50 watt 67/68 build

[valco51]

I Have been working on my 50 watt 67/68 plexi build. I picked up one of Mastrans new 50watt chassis. I had bought the transformers several years ago form Brian. A lot of parts came from E-bay, Valvestorm, Watts tube audio, resistors from Metro member wrea398. I followed the instructions from MetroAmp. What a help that was. Thanks to all of you that have shared you knowledge and time. I have spent a lot of time reading posts and hitting the search button in order to get this amp up and running.
How does it sound. First of all the amp is quiet. Hard to tell its on unless I plug in the guitar. Over all the amp has a great clean sound but with little breakup. This is with the volume at 7. Not sure i want to push it to 10 just yet. I was hoping for more sustain and breakup. The neg. feedback resistor is 47k going to 8 ohm lug. Bias at 38ma. The amp is running at 440 volts.
http://s563.photobucket.com/albums/ss74 ... lexibuild/" onclick="window.open(this.href);return false;

[OnTheFritz]

Give it some break in time, and a V2A cathode cap will add gain. I built a JTM-50, and that did the trick.

[valco51]

I went back in and made some changes. I split V1 added 2.7k/.68cap. It made both channels more useable. I tried the .68cap along with other sized caps on the V2A cathode. I thought they took away some of the sweetness to the sound of the amp. So I left it without a cap on the V2A cathode. I have a 7H choke (MM JT45c) in the amp. I bypassed the 7H with a 3H choke (MM Mar100c) to see it would improve the sound. The 3H added some brashness the the sound and not as smooth sounding. I'm sticking with the 7H for now. I tried a few treble caps on volume 1 but they were too bright for me. I am going to leave it as is and see what time will do for it.

[neikeel]

Must confess I like the 680nF on V2a with the 8ohm/47k NFB and 5000pF ceramic cap on the bright cap. If you play with your PPIMV you will find that sweet singing sustain/feedback point that makes these a joy to play. I prefer the 3H choke too so maybe we just like different things . I prefer smaller PI caps too (0.047uF is a good compromise between the 0.022 and 0.1 IMO and Mustards are easier to find!)
My Marstran equipped switchcable 1987/2004 is definitely a keeper.

[valco51]

Well I got listening to plexi builds on YouTube and my amp didn't sound close to what I was hearing. After looking at more pictures of other amps to see what was different, I found that I had both sets of wires coming off the 68k resistors going to the input jacks reversed. I switched them to the correct setup for inputs 1and 2. While I was in there I changed out the V1 330uf for a 220uf per lead specs. The amp started to give up the goods with more distortion. Then I changed the 250pf/56k for a 560pf /330k per the lead specs. That was the ticket. The amp started to come alive and notes had some nice bloom to them. It might be there was some thing wrong with either the 250pf or the 56k resistor. The 56k resistor had drifted up to 60k. I went back and tested the .68uf cap on the V2A cathode. With the other changes made and the .68uf now sounded better with it in. I tested the 3H choke again but I still prefer the 7H. Also I tried out another batch of treble caps but still like the amp with out one. Other than some tube swapping I am going to leave it as is for awhile.
By the way, I bought only 3ft of the pink wire and I cut and fit each wire and had didn't have any wire left. 5ft would of been better in order to cut to the 6 inch lengths per the kit instructions when wiring up the board.. Then I would of had enough to do the other two wires in pink and had some to spare.