Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: taylorc315 on April 20, 2015, 07:03:33 am
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Hello,
I'm new at building amps and was wondering if any one could tell me the color code of a Hoffman 50 watt power transformer? I'm having trouble with the heater section and what and where the wires attach. there's blue,2 blacks one smaller gage, orange & brown that I have left. any info would be great.
Thanks
Tracy
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The little black wire is probably on the primary. The bigger black wire is probably one of the heater wires. Use your ohm meter to check for low resistance between the orange wire and a black wire. Whichever black wire shows low resistance is the correct heater wire.
Once you are sure which black wire belongs to the filament winding, the orange and black wires will become your filament string, and the brown (center tap) will usually connect to chassis. But if you are using a Hoffman board just tape off the brown wire and use the two 100Ω resistors on the board as an artificial center tap.
Page two of this pdf shows exactly how to connect all wires of the PT.
http://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf (http://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf)
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This is great info.. there's a blue wire also is that the bias tap?
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This is great info.. there's a blue wire also is that the bias tap?
Look at page 2 again.
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Ok sorry, I do have the blue hooked to the switch, the orange and black go to the board at the 100 resistors the smaller black goes to the filament which is?
Sorry I'm new at this and not the best schematics reader. I know that once I get this figured out (with help) I'll see the light.
Thank You
Tracy
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Did you use an ohm meter to check resistance like I suggested?
The smaller black wire is probably in the primary winding. Check for continuity between the small black wire and the blue wire to confirm this.
Now, if your mains AC voltage is 120V then connect the little black wire to the line cord white wire. Again, this is clearly shown on page two.
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Thanks Again, I think that I have it, I will check everything when I get home. thanks for your patience and info.
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I'm going to chime in and strongly suggest you build a lamp bulb limiter before you fire that transformer up the first time. Even as slap dash I can be about some things, I use mine religiously EVERY time I change anything. Especially if you're not dead sure you've got it wired right, that limiter will keep you from burning up the transformer. I put a 60W bulb in mine because I build mostly low watt amps. You'll probably need a 100W in yours.
And be careful, this is an addictive habit. :icon_biggrin:
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Thank you. Yes the current limiter was the first thing I built, it already has saved me. I do have a question, the lamp in the unit I'm building is a neon lamp, it calls for 120v but the layout is calling for 6.3v I do see the hook up for the neon bulb but so far I have been struggling. I know it's right in front of me but the light switch has yet to switch on in my head. I will probably desolder everything and start from scratch. I am using a load station and plugging it into the speaker jack, is this correct? any info would greatly appreciated.
Thanks
Tracy
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Look at page 2 of this pdf again...
http://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf (http://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf)
One side of your neon lamp will connect to the blue wire at the on/off switch. The other side of your neon lamp will connect to the white wire of the power cord.
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Thank You,
I have to be close, This is frustrating and exciting all at the same time. I really appreciate all of your input and sorry to say I will probably asking more questions.
Thanks Again
Tracy
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What am I doing wrong? Everything looks good but for some reason the tubes will not heat up. Very frustrating.arrgghh
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Well, this is probably the last thing you want to do (instead of actually working on your amp) but how about downloading ExpressSCH? It's a free download, pretty easy to learn, and it'll do a couple things. Learning how to read and draw schematics, IMO is crucial... maybe just because it's been such a huge help to me.
It will also let you draw your schematic "as built", the way you have your PT hooked up, and let us see where the problem is.
Anyway, to get back to this
What am I doing wrong? Everything looks good but for some reason the tubes will not heat up. Very frustrating.arrgghh
You've got your heaters wired wrong. They're so dead simple you'd think it's impossible, but I did just that on my current build. I hooked up the filament wires on the wrong side of the 100 ohm resistors that go to ground, and so gosh, they just wouldn't glow with about 2 volts going through them. (in case anyone wonders how on earth, I had the 2 resistors going to the humdinger pot for a "safety fuse"... one more duhhh moment for me)
Anyway, it's something simple. Don't get frustrated, it's all part of the fun!
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Thank you very much.. Schematics are an Achilles heal for me, I will check out the download, If I have to I will start from scratch, like I said before this is exciting and frustrating all at the same time. Kind of like golf.
Thanks Again
Tracy
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Do you have a meter?
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Yes I have meters and oscilloscopes, it's got to be something simple that I'm just not seeing. :{
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Yes I have meters and oscilloscopes, it's got to be something simple that I'm just not seeing. :{
Does your neon lamp turn on and off with the power switch?
Check some AC voltages on the PT. What do you measure BETWEEN the orange and black filament leads? What do you measure BETWEEN pin 2 and pin 7 of each EL34 tube socket?
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I've got the tubes to light up. Yeah.yeah.yeah. yes the lamp turns on it seems that I've got that part of the problem solved. I've got 6.5 or so depending on the bias.. I have a couple of other issues that I have to check out. I think I need to go through the input jacks. If it ain't one thing it's another. I do appreciate all your help and input I'm sure that I will be asking more questions. Thanks again for everything.
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I've got the tubes to light up.
What did you do?
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I ran wires from the heater to 7&2 on the tube socket is that correct?
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I decided to start from scratch, I was not happy with the way I had everything wired.trial and error I've found to be the best way for me. Thanks for all your help, I'm sure I will be asking more questions.
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OK everything looks good, I have the brown wire taped off I still have the thinner black wire that has 110 v coming out of it can't find where it goes. Am I blind?
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Read this thread again.
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The little black wire is in with the heater wires bundle it should be connected to the neutral side of power?
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It looks like the black wire goes to ground... That's not right is it? That is a dead short right?
