Plexi 50 testing, getting weird pin values

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Major Sparky
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Plexi 50 testing, getting weird pin values

Post by Major Sparky » Fri Jun 08, 2012 5:26 pm

Hello, I just finished building my 50 watt 1987 Lead Plexi kit I got from triode electronics. I followed the guide from metro http://www.metroamp.com/downloads/50_WA ... CTIONS.pdf" onclick="window.open(this.href);return false; and this layout from triode, http://site.triodestore.com/50WPlexiLayout.pdf" onclick="window.open(this.href);return false;.

I started doing the tests listed and I wasn't getting the right voltages. Here's what I had, these were all taken with power on and standby off.

v1 3.7, N/a, 0, 3, 3, .5, N/a, 0, 3
v2 3.7, N/a, 0, 3, 3, 2, 2.14, -.4, 3
v3 3.7, N/a, 0, 3, 3, 2.2, N/a, 0, 3
v4 N/a, 3, 2, 2, -191, 2, 3, N/a
v5 N/a, 3, 2, 2, -191, 2, 3, N/a



I noticed that under the table where it lists the expected values in the guide that it says that the measurements were taken with the standby on as well. So I flipped on the standby and started taking measurements. Most things stayed the same, but pins 3, 4, and 6 for v4 and v5 all jumped up to around 490. I started to take the measurement for pin7 on v5 and I think I might have accidentally touched pin 6, (i'm not sure) and I got a big spark. I went back and saw if any of the values changed and the values for pins 3, 4, and 6 for v4 and v5 all dropped to about 250. I don't know what happened. I only checked the pins on v4 and v5.

Anyway, obviously there was something wrong from the beginning but I think I might have made things worse. Can anyone tell me what might be wrong? I can post pics of the amp if that will help.

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Re: Plexi 50 testing, getting weird pin values

Post by axeman » Fri Jun 08, 2012 5:59 pm

Post some pictures and we will walk you through it.

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Re: Plexi 50 testing, getting weird pin values

Post by toner » Fri Jun 08, 2012 6:03 pm

Major Sparky wrote:...I went back and saw if any of the values changed and the values for pins 3, 4, and 6 for v4 and v5 all dropped to about 250...
This isn't a good sign but it depends on the circumstances.

First off, the amp needs to be in play mode (not in standby) to check all the voltages. Second, leave all tubes out at first when testing (in case you have them in).

Touching pin 7 and 6 on a power tube socket at the same time will definitely cause a spark, Major Sparky (sorry)! This can also blow a fuse so check both the mains and HT fuses for continuity with a meter and replace any that are bad with the correct value.

There are some obvious differences between the Metro and Triode layouts so pictures may help, as mentioned.

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Re: Plexi 50 testing, getting weird pin values

Post by Major Sparky » Sat Jun 09, 2012 12:18 am

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You were right about the blown fuse. The HT fuse blew. Also, for the bias pot, they gave me an actual pot instead of a trim pot, I'll most likely replace it soon.

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Re: Plexi 50 testing, getting weird pin values

Post by Reeltarded » Sat Jun 09, 2012 5:37 am

I go nuts adding junk later, but I advise you to go back over all your wiring. Mind your dress!

The red and green power tube grid feeds should be twisted about 3 turns per inch. The heater wiring pretty much the same. The wires that are signal from the pots to the board should drop straight to the deck, then continue along the chassis until they arrive at their terminations. Heaters are better flying above the grids and cathodes if you have to have something in suspension, bit it's all sort of meshed together there.

There are a couple other points... don't hate me. Just sayin'..
not kicking the dead horse

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Re: Plexi 50 testing, getting weird pin values

Post by neikeel » Sat Jun 09, 2012 5:56 am

Reeltarded wrote:I go nuts adding junk later, but I advise you to go back over all your wiring. Mind your dress!

The red and green power tube grid feeds should be twisted about 3 turns per inch. The heater wiring pretty much the same. The wires that are signal from the pots to the board should drop straight to the deck, then continue along the chassis until they arrive at their terminations. Heaters are better flying above the grids and cathodes if you have to have something in suspension, bit it's all sort of meshed together there.

There are a couple other points... don't hate me. Just sayin'..
Yes getting it tidy first time is easiest - I know that it is easy to rush as you want to plug and play ASAP!

What I cannot see is the short link wire between the two runs of diodes on the rectifier - it may be under the board.

Normally for a first time builder I strongly recommend the bus wire is on top of the board so you can see where it goes. Also using a perforated board with all of the wires come through the holes so that (a) you can see which wire is which and (b) that they have not become disconnected.

The neatest cleanest wiring that avoids cross talk, parascitic oscillation, ground loops and hum is not necessarily the shortest route :wink:
Neil

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Re: Plexi 50 testing, getting weird pin values

Post by Reeltarded » Sat Jun 09, 2012 8:30 am

Efficiency on an amp wired like that.. hmmmm.. HmmMmMmmMmmMmmmmmmmmMMmMmMmmmmMMmm -fritter-

Extraneous signal isn't just annoying. It robs your amp of efficiency. No sustain. Flat. All the stuff Neil said. I'd imagine that the dress just in the preamp section and not having directly socket mounted RF suppression there will be using 20% in the undetected bands above what we hear at idle alone, with the gain off. Just a guess.

