Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Shack on March 28, 2020, 05:22:47 pm

Title: can a JCM 800 preamp be voiced to sound more like a Friedman
Post by: Shack on March 28, 2020, 05:22:47 pm

I love this high gain jack , but it is kind of trebly and doesnt play well with pedalboards. The low jack works well with my pedalboard but dont have the crunch.....just asking

Title: Re: can a JCM 800 preamp be voiced to sound more like a Friedman
Post by: sluckey on March 28, 2020, 05:45:09 pm

Convert it to a Plexi 1987 preamp. Plenty of high volume crunch, no screech.

Title: Re: can a JCM 800 preamp be voiced to sound more like a Friedman
Post by: Shack on March 28, 2020, 06:04:05 pm

Ah I thought that was the key.....this amp sounds good, but im not a metal or 80s kind of player....wonder if I can do that on this board, im thinkin yea

Title: Re: can a JCM 800 preamp be voiced to sound more like a Friedman
Post by: Shack on March 28, 2020, 06:06:27 pm

Every you tube video I see with a Friedman , the amp sounds so good

Title: Re: can a JCM 800 preamp be voiced to sound more like a Friedman
Post by: Bantam on April 05, 2020, 07:20:29 pm

The old Cameron mod thing was to add effects loop.

Title: Re: can a JCM 800 preamp be voiced to sound more like a Friedman
Post by: MFowler on April 06, 2020, 11:14:04 am

Remove the treble cap on the volume pot or put it on a switch to remove the highs.


Put in an interrupt type effects loop between master pot wiper and PI entrance.


Mark

Title: Re: can a JCM 800 preamp be voiced to sound more like a Friedman
Post by: darkbluemurder on April 08, 2020, 05:46:03 am

Yes you can.


1) Add a 220k resistor from the gain pot input to ground.
2) Change the Rk on the second gain stage from 10k to 2k7 or 3k3 and add a .68uf cap.
3) Remove the 470pf cap on the 470k/470k voltage divider between the 2nd and 3rd gain stages.
4) Add a 470pf cap to the plate resistor of the third gain stage.
5) Add a 2200pf cap to the cathode resistor on the cathode follower.


If there is too much bass, adjust the coupling cap after the first gain stage. Likewise if there is too little bass.


Good luck,
Stephan

Title: Re: can a JCM 800 preamp be voiced to sound more like a Friedman
Post by: Shack on April 27, 2020, 04:36:28 pm

Thank you for all the replies......but what I did is change the preamp to a Plexi style. after the preamp section the only difference from this jcm800 style build is there is a different presence circuit , and I havent added a master volume yet, I cant believe how loud it is with princeton reverb iron and 6v6. I may change the feedback resistor from 100k to 27k to clean it up some but I havent played it much yet.

Title: Re: can a JCM 800 preamp be voiced to sound more like a Friedman
Post by: martinomma on October 15, 2020, 05:47:55 pm

Quote from: darkbluemurder on April 08, 2020, 05:46:03 am

Yes you can.


1) Add a 220k resistor from the gain pot input to ground.
2) Change the Rk on the second gain stage from 10k to 2k7 or 3k3 and add a .68uf cap.
3) Remove the 470pf cap on the 470k/470k voltage divider between the 2nd and 3rd gain stages.
4) Add a 470pf cap to the plate resistor of the third gain stage.
5) Add a 2200pf cap to the cathode resistor on the cathode follower.


If there is too much bass, adjust the coupling cap after the first gain stage. Likewise if there is too little bass.


Good luck,
Stephan

it's very bad!!!!!!!!!!