standby lol I'm gonna go through that checklist tomorrow.
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The lightbulb is glowing halfway or less. No voltage past the first filter cap. The 5k5w reads 5k across it. I'll read pin 4 to ground when I go back out to the shop. If any of this tells u guys anything the.n please carry on with the advice. Thanks!
The 5k resistor almost has to be not connected at one end or the other. If it's connected to the first cap, the other end should be reading almost the same voltage if your tubes are pulled.
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Recheck. If you have +220 v and +0 volts as indicated, yet the 5K resistor is good...that's hard to believe. One way this could happen is if the e-cap connected to the junction of the 22K and 5K is a dead short. (Brand new part?) That would mean 220/5000 = .044 amps through the 5K, .044 * .044 * 5K = 9.68 watts. That 5K resistor would be searing HOT. You would probably smell it, it could take skin off your finger. The other way is if the solder joint at the east end of the 5K is completely nonexistent.
Now that is relative to the FENDER parts board. The Hoffman board has a change:
Doug's board takes that node, the junction of the 5K and the 22K, and runs that node through separate 470 ohm "screen" resistors for each 6V6. A good idea. That/those connections aren't being made in your case. IMO that's all this is.
Fender:
(http://i172.photobucket.com/albums/w32/ttm4/5E3_1_zps0fweolvz.png) (http://s172.photobucket.com/user/ttm4/media/5E3_1_zps0fweolvz.png.html)
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(http://i172.photobucket.com/albums/w32/ttm4/5E3_2_zpshlturofn.png) (http://s172.photobucket.com/user/ttm4/media/5E3_2_zpshlturofn.png.html)
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Ok I have 470 ohms at pin 4 with the power off. Every part in this chassis is brand new, except I used vintage sockets. The 5k measures 4.967k with my fluke meter. Just in case I replaced both 470 and the 22k and the 5k resistors. The end of each 470"is running to each pin 4 as they should be. After all this I still measure around 220vdc from pin 3 to the + side of the first filter cap. The negative leg of the fc measure 0vdc. And so does the rest of the joints on the board. No voltage DC on the pins of the preamp tubes and the 5k resisistor does not sear or get too hot. I ran jumper wirs on top of the board just in case something happened to the wires under the board But that made no difference either. I am about ready to gut it and just start over.
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Are you saying you have 470ohm on pin 4 to ground? If so, there is a short to ground at the junction of the 470R resistors, where there is supposed to be screen voltage.
Ok I have 470 ohms at pin 4 with the power off.
I am about ready to gut it and just start over.
Don't do that, once you figure it out, you'll have that experience under your belt forever. Nothing replaces experience.
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Wild.
Option #1: Put it aside for a few days and clear your head. It's gotta be something you will later be embarassed about goofy.
Options #2: Start from the other end. Turn power off and let it sit for a few minutes so any caps drain. Clip one of your ohmmeter leads to the "hot" side (eg; power supply side) of a plate resistor of a preamp tube and start measuring towards the power supply with the other lead. Find something, somewhere where you get some non-infinity reading.
Any money says you have a goofy error and you're just seeing wrong every time. It happens, trust me.
Are you saying that, laid out as the dwg above with the purple lines, you have 220 volts on one side of the big 5K and NOTHING on the other side? Forget any connecting wires, strictly on one side and the other side of that one resistor.
not possible!
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Are you saying that, laid out as the dwg above with the purple lines, you have 220 volts on one side of the big 5K and NOTHING on the other side? Forget any connecting wires, strictly on one side and the other side of that one resistor.
not possible!
Sure it is. Sounds like he has a short to ground on that end of the 5K resistor. That's why I wanted the resistance check from pin 4 to ground. Also why I wanted to know if that resistor was hot. I believe there is a short and that also accounts for the B+ only being 220V.
If you don't have a wiring error on the left side of that 5K resistor then it's likely that the second filter cap is shorted or installed backwards. Disconnect or just replace the second filter cap. Get better.
One clear hi rez pic of that part of the board would likely solve this problem PDQ.
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Wild.
Are you saying that, laid out as the dwg above with the purple lines, you have 220 volts on one side of the big 5K and NOTHING on the other side? Forget any connecting wires, strictly on one side and the other side of that one resistor.
not possible!
HAHA, that reminds me of years ago, when I had HVAC techs in the field. One guy called in for consult, saying that he thought he had a shorted transformer. He said, "This transformer is sending the hot straight through to the neutral!". I asked him to define neutral to me, which he did, and then said, if the neutral is hot, either you have no neutral, or you have FIRE! Do you have fire? "No". Then find out where your neutral is broken. He did a wire repair, and I had a happy customer.
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"Sure it is. Sounds like he has a short to ground on that end of the 5K resistor."
If that was the case, then he would have 220/5000 = .044 amps flowing through his 5K resistor
.044 * .044 * 5000 = 9.68 watts. His 5K resistor would be BBQ-grade HOT.
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"Sure it is. Sounds like he has a short to ground on that end of the 5K resistor."
If that was the case, then he would have 220/5000 = .044 amps flowing through his 5K resistor
.044 * .044 * 5000 = 9.68 watts. His 5K resistor would be BBQ-grade HOT.
