BYOC 50 Watt Plexi Build Help!

Get support and show off your MetroAmp 50 Watt kit builds.

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BYOC 50 Watt Plexi Build Help!

Post by aroundthefur444 » Sun Jan 08, 2017 3:58 pm

Hi everyone, I just finished wiring up my BYOC 50 watt Plexi Kit. Everything was going great, the light turned on when I powered it up, but when I went to take the test measurements for voltage, I was not getting the expected 490 V on the 1st 32uF capacitor - I was getting like -60mV? I put all the tubes in and they are all glowing OK (tubes are new and from my Marshall JCM900, so I know they are OK). However, I am getting NO sound from the amp at all. Speaker cab and speaker cable are OK - tested with my other amp. When I tried to bias the power tubes, the plate voltage is 655V (I am using EH6CA7 tubes). The bias reading on pin 1/8 do not make any sense - I think less than 1 mV.

I downloaded the Metroamp 50 watt instructions and took all the voltage measurements found in there (with power and Standby set to ON). They are way off:
Tube:Pin1; Pin2; Pin3:Pin4:Pin5:Pin6:Pin7:Pin8:Pin9
V1: 124mV; 0; 0; 3.56VAC; 3.56VAC; -133mV; 0; 0; -1.3mV
V2: 124mV; 0; 0; 3.56VAC; 3.56VAC; -133mV; 0; 0; -1.3mV
V3: 124mV; 0; 0; 3.56VAC; 3.56VAC; -133mV; 0; 0; -1.3mV
V4: 0; 3.57VAC; -131mV; -132mV; 13.6-22.4mV (bias pot min and mx); -136mV; 3.56VAC; 0
V5: 0; 3.57VAC; -131mV; -132mV; 13.6-22.4mV (bias pot min and mx); -136mV; 3.56VAC; 0

Off side of MAINS switch = 126VAC
3A rear panel fuse = 30.4mV on both sides
500A chassis fuse = 345.9V (circuit board side) and -156.7V on other side

Any help would be very much appreciated! This is my first attempt at an amp build.

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Re: BYOC 50 Watt Plexi Build Help!

Post by neikeel » Tue Jan 10, 2017 9:07 am

Not familiar with their amp kits (made a couple of pedals). We will help if we can. definitely post some really clear well lit photographs (links to a site like the cloud or photobucket).
We will have a look, ask some more questions and get her up and running.
Odd to get different readings either side of a fuse (unless it is blown). Your heaters are a bit high (6.3vac is target)
From first pass there is a wiring mistake around the HT/rectifier area but need:
Layout diagram of their amp
The schem that you were using)
Pictures of your work (quality of soldering, lead dress etc).
Hope you used standard Marshall wiring colours - it is SO much easier. :wink:

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Re: BYOC 50 Watt Plexi Build Help!

Post by danman » Tue Jan 17, 2017 4:32 pm

All of those DC voltage readings that you are seeing in mv and -mv make it seem that you have no voltage at these points at all. My meter will throw out those very low readings also when no voltage is present. It is odd that you are showing voltage on the circuit side of the HT fuse but nothing on the other. The 345v that you do see is very low though as this should be full B+ voltage through the HT fuse. The mains fuse will be an AC voltage reading but the HT fuse will be a DC voltage so be sure to set your meter accordingly. Your meter appears to be working because your AC voltages are reading fine. They are a touch high but this is likely because the amp is not drawing current. I cannot understand how you are getting 650vdc at the plates but zero voltage at all other nodes though. With zero voltage at the first filter cap, you should not be getting a DC voltage reading at the plates. Like neikeel mentioned, we need some pics, but it would seem that you have a wiring error between the PT secondary leads and your first filter cap. Remember that everything from the PT to the rectifier diodes will be AC voltage and everything after the rectifier diodes is DC.

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