Plexi50 / JCM800 with hot switch mod build

[Crate50watt]

Hey everyone. I rebuilt an old non functioning silvertone console record player over the summer as a project and I enjoyed it. After completing that, I started looking at building my own amplifier. I've always loved the way the JCM800 sounds, so I thought I would try to put one together. I have the whole thing wired up and am just waiting on some tubes, break in, and bias. Then playing, then selling. As soon as this one sells, I think I will build an 18 watt tmb, in a small combo cabinet.
But for now, here is my modified version of Dougs plexi 50 drawing. Modified because I am too cheap to put all of the extra turrets in! I also added a couple photos of the turret board and amp chassis.

[BetterOffShred]

Looks cool man.      Hit us with some audio clips when it's done!
-Brett

[sluckey]

On the board layout... You have V1-8 connected through a 2.2K to the volume control. I don't think you want to do that.

[Crate50watt]

Hey sluckey thanks for the input. I noticed this discrepancy while I was wiring it and changed it. However, I didn't get around to changing the diagram since I already wired it.
I finished it up today and plugged it into my current limiter, fired it up and I've got no sound. I'm going to see if I can't track down the reason tomorrow. Hopefully it's just something simple.

[sluckey]

In a later drawing you have the coupling cap from V1 pin 1 connected to ground.

[Crate50watt]

In a later drawing you have the coupling cap from V1 pin 1 connected to ground.

That was part of the same spot I had messed up with that 2.2k resistor going to the wrong spot. I noticed that as well while I was wiring, so that is corrected, but I'm certain I have mistakes somewhere else. I would be better off using the original drawings from now on, no doubt! Here is an updated version with the changes i made.

[Crate50watt]

Here is a couple photos of it as it stands.

[sluckey]

A good next step would be to measure voltages for all B+ nodes and tube pins.

[Crate50watt]

A good next step would be to measure voltages for all B+ nodes and tube pins.

Ok, I went through with Dougs schematic and traced everything wire for wire. There were a couple of things I noticed that I wasn't sure would make a difference or not. One is that on the schematic it shows no connection on pin 6 of either output tube, but on the drawing it shows a resistor between pins 5 + 6 with the wire hooked up to pin 6.
On the board it shows the bias tap coming off of that 220r resistor. I can just jumper that over two turrets to the diode turret, correct?
The B+ voltages I measured are :
A - 480
B - 480
C - 345
D - 305
E - 297
PT REDS - 358
Pins 4+5 , 9 on v1 v2 v3 are all 3.2v
Pins 2,7 on v4 v5 are also 3.2v
Pins 1+6 on v1 are 190v
Pins 1+7 on v2 are 150v
Pin 6 on v2 is 305v
Pin 1 on v3 is 215v
Pin 6 on v3 is 205v
Pin 4 on v4 and v5 are 475v

This thing is dead silent. Its almost like its not turning on at all, or like the guitar cable is shorted out. I made sure to check that! I went back through and double checked the components to make sure they were grounded as well.

[Crate50watt]

Also on the drawing it shows pins 1+8 on v4 and v5 connected to those 1r resistors. On the schematic it doesn't show anything for pin 1. Does that make any difference?

[sluckey]

On the board it shows the bias tap coming off of that 220r resistor. I can just jumper that over two turrets to the diode turret, correct?

That's correct, well, except that 220r is really 220,000r.

Also on the drawing it shows pins 1+8 on v4 and v5 connected to those 1r resistors. On the schematic it doesn't show anything for pin 1. Does that make any difference?

Pin 1 must be connected to pin 8 as shown on the layout drawing.

You've only listed the big voltages for the tubes. But we need to see all tube pin voltages for all tubes, even if it is zero. Pin 1/8 of the power tubes will be very small, on the order of 50mV. And pin 5 of the power tubes should be a negative voltage.

[Crate50watt]

Ok sluckey, rookie mistake. I'll get all pins all tubes in a flash and get back. Thanks!

[Crate50watt]

Ok, here are the voltages I measured at each tube and pin. Let me know if I am still missing something.
Also, I strummed my guitar with my voltmeter on pin6 of v1 and saw it rise so I assume my signal is making it through that tube. Then I used the voltmeter to follow the ac signal of the guitar all the way through to the .022uf at pin2 v3, but there it changes to dc voltage?
Also, correct me if I'm wrong but, shouldn't you be able to strum a guitar, and watch for the voltage to spike and follow that signal at each (signal) component through an amp?

[sluckey]

Voltages look OK except for V4 and V5. Pin 5 voltage is too high and pin 8 voltage is too low. Those output tubes are barely conducting.

Use the bias pot to set the pin 5 voltage to about -35v. Then recheck all voltages for V4 and V5.

[Crate50watt]

Voltages look OK except for V4 and V5. Pin 5 voltage is too high and pin 8 voltage is too low. Those output tubes are barely conducting.