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READ THIS THREAD AGAIN.
You had it working at one time.
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Yes I have the tubes lighting up, maybe I have more than one problem. like I said everything looks good now, but no sound from the amp:( I've double checked the input and output jacks they seem ok, I will continue to troubleshoot everything and hopefully find an issue. Thanks again
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Which amp you are building and who's layout are you following? Make some voltage measurements. AC voltages out of the PT, B+ voltages on each filter cap, and tube pin voltages. Post your voltage readings here. Post some hi-rez pics also.
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Wiring diagrams are in the library for all the transformers I sell
The links are right next to every transformer on the page you were on when you ordered the transformer
http://hoffmanamps.com/MyStore/catalog/Transformers.htm (http://hoffmanamps.com/MyStore/catalog/Transformers.htm)
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I will do that as soon as I get home, I am using the 50 plexi Hoffman layout thanks again for all your input I know it's probably frustrating believe me but I really do appreciate your help.
Thanks
Tracy
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Wiring diagrams are in the library for all the transformers I sell
The links are right next to every transformer on the page you were on when you ordered the transformer
http://hoffmanamps.com/MyStore/catalog/Transformers.htm (http://hoffmanamps.com/MyStore/catalog/Transformers.htm)
By the way
There is no such thing as this
Hoffman 50 watt power transformer
Go to the page link above and find the transformer you have and follow the wiring diagram link
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blue wire 110 volt AC. black wire 110 volt AC. Red wire number 1 317 volt AC. Red wire number to 290 volt AC. Large black wire 3.2 volts ac. Orange wire 3.2 volts AC. Brown wire 0 taped off send black wire 64 votes ac.power tube number 1 0 volts AC 0 volts DC pin number 2 3.2 volts ac zero volts DC.pin's 3-6 zero volts ac pin 3 -.929vdc pin 4 -.930vdc. Pin 5 -.656 vdc pin 6 -.645 vdc. Pin 7 has 3.2 vac zero volts DC. Pretty much the same for tube 2.... I am only getting 1 volt or so out of the filter caps, that don't seem right does it? I have all the preamp tube numbers if you want them,but I think we already have an issue with the filter caps. Thanks for your help
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Recheck your wiring. Time for some good hi-rez pics.
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I will take them asap!
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I'm having trouble attaching pics i will try to send from my phone to my computer and see if that will work. I'm trying to send from my phone and it's not working for me.
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Here's a couple of links with info on how to add attachments and how to shrink the size of pictures.
http://el34world.com/Forum/index.php?topic=4207.0 (http://el34world.com/Forum/index.php?topic=4207.0)
http://el34world.com/Forum/index.php?topic=464.0 (http://el34world.com/Forum/index.php?topic=464.0)
Brad :icon_biggrin:
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I know this is not the best, but I will get better at this stuff
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More pics
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Hard for me to see much in the pictures.
They are pretty tiny on my 23 inch onitors
They are 250 pixels wide
You can make your pictures 1024 pixels
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I knocked them down to 250 in the pixel maybe to much?
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1024 pixels is much better for everyone
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Thank You
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More Pics
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You just took your tiny pics and digitally blowed them up then resaved as jpg! That ain't hi rez. The big pics are even worse than the original tiny pics.
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What do you suggest? I tried to load them without adjusting them but it wouldn't let me upload them.
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Do you have a e-mail address that I could send the pictures to you?
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What do you suggest? I tried to load them without adjusting them but it wouldn't let me upload them.
I suggest you start with the original BIG pics and downsize them to an acceptable size for the forum. Never take a small pic and upsize it. Too much detail will be lost.
Even better, upload your BIG pics to one of the photo sharing sites and just post a link to the actual BIG pics.
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What ever program you used to reduce the size must have let you set the size?
Just set the size down to 1024 pixels, not 250 pixels wide
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Hi Taylor, not to overload you with advice, but if it were me I'd unhook all connections from that PT. If you know which 2 get hooked to the wall voltage, hook those up, let everything else free. Tape off the ends of all them except the ones you're going to measure with your voltmeter.
Then, using extreme caution and alligator clips, take measurements of wires you think are pairs, like the secondary high voltage, etc. Hook up your test leads first, then apply power. With everything hooked up to filter caps and all that, it's pretty easy to get confused. Be very careful and look at everything twice before you throw the power switch on. Do not forget the "one hand behind your back" rule.
I assume when you said Hoffman 50 watt, you meant this one, the Marshall 50 watt?
Another thing I think you may be doing, just by looking at your voltage measurements, is forgetting to switch between AC and DC reading on your meter. Hope this helps, stick with it, and be slow and careful!
(http://el34world.com/Transformers/images/MPT50.gif)
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Thanks John, I will do that also yes it is the Marshall 50 watt. yes I have been switching from ac to dc but I'm still learning where the dc volts are and where the ac volts should be. still studying the schematics slowly getting it. I will keep trying to get some pics loaded. and about the advice I need to be overloaded
Thanks
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If that PT is like the ones I'm familiar with, your primary wires will be coming out of one hole, and the secondary will be coming out of the other hole.
AC volts are the ones you measure *before* the rectifier. Anything after the rectifier (either the diodes or a tube rectifier) will be DC. When you're measuring your filter caps, that's DC.
When you're new, even the simplest things can be confusing. So again, go very slow, be very careful. Do NOT rest one hand on the chassis while you're measuring voltages.
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Why does the diagram show the black wire between the red board diodes look like it goes to ground?
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That black wire has 110 volts ac coming from it.
