Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Vdelli on September 13, 2018, 01:24:12 pm

Title: New Build Question/ EF86 Channel - JCM800 + Boost+Power Scaling
Post by: Vdelli on September 13, 2018, 01:24:12 pm

Hi,

I want to build a new amp...after building a few 1 channel amps one of them with an switchable boost. I wanna try to build a 2-2,5 channel amp.
My idea is an EF86 channel, something like Matchless nighthawk or the 18w Marshall EF86 Channel, that goes from clean to "vintage" overdrive but with NFB and one JCM800 channel with a boost.
The boost is something similar to a tube screamer but it works with one 12ax7.


For Power Scaling I will use the London Power SV1 (Power Scaling PCB) and the RBX (Bias Power Supply)
I used this in my deluxe reverb style build before an it works pretty good!




I draw a full schematic (first time that I do this) for this idea.


What do you guys think about that and is that switching of the channels correct?

Should I use LDR´s to mute the hi gain channel?

Will there be a "plopp" if I switching?



with best regads from Germany!

Title: Re: New Build Question/ EF86 Channel - JCM800 + Boost+Power Scaling
Post by: shooter on September 13, 2018, 08:45:34 pm

I believe you will POP.
got lost on the middle gain stage, the other 2 seemed doable
I drew up a 3ch, I did a bread-board build of each ch on it's own, was great, I figured out I didn't like 2 of the ch's so I now have a sweet SE '88 18W'r with a modified Bogner lead ch for the pre.

take the time and test each stage
I have to go back n look, but check phasing, that might be an issue

Title: Re: New Build Question/ EF86 Channel - JCM800 + Boost+Power Scaling
Post by: Vdelli on September 14, 2018, 02:26:32 am

Thanks, shooter!

The middle schem is a tube screamer based on a 12ax7. My idea was to switch it in an out, as boost.
The 3rd schem is a really standard jcm800.


What do mean with phasing?


Bes regards


 

Title: Re: New Build Question/ EF86 Channel - JCM800 + Boost+Power Scaling
Post by: VMS on September 14, 2018, 02:45:30 am

I think phasing is only an issue when playing two channels parallel and at same time but since you are switching channels it shouldn't be an issue.


You may need to add 2M2 resistor to V1 like you have on V2a, not sure.

Title: Re: New Build Question/ EF86 Channel - JCM800 + Boost+Power Scaling
Post by: Vdelli on September 15, 2018, 03:48:00 am

Okay!
I think my biggest Problem here is popping when switching and maybe u can hear the unused channel in the used channel....I read a lot, there are a few options that can be used...Optocouplers or transistors called AQY210. I understand how the optocouplers work but I doesn´t understand how the AQY210 work.

I looked at some schematics like slo100, mike soldano use optocoupler to mute the unused channel after the 2nd or 3rd gainstage.