Use the bias pot to set the pin 5 voltage to about -35v. Then recheck all voltages for V4 and V5.

Ok, adjusted bias pot. v4 and v5 now read :
pin1 - 40mvdc
pin2 - 3.2vac
pin3 - 430vdc
pin4 - 425vdc
pin5 -   -35vdc
pin6 -    -35vdc
pin7 - 3.2vac
pin8 - 40mvdc

[sluckey]

Does it play music now?

[PRR]

> dead silent. ... like the guitar cable is shorted out.

Even a shorted guitar cable, you would have small hiss/hum from the preamp stages, adjustable with the volume and tone knobs.

DEAD silent is often a mis-connection at the speaker jack.

[Crate50watt]

Does it play music now?

It does not. It's still dead silent. Not even any of that air moving sound or hum.

> dead silent. ... like the guitar cable is shorted out.

Even a shorted guitar cable, you would have small hiss/hum from the preamp stages, adjustable with the volume and tone knobs.

This is the same thing I was thinking. Or at least if the cable was bad it would make that scratchy noise. And theres none of that. I ohmed out the speaker cable and it's good, I check continuity on the speaker jacks as well and theyre connected to the OT secondaries, like they should be.
I have signal going through v1 and v2. I can measure the ac with my meter. I lose it at the grid on v3. Right there if i leave my meter on ac it bounces around all over and I have to switch it to dc to get a voltage reading. On the other side of that grid capacitor I can see my guitar signal fluctuate with the strum. Is it possible that one of these brand new jj 12ax7s are bad?

{edit - fix quote tag - PRR}

[Crate50watt]

Oooolll righhhttt. I am up and running! If you never made any mistakes, you would never have anything to learn from!
After using a multi meter to track the guitar signal all the way through the amp, I realized the tip and the sleeve of the speaker jacks were touching, causing the signal to go straight to ground. Oops. I separated the two parts and bam! Hopefully this doesn't end up frying anything.
I have it burning in meow and after a couple hours I'm supposed to adjust the bias again correct?
Tomorrow I will throw up some pictures and some audio with my two 2x12 cabs.
Thanks for the help team1

[Crate50watt]

Here are some sound clips from this amp. My playing isn't the greatest!
https://drive.google.com/open?id=0B327xnLcwKv5TEYzVzk3YjE2Q2M
^ clean on rhythm and treble

https://drive.google.com/open?id=0B327xnLcwKv5TzFrbE5aNU92X2M
^ a little grit on rhythm and treble

https://drive.google.com/open?id=0B327xnLcwKv5QW16SXd1cm5CYVE
^ lead on rhythm and treble

I though I had made a shred audio file but apparently I didn't so I'll attach one later.

I am having trouble getting my google drive to download today/right now so I'm hoping that these links will work. 

Also, the audio in these is being recorded on a cell phone, in front of two WGS 12's. One is a veteran 30 and the other is a et65. I am playing through a mexistrat with dragonfire active single coil pickups with tusq nuts and saddles.

[tubenit]

 Congrats on getting it up and going!  Thanks for sharing your success.  Appreciate hearing the sound clips.

 

with respect, Tubenit

[Crate50watt]

Here is a couple pictures of the finished product. I still need to figure out a way to mark the speaker selector impedances and fuse terminal values, but this cabinet i ordered from mojo is siiiiiiick looking.

Here is a sound sample with this amp through a 12" jensen
http://www.driveplayer.com/#fileIds=0B327xnLcwKv5ZUtmb1cySW1oUWM&userId=106691448903782353034

Here is a sound sample of this amp though a 212 extension cab with a wgs veteran30 and a wgs et65.
http://www.driveplayer.com/#fileIds=0B327xnLcwKv5cnNOOEdMREdMZTg&userId=106691448903782353034

Hopefully you guys don't mind some sloppy ass playing!

[sluckey]

Really nice looking amp. I would be nervous about the lack of ventilation though.

[Crate50watt]

Really nice looking amp. I would be nervous about the lack of ventilation though.

I have played it over the course of the last few weeks, sometimes for an hour or so straight (maybe at 25-50%) and had not noticed any issues with overheating yet... Is there something I could pay attention to, that would indicate it is getting too hot in there? Besides of course putting my hand over the back vent and feeling for heat?
The cabinet is a Marshall style head cabinet so it is the same design they use, so I would assume the venting is adequate?

[sluckey]

I'm just a nervous type.

[PRR]

> lack of ventilation

20 square inches of open mesh top-back. Some gaps at front and back panels, maybe 4 sq.in. I've seen that work OK for an amp this size.

For 300 long stadium gigs a year, or for Burning Man, I would boff more holes in it. (I'm nervous too.) But for casual play it is probably ample.