Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: roseblood11 on February 16, 2017, 08:04:31 am

Title: JCM 800 - 2204 with 2*EL84 15w combo
Post by: roseblood11 on February 16, 2017, 08:04:31 am

Has anybody here built such an amp? I wonder if it's worth the effort...
My idea is to modify a Fender Blues jr to 2204 specs, or maybe to build the circuit on turret board and just to use the parts from the Fender.
My Blues jr has most of the BillM mods, so it might be a quite good starting point: Heyboer TO22 OT, cathode follower, adjustable bias, presence pot, power supply stiffening and an Eminence CV75 speaker. The most difficult part will be to move v2b (which is the gain stage after the reverb in the Blues jr) to a earlier position and use it as 2nd or 3rd stage of the 2204. Reverb wil be removed completely.
Would you make changes to the phase inverter, power amp or power supply, or just modify the preamp and see how it sounds?

This is close to my Blues jr:
http://mhuss.com/BluesJr/inputMod.jpg (http://mhuss.com/BluesJr/inputMod.jpg)
http://mhuss.com/BluesJr/output.jpg (http://mhuss.com/BluesJr/output.jpg)

JCM800: http://mhuss.com/MyJCM/JCM800_2204.gif (http://mhuss.com/MyJCM/JCM800_2204.gif)

Regards, Immo

PS: a turret board for this kind of amp would be great - Blues jr conversion 2.0 ...

Title: Re: JCM 800 - 2204 with 2*EL84 15w combo
Post by: Leevi on February 17, 2017, 09:52:26 am

There is certainly lot of room for changes but I would start with a basic 2204 and check then
how it sounds. One option is the change the power amp cathode biased which means that you are getting closer to VOX sound.
/Leevi

Title: Re: JCM 800 - 2204 with 2*EL84 15w combo
Post by: MFowler on February 17, 2017, 03:10:43 pm

Yes I have built an 18w JCM2204 with EL84 power tubes.  This is a new built so it is in a small cab Marshall type cabinet 1x12, new chassis and new board. With this one you want the grids to 5k6 or higher and change the 220k/220k to 470k/470k. I use a 47pf silver mica cap across the 100k/100k resistors to stop fizz. I had to increase voltage to get the amp to sound it's best.  This was a recycled chassis so it's no neat gut shot in photo below.


I also built an 18w EL84 tubes in a Blue Jr cab but with new chassis, Marshall type 18w transformers but the circuit is Dumble style.