Plexi rebuild problems

[Auke Jolman]

I decided to overhaule my homebrew plexibuil and use the layout as posted here on the website, with a few diifernces:
- I use a GZ34 rectifiertube instead of diodes
- I implemented a PPMV
- I use a 10K resistor as the first dropping resistor

After completing the board and all of the wiring and checking if everythings is ok twice, I fired it up. No sound at all and 1 tube redplating. I quickly turned it of. I took out the PPMV and used the 2 220K resistors instead for the bias voltage, because I thought that I made a mistake in the wiring of it. But the result is still the same.

I than started checking component placement and wiring, but onfortunaly I can't find what I've done wrong.

I decided to take out the powertubes and do a voltage check: A 398, B 392, C 259, D 141 and E 130. The bias voltage reads -39 V. A picture of the amp is included also. In my attempt tofind out what is wrong, I hardwired a jack at the middle lug of the treblepot and test the preamp. Still no sound.

Can anybody give mo some advice on how I can find out what I did wrong?

[SILVERGUN]

Are you reading the -39VDC at both tube sockets - pin 5?

My goal would be to fix one issue at a time,,,imagining that you have 2 separate unrelated issues.
1) red plating
2) no sound

Lets get that one tube to stop red-plating first....that's an expensive light bulb. 

More close up pics in better focus will help anyone who is trying to see what you might have missed.

[eleventeen]

Fully agree with Silvergun. You have to knock out the redplating before you can legitimately leave the amp on (for any period of time) to check for "no sound".


Only the obvious comes to mind, wiring errors. Measure bias volts on both tube sockets with the output tubes pulled out. Wiggle things, be sure you don't have something goofy like a broken wire or a cracked or loose or oxidized tube-socket terminal or bad solder joint.

[Auke Jolman]

Thanks for the replies sofar.

I've got a link here where I put some closeups: https://jaapjolman.nl/public.php?service=files&t=079b596281865c96503a673960a49533

The bias voltage is now set at -38 V measured at the joint of the 220K resistors. At pin 5 I've got -31V and 4V at pin 5 at the other socket!?!?!? There is a closeup available

I tried wiggeling and pulling at wires and did some testing with my ohm-meter. Could not find anything out of order

[jojokeo]

photos are very hard to look at and follow the wiring for everything. But - you are using a LOT of shielded wiring even for shorter low impedance runs which really isn't necessary. The more shielded cable you use and in short runs can complicate things a lot & be possible to make a loop kind of mistake. But - did you actually test every shielded wire that you made prior to soldering it in to absolutely assure that you didn't accidentally get one too hot to where you might have melted the insulation on one of them? This will short out your signal in a hurry and happens easier than you may realize? I caught this on a recent re-build before soldering in a cable but decided to try it anyway - just incase I was getting a false reading. But sure enough, I had to replace it, then all was good under the hood.

[SILVERGUN]

Your bias voltage comes flying in on that red wire and should just get equally applied to each of those black wires that go over to pin 6 and through the resistors to pin 5

Until that happens, all bets are off

P.S. just listen to sluckey,,,here's here to save the day 

[sluckey]

and 4V at pin 5 at the other socket

That's why that tube is redplating. It's either wired wrong or maybe that coupling cap from the PI plate is leaking. Disconnect the cap and see if the voltages changes to -31. If so, replace the cap. If not, keep looking for the wiring error.

BTW, I noticed some very dangerous wiring on pin 3 of one of the output tubes.

[eleventeen]

If you have -35 or so volts on one output tube pin 5 and +4 volts on the other one, that is your redplating culprit, no question about it. You need negative volts on those grids or those tubes will overcurrent, runaway, redplate, whatever you want to call it. Blow up. That negative voltage throttles back the current through the tube to a controlled level. You need the same (negative) voltage pretty much on both tubes, pin 5. Whatever reason that one tube is getting +4 instead of the -35 or -40 it is supposed to get is your problem. Whether it is because of a wiring error, broken wire, or phase of the moon, you have to resolve that before proceeding. It *looks* like you have that wired right---maybe you just need to reheat some solder connections.

I notice that your input grid resistors on your 6L6 output tubes are green-brown-red----which is 5100 ohms---versus brown-green-red = 1500 ohms. Now myself, I would not think that would matter in any way (and certainly would not have the slightest anything to do w/your redplating) But it is sort of a WTF if you are asking me "what might I have done wrong?" if you get my drift. Just observing from afar, you know.


