Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on November 20, 2011, 08:45:17 pm

Title: adding a gain stage to a JCM 800
Post by: Colas LeGrippa on November 20, 2011, 08:45:17 pm

Hi, I have built a new SE amp with the filtering and preamp section of a JCM 2204 that sounds amazing ( soon on youtube , I'll post a new topic when it's there ). I would like to add a switchable gain stage for
heavy rock / metal purpose. I have checked in the previous posts here, there and everywhere without finding something appropriate. Would you ( boys and girls) have something simple, easy and efficient to propose ? I can easily add one more 12AX7 to the chassis as the amp was originally designed for a push/pull amp and the PT is strong enough to supply heaters.

Best regards !

Colas.

Title: Re: adding a gain stage to a JCM 800
Post by: sluckey on November 20, 2011, 10:22:51 pm

Here are a couple ideas. Be careful. HIGH GAIN!!!

Title: Re: adding a gain stage to a JCM 800
Post by: tubenit on November 21, 2011, 05:54:16 am

This would be switchable.  I have not built it, but I thought I'd offer it as a design to consider.

You would have a boost on paralleled V1 with switchable cathode caps

You would have a boost with the pre amp boost (PAB) on the tone stack

You would have a boost with the switchable Dumblish type OD section

I attached an editable SCH version also.

With respect, Tubenit

Title: Re: adding a gain stage to a JCM 800
Post by: tubenit on November 21, 2011, 06:20:25 am

IF you want a somewhat more relaxed not as intense OD, then this one would be worth considering.  I used this OD topology on a friend's amp that he loves.

This sound clip has this type of OD to it. It's articulate but not over the top.

http://www.soundclick.com/player/single_player.cfm?songid=10391428&q=hi&newref=1 (http://www.soundclick.com/player/single_player.cfm?songid=10391428&q=hi&newref=1)

With respect, Tubenit

Title: Re: adding a gain stage to a JCM 800
Post by: Colas LeGrippa on November 21, 2011, 07:01:51 am

my lord ! this gives me many hours of pure pleasure!

thanks a million.

Colas

Title: Re: adding a gain stage to a JCM 800
Post by: Boots Deville on November 21, 2011, 07:45:02 am

This link: http://i.imgur.com/40nC3.jpg (http://i.imgur.com/40nC3.jpg)
is to a layout I have squirreled away in the same folder that I have the AFD_Preamp image Sluckey posted, but I don't remember if I got them from the same place.   I haven't done anything with it, so I can't vouch for it's accuracy.  I also don't know if it matches that schematic, but if you decided to go the AFD route, maybe this layout info will help.

Cheers!

John

Title: Re: adding a gain stage to a JCM 800
Post by: bakerlite on November 22, 2011, 03:14:08 pm

That looks interesting,
Is there a shematic knocking around for that fella?

Quote from: Boots Deville on November 21, 2011, 07:45:02 am

This link: http://i.imgur.com/40nC3.jpg (http://i.imgur.com/40nC3.jpg)
is to a layout I have squirreled away in the same folder that I have the AFD_Preamp image Sluckey posted, but I don't remember if I got them from the same place.   I haven't done anything with it, so I can't vouch for it's accuracy.  I also don't know if it matches that schematic, but if you decided to go the AFD route, maybe this layout info will help.

Cheers!

John

Title: Re: adding a gain stage to a JCM 800
Post by: JayB on November 22, 2011, 06:02:32 pm

Quote from: tubenit on November 21, 2011, 06:20:25 am

IF you want a somewhat more relaxed not as intense OD, then this one would be worth considering.  I used this OD topology on a friend's amp that he loves.

This sound clip has this type of OD to it. It's articulate but not over the top.

http://www.soundclick.com/player/single_player.cfm?songid=10391428&q=hi&newref=1 (http://www.soundclick.com/player/single_player.cfm?songid=10391428&q=hi&newref=1)

With respect, Tubenit

That's a really cool sounding amp. Reminds me of some real old Black Sabbath tones, sort of the same character.