That's why I asked two days ago if the resistor was hot.
He just measured 470Ω resistance from each screen grid to ground. That surely indicates a short to ground on that end of that resistor.
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Do you think it's possible that his 5Y3 has an internal short from plate to heater??
Then, conceivably, he could be measuring from his 6V6 pin 4's to ground and reading a path from 5Y3 heater to a 5Y3 plate through to the CT of the HV winding which is grounded..?
MF76, pull out your 5Y3 and see if the ohms readings you took from your 6V6 screens (pin 4) change.
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How do you account for the 220vdc?
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I have 220vdc through the lightbulb limiter to the 1st filter cap
isn't the lamp limiter AC?
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If one plate of the 5Y3 was shorted but the other good, he could indeed have a half wave rectifier. :laugh:
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If one plate of the 5Y3 was shorted but the other good, he could indeed have a half wave rectifier. :laugh:
If one plate was shorted there would be big AC voltage on the B+ rail and there would be another set of obvious symptoms like unhappy filter caps and unhappy PT. If one plate was OPEN that would make a half wave rectifier.
If you believe what MF76 originally said then the obvious possible problem would either be open circuit between the first and second filter caps or short to ground at the second filter cap. Likely suspects would be 5K resistor, shorted second filter cap, wiring error, missing board jumper, open circuit, or something under the board shorting the second filter cap. Should be very easy to spot with an ohm meter or just looking.
Later on, MF76 gives some conflicting info. He says "Ok I have 470 ohms at pin 4 with the power off." My interpretation is that means 470Ω resistance between pin 4 and ground since I had asked him to make that specific measurment. That implies a dead short at the junction of the 470Ω screen resistors and the second filter cap. He also says "The 5k measures 4.967k... the 5k resisistor does not sear or get too hot". One of those statements must not be correct. A 5K/5W resistor with 220v on one side and zero volts on the other side would be very hot!
It will be interesting to see what the real problem is.
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"It will be interesting to see what the real problem is."
That's exactly how I see it. Something isn't adding up. Someone on this forum had a semi-shorted GZ34 a while back which is hard to believe, but on the other hand, it's probably a lot more possible in an indirectly heated recto tube than a plain ordinary 5Y3, maybe the most bulletproof tube in existence.
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MF76, do you have it working yet?
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I will start working back on it this coming week. I am a gigging musician so I only have time to work on the amp Mon-Weds. I will keep this updated with what I find. I agree with you all so far as this has to be a goofball overlooked mistake. I will try everyone's advice and comment with results. Gonna try and add some pics too. Maybe that will help out.Thanks so much to everyone that is helping me on this.
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PICS
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Honestly, if that 5K / 5W resistor really measures 5K (which you measure from one LEAD to the other LEAD) and you have 220 volts on one side of it and nothing on the other side of it, you must have a cold or otherwise bad solder joint on one or both sides. The input side LEAD DOES seem to be making contact. The output side, NOT. I guess it would also be possible that you did not trim the lead short enough and it is poking through the parts board and contacting the chassis, but then, we would have home-fried resistor.
Take note: you are probably making your ohms measurement across that resistor from the LEADS. Not from the eyelets. Likewise, you are probably making your volts measurements from the resistor leads. Not the eyelets. True? Doing this assumes the intervening solder connections are made. Maybe they are not.
First thing, with power off & caps discharged, measure ohms from the hot side of the resistor (where you have the 220) to the junction of the two 470 ohm screen resistors. If that is open, then as I am suggesting, you have one or more bad solder joints. If it is "0", then it's time to retire to some alternate universe. That measurement should be 5K, the same as across your resistor.
Take your ohmmeter and assume that NO eyelet connection is good. In other words DO NOT ASSUME that you have zero ohms between two component leads that happen to go to the same eyelet. That assumption assumes the intervening solder joint is good. Clear? You cannot assume that here.
Reflow the connections with a bit of solder so as to get some new flux into the eyelet. Better would be to completely remake the eyelet connections, you want nothing iffy about them. At a dead minimum, while you have the solder molten get a little screwdriver under the body of the resistor and gently pry it up a little so that you get a little bit of abrasion going between the output lead and the eyelet. I can understand that if you don't have a solder sucker and you pull that resistor lead OUT of the eyelet it may be tough to get it back in. Even though it makes no sense that an untrimmed lead on that resistor is somehow contacting the chassis, leave the resistor up a bit from where it is now just to eliminate that impossible possibility.
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Also make this resistance reading. See pic...
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I got -2.5 ohms measured from ground to the end of the 5k resistor on the same side I that hadn't been getting any voltage at. I measured with the power off as advised by Sluckey. I'm going to resolder all the joints now to see what happens.
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When I measured from the hot side of the 5k resistor,where I am measuring 220v, with the power off to the junction of the 470 ohm screen resistors I actually am getting a reading of 4.87 ohms.
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How much space do you have between your board and chassis. It looks like about 2mm. Try removing your hold down screws and lift the board up and make sure you do not have a lead on the filter cap touching the chassis.
Dental mirror to look if you have one. It is easy to cut those cap leads too long.