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If you're on the primary windings, they're what get hooked up to your on/off switch. So, if that switch is on, and hooked up right, that black wire *on the primary side* will have about 120 volts ac on it. I'm assuming you're on 120 wall voltage, not 220/240 European.
The black wire on the secondary side is the center tap for the high voltage winding. Those 2 red wires get hooked to your rectifier. If you're using diodes, a red wire will go to each one. That black wire then gets bolted to ground. The PT is probably a 325-0-325 or something like that. The red wires are the "325" and the black wire is the "0".
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This is great information I will make sure I have the right black wire before I run it to ground. Thanks John.
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When I try to ground the black wire it lights up my current limiter. This is the wire that is in the bundle with the red wires. It is showing around 66 vac with the meter. I have the black and orange wires connected to the heaters,I just can't figure out where it goes.
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Hmm. Maybe the black wire bundled with the red wires actually goes to the heaters, and the black wire you have on the heaters actually is the CT? Since you're using a lamp limiter, switch them and see what happens. You're disconnected from the filter caps, correct?
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The black that I have hooked up to the heater is right at 3.2 vac and when I check across the orange and black I get the 6.3 vac that the heaters call for. Very confusing, in the other bundle that has the black and blue wires, I have the blue wire hooked up to line voltage through the switch and fused and the black wire to the neutral side, this is how it shows in the diagram that is correct right? I will recheck everything to make sure that I'm not just doing something stupid and simple.
Thanks
Tracy
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Your PT is connected properly. And that third black wire must connect to ground as shown on the layout. The fact that your lamp limiter lights up when you connect the black wire to ground does not indicate that the black wire is faulty. It indicates that something else is not connected properly in the high voltage supply.
I see a green/yellow wire connected to the bottom side of your rectifier diodes. Where does the other end of that wire connect? Hopefully it connects to a fuse. Most people would use a red wire for that.
PS... Pull all your tubes until this is resolved.
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Yes the green/yellow wire goes to the fuse, sorry there was so much red and black that I switched it up a little.
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The color of the wire is not the problem. But that wire will have high voltage dc on it if you ever get this thing working. Red is a color that most people associate with high voltage dc. Green with a yellow stripe is most often associated with low voltage heater center tap or AC mains safety ground.
Measure resistance from that green/yellow wire to chassis. What do you get?
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I'm at work right now, as soon as I get home I will measure the resistance and post, also I will change that wire to red.
Thanks
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Resistance is 0 from green/yellow to chassis.
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Resistance is 0 from green/yellow to chassis.
That's definitely a problem! Something ain't wired right. You should be able to find that just by looking.
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I'm on it,,I will find that damn thing. : )
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Does 35k sound better? I'm checking the resistance with the power on and the stand by switch on.
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DON'T CKECK RESISTANCE WITH THE POWER ON!
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Still learning, with no power I have 10.47 m ohm's
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So what changed between the time of the zero reading and 10.47M reading?
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Still had it hooked up to the fuse through the standby switch with the switch in the off position.
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Still had it hooked up to the fuse through the standby switch with the switch in the off position.
That should be a high resistance reading regardless of which position the standby switch is in.
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Maybe I need to look at how I have the stand by switch and fuse hooked up.
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Maybe I need to look at how I have the stand by switch and fuse hooked up.
Yes. That green/yellow wire should go to the fuse. The other side of the fuse should go to the STBY switch. The other side of the STBY switch should return to the board at point "A".
But you should also insure that your choke and all cap cans are wired to the board correctly.
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Thanks I will make sure that I have everything wired correct
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Should I have continuity between the black and blue wires?
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The wire that is with the red and white wire's that goes to pin 3 of the power tubes,looks like it goes to board A,is that the filter caps? It seems to be what is causing the limit switch to light up.
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You really need someone to stand by your side while you do this. You are in waaayyy over your head.
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You're probably right about having someone by my side, but as far as waaayyy over my head, I've done a lot of things that may have been way over my head, but have learned that nothing is more satisfying then getting something accomplished that might have been more challenging than you thought at first. I will figure this out, I do appreciate all the info that I have received from all that have responded.
Thanks
Tracy
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My replies tend to be short and direct. I don't mean to discourage you. I should have explained that when I said "need someone to stand by your side" that I meant someone with some electronics background and some experience with building amps. Someone that could see exactly what your questions mean and to give an immediate answer. Someone that can put their hands on the amp, rotate this way and that way, examine the wiring, and say "this wire really needs to go here" or "you must put a jumper between the bias range resistor and the AC input of one of the rectifier diodes" and explain why on the spot.
A lot of amp building issues can be solved over the net, but problems that would take an experienced technician a few minutes to solve if he had the amp in hand can and often do take days to solve over the net just due to time lag between posting replies. And when there is a lack of basic electronics knowledge involved, simple issues such as "how to connect a transformer" become major issues.
Things like your last question (The wire that is with the red and white wire's that goes to pin 3 of the power tubes,looks like it goes to board A,is that the filter caps?) would be so much easier to understand and respond to if you attach a high resolution picture that clearly shows exactly what you are looking at. I think you are referring to the OT primary wires but I'm not clear on what the actual question is. That question could be answered in a few seconds by someone at your side, but over the net, and with no picture, it's probably gonna be a long time before anyone knows for sure what you are asking.
I really want to help with your project (heck, we all do), but we really need more info and pics to do so.
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No worries, I wish that there was someone around here that I could lean on, heck I would even travel just to gain more knowledge, I will defiantly get some better pics loaded somewhere so that you may be able to see what I have going on. as far as the wire it's the one that connects to board A with the choke.
