Problems Solved! 5f2a Princeton Build Issues

thewestwon

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553
10/30 Case closed. Thanks for all the help!

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So, I'm building a 5f2a Princeton from the Weber Kit, and I've run into a bit of a roadblock.

I have traced everything on the schematic/layout multiple times and I'm sure (sure as I can get) that I have it wired correctly. I have double checked the speaker, which was pulled out of my Super, and swapped tubes with known good ones. Here are my voltages:

Heaters at lamp: 3.25 vac/3.25 vac

Copper cap rect

pin 2: 5.3vac
pin 8: 373v
pin 4: 340 vac
pin 6: 340 vac

6V6 GT

pin 2: 3.25vac
pin 3: 362v
pin 4: 337v
pin 5: 0
pin 7: 3.25vac
pin 8: 22v

12ax7

pin 1: 195v
pin 2: 0
pin 3: 1.5v
pin 4: 3.25vac
pin 5: 3.25vac
pin 6: 185v
pin 8: 1.5v
pin 9: 3.25vac

The issue is that I have no output at all. I heard a couple crackles with measuring pin 3 of the 6v6 so I powered down and re-soldered all the tube connections thinking I might have a bad connection. Now no crackles, but still no output.

Out of the box, both the lamp and fuse were DOA, so I guess it wouldn't surprise me if something else was bad. I measured all resistors before I put them in and they were to spec. I just don't really know where to go from here. Here are some pictures in case somebody can see something. Let me know if anything else will be of assistance in troubleshooting. Appreciate the help.



 
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Steppin' Wolfe

Silver Supporting Member
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5,489
thewestwon, P8 on that rectifier should be read in DC....that is your B+. You obviously have maybe 365VDC there because you have good plate voltages on the 66 and the 12A_7, right?
 
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thewestwon

Member
Messages
553
I corrected my first post. I do have 373vdc on pin 8/B+.

Ok so looking at the layout below, I have voltage on pin 6 of the 12ax7, and I can see that voltage on the south side of the .022uF cap. There is no voltage on the north side of that cap where 6v6 pin 5 connects. However, when reading around other forums, 0v here seems normal. Here's a link to one where someone points out 175ma here as high. I'm far from the expert...basically using forum science here.. any thoughts?? I also posted the 6g10 Harvard schematic with voltages, which is what it seems like I should be using as reference. Out of curiosity, where were you seeing the 19VDC called for there? Thanks again for the help!

http://music-electronics-forum.com/t14560/ (last post)
 
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Steppin' Wolfe

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5,489
My apologies....mixed up on what is going on there....and what amp circuit you have there. I was thinking about the bias grid p5 I should have been thinking about the cathode voltage ..and you obviously have the correct voltage there....22VDC. NO excuse for my error...but the muscle relaxer for my back must have kicked in.
I'll give it another try...more carefully this time.
Crackle when testing voltage at P3, 6V6....that measurement should cause a light Pop!. AS you measure plate voltages on back through the circuit, the pops should increase in volume with the input stage plate being the loudest...because it is amplified through the entire circuit. IF you are not getting any noise...pop....when you take these plate voltage measurements, then either there is no output or the amp is in oscillation at a frequency too high to be heard. Note: that plate voltage measurement from the output back to the input is a tried and true indicator of where a problem might be....wherever there is no Pop! indicates that there is no signal being processed there. So, if there is Pop on the 6V6.....you have problems in the output. You don't know if there are other problems towards the preamp until you get a Pop on the output and then you can move to stage that preceeds that output..and so forth toward the input.

Pin 5 on the 6V6 in this cathode biased amp.....
That is the control grid. The input signal that is inserted here controls the 'biasing'.....it will be negative in relationship to the cathode. And...the voltage there will be dependent on the signal strength and frequency of the signal...as well as I can understand things. This is the essence of cathode or 'self biasing' circuits versus 'fixed biased' circuits. The voltage differential between that control grid in a fixed bias amp is constant....established by the bias voltage applied to pin 5...a negative voltage while the cathode is constant at ground. IN that cathode biased circuit, when the guitar signal is amplified by the circuit and introduced to that control grid---pin 5---, that signal will have a higher voltage than the circuit shows at 'idle'. This is why a cathode biased amp is working harder at idle than it is with a signal being processed through it....the 'negativity' of the voltage on p5 is greatest at idle...and therefore the tube is dissipating more energy at idle than it will when the control grid voltage becomes 'less negative' in relation to the cathode. I will confess that I have never had the need to check that voltage on p5 in such a circuit.....but I will the next time I have one open.
PIcs are dark...but I don't know if I could discern any problem or not. Hands on is best...obviously.

