Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ncusack on May 27, 2011, 12:43:19 pm

Title: Help Mating Pre to 18W Power section
Post by: ncusack on May 27, 2011, 12:43:19 pm

Hey Everyone,

So i've been trying to sigure out a way to make a 2203 with a series FX loop and think i've found what I need. I've attatched the .sch of the pre. What im looking for is a bit of help with the power amp section. What PI values would you use to drive a pair of EL84's with this Pre. Also if you can suggest a suitable B+ rail to make this baby work that would help too. I have a hammon 270EX on hand that i think will make the cut for this amp using a 5u4 rectifier but im not sure.

Any assistance here would be greatly appreciated.

Cheers

Title: Re: Help Mating Pre to 18W Power section
Post by: phsyconoodler on May 27, 2011, 01:49:05 pm

Simply copy the PI values for a Marshall 18 watt amp including screen resistors and grid stopper values.Decide whether or not you want negative feedback and build it.
  Voltage will be lower on the preamp because keeping the B+ low enough for EL84"s is necessary,but it will likely sound great.
  If you want to experiment with dropping string resistors to get the preamp voltage up,go for it.

Title: Re: Help Mating Pre to 18W Power section
Post by: TIMBO on May 27, 2011, 04:00:17 pm

Hi ncusack, The schem below was a attempt to mate the mesa dual rect with the 18w.I was trying to keep the power section in the 18w voltages and bumping up the voltages in the pre to close to the mesa. This did work but sadly the amp has not sprouted wings yet, work in progress.  :icon_biggrin: Thanks

Title: Re: Help Mating Pre to 18W Power section
Post by: topbrent on May 27, 2011, 05:44:58 pm

Take a look at the Hoffman stereo projects for inspiration.  Build half of it and it is probably close to your goals.

http://www.el34world.com/projects/StereoPreAmp1.htm (http://www.el34world.com/projects/StereoPreAmp1.htm)

http://www.el34world.com/projects/StereoPowerAmp1.htm (http://www.el34world.com/projects/StereoPowerAmp1.htm)

(http://www.el34world.com/projects/images/stereopreampschematic.gif)




(http://www.el34world.com/projects/images/stereopowerampschematic.gif)

Title: Re: Help Mating Pre to 18W Power section
Post by: Matty_V on May 28, 2011, 12:14:22 am

I just tried building something similar, trying to spank a pair of el84s with 3 gain stages plus cathode follower and the result was out of control loud, nasty noise. Totally unusable. I went back to the drawing board and here's where it is at the moment. I really like it. I'm throwing most of v2 away and its still quite mean, but also cleans up nicely.