Thanks
Tracy
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There are actually 4 things that must be "electrically" connected to point A on the board. The primary center tap wire of the OT, one wire from the choke, a wire from your blue filter cap, and a wire from the standby switch. There are several ways to physically connect these 4 components but they must be electrically connected.
I want to repeat this... Don't put any tubes in the amp until you have the power supply working. That includes the bias supply too.
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heck I would even travel just to gain more knowledge
What state/country are you in?, someone here might be your neighbor :icon_biggrin:
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I live in Cedar Rapids Iowa, that would be great, like I said I would be willing to take a road trip as long as wasn't to crazy far.
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Taylor, I backed out when Sluckey chimed in because he's forgotten more than I'll ever know, and he's really good at getting you (me) to think about what you're doing. :)
Anyway, I was looking at your pics, and it appears you have lots of extra wire coiled up coming from your PT, which is good. That allows you to disconnect and start over, should you want to do that. If it was myself, that's what I'd do. I have, in fact, more than once. When I'm hooking up a PT, even a simple one, here's what I do:
Hook up the primary to the off/on switch. Plug into limiter, test secondary AC voltages.
Connect secondary center tap, connect secondary high voltage wires to rectifier, either diodes or tube. Test DC voltages coming out of rectifier.
Connect heater CT and heater wires. Check AC voltages.
Connect high voltage DC wire, coming off your rectifier, to your first filter cap. Check voltages. At this point I still don't have anything connected past that first filter cap.
Try that if you've not found the problem yet, and let us know what you've got. Cheers!
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Thanks John, I am going to start at the beginning, and do exactly what you suggested. And again I appreciate everything you guys have done for me and the knowledge that I have gleaned from you.
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I think that I might have damaged the pt I only have around 17/18 vac coming out of one of the red wires. I'm going to order another one and start over, I don't know what if anything I did but obviously I have done something. Thanks I'll be back.
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If you had it hooked up to your limiter, it shouldn't have gotten damaged. Usually damage is accompanied by a lot of smoke and a nasty smell. How much AC voltage do you measure across the 2 red secondary wires with the center tap grounded?
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No smoke or smell at all, I will check ac voltage across the red wires and see what I come up with. Thanks
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I've got 745vac across the two red wires, that seems about right doesn't it? Nevertheless I have another pt coming
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Yes, it does. Don't be in a hurry to hook up that other you've got, I don't think there's anything wrong with this one. So, now that you've got your 745 across the 2 red, you know where they get hooked up, right?
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Yes I have diodes, so I will connect them to the diodes.
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Excellent!
Plug into limiter, measure the DC voltage coming out. If anything is weird, then the problem is downstream of the rectifier (probably).
As Sluckey instructed, check the connections on your filter caps and all that. If your limiter shines bright without tubes in, you've got a short... or something miswired... after the rectifier/diodes.
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Plug into limiter, measure the DC voltage coming out. If anything is weird, then the problem is downstream of the rectifier (probably).
As Sluckey instructed, check the connections on your filter caps and all that. If your limiter shines bright without tubes in, you've got a short... or something miswired... after the rectifier/diodes.
Yes, agree.
Brad :icon_biggrin:
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I have 509-512 vdc coming from the filter caps, the DC side of the diodes is at 512volts. Also nothing is lighting up the limiter which is good. So maybe the problem is in the tubes. Should I just insert one at a time and see if I get the limiter to light up?
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What I don't understand is that I had resistance on the DC side of the diodes yesterday but today I have nothing.
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Don't plug the tubes in until you verify that you have approx. -40v (or more) on pin 5 of BOTH output tubes.
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Thank you
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I have -46.5 on both tubes
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So far, so good. Now what do you measure across your filament wires?
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6.9 vac
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Cool! Your PT sounds fine. What are you going to do next?
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I was thinking that I would put the tubes in one at a time, just to make sure that I have them connected right.
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Also I need to figure out the resistance on the DC side of the diodes, I had resistance the other night, and the next morning I didn't have any resistance.
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If your voltages are good, don't worry about the resistance.... what are you taking resistance readings on anyway? Put your power tube in, plug into limiter, see what happens.
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If your voltages are good, don't worry about the resistance.... what are you taking resistance readings on anyway? Put your power tube in, plug into limiter, see what happens.
Yes, yes, yes!
Don't over think it.
Brad :icon_biggrin:
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Tubes in power on, everything looks good. Do I need to check anything else as far as voltages? Or do I try to
see if I get any sound out of it?
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Hooked up to a speaker cab nothing. I'm going to check the input/output jacks they seem OK but I will go over them.
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Ok, no sound, but no smoke.
Now take voltage readings on every tube and post them.
Brad :icon_biggrin:
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V5..pin 1-0. Pin2-3.25vac. Pin3-505vdc.. Pin 4-510vdc. Pin 5-40.7vdc. Pin6-40.7vdc. Pin 7-3.25vac. Pin 8-0.. V4. Pin 1-0. Pin2-3.2vac. Pin3-518vdc. Pin4-517 vdc. Pin5-41.3vdc. Pin6-40.6vdc. Pin7-3.23vac. Pin8-0..V3. Pin1-593vdc. Pin2-598vdc. Pin3-573vdc. Pin4-3.2vac. Pin5-3.2vac. Pin6-601vdc. Pin7-628vdc. Pin8-566vdc. Pin9-3.15vac. V2. Pin1-605vdc. Pin2-594vdc. Pin3-0. Pin4&5-3.2vac. Pin6-595. Pin7-693vdc. Pin8-92.8vdc. Pin9-3.1vac. V1. Pin1570vdc. Pin2-357vdc. Pin3-8.9. Pin4&5-3.17vac. Pin6-578vdc. Pin7-353vdc. Pin8-29.2vdc. Pin9-3.2vac... I see some discrepancy in some of the voltages, does this tell you anything? Thank you!
