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 Post subject: Custom Plexi Build
PostPosted: Sat Sep 05, 2020 10:12 pm 
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Hi All,

This will be my first amp build, but I have built lots of pedals and modified a Fender Champion 600 amp previously. I'll update this thread as the build progresses. Received my kit in the mail today and got started. My tubes and speakers are here too and my head cab and extension cab have shipped and should be here Wednesday from Guitar Cabinets Direct.

Tomorrow, I am going to go through the resistors and caps to check values. I am also going to give a go at identifying the outside foil on the Mallory caps.

One thing I did notice so far, the 300r resistor for the 6v6 cathodes is 5 watts in my kit. It's identified as 10 watts on the schematic and layout. Will this present an issue?

I also measured all the pots while installing them and selected among the the 1M values for the positions I thought made the most sense. Lowest in Normal Channel Volume, 2nd lowest in Plexi Channel Vol; highest in Gain, and 2nd highest in Bass.

-Dan

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 Post subject: Re: Custom Plexi Build
PostPosted: Sun Sep 06, 2020 2:21 am 
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Hi Dan, welcome to the forum!

I never thought about grading the pots as you have done. They are all fairly close to the expected values, though, as tolerances in the 1M range for tube equipment goes. I don't think it will make much difference which one is installed where in the amp. The way you have them is good. The only place where the value could be a bit critical is the bass control since it's part of a tone stack tuned for a certain frequency response.

That 300R 5-watt cathode resistor is probably OK if that's what you got with the kit. Once you get the amp working just check its temperature. If it's too hot to comfortably touch, then the wattage should be higher.

It's good to identify the outside foil on the capacitors. It's easy to do and there aren't that many in the amp so it shouldn't take too long. Installing them the right way will help keep the hum and noise to a minimum.

Your chassis is looking good. Best of luck with the rest of your build!


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 Post subject: Re: Custom Plexi Build
PostPosted: Sun Sep 06, 2020 11:57 pm 
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Thanks Mitch!

I made some more progress today. Wired up the heaters, installed the OT and PT, and started on the rest of the power circuit. Noticed that I had installed the grommets for the OT wires the wrong way, so fixed that. I also checked all resistors and caps and tested for the outside foil and marked accordingly.

I noticed a couple inconsistencies in the layout/schematic. The layout omits the .01uf Ceramic and 220K 2W metal oxide from the standby switch (but these are on the schematic). In the Phase Inverter long-tail pair the schematic and included parts have a 15K but the layout shows 10K. The layout also shows the 5K6 resistors as what I would expect to be metal film, but the included ones are carbon film (not a big deal).

More to follow!

-Dan


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File comment: Heaters, OT/PT install, starting on power wiring
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 Post subject: Re: Custom Plexi Build
PostPosted: Mon Sep 07, 2020 9:25 pm 
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Finished the power wiring and tested voltages. All good so far and the indicator light works. On to the board next.

Voltages:
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


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 Post subject: Re: Custom Plexi Build
PostPosted: Wed Sep 09, 2020 6:06 pm 
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Thanks for the update and pictures. The chassis continues to look good. The voltages look good, too, at this stage of construction without the board and tubes installed yet.


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 Post subject: Re: Custom Plexi Build
PostPosted: Sat Sep 12, 2020 3:44 pm 
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Thanks Mitch!

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


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 Post subject: Re: Custom Plexi Build
PostPosted: Sat Sep 12, 2020 7:14 pm 
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Location: Shawnee, OK
Nice build. I wish my first and second builds looked that good :!: :thumbsup: :thumbsup:

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 Post subject: Re: Custom Plexi Build
PostPosted: Sun Sep 13, 2020 1:07 am 
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The board looks good. I assume the turrets that aren't full of solder yet are so you can attach the fly off wires.
The white cabinet with the gold piping and Bluesbreaker grill cloth looks amazing. Really nice workmanship.

Yes, the extra gasket material for the speakers is so you can front mount them.


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 Post subject: Re: Custom Plexi Build
PostPosted: Sun Sep 13, 2020 7:54 am 
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Thanks for the feedback, I appreciate another set of eyes!
I was going to fly the wires from the bottom of the turrets on the top side of the board. When I was adding the resistors and caps, it seemed like some turrets wouldn't fill up all the way with solder. All of these seem to have a solid connection and aren't dry joints. I also checked the pictorial reference thread for the plexi and it looked like many of those turrets were the same.

I can go back and add more solder if this is an issue.

