single channel rack preamp project (marshall 2204 mod)

[grrrunge]

I'm thinking about starting my journey into the world of amp building by building a rack version of the Marshall 2204 preamp with a few mods around.

For a start, i'd like to keep it a single channel preamp as is, but with only one input, as i'm planning on using it in my rack switching setup, in order to avoid having to repatch if i feel like using the "other channel".

Then it sounds like the 2203/2204 tone mod from here: http://www.schematicheaven.com/mods/22032204mods.htm does nice things to the preamp section (more gain, fatter tone etc).

As the mod suggests three different plate resistor values for v1b, i thought of adding a three way switch, letting me choose between three "amounts" of gain from the faceplate.

The faceplate layout should then be input jack, hi/lo input switch, classic/crazy/insane gain switch, gain dial, bass, middle, treble and master volume.

The original marshall scheme i copied is attached here, as well as my redrawn modded version of it.

Since it is my very first attempt to accomplish anything within this field, i would really much like some help on this one, mostly just checking out the diagram, to point out any errors before i start building this thing.

how high a voltage should i feed the circuit with, and can anyone recommend me some good power trannies? also i need to construct some sort of PSU circuit. How should i do that?

Lastly I need to know whether i can just add a jack at the output end (as i did on the scheme), to make the preamp run at unity gain, or if i need some sort of output buffer/whatever stage before feeding the preamp into my rack switcher (TC G-System) in order to "not kill" it

[grrrunge]

bump

[Konny]

Hi grunge,

first, the typical resistor in series with the V1 grid is 68k for normalinputs and 33k for a high input.
Bypassing that resistors gives more highs and depending on the capacitance perhaps more high mids, BUT it´ll also let radiowaves pass into V1 and soon you´re listening to radio moscow

Then the 100p on the grid to the anode on V1 isn´t neccessary if the amp is build well. I was implemented in the circuitry when Marshall started using PCBs to avoid oscillations from bad routings.
Removing this cap will give ya a bit more highs.

Why did you change the bypass cap on the cathode of V1?

Cheers,
Swen

[grrrunge]

Hi grunge,
first, the typical resistor in series with the V1 grid is 68k for normalinputs and 33k for a high input.
Bypassing that resistors gives more highs and depending on the capacitance perhaps more high mids, BUT it´ll also let radiowaves pass into V1 and soon you´re listening to radio moscow

Fabulous i like russian radio.. maybe listening that could brush up on the two hours i spent learning basic russian terms in high school

Then the 100p on the grid to the anode on V1 isn´t neccessary if the amp is build well. I was implemented in the circuitry when Marshall started using PCBs to avoid oscillations from bad routings.
Removing this cap will give ya a bit more highs.

So, i could safely remove that one, if i do a proper turret board layout, and a pretty wiring job?

Why did you change the bypass cap on the cathode of V1?

Changing the bypass cap on the V1 cathode was one of the mod's from the mod article mentioned in my first post.

Locate the .68uf cap (C1) which is found on V1A and replace it with a .47uf cap. This alters the midrange response and adds a "heavier" tonal quality to the amp.

that sounded pretty nice to me, so i did that on the schem

furthermore: what about the output level of this thing? is it safe to connect it to unity gain devices as is, or should the output be modified somehow?

[grrrunge]

bump again

[7string]

Hi grrunge,

Hoffman has a project page with a similar preamp.

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

Also check out these Fender style preamp projects.

http://www.frontiernet.net/~jff/af2b...ss_Guitar.html

http://moosapotamus.net/IDEAS/F2B/alembic.htm

All these are stereo, but I think you will be fine with a mono pre and let your TC do the stereo split (or not). The plate resistor switch sounds like a bad idea IMHO. Running high voltage wires to a switch and back could cause oscillation problems.

You say you have a MIDI switcher? Build both pres and let it switch between inputs. Use the Fender for clean, 2204 low sensitivity input for medium, and the 2204 high sensitivity for high gain.

Start with following the Marshall schematics exactly, then add the mods one at a time until you find what works best for you. Nobody on the internet can tell you what will sound best with YOUR music, guitar, playing syle, speakers, rack setup, etc.