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V5.
Pin 1- 0.
Pin 2- 3.25vac.
Pin 3- 505vdc.
Pin 4- 510vdc.
Pin 5- 40.7vdc.
Pin 6- 40.7vdc.
Pin 7- 3.25vac.
Pin 8- 0.
V4.
Pin 1- 0.
Pin 2- 3.2vac.
Pin 3- 518vdc.
Pin 4- 517 vdc.
Pin 5- 41.3vdc.
Pin 6- 40.6vdc.
Pin 7- 3.23vac.
Pin 8- 0.
V3.
Pin 1- 593vdc.
Pin 2- 598vdc.
Pin 3- 573vdc.
Pin 4- 3.2vac.
Pin 5- 3.2vac.
Pin 6- 601vdc.
Pin 7- 628vdc.
Pin 8- 566vdc.
Pin 9- 3.15vac.
V2.
Pin 1- 605vdc.
Pin 2- 594vdc.
Pin 3- 0.
Pin 4&5- 3.2vac.
Pin 6- 595.
Pin 7- 693vdc.
Pin 8- 92.8vdc.
Pin 9- 3.1vac.
V1.
Pin 1- 570vdc.
Pin 2- 357vdc.
Pin 3- 8.9.
Pin 4&5- 3.17vac.
Pin 6- 578vdc.
Pin 7- 353vdc.
Pin 8- 29.2vdc.
Pin 9- 3.2vac.
Edit; I rearranged the voltages into a column to read them a little easier. Brad :icon_biggrin:
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If those voltage readings are correct.........
Turn the amp off, unplug it and don't plug it back in and turn it back on until you fix some wiring!
V3, V2 and V1 must be wired up wrong. Something is very wrong for the grids and K's to have the high plate B+ dcv on them.
On 12 _ _ 7 tubes pin's 1 and 6 are the plate, pin's 2 and 7 are the grid and pin's 3 and 8 are the cathode.
The grids and cathodes (K) should not have high dcv on them like the plates. (Cathode followers and concertina PI will have K dcv the same/very close too the plate dcv.)
And I take it you meant you have around negative -40.7dcv on V5/V4 pins 5 and 6?
Brad
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Thank You, I will go through the wiring on V1-V3 and see what is going on. Yes negative 40.7vdc
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I have 509-512 vdc coming from the filter caps, the DC side of the diodes is at 512volts.
This is what you measured 3 days ago with no tubes plugged in. The voltage reading at the rectifier diodes will be the highest voltage in the entire amp. 512V is very believable for a Hoffman Plexi 50 with no tubes.
But now you have several readings that are considerably higher than that. 598, 566, 628, even 693! These readings are impossible! You are doing something very wrong when you measure voltage. I suspect you may be misinterpreting your meter or your meter is just no good. Maybe your 693V reading is really .693V or 693mV. There's a big difference.
The point is these reading can mislead someone that is troubleshooting your amp. They are so badly wrong that you need to take another complete set of readings and be sure they are accurate. At this point the only thing that is clear is the amp is not wired correctly. Your only reliable symptom is "no sound" and that can be any of hundreds of problems. Probably even multiple problems.
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Thanks Sluckey,
I will take the readings (using a different meter) again and repost, I'm sure it's operator error.
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You have good "standards" out in the world to both calibrate a meter and it's operator, a car battery is good for DC, (12 - 14.8vdc), your house wiring for AC, (115 - 125Vac). might start there to verify your equipment.
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It's possible his meter is reading the DC and AC both. I had one do the same thing that had me scratching my head a while back.
A different meter is a good idea.
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OK here we go....
V5
Pin 1. 32mvdc
Pin 2. 3vac
Pin 3. 429vdc
Pin 4. 424vdc
Pin 5&6.-39vdc
Pin 7. 3vac
Pin 8. 32mvdc
V4
Pin 1&8 30mvdc
Pin 2. 3.25vac
Pin 3. 424vdc
Pin 4. 419vdc
Pin 5&6 -39.8vdc
Pin 7. 3.28vac
V3
Pin 1.327vdc
Pin 2. 369vdc
Pin 3. 12.68vdc
Pin 4&5. 3vac
Pin 6. 328vdc
Pin 7. 372vdc
Pin 8. 12.6vdc
Pin 9. 3vac
V2
Pin 1.162vdc
Pin 2. 0v
Pin 3- .95vdc
Pin 4&5. 3.2 vac
Pin 6. 289vdc
Pin 7. 161vdc
Pin 8. 163vdc
Pin 9. 3vac
V1
Pin 1. 156vdc
Pin 2. 0v
Pin 3- .93vdc
Pin 4&5. 3.17vac
Pin 6. 203vdc
Pin 7. 0v
Pin 8. 1.78vdc
Pin 9. 3vdc
Pins 3 in V2&V3 are point 95 and point 93 vdc. Hopefully this looks better. Thanks again for your patience guys.
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Looks OK except for V3. Please recheck those pins, in particular, pin 2, 7, 3, and 8.
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Recheck
V3
Pin 1. 340vdc
Pin 2. 388mvdc
Pin 3. 13.3 vdc
Pin 4&5. 3vac
Pin 6. 343 vdc
Pin 7. 389 mvdc
Pin 8. 13.15 vdc
Pin 9. 3 vac
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Check the values of your cathode resistors on V3. (only a guess on my part)
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I will do that, Thanks
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As a quick check... Turn power off. Measure resistance from V3 pin 8 to chassis ground. Should be about 15KΩ. What do you get?