Thanks

-Dan


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 Post subject: Re: Custom Plexi Build
PostPosted: Sun Sep 13, 2020 4:09 pm 
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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


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 Post subject: Re: Custom Plexi Build
PostPosted: Sun Sep 20, 2020 9:39 pm 
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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


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 Post subject: Re: Custom Plexi Build
PostPosted: Sat Sep 26, 2020 8:54 pm 
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All finished up and ready for the smoke test! 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


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 Post subject: Re: Custom Plexi Build
PostPosted: Sun Sep 27, 2020 10:05 pm 
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Went slow with the start up procedure and brought up voltage slowly on the variac with a light bulb current limiter and dummy load. No problems. Removed light bulb limiter, all good. Installed tubes, no problem.

Hooked it up to the speaker cab and plugged in a guitar, all good. Everything works as it should. Sounds good and is pretty loud.

I'm going to burn it in this week on the dummy load and afterwards roll some tubes. I'll prob play it for 2 or 3 weeks before installing the VRM. Right now I have the SS rectifier in, Mullard reissue long plates in v1 and v3, a Mullard reissue shortplate in v2, and JJ 6V6s in v4 and v5.

Voltages:
Mains 121.4
B+ 386
B+1 370
B+2 281.4
B+3 242.3
B+4 227.5

V1
1- 172.8
3- 1.408
6- 131.9
8- 0.775

V2
1- 141.8
3- 0.982
6- 240.8
8- 142.9

V3
1- 181
2- 19.34
3- 31.17
6- 184.8
7- 18.75
8- 30.95
I think the vintage chart in the manual has pins 7 and 8 reversed for v3

V4
3- 376
4- 370
8- 25.13

V5
3- 373.3
4- 376 (maybe 370, can't read my handwriting)
8- 25.13

-Dan


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 Post subject: Re: Custom Plexi Build
PostPosted: Mon Sep 28, 2020 4:09 pm 
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Thanks for the update and pictures. The amp and speaker cabinet look great! Congratulations on a successful build.

It's always good when an amp works the first time. Working carefully, keeping things neat, following the instructions, checking your work, and rechecking it numerous times pays off!

Good idea to run the amp for a while before installing the VRM or doing any other mods.

The voltage at pins 3 and 8 of V3 should be the same. The voltages at 2 and 7 should be fairly close to each other, too. It looks like 7 and 8 are reversed on the chart on the schematic in the Support section here, so they are probably reversed in the manual, too.


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 Post subject: Re: Custom Plexi Build
PostPosted: Mon Sep 28, 2020 8:14 pm 
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Thanks Mitch! The voltages probably were the same on 3 and 8 of v3 but may have been a minute or two between measurements. It's thunderstorm/hurricane season here in Florida and there was a bad storm while I was working on the amp and the Mains were fluctuating a bit. Happens all the time in my neighborhood during hard rains.

Anyways, hope to report back in a bit with some clips and updates.


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 Post subject: Re: Custom Plexi Build
PostPosted: Sat Oct 03, 2020 1:14 pm 
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I wanted to provide a quick update and ask for some thoughts/suggestions. I burned in the amp on a dummy load for about 25 hours total (over a few days) by playing music from an iPad into the input with the channels jumpered.

When I initially tested the amp, I was playing through just the normal or just the plexi channel, not both. When I play either channel there are no issues, I can dime the gain and volume and it reacts like I would expect. I have never owned an amp before with channels that you can jumper, so I am not sure what I should be expecting. Overall the amp does seem quite bright (even with a Les Paul and very bright with a strat). It is manageable when playing either channel. When jumpered, it seems much more bright and I can only run the channels with moderate gain/vol otherwise it runs off into feedback like crazy. At levels where I can control the feedback notes will sustain indefinitely, but there is very nasty distortion happening that is very present/resonant on lower notes.

I am plugging straight into the amp with a 20ft cable, no pedals. Maybe I need to reverse the leads of the OT primary? I don't otherwise have any "classic" symptoms of the leads being reversed like squealing, but I understand it can manifest more subtle than that. Is this a lead dress issue? I can chopstick around if needed.

I tried to at least eliminate the speakers being an issue. I tightened all the screws on the back of my cab, did not really help. I removed the middle piece to make it semi-open back. Helped a little, but I think that is because the bass is less dominant. Loosened the screws mounting the speaker a 1/4 turn each, no change. I also tested through an egnater tweaker 1x12 cab with a broken in G12H30. The highs were much more in line with what I expected, also a lot less bass than I would have imagined (it is a vented cab). But the nasty distortion is still present, though not as prominent.

