Single input Plexi - separate 68k input grid or one?

[jasonvilla]

Hi all, can I use just one 68k or 33k input grid resistor to feed both triodes or do I need to separate them?
Thanks!

[shooter]

typically each grid gets it's own, preferable as close to the grid as feasable

[sluckey]

I use a single 33K and jumper pins 2 and 7 on the socket. Like this...

     http://sluckeyamps.com/dual_lite/dual_lite.pdf

[jasonvilla]

I use a single 33K and jumper pins 2 and 7 on the socket. Like this...

     http://sluckeyamps.com/dual_lite/dual_lite.pdf

Awesome, that’s exactly what I want to do. Thanks!

[sluckey]

This may more closely resemble your circuit...

     http://sluckeyamps.com/6v6plexi/6v6plexi.pdf

[jasonvilla]

That’s what I’m going off of, but instead of the two 68k I’ll just use one 33kish input grid resistor. And instead of the two 1MA volume pits in using a single dual gang volume pot.

[Willabe]

And instead of the two 1MA volume pits in using a single dual gang volume pot.

That might not work because of the 2 channels tone.

You want to be able to dial in a balance of treble and bass tone between the 2 channels.     

[jasonvilla]

Well I hope I like it as I only have room for one volume pot!

[sluckey]

I bet you would like dual concentric post better.

[jasonvilla]

Well I built it as I described and it works but not that into it. It’s way too dark/almost muddy even with the treble dimed and the bass off. Gonna make sure everything is hooked up right and check the voltages... then I might take the 22uf off the dark channel... and then maybe just make the darn thing a 2204 haha

[sluckey]

All you gotta do is separate the volume pots! 

[jasonvilla]

Haha well I wish I had separate volume controls because I would have found my problem sooner. The bright channel doesn’t have any audio... just weird sputtering noises that come in and out. I can’t figure out what’s wrong! Reflowed everything. Changed the coupling cap. Bypassed the volume pot... :( Not sure what the heck is wrong.

[sluckey]

Disconnect the 470K mixing resistor from the normal channel so you are only working with the bright channel. Not many components in the bright channel. Should be easy to find. Be sure you have a jumper between pins 4 and 5.

[jasonvilla]

I did disconnect the normal channel! That’s how I found out the bright channel was doing nothing. Would it be possible for the heater to only be working one side of the tube? I tried three different 12ax7s with the same results

[sluckey]

Would it be possible for the heater to only be working one side of the tube? I tried three different 12ax7s with the same results

That's why I mentioned the jumper on pins 4 and 5!

You already replaced the coupling cap. There are only 3 or 4 components left. And a half dozen solder joints. Shouldn't take long to replace everything if need be. What voltages do you have on the plate and cathode?

[Colas LeGrippa]

I strongly suggest you put a low (minus 6 db) and a high input per channel. Some humbuckers p/u  or high output p/u will have a better tone in the low imput which is in fact a voltage divider made of 2 x 68k resistors, one connected to grnd via the high input contact to gnd. What I did in my last build is put a high/lo sensitivity switch. I can clearly hear the difference flipping the switch hi or lo while my guitar ( gently weeps ?  ) stays connected.

[Colas LeGrippa]

...and Sluckey is right concerning the grid stoppers. On the tube socket if possible. This is a must on the power tubes. Right after the grid leaks / the 100 or 220k  bias resistors.

[jasonvilla]

Im still stumped!!! 6.44VAC across pin 4/5-9. Pin 8 - 1.4vdc. pin 7 -99mv. pin 6 - 189vdc

[sluckey]

Can you post some hi-rez pics that clearly show the tube socket, input jack, volume control, and components for the bright channel? Several different angles

[jasonvilla]

Pics

[jasonvilla]

Pics_2

[jasonvilla]

Pics_3

[jasonvilla]

Took some more measurements -
Ohms
pin1 - starts at 140k and keeps going up
pin 2 - 33.8k
pin 3 - 812ohm
pin 4 - .7ohm
pin 6 - starts at 140k and keeps going up
pin 7 - 33.8k
pin 8 - 2.6k
pin 9 - .7ohm

Voltages
Input jack tip (-74mv)
V1
pin 1 - 120v
pin 2 - -70mv
pin 3 - .9
pin 4 - 5mv
pin 5 - 5mv
pin 6 - 165v
pin 7 - -70mv
pin 8 - 1.9v
pin 9 - -0.1mv

V2
pin 1 - 180v
pin 2 - meter freaks out - something in mv
pin 3 -
pin 4 - -0.3mv
pin 5 - -0.3mv
pin 6 - 234v
pin 7 - 167v
pin 8 - 166v
pin 9 - 0

V3
pin 1 - 187v
pin 2 - 27v
pin 3 - 49v
pin 4 - 0
pin 5 - 0
pin 6 - 185v
pin 7 - 29.9v
pin 8 - 49v
pin 9 - 18mv

V4
pin 1 - 0
pin 2 - wonders around a few mv
pin 3 - 402
pin 4 - 393
pin 5 - -34
pin 6 - NA
pin 7 - wonders around a few mv
pin 8 - 0

V5
pin 1 - 0
pin 2 - wonders around a few mv
pin 3 - 402
pin 4 - 391
pin 5 - -34
pin 6 - NA
pin 7 - wonders around a few mv
pin 8 - 0

Thank you!

[sluckey]

Great pics. I can clearly see the problem. It's that tangled mess of component leads and that sloppy soldering job. That's not just unreliable, it's unsafe. My best recommendation is to pull all that stuff out and rebuild. Maybe look at some of the thousands of successful build pics on the net and try to make yours look just that neat.

[davidwpack]

That's sorta the layout theme in my Silvertone. I don't play the Silvertone anymore.

[MakerDP]

Gotta agree with Sluckey again on this one... that layout and lead-dress will only lead to problems at the onset and make finding and fixing them extremely difficult.

If you want a great example of a single-input Plexi design then have a look at the October over at AX84.com...
http://ax84.com/october.html

Schematics, clear layouts, BOMs, chassis drilling templates... it's all there... along with three power variants - 2W, 20W and 50W. It's also an amazing sounding amp!