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Will do asap...Thanks
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I've got .484 m ohm's
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I've got .484 m ohm's
That indicates you have a 470K as bias resistor probably. It should be 470 Ohm, not 470,000 ohm. That is the resistor right off the cathodes, which connects to a 15K then to ground. That explains the 480K, or .480M. Check that resistor..
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I'm on it I will check it out. Thanks
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I changed out that resistor, I now have 15k out of pin 8. Thanks guys is there any thing else I should look at?
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I changed out that resistor, I now have 15k out of pin 8. Thanks guys is there any thing else I should look at?
Make sure you have the grid leak resistors (1m) from pins 2 and 7 to the junction of 470ohm-15Kohm, and try it out.
That resistor was the reason for little to no sound. The Phase inverter was biased into shutoff, and if you really pushed it, it would likely moan and groan. The amp should make noise now...
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Will do. Thank you
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Still nothing.. The power tubes were getting pretty hot right away it doesn't seem right, darn it!
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Still nothing.. The power tubes were getting pretty hot right away it doesn't seem right, darn it!
Did you set bias. If not the tubes can come up and immediately begin to red plate. You need to check every connection, and resistor value throughout. It is a common mistake to pop in a certain "k" resistor for a certain ohm resistor.
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I will go through everything make sure that I have all the right resistors in,check the bias. I'll post what I come up with. Thanks again
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I did find another 470ohm in a 470k that is connected to V2 pin 2. Changed it to the correct resistor, now when I power up and switch off stand by the limiter slowly starts to glow??? If it ain't one thing its another
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I did find another 470ohm in a 470k that is connected to V2 pin 2. Changed it to the correct resistor, now when I power up and switch off stand by the limiter slowly starts to glow??? If it ain't one thing its another
If that was a 470 and you replaced it with a correct 470K, that makes sense, 470 ohm would've not allowed enough signal on the V2a grid, so that would make a silent amp... now find out where the short is, look around your work where you changed the resistor. set bias on your output tubes, and that may stop the glowing, but a slight glow would be normal when the current draw rises to full.... if it isn't bright, it's just a small load, not a direct short.
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OK great, it's just a very lite glow and it seems to start as the tubes warm up. I will double check everything and check voltages Thank you once again.
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OK great, it's just a very lite glow and it seems to start as the tubes warm up. I will double check everything and check voltages Thank you once again.
That's very normal, it means the tubes are conducting. Time to try it off the limiter. then post results.
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Will do. Thanks again
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Time to try it off the limiter. then post results.
Be ready, just in case, to pull the plug. (We all do the same with the 1st no limiter flight.)
Brad :icon_biggrin:
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Well powered up everything looks OK, no smoke : ) bias is set at 35vdc. Tubes still seem to get hot when I switch the stand by switch to on. I'd there any thing I should look at?
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Well powered up everything looks OK, no smoke : ) bias is set at 35vdc. Tubes still seem to get hot when I switch the stand by switch to on. I'd there any thing I should look at?
Not sure what you're reading at 35vdc, but your bias setting should be done to adjust the milliamps of current read at the cathodes of the power tubes. The way most of us do this is by reading the voltage drop across a 1ohm resistor that goes from the cathodes to ground. That voltage drop, read in mvdc will equal the current, in milliamps dc, the tube is drawing through the resistor, since it's value is 1. that is the reason a 1 ohm is used, to make reading amps with a volt meter easy
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OK I will check what I get across the resistor and post. Thanks
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OK I checked across the 1ohm resistor's and set the bias at 35mvdc. I did plug back into the limiter just to be safe, it seems that the bias was the cause of the light glowing.
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OK I checked across the 1ohm resistor's and set the bias at 35mvdc. I did plug back into the limiter just to be safe, it seems that the bias was the cause of the light glowing.
For reference, it is a good procedure to always adjust bias voltage to max (Which means as far negative as you can) before installing tubes to test. Then when the tubes are installed, they are at the least current draw setting, and then adjust them to proper idle current first thing. No need for the limiter to set bias, in fact that could make the setting incorrect. Check it again after you take it off. does the amp work? do you get output?
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Well I did get a little output. Very little also noticed that the tube got very hot. I took the tubes out and set the voltage as far as I could around -56v and then set it at 35. I plugged into a speaker cab and did get some sound out of it. Right when I saw the tubes start glowing I shut it down.
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Well I did get a little output. Very little also noticed that the tube got very hot. I took the tubes out and set the voltage as far as I could around -56v and then set it at 35. I plugged into a speaker cab and did get some sound out of it. Right when I saw the tubes start glowing I shut it down.
Do you mean you set it to 35ma per tube? How did you derive that number? What is your plate voltage? (varies with bias current). (Plate voltage) times (idle current), (Voltage dropped across 1 ohm) equals idle wattage of plate and screen combined.... so plate is less than this calculation, since screen current is not subtracted. The tube data will tell you how much wattage the plate can dissipate, if you run more than that, the tubes will redplate, because you are adding heat, or wattage, faster than the plate can get rid of it.
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I checked the voltages across the 1ohm resistors to ground and set it at 35mvdc And I checked the voltages at pin's 6&5 which gave me -46vdc. I'm still learning how to do the math part of amp building.
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I checked the voltages across the 1ohm resistors to ground and set it at 35mvdc And I checked the voltages at pin's 6&5 which gave me -46vdc. I'm still learning how to do the math part of amp building.
