18 Watt Plexi Build

[dbharris]

Hi All,

This will be my first full amp build. I went with a Trinity kit for their Custom Plexi with 6v6 power section and either plug in SS rectifier or 5AR4 tube rectifier. I have a thread in the Trinity forum, but it seems sort of dead over there so I am cross posting some pics here too. If anyone spots something wrong or that could be done better I would appreciate your thoughts!

I am trying to mimic the lead dress from a picture of a JMP 50 (metal plate but same circuit as a Plexi) from Dave Hunter's book. Obviously this amp is not a clone of that, so there will be some differences in placement of parts, etc.

I ordered a bunch of new tubes to roll through the amp, some long plate and short plate modern Mullards, a JJ long plate, and an EHX 7025. I also have some ANOS hammond and RCA tubes and a vintage Mullard long plate that I might try. Power tubes are JJ 6v6s.

I've wired up the heaters and power supply. I will start working on the board this weekend most likely.

So far, indicator light works and my voltages are:
Mains and across the IEC 121.2vac
Across pins 2 and 8 of the Rectifier 5.54vac and across pins 4 and 6 624vac(V6)
Across pins 2 and 7 of the 6v6 sockets 6.91vac (V4) and 6.94vac (V5)
Across pins 4&5 and 9 of the 12ax7 sockets all 6.91vac

-Dan

[pompeiisneaks]

Looking good!

I'm guessing you've done soldering before? My first solder joints didn't look anything like those

Keep us posted!

~Phil

[Colossal]

Good job! You are off to an excellent start. Enjoy the ride.

[dbharris]

Thanks guys! Phil, yeah I have made a bunch of pedals and modified a little single ended amp, so I'm ok with an iron. I also just upgraded to a Hakko iron and it is soooo much better than the old radio shack one I used for a decade haha.

-Dan

[dbharris]

So, my cabs came in this week and I loaded up the extension cab with 2 12 inch WGS Green Berets. I didn't use the extra foam pieces that came with the speakers, I assume that is if you are front mounting them? Trying to run some music through it to break them in while I finish up the amp. I have the cab in fully closed back mode right now. Will experiment later on with semi-open back too.

I got the cab and head cab from KC at guitar cabinets direct. He was a pleasure to work with and they look great! Hot White Bronco with gold piping and Bluesbreaker grill cloth.

I also got the board populated. My first time with turrets. I think it went pretty well so far. I had previously tested all the Mallory caps for outside foil and marked them accordingly.

-Dan

[dbharris]

Got most of the tube side wiring done. Hoping to fire her up next weekend. Need to build a light bulb limiter and dummy load this week too.

Since the parts came with an extra dpdt switch, I may wire up an option for global negative feedback.

-Dan

[dbharris]

Not quite finished yet....

I made my dummy load box and tested. Realized after that the 4 and 16 Ohm jacks are in parallel, so I could have just used one. Also, tested this out with a 15 watt 2x6v6 amp, not quite dimed but about 90%. The enclosure got pretty warm after about 25 minutes. Wouldn't want to keep my hand on it more than 10 seconds. So, I ordered some heatsinks and thermal paste. Should be here tomorrow. I was a little surprised by how warm it got, since it should be able to handle 200W at 4 Ohms, how I was testing it. Hoping I cant get it up to at least an hour of use at a time for burning in the amp.

I also drilled out my head cab, which I am embarrassed to admit how long it took me lol. Once I figured out to make a cardboard template, it went smoothly. I also applied some copper tape to shield the inside of the cab.

On the actual amp, I finished wiring the grid resistors on V1. Wired up all the input jacks (really glad there were not bright hi/low jacks too), and the Normal Channel. I started on the Plexi Channel as well.

Decided I am going to wait on testing out GNFB until I have the amp up and running. Since this amp sends the Normal Channel to the inverting input of the phase inverter and the Plexi Channel to the non-inverting input, I was not sure if applying GNFB in the normal Marshall way might produce positive feedback as to the Normal Channel. So, I guess I will jump it in later and see.

-Dan

[dbharris]

More pics:

[Colossal]

Excellent!

[Paul G.]

On your PI you've got a lead from the grid and plate running parallel and touching. This may add noise. It's a good idea to have a little space between them.

Less likely, but something to look at is the couple of places where plate and cathode wires are also right up against each other. It is never a bad idea to keep signal wires slightly separated.

Of course, your build may be quiet, so you may be fine, just mentioning best practice.

[dbharris]

Thanks for the advice, I really appreciate it! I will work on the lead dress in those areas before I add anymore wiring.

-Dan

[dbharris]

All finished up and ready for the smoke test! I tried to adjust my lead dress from the suggestion above and flew the grids of the PI up higher and tried to re-work V1 and V2 a little bit also. I am going to build a light bulb limiter tomorrow morning before triple checking all connections again and going through the start up procedure. I have a variac and plan to bring up the voltage slowly the first time to form the caps.

I also got some thermal paste and heat sinks for my dummy load box. I put the paste on the bottom of the resistors where they touch the enclosure and under the heat sinks where they touch the enclosure. I didn't realize that the thermal paste wouldn't also adhere the heat sinks, so I ended up adding some glue afterwards. It works great now, barely got warm with my same setup as before.

Let me know if you see any errors or anything that can be improved.

Thanks!

-Dan

[xtian]

Whut? Did you just paste heat sinks to the outside of the enclosure?

[dbharris]

Well, sort of lol. If I need to re-do that I am happy to, but I wasn't sure how else to attach. There wasn't any flange to screw them on.

I applied thermal paste first, like for a pc processor and then glued around that.

What's the proper method?

Thanks!

[pompeiisneaks]

Generally you need tight physical contact with the area to be cooled.

You'd want some kind of screws through the heatsinks into the chassis of your dummy load.

~Phil