Botched 1979 JMP... advice?

[Opry Audio]

I was a big dummy and just needed to make a bias pot adjustment. The bias actually works very well now, and the amp sounds excellent.

Just a few more thoughts before I figure out the switch situation. Channel 2 sounds much darker than channel 1 does... is this by design? It almost sounds too dark. I’m getting some oscillation in channel 1 at high volumes but I need to take it off my bench to see if it’s the lights / lack of shielding

[shooter]

the quick n dirty "fix" for dark sound, decrease the cathode Ecaps, starting with 1st gain stage, example if it's 25uf, go to 10, evaluate, repeat.  once that's done, coupling caps.  but the real answer lays in an oscope, a sig-gen that you can set for specific frequencies and "see" where treble ain't n bass is to much or visaversa.  I typically use 80hz, 200hz, 400hz and 1k (4k when i'm really bored :). 

[Opry Audio]

Well, just to assume it is correct, what I would say about it is that I would probably not use it on it’s own. Jumpering both channels together makes magic happen.. so I will keep it as is. I think that’s probably it’s intended function.

I’m still a bit confused about this switch.. according to the sheet sluckey linked, my switch should be lighting up. I have a feeling that there is a connection to ground that it needs but I’m not sure which terminal. I don’t want to short out the mains voltage

[sluckey]

Channel 2 sounds much darker than channel 1 does... is this by design? It almost sounds too dark.

Look at the schematic of the Hoffman Plexi50. It's pretty obvious that CH1 is very dark and CH2 is very bright. That's just the way a plexi is. Neither channel really sounds great. But magic happens when you jump the channels together and use the volume controls to blend the dark and the bright together.

[Opry Audio]

Thanks for your help everyone, this amp came out great!

Any ideas about the rocker switch? Apparently the LED needs a ground connection but not sure where. The sheet that was linked is for a switch that doesn’t require it. This one has 6 tabs on it. Last thing I have to do before I close her up! :)

[shooter]

That's just the way a plexi is.

 
thought it was the jcm 900 amp


+1
took me some time to figure out the blend on a plexi, but when I did, i stole the preamp 3 more times

[sluckey]

Do not connect a ground wire to any lug on that power switch! There should be some numbers for the switch lugs. Look closely. Draw a pic of the switch lugs showing the numbers. Post the pic. Or post a good closeup of the actual switch so we can see the numbers.

[Opry Audio]

Here’s a pic. Sorry it’s gonna be sideways.. pics always do that from my phone to threads here.

Neutral is wired to 12
Blue PT primary jumpered between 11, 13

Hot going to 25
Fuse and brown Pt primary on 24

26 is vacant

[sluckey]

You only need four wires connected to the switch. What are the two extras connected to?

[Opry Audio]

The other two is for the polarity switch. I cut out the cap so it’s not doing anything

[Opry Audio]

After jumpering 26 to 24, it lights up. Case closed  awesome build, I might try this one again. Thanks y’all! I finally built something good haha

[Opry Audio]

Just curious but is there anyone who’s built a plexi into a bassman chassis?

Will it fit the transformers needed for this kind of build? I would love to build this amp again but as a combo. I’m really glad I got a chance to build this with JMP drake iron.. probably why it sounds so good. Any recs for transformers that would be similar?

[SILVERGUN]

Just curious but is there anyone who’s built a plexi into a bassman chassis?

Will it fit the transformers needed for this kind of build? I would love to build this amp again but as a combo. I’m really glad I got a chance to build this with JMP drake iron.. probably why it sounds so good. Any recs for transformers that would be similar?

...to help you get an answer:
Which Bassman chassis?

[shooter]

fwiw;
I put this 5 tube, 6 knob, 4 switch, handyman special in a 12"X8"X3" and had room to dance, IF i was neater 
do the square foot math to be sure 

[Opry Audio]

Sorry I meant a tweed bassman chassis. It has the right amount of input jacks and tube socket holes... haha