Good, now multiply that 35ma by plate voltage. that gives you wattage the tube is drawing. Think about this. .035A x 500V = 17.5W while .035A x 400V = 14W. Since the plate dissipates watts (directly related to heat), that same .035A setting can give quite a range of wattage. The watts is what matters in bias settings.
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Will do, I will post what I get. Thank you
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I have 487vdc at the plates and I have 30mvdc at resistor's I come up with 14.61 watts.
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I have 487vdc at the plates and I have 30mvdc at resistor's I come up with 14.61 watts.
Those are EL34? if so 17.5W would be 70% of the rated 25W dissapation. 14.61 would be a bit cold, but certainly should make lots of sound... how is it sounding? if it's weak, then you have to trace down why it's getting attenuated somewhere in the circuit.
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Yes EL34's, very little sound as in I have to turn it up all the way to get anything. with 14.61watts the tubes shouldn't get so hot should they? maybe because of having to turn all the way up had something to do with it. I will check everything again, any suggestions?
Thanks once again.
Tracy
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There's a wiring error somewhere. :icon_biggrin:
Double check the filter cap connections, make sure each "node" on the power supply rail is going to the right one. (made that mistake)
Check your grid leak resistors on the power tubes, make sure they're not say 2.2k instead of 220k (or whatever the schem says)
Are your power tubes actually glowing red, or just getting hot very quickly? Because power tubes do get much much hotter than your preamp tubes.
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Yeah they are glowing red, I will check the resistors, I did notice that the voltage from board A,B,and C was much higher than D&E don't know if that could be an issue. I will check them again after work but they seemed to be at least 100vdc difference.
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Hmm. I'm tapped out of ideas (my knowledge is a thimble compared to Sluckey's bucketful) :laugh:
I can't understand why your power tubes are redplating when, if your math is right, they're only dissipating ~ 14 watts per tube at the plates. You're measuring across a 1 ohm resistor? Are you sure that's a 1% tolerance 1 ohm, like Doug sells?
This is why I like cathode bias. :icon_biggrin:
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Yes I got everything from Doug. I will check again and post as soon as I get home from the work thing.
Thanks
Tracy
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That darned work thing! It keeps my ouput to 2 a year, if I'm lucky.
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I have 5.6k resistors on the tubes, that's what it calls for. It stands to reason that board A has more voltage now that I look at it duh on my part. I do have the bias tap wired to the 220k 3 watt resistor from the pt red output. And the feedback wired from the 8ohm switch to the 47k resistor as it shows in the diagram. I'm going to keep looking this damn thing over maybe I'm just not seeing something.
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Should I be getting 470+vdc out of all tabs on the filter caps?
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Should I be getting 470+vdc out of all tabs on the filter caps?
No, look at your schematic, there is the first node, highest voltage, then choke, which drops some voltage, so it's cap would be just a little lower, then dropping resistor, and another cap, which should be lower voltage, (assuming a normal loading with all tubes in place and functioning). Then another dropping resistor and cap, with lower voltage. Each power supply resistor drops the voltage a bit.
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OK great that's what I'm getting. Thank you
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Thought I might have found a problem, it was in the preamp tube thought I had wire's touching but wasn't it. This is kind of like golf very humbling. Still red plaiting damn.it
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This is kind of like golf very humbling.
But way more fun.
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Okay, you've probably already done this and posted the answers, but if not: with the amp turned off (and unplugged), measure resistance from each pin of the power tubes to ground. On both tubes, the pins should measure pretty much the same, but probably not exact.
And, you're not using pin 1 on either tube as a tie point, are you? IIRC the EL34's have an internal connection with something there.
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Yes I have pin 1 tied in with 8. Maybe Bingo?
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That might be the problem! BUT, I don't know enough to say for sure, so wait until someone else chimes in.
*edit* No, that's not it. It is supposed to be tied to the cathode.
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Dang it
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I will check the resistance on all.pins and.see what I get and post.
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V4
Pin1&8=1.2ohm
Pin 2=50.1ohm
Pin 3=O.L
Pin 4=O.L
Pin 5=297.2k
Pin 6=291.6k
P 7=50ohm
V5
Pin 1&8=1.4ohm
Pin 2=50ohm
Pin 3=O.L
Pin 4=O.L
Pin 5=297.1
Pin 6=291.5
Pin 7=50.1
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Check the little 1 ohm resistors again. When you set your DMM to check ohms, touch the leads together first; that should give you a 0.00 reading. Those resistors should be reading 1 ohm, or 1.01 at the most.
Otherwise, the one is 20% off and the other is 40% off. That'll give you bad math for sure.
And hey, somebody that knows more than me (which is everybody)... should pin 6 be showing those resistance readings?
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should pin 6 be showing those resistance readings?
Yes. The 5.6K grid stopper is mounted between pin 5 and pin 6.
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Thank you I'm on it.
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I get 1.0 on one and 1.1 on the other, it's not 1.01 does that make a difference?
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John, remember I'm on here so I would be the one that doesn't know anything ; )
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I don't worry about a 1Ω 1% precision resistor being exactly 1Ω. If it reads close to 1Ω, I always assume it is correct. Such things as lead resistance, connectors and even the pressure you put on you probes can cause a seemingly out of tolerance reading.
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Thanks sluckey,do you have I suggestions as to the red plating in the tubes? What I can check?
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Power tubes (may have been damaged earlier), voltages on power tubes, especially the - bias on the grids, wiring to the power tubes, components in the power tube circuit.
Disconnect the NFB wire. Does that help? Leave it disconnected for now.
Pull all the little tubes and leave them out until the red plating condition is solved. Post a few hi-rez pics of the power tubes and board near the power tubes. We need to be able to clearly see a wire that leaves the board and connects to a pin on the power tubes. We need to be able to see resistor color codes and cap values.
