First DIY Plexi 1987 - B+ issues (no voltage drop @ v1 - v3)

[kdmay]

So I've spent the last 2 weeks building my first amp - 1987 Metro kit, and I'm very pleased with how it's come together.

Anyway, fired up today for the first time and got stuck on Testing Part 1.

Initial fire up had high rectifier readings, then read some posts about the 240v wiring (I am in Australia) and quickly connected the yellow wire in place of the orange wire.

Second fire up and rectifier readings are + 458v; AC 332v; AC 331c. I'm happy with this, as I'm sure there will be a drop @ + once the tubes are in and pulling current.

Next, the heaters all look reasonable. Interestingly, when I had the orange wire incorrectly hooked up I was getting a perfect 3.10 - 3.20, but now with the yellow wire I am getting 2.80 - 2.90. I am not too stressed about this, as I've read that this should be ok. But let me know if it's too low!

Next, the EL34s all look fine. Again, a little high but I'm sure they will drop a little and into the range as per the voltage chart. Refer below.

Onto the problem: the preamp tubes. Refer to the readings below. It's like the B+ line isn't experiencing any voltage drop going through the 10k/8.2k resistors. There is a voltage drop as a result of the 100k plate resistors if you look at V2 Pin 6 vs all the other plate voltages, but it's strange that the B+ line is retaining the same voltages from the rectifier up to each of the plate resistors.

I am going to buzz everything out once again, but would appreciate any tips.

Below are a few shots and a link to my Photobucket album with all progress shots, and the voltages as I am reading.

Thanks!!



http://s641.photobucket.com/albums/uu13 ... i%20Build/" onclick="window.open(this.href);return false;

[Roe]

pop in preamp tubes and see if the voltage drops

[kdmay]

pop in preamp tubes and see if the voltage drops

Thanks, I thought of this but was concerned about killing the nice Mullards supplied!! I did think that the voltages would drop as the tubes pulled current, I was just confused as to why the instructions stipulated the plate voltages at the pins without the tubes in. I also didn't think they'd drop that much, but we'll see after I give it a try.

I just spent the last 90 mins buzzing every single junction on the damn schematic and I didn't find a fault.....at least I don't think so.

[kdmay]

Well, I popped in the preamp tubes and it all came together. Every reading was almost bang on the voltages in the chart.

Popped in the EL34s, set the bias (need to change the bias resistor as full bias yielded just under 35ma for one and 32.8ma for the other), but it was tonal bliss from the first chord.

Initial observations are that the bright channel was quite bright, but I jumpered the channels to blend some of the darkness from channel 2 and now I understand why this is done (never have before).

Didn't get much of a chance to crank it past 4, but damn it is loud. Like seriously loud. Much louder than my 50w plexi style Australian built combo. I reckon 50w is very conservative, I was running it through a single v30 and started to get a little concerned.

There are some serious clean tones to be had with it, and the power amp break up is just AC/DC classic rock in a box!

Anyway, will spend some time playing and letting the caps and trannies settle in, then may look at some minor mods down the track (like altering the bright cap etc), but I reckon the first mod will be the Lar/Mar PPIMV!!

Thanks to Metro - one outstanding amp that I can have the pride in saying I built myself!! Let's hope the shop opens again soon, I could never have done it without the kit and posts on this forum.

Cheers