Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on February 26, 2013, 04:16:42 pm

Title: 18 WATT PLEXI QUESTION.
Post by: phsyconoodler on February 26, 2013, 04:16:42 pm

I just built an 18 watt plexi style amp and the original layout calls for a voltage divider from the volume control to the grid of the power tube.A 470k/470k voltage divider like on plexi's and JCM800's.
  My question is what does this voltage divider do?It sure looks like a treble peaking circuit to me,but the 18 watt version doesn't use the 500pf bypass cap.
  I'm thinking it's just to pad the volume gain a bit.I built the amp without the divider and it sounds fine,but I would like to know what that divider is supposed to do.Anything to be gained by installing it?I know I could just do it,but it would be cool to know what it's for.
  Anyone know?

Title: Re: 18 WATT PLEXI QUESTION.
Post by: stratele52 on February 27, 2013, 10:41:23 am

Am I wrong  ? I did not see what you mean on those schematic .

Could you show shematic you see that.

Title: Re: 18 WATT PLEXI QUESTION.
Post by: phsyconoodler on February 27, 2013, 02:13:50 pm

Here you go.

Title: Re: 18 WATT PLEXI QUESTION.
Post by: sluckey on February 27, 2013, 02:55:27 pm

Quote

My question is what does this voltage divider do?

Referring to the Ceriatone layout, it simply divides the signal in half, nothing more, nothing less.

Title: Re: 18 WATT PLEXI QUESTION.
Post by: stratele52 on February 27, 2013, 02:56:10 pm

Thank's phsyconoodler,

Here I found why I can't see what you mean ;

"   I just built an 18 watt plexi style amp and the original layout calls for a voltage divider from the volume control to the grid of the power tube.  "
_ phsyconoodler

Volume don't go to power tube grid , but to  ECC83

Title: Re: 18 WATT PLEXI QUESTION.
Post by: phsyconoodler on February 27, 2013, 06:53:26 pm

Duh! I meant the preamp tube,silly me!

 It really doesn't need to have it in there as the amp sounds stunning without it. I'm still going to graft it in to her what it sounds like before I decide to button it up.

 It's not a Ceriatone layout,someone at 18 watt.com did it a few years back. Zaphod Phil I think.
  I think he modded an existing ceriatone layout.

Title: Re: 18 WATT PLEXI QUESTION.
Post by: Scsoul on February 27, 2013, 08:22:08 pm

     That's ZP superlite iv built a few they are excellent, also did one with el34
and a 50watt marshall output transformer for a friend and he just loves it,  seems like
18watt.com is gone? I cant log on anymore? anyone know whats going on?

Title: Re: 18 WATT PLEXI QUESTION.
Post by: phsyconoodler on February 27, 2013, 09:27:00 pm

They revamped the site and it's still a mess.The activity there has trickled down to almost nothing.

 I've built the 18 watt plexi before but always used the 470k/470k on the volume.This one is the best yet.

Title: Re: 18 WATT PLEXI QUESTION.
Post by: plexi50 on February 27, 2013, 09:31:30 pm

I did that on an old bassman years ago and the amp had a deep and full robust to it. I just ran a wire to the grid of V2 pin 2.

Title: Re: 18 WATT PLEXI QUESTION.
Post by: DummyLoad on February 27, 2013, 09:41:19 pm

just wondering if the attached revision is how the PS should be wired. seems odd to put a 220K 1W R in series with entire load.

--pete

Title: Re: 18 WATT PLEXI QUESTION.
Post by: phsyconoodler on March 01, 2013, 09:13:50 pm

It's a sag resistor.I didn't do it that way.The tone is to die for with single coils or humbuckers.
  I used a Deluxe Reverb OT on this one.
My amp uses a tube rectifier and no sag resistor.I have about 330v on the plates.