Technically, it's as much about what you can't hear as what you can. I find myself tweaking things out, and more rarely tweaking stuff in... sorta.
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Re: Plexi 50 testing, getting weird pin values

Post by Major Sparky » Sat Jun 09, 2012 9:55 am

So you guys recommend that I should go back and re wire everything? I was kind of rushing this because I was excited to try and hurry up and play. But I can go back today and rewire things. Does anything else seem out of place?

I'm a complete idiot. While going back and un wiring the board from the chassis I saw a lot of things I just forgot to do. I'm gonna re wire it all as best as I can and take it slow this time. One question though, is it more important for the wire to be short, or neat?

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Re: Plexi 50 testing, getting weird pin values

Post by neikeel » Sat Jun 09, 2012 10:46 am

Major Sparky wrote:So you guys recommend that I should go back and re wire everything? I was kind of rushing this because I was excited to try and hurry up and play. But I can go back today and rewire things. Does anything else seem out of place?

I'm a complete idiot. While going back and un wiring the board from the chassis I saw a lot of things I just forgot to do. I'm gonna re wire it all as best as I can and take it slow this time. One question though, is it more important for the wire to be short, or neat?
It might be the best for the long term.
I do not like those snipped floating yellow/white wires off your PT particularly.
Is that wire link on the rectifier underneath?
Neil

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Re: Plexi 50 testing, getting weird pin values

Post by Major Sparky » Sat Jun 09, 2012 10:58 am

Yes the wire is underneath the board. I did all the bus wire underneath the board.

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Re: Plexi 50 testing, getting weird pin values

Post by toner » Sat Jun 09, 2012 1:19 pm

Major Sparky wrote:...One question though, is it more important for the wire to be short, or neat?
Neatness is nice to a point but the key is knowing where potential problems exist. This only comes with experience or a lot of reading though.

I don't think you need to rip everything out and start over but make sure the under-board jumpers are correct first. You can test those by checking continuity with a meter between each connected turret (the dotted line wires in the Triode layout). Depending on what you "forgot to do", that may save you some time.

Your wiring on the pot side of the board is probably okay.

Most of these have been mentioned but here are some key things to do:
  • tape off or shrink-wrap the cut PT wires
  • make a new twisted pair from the board to pin 5 of the power tubes and run it under the board away from other wires as much as possible
  • your heater wires aren't that bad but keep them flat against the chassis and away from other wires that run to the tubes. Put the wire on V3 pin 9 under the wires that go to pins 6, 7 and 8.
  • lift the green grid wires (preamp tubes pin 2 and 7) up away from the chassis and other wires (especially the heaters)
  • check that the bias pot affects the negative voltage on pin 5 of power tubes and set it to the max neg voltage before installing power tubes

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Re: Plexi 50 testing, getting weird pin values

Post by hdahs » Sat Jun 09, 2012 2:28 pm

You are not an idiot. Just anxious to hear the fruits of your labor, I suspect. In regards to neat vs. short. it is a little of both. There are tons of pictures on this site. Check them out and you should get an idea how things should look. TAKE YOUR TIME!!! I can't stress that enough. I fix amps for a living, and I still took plenty of spare time over a few weeks to build my first amp, but it was worth it. The amp sounds great and is quiet as a mouse. I'm not sure what kind of wire Triode provides with their kit, but the pre-tinned 22ga wire, that you can get from Valvestorm, is not only a joy to work with, but really easy to bend and shape to be as neat as possible.

Good luck and keep us posted!

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Re: Plexi 50 testing, getting weird pin values

Post by neikeel » Sat Jun 09, 2012 2:36 pm

hdahs wrote:You are not an idiot. Just anxious to hear the fruits of your labor, I suspect.
Good luck and keep us posted!
Absolutely not an idiot at all, doing this stuff is balance of time well spent vs overanalysing and being anal about details that do not matter.

They do not have to be pretty but certain rules need to be followed, plus a tidy layout is easier to trouble shoot and if you get it right quieter and will sound better.

Would love to see and hear it when you have tided a little and checked your work 8)
Neil

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Re: Plexi 50 testing, getting weird pin values

Post by Major Sparky » Sat Jun 09, 2012 10:07 pm

Well I rewired most of it. Most of the wires on the pot side of the amp are still not up against the chassis, but that's because I didn't really have enough wire to do that. And there's still a bit of a mess around the power tube sockets, but I really don't know how to make that neater because everything seems kind of cramped with the triode layout and the chassis I got from mojotone.

Anyway, the values aren't any better. for pin 5 of v5 & v4 the reading is about -215. All the ac voltages are at about 2.9-3. Pins 1 & 2 of v1, v2, v3 and pins 3,4, and 6 of v4, and v5 are about -.16. Everything else is about 0.

I'm getting continuity everywhere I check so I don't know whats wrong. Pics will be posted shortly.

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Re: Plexi 50 testing, getting weird pin values

Post by Major Sparky » Sun Jun 10, 2012 1:10 am

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Alright, I have bigger problems now. After my last post I noticed that there were a few small connections I forgot to make on the board. So I fixed that. Then I plugged in the amp and turned the power switch on and the was a hum and then the Mains fuse blew. I replaced it and then tried to check the ac voltage going to the hot side of the power switch and there was another hum and a spark in the mains fuse holder when I touched the voltmeter to the switch.

I haven't done anything else to the amp because I don't know what's going on.

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