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I should know this,. But what is NFB? Thanks. I will try to get some decent pictures posted as soon as I can.
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There's a wire that goes from the amps speaker output jack to a R then it goes to the PI. (NFB = negative feed back)
Brad :icon_biggrin:
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R=? PI=? Sorry still really new at all the hyphens. And thanks again
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Ok, sorry. R = resistor, PI = phase inverter. Sometimes LTPI = long tail phase inverter. C = capacitor.
E-cap or e-cap = electrolytic capacitor, B+ filter cap or cathode (= K) bypass cap. CCR (no not the band :laugh: ) = carbon composition resistor, MFR = metal film R. The plate of a tube = anode = A or P.
Switches, SPST = single pole, single throw = on/off switch. SPDT = single pole, double throw. DPDT = double pole double throw. It goes on as there are any # of poles and throw combinations. Also the switch will be listed as maybe center off or on/on/on....... (The pole is the 'common' of the switch and the throw is what the switch switches to.)
Also you will see NFB as -FB or FB. (There's also positive FB.)
Brad :icon_biggrin:
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CCR are you sure its not the band : ) To funny! Thank you very eye opening. I pulled the pre amp tubes now when I take readings on the power tubes I get pretty much nothing, is that normal?
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I believe that I may have a problem Houston, I don't have any resistors connected to my output jacks. The diagram that I have and maybe I'm reading it wrong, which is entirely possible doesn't show any.
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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 :icon_biggrin:
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Can you post your diagram?
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It's page 2 in the library of information plexi 50 watt
That's the easiest way for me to post because I suck at computer stuff
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I pulled the pre amp tubes now when I take readings on the power tubes I get pretty much nothing, is that normal?
Say what!!!
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Yeah, I have 0 volts on pins 1&8
3.4 vac on pin 2
5.74vdc on pin 3
5.74vdc on pin 4
-49 vdc on pins 5&6
Great huh : (
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Here, red arrows, shows NFB wire from OT (output transformer, secondary side) speaker output jack, going to the series NFB R.
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OK I have the 8ohm wire from the output going to the 8ohm selector switch from the selector switch to the resistor from the resistor to the 5k pres switch.And thank you again
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About not having power to the power tubes, I had a blow.5 fuse. It's good now
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I found a resistor that I soldered in the wrong way, it's the 330/16v that is in with the 820. That's probably not good. I switched it around don't know if that is my issues but it has to caused some of my headaches.
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it's the 330/16v that is in with the 820
Are you talking C1a - R7a, coming off pin 3 of V1a? That would be a cap - capacitor, in || - parallel, with 820 ohm R - Resistor. If so, you might want to replace it, caps usually don't do well when they're hooked up backwards, and, ya, that will give you problems. Do you know WHY your fuse blew?
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No I don't, I can only assume that I did something to cause. Of course we all know what assuming does.; ) I will replace and see what happens. Thank You
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https://www.flickr.com/photos/131879876@N08/17937088129/in/album-72157651167916813/ (https://www.flickr.com/photos/131879876@N08/17937088129/in/album-72157651167916813/)
Please see if this works.Pictures
Thanks
Tracy
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Doesn't work. Requires log in.
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try tra412000 then taylor for pass
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Clicked on link says, 'page not found'.
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Try tra412000 and use password taylor or maybe if you could go to flicker and search for taylorc315.
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Any Luck?
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I see the pics.
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I've traced all the wires and can't find anything. Does the wire that laces the turrets need to be a certain gage? Were you able to see through the mess at all?
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I've traced all the wires and can't find anything.
The problem(s) is still right in front of you.
Does the wire that laces the turrets need to be a certain gage?
I use 22AWG solid tinned buss wire.
Were you able to see through the mess at all?
No. Well, I can see certain areas OK but other areas of interest are not visible. I would have to have my hands on that amp to see what I need to see in order to fix it.
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You do see some issues? Is there any particular areas that I can maybe see if I can get better pictures?
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I.also don't have a problem sending this to you if that is an option. I just need a lot more knowledge and guidance. I am going to start another one and take it step by step, I got into a bit of a hurry with this one. I'm not going to make the same mistakes. I used 20 gage copper wire instead of the tinned that you use, is that an issue?
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I am going to start another one and take it step by step,
I don't think you need to do that, just de-construct this one. You might just find the mistake when you're taking stuff apart. Use it as a learning experience. Highlight each connection/wire on the schematic as you take it off. For desoldering, braid is your friend. :)
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I've deconstructed the amp and now just waiting for some tinned buss wire to start over. I think that the wire I used was not adequate for the job.
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I've deconstructed the amp and now just waiting for some tinned buss wire to start over. I think that the wire I used was not adequate for the job.
I don't think the wire was a factor. Please start a new thread when you post about the rebuild. Good luck.
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Thank you Sluckey. I will
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Hey I'm back : ) I re did everything I compared to my 72 Marshall as far as voltages it seems that everything looks good. I was wondering if my problem could be with the input and/or output jacks? I compared with the old Marshall but it doesn't have the same type jacks. Thanks
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I also uploaded more pics at the flicker account.
Tra412000.... Taylor hopefully this still works
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May I suggest you start a new thread? Since you have totally rebuilt this amp it's kinda like having a different amp with different issues. Probably most of the 200+ messages in this thread are not pertinent to this rebuild. We need to know your new issues, voltages, pictures, etc.
EDIT... Also post the link to your new pics so we don't have to dig back through 200+ messages to find it.
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Will do