Agree with Sluckey, clean up those wire stubs on your output tube sockets and maybe swab a Q-tip soaked in alcohol on those to get some of that dirt or crud or whatever it is out of there. 


It is not entirely clear to me how you have run your OT primary leads. I of course can only judge from a pix which is only a "window" into how you've done things. To wit: On one output tube, pin 3, you have a blue wire. That is fine and proper and other than wanting to be trimmed back, is good. That blue is the right color wire I would expect at one end of the OT primary. So then I want to see a BROWN wire going to the other output tube pin 3. On your pix, it is green. Hmmm. Maybe you spliced the wire and used some green heatshrink. But then the wire that really IS BROWN, is somehow headed off in rather the wrong direction. Where is it going? Seems odd to me.

[Auke Jolman]

I took the board out to see underneath.... there was a shortage. This fixes the redplating! I set the bias to 32 mA, measured over the 1 ohm resistors as mV.

In the same proces I changed out the shielded wires goint to and from the normal and bright vol pots. Stil no guitar signal coming through. I did however notice a crackling sound when I turn the presence-pot like there's some DC floating there.

I also changed the wires going from the board to the treble-pot. When I wiggeled those wires the amp hummes a bit.

I've used an OT which has a blue and green wire instead of brown (http://www.tonefactory.nl/output-trafo-ot1).

I'm using EL34's Pin-4 are 1K and Pin-5 are 5,1K

[sluckey]

I decided to overhaule my homebrew plexibuil

Did the amp work before you overhauled it? If so, then you most likely have mis-wired it during the overhaul. Keep looking. Just because you haven't spotted the error yet does not mean there is no error. Sometimes it takes looking at it over and over, comparing the schematic , the layout, and the actual amp.

[Auke Jolman]

It is an almost complete overhaule. I made a new board and rewired the preamp tubesockets and pots.

I will get back to it tomorrow.

I would like to thank everybody who chimed in and if somebody has some more bright ideas, Iḿ al ears.

I've got a question about leaddress. How is it done best. Is there some reading material around? For example the wires leading to the pots, do I have to make them as short as possible or is it ok to lead them to the side of the chassis and than run them allong the side to the pots?

[jojokeo]

It is not entirely clear to me how you have run your OT primary leads. I of course can only judge from a pix which is only a "window" into how you've done things. To wit: On one output tube, pin 3, you have a blue wire. That is fine and proper and other than wanting to be trimmed back, is good. That blue is the right color wire I would expect at one end of the OT primary. So then I want to see a BROWN wire going to the other output tube pin 3. On your pix, it is green. Hmmm. Maybe you spliced the wire and used some green heatshrink. But then the wire that really IS BROWN, is somehow headed off in rather the wrong direction. Where is it going? Seems odd to me.

I saw the same thing on my laptop. I was making corrections/comments w/ paint program but then aborted. Too much trouble and time for me at the moment. Then to read about the color changes - BUT I still see a brown wire that sure looks like it ought to be from the OT!? Oh well, that's why internet troubleshooting is a beeyotch.

[6G6]

If you are looking at the same things, the same way and they keep looking OK,
you begin to get used to the way they look and they seem more OK.
If you have been working, say from input to output, try starting at the output and working backwards.
It's funny how sometimes a different perspective will change things.

[jojokeo]

Good point 6G6, I do this all the time also. Best just to take things slowly and carefully when troubleshooting. And never forget the golden rule - NEVER skip steps. It will always bite you in the but sooner or later.

[Auke Jolman]

It works.

After checking and checking I discoverd I had the wrong value resistor from pin8 100 ohm instead of 100K. But still nothing coming out of the speaker. I decided to test the preamp by connecting a outputjack on the middle lug of the treble pot... the preamp worked! So the problem had to be in the PI. Still no dice so I put it to rest for a while. After an hour or so, picked it up again and suddenly I saw what I did wrong. I connected the wire from pin7 of the PI to the wrong side of the 0.1 cap

Still got some fine tuning to do, but I'm really excited over it and foremost I'd liked to thank all of you for giving me directions and tips.

Still got some questions about the do's and don'ts with respect to leaddress.

[tubenit]

Congrats on finding the solution!  And THANK you for posting that you got it fixed and how.  I've made mistakes like that many times.

With respect, Tubenit