At this point...since as I understand it you are not getting any Pop!s when testing plate voltage, I would suggest checking the OT resistance..primary and secondary..plus the wiring of the OT leads. IT is odd ime to have some noise when checking voltages and then have that noise, which is a diagnostic indicator of sorts, go away is troubling.
 

thewestwon

Member
Messages
553
Will dig into the trace on Pin 3 shortly. If I'm understanding things correctly, my signal flows from [edited] input -> grid stoppers -> 12ax7 pin2 -> 12ax7 pin1 -> coupling cap -> volume/tone -> 12ax7 pin 7 -> 12ax7 pin6 -> through other coupling cap -> 6v6 pin 5 -> out 6v6 pin 3 to OT, which would be the path to hunt for pops?

And awesome information. Super helpful!

Edit: So I am able to hear a faint popping while testing pin3, but the pop I was describing previously was loud. Like the sound of a speaker when you hook it to a 9v.
 
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Steppin' Wolfe

Silver Supporting Member
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5,489
Input->V1P2(grid)->V1P1(plate)->volume and tone->V1P7(grid)->V1P6(plate)->6v6P5(control grid)->6V6P3(plate)->Ot->speaker->ears->human brain for recognition and pleasure. (;^)
 

thewestwon

Member
Messages
553
Input->V1P2(grid)->V1P1(plate)->volume and tone->V1P7(grid)->V1P6(plate)->6v6P5(control grid)->6V6P3(plate)->Ot->speaker->ears->human brain for recognition and pleasure. (;^)
Wow. Yes, that's what I traced on the diagram. I don't know what in the heck I typed out there...definitely not what I was seeing and thinking, haha. That made zero sense.
 

Steppin' Wolfe

Silver Supporting Member
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5,489
You have a Pop! on the 6V6 P3....what about the two plates on the 12AX7? P1 should be your loudest Pop!
 

thewestwon

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Messages
553
I do have a faint 'pop' at P3. No pop on 5 of the 6v6 nor any pops on the 12ax7.

No pop at pin 5 seems odd to me since I can hear it at 3. I tried three different 6v6s with the same result. There's no visual indications the socket is faulty, but do you suppose that could be a player here?
 
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Steppin' Wolfe

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5,489
Is there any noise at all coming out of the speaker...some hiss, maybe...very faint? IF you do hear some sort of sound at the speaker, you might think that the output is working and you don't have any signal coming to the 6V6 for some reason.
FWiw, this statement: "I have traced everything on the schematic/layout multiple times and I'm sure (sure as I can get) that I have it wired correctly." is atype of mental situation that can cause one to overlook things. I thin a fresh approach after a rest might be in order. When you come back to the project with a fresh mind, trace through things without any preconceived notions...like ''I am as sure as I can be". There is a great book..."Zen and The Art of Motorcycle Maintenance" by Robert Pursig that points out just such a situation in which our minds can 'blind' us from seeing an error in our own work.

Now, if you had a signal generator and an oscilloscope, you could track the signal from the input to the point at which it fails. The failure of the circuit to exhibit a Pop on pin 6 of V1 could be a clue that the path from that tube to the 6v6's plate is not 'made'....there is a failure somewhere in that area.
 

thewestwon

Member
Messages
553
Totally with you on overlooking things. I put a lot of effort into making sure everything was right from the get-go.."measure twice" mentality.. and did a complete trace before assembling and soldering the final leads that have to be done in place. I totally acknowledge the possibility that I have missed something and am just not seeing it at this point, though. This is my first build (after only a few repairs previously), so I'm far from experienced, although not clueless in what's involved and the engineering behind it.

It's the old lady's birthday weekend (at least as soon as she gets home from work). So, it'll be put away until Monday. At that point, I'm thinking my best but will be to pull the board and step back in with fresh eyes.

There is slight noise when the amp is powered up (and I mean slight...could be my imagination), but I don't notice any speaker hiss anywhere in the volume sweep when it's on.

Again, really appreciate the help!
 

Steppin' Wolfe

Silver Supporting Member
Messages
5,489
IF the output section working but the preamp/PI are not sending signal to the output, then the volume pot will not give you any clues. That is one of the things tha the Pop! test will tell you....where is the signal NOT being processed. IF there is no Pop on V1P6, then the PI is not sending signal to the output tube. IF you get a Pop on V1P6 but not on V1P1, then the preamp is not sending signal to the driver....the second triode in V1.
 

thewestwon

Member
Messages
553
10-4. So based on the limited amount of info we've collected, do you think there's anything to a pop on 6v6 P3 but not P5? Or should I be focusing my troubleshooting on the preamp/PI? Or is the no pop on 6v6 P5 directly related to an issue in the preamp/PI (as in you wouldn't expect to hear it if there was an issue in the PA/PI). I suppose it ultimately doesn't matter since I'm pulling the board and re-checking everything; I'm more curious for future knowledge.
 