I swapped tubes around too in the preamp and do not think that is the issue. I don't have an extra set of power tubes, but can order if that is advisable to eliminate it as a possibility.

Thanks in advance!

-Dan


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 Post subject: Re: Custom Plexi Build
PostPosted: Fri Oct 09, 2020 5:47 pm 
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To report my progress since my last post, I have moved wires around with a chopstick quite a bit with no significant changes. Sometimes by moving wires I could induce more background hum, etc. But it did not impact the amp having nasty distortion riding under bass notes or it wanting to feedback with channels jumped.

I swapped OT leads for the primary. When I did this the amp did not squeal/howl, etc. It did seem like it wanted to feedback easier and more brittle/high pitch sounding when channels are jumped. So, I swapped them back as they were.

I took another set of voltage readings, a couple notes...I have swapped preamp tubes a bunch so this is a totally different lineup than when I took the initial voltages on fire up. Also, when I first took voltages, I was on a dummy load. This time I was on the 1x12 G12H30 cab. When I took the voltage for pin 3 of V4, it induced squealing and the voltage was moving quickly around in the range of 393-399VDC. No behavior like that on pin 3 of V5. I did not take voltages with OT leads swapped.

I am going to visit my parents for the weekend. When I get back, I'll make a short recording of the sound. I am thinking it may be crossover or blocking distortion and I may need to try the Paul Ruby zener diode clamps.

V1
1 - 170.6
3 - 1.478
6 - 129.1
8 - 0.799

V2
1 - 143.3
3 - 0.973
6 - 241.3
8 - 144.1

V3
1 - 173.1
2 - 19.17
3 - 32.14
6 - 181.6
7 - 18.94
8 - 32.14

V4
3 - 393-399 (squeals when tube pin touched by DMM probe)
6 - 366
8 - 24.42

V5
3 - 369
6 - 366
8 - 24.42

Thanks!

-Dan


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 Post subject: Re: Custom Plexi Build
PostPosted: Sat Oct 17, 2020 7:57 pm 
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Alright yall, I'm back and hoping this post may help shed some light on where to look to fix this issue. After much digging on several forums it seemed like I was going to need a scope to properly diagnose the problem, so I took the dive. Here are some screenshots from the scope. It appears to me, I am dealing with crossover distortion. But, not sure what to do about it.

I ran a 1K sine wave into the input of the amp, from an app on my tablet. I'm commenting the amp settings in the caption below each picture. Scope was on on the output jack. I'm attaching pictures at clean settings and then the setting where crossover distortion is most apparent. Past that it goes very square wave.

On the Plexi channel and with them jumped, I can control/effect that hump in the trough with the treble control.

After this I did probe a few different parts of the signal, and I believe this problem first shows up pins 3 of the power tubes.

Let me know if there is anywhere else you would like to see scoped out or anything different, etc. I have a lot more screenshots at volume settings 2, 4, 6, 8, and 10 for a few configurations.

Thanks for any insight!

-Dan


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File comment: Normal Vol 2, Tone 5
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File comment: Plexi Vol 2, Gain 5, Tonestack 5
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File comment: Plexi Vol 6, Gain 5, Tonestack 5
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File comment: Channels Jumped Vols 2, Gain either 5 or 10, Tonestacks 5
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File comment: Channels Jumped Vols 10, Gain either 5 or 10, Tonestacks 5
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 Post subject: Re: Custom Plexi Build
PostPosted: Mon Oct 19, 2020 1:07 pm 
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Location: Shawnee, OK
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Dan, I don't know if this relates to your problem, but I noticed that a lot of your wiring on V1,V2 and V3 parallel each other (plate, cathode, screen and shielded) and some cross over the black heater wires. The parallel wires in particular concern me because if I recall correctly wires crossing 90 degrees are usually OK, but it is recommended that wires carrying signals DO NOT run parallel. Also DC crossing AC (heater wiring) is probably a bad thing, especially DC wires carrying signal. So, you might try adjusting your lead dress. Think vertical plane, too, instead of just one horizontal plane. Wiser heads may have more insightful advice :!:

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 Post subject: Re: Custom Plexi Build
PostPosted: Tue Oct 20, 2020 6:19 pm 
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Location: Shawnee, OK
viewtopic.php?f=7&t=6120 Here is a thread where Coco and mitch m comment on lead dress

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