Steppin' Wolfe

Silver Supporting Member
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5,489
I focus on the plates for the pop test thing. AS you can see, the plate is the point in this circuit from which a signal leaves one stage for the connection to the next stage. There should be some reaction to the voltage measurement contact on the cathodes, too....but the plates are where I listen for the clues to signal production or failure.
 

pdf64

Member
Messages
7,937
My hunch is that somewhere along the signal chain there's a mistake, the signal path is either getting broken (open circuit) or shorted to ground.
With the amp isolated from the line and de-energised, what is the resistance between the 6V6 control grid pin5 and ground?
What is the resistance between the 6V6 control grid pin5 and the coupling cap that connects to V1b plate pin 6?

BTW, on that linked MEF thread, Mark made a typo
'175mA on pin 5 seems high, should be "0v"
should have been
'175mV on pin 5 seems high, should be "0v"
 

thewestwon

Member
Messages
553
Hm so after an extended break from this, I pulled it out to start diagnosing the issues today. I pulled the board and retraced everything again..still didn't see any issues compared to the weber layout. I dropped it back in, rewired the board to the tubes and input, double checking the layout/schematic multiple times. Same as before, which was a light crackle on v2p3 (I would not call what I'm hearing a loud pop, though) and a faint hum from the speaker.

Per pdf64's questions that I'm not answering in a timely fashion:
6v6 p5 and ground shows OL on all settings and
6v6 p5 and the coupling cap (v1p6) shows OL, as well.

So there's that...problem found. I do show resistance between 6v6p5 and the other side of the coupling cap (that would be the north side when looking at the schematic. I pulled that coupling cap (the leftmost .o22uF in the layout) out and it seemed to charge/discharge on my meter by switching the leads. Then we get to the fun part. I was testing continuity on the (left on schematic) 100k resistor to v1p1 and was getting OL. Pulled that left 100k resistor and it was bad. It's not burnt or anything...I'm definitely curious as to whether it was a cause or a symptom of my issues. Of course, I don't have a spare laying around, so we'll be on hold again while I wait to pick up a replacement.
 
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Steppin' Wolfe

Silver Supporting Member
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5,489
That is a plate resistor....power supply to that plate...V1P1. The voltage test....the 'pop' test is a votlage test....would have shown this....and you would have seen no voltage on that plate. That is a problem.
But....in review, I see that you found 195VDC on that V1P1????
 

thewestwon

Member
Messages
553
Good point, there. So that measurement was taken basically the first time I powered it up, but as far as I recall, yeah that was correct. Although various things have been pulled and re-soldered between then and finding that 100k had failed.

It appears that I have some Gremlins running around in my Princeton. My resistors will be here Thursday, so I'll throw a new one in and do another full set of voltage measurements to evaluate where we're at with all of that. My hope is that something was incorrect (and caused the 100k to fail) and got fixed with pulling and re-installing the board, although for the life of me I can't imagine what it was...probably wishful thinking, though.

I am learning a lot, though, so I suppose the ultimate goal of the kit has been accomplished...it'd sure be nice to get it running, though.
 

thewestwon

Member
Messages
553
Voltages after 100k replaced:

Copper cap rect

pin 2: 5.0 - 5.3 vac...not stabilizing
pin 8: 368v
pin 4: 330 vac
pin 6: 330 vac

6V6 GT

pin 2: 3.15vac
pin 3: 356v
pin 4: 329v
pin 5: 0
pin 7: 3.15vac
pin 8: 20.2v

12ax7

pin 1: 195v
pin 2: 0
pin 3: 1.3v
pin 4: 3.18vac
pin 5: 3.18vac
pin 6: 194.8v
pin 8: 1.3v
pin 9: 3.18vac

I started the pop test:
heard a very loud pop on 6v6p3 now. No pop on 6v6P5 on up the coupling caps to v1p1 or v1p6, and of course no sound is passing through.

I'm kind of at a loss where to look, now. It seems to me like the issue is in the 6v6, right? If I have a pop on p3 but none on p5 that tells me electrons aren't passing from grid to plate?
 

robrob

Member
Messages
408
Your voltages look pretty good to me but it looks like the volume pot's output wire is connected to pin1 (left), the ground connection. Is that just an optical illusion?
 






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