I would leave some room and an extra tube hole just in case you need to add another gain stage after the tonestack. Maybe you should do a not-so-pretty prototype to work out the bugs first.

[grrrunge]

Hi grrunge,

Hoffman has a project page with a similar preamp.

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

Also check out these Fender style preamp projects.

http://www.frontiernet.net/~jff/af2b...ss_Guitar.html

http://moosapotamus.net/IDEAS/F2B/alembic.htm

All these are stereo, but I think you will be fine with a mono pre and let your TC do the stereo split (or not). The plate resistor switch sounds like a bad idea IMHO. Running high voltage wires to a switch and back could cause oscillation problems.

You say you have a MIDI switcher? Build both pres and let it switch between inputs. Use the Fender for clean, 2204 low sensitivity input for medium, and the 2204 high sensitivity for high gain.

Start with following the Marshall schematics exactly, then add the mods one at a time until you find what works best for you. Nobody on the internet can tell you what will sound best with YOUR music, guitar, playing syle, speakers, rack setup, etc.

I would leave some room and an extra tube hole just in case you need to add another gain stage after the tonestack. Maybe you should do a not-so-pretty prototype to work out the bugs first.

Nice! Thanks alot!!

What about B+ voltage for the 2204 pre? any suggestions?

[7string]

You should keep it under 200vdc at the plates, more like 100vdc at V1's plates. A Hammond P-T269EX is 190-0-190 CT which will give you around 266vdc B+ after the bridge rectifier. You can knock that down some by increasing the 2 watt reistors between the filter caps or the plate load resistors. I would try to get close to the voltages shown on the 2204 schematic. But, anything within reason should work.

[Bruce / Mission Amps]

You should keep it under 200vdc at the plates, more like 100vdc at V1's plates. A Hammond P-T269EX is 190-0-190 CT which will give you around 266vdc B+ after the bridge rectifier. You can knock that down some by increasing the 2 watt reistors between the filter caps or the plate load resistors. I would try to get close to the voltages shown on the 2204 schematic. But, anything within reason should work.

I'm betting you meant a full wave rectifier.
A bridge rectifier across the outside leads of that PT, with a capacitor input filter, will give you about 550vdc.
If you use a full wave rectifier with a capacitor input filter you'll get about 290vdc.
If you use full wave rectifier and a choke input filter followed by a 22uF to 47uF cap, you'll get around 180vdc.
It will also be a very low ripple + rail, perfect for a two tube preamp with 82K to 120K plate load resistors.

[grrrunge]

I'm betting you meant a full wave rectifier.
A bridge rectifier across the outside leads of that PT, with a capacitor input filter, will give you about 550vdc.
If you use a full wave rectifier with a capacitor input filter you'll get about 290vdc.
If you use full wave rectifier and a choke input filter followed by a 22uFto 47uF cap, you'll get around 180vdc.
It will also be a very low ripple + rail, perfect for a two tube preamp with 82K to 120K plate load resistors.

Wow - that's an insane amount of talk that i need to have explained further Is there any resource on the web specifically about designing power supplies for tube preamps or just tube amp stuff in general?

Really - I need stuff for starters here - am i in way over my head with building a preamp?

[7string]

Thanks for having my back Bruce. Misinformation can be a very bad thing. I'll keep that to a minimum.

Here is some good reading.

http://www.valvewizard.co.uk/

A preamp could be a good first project. The Hoffman link had a schematic and layout all ready to go. Maybe just delete one channel and get a smaller PT.

Safety is most important. You can really get hurt even killed with this stuff. Just have fun building and don't rush. You learn the most when trying to solve these problems as well as trouble shooting a new build.

[kg]

Wow - that's an insane amount of talk that i need to have explained further Is there any resource on the web specifically about designing power supplies for tube preamps or just tube amp stuff in general?

Really - I need stuff for starters here - am i in way over my head with building a preamp?

this might help:

http://ken-gilbert.com/power-supply-...nry-pasternack



good luck!

ken

[Bruce / Mission Amps]

I just got back here and looked at the LIVA pdf file and noticed that the coupling cap from the first high gain preamp stage is shunted right to ground! ha ha... no sound but the amp will be very quiet.
That whole switch and input jack thing is wrong anyhow, well, I don't see how it would work unless switched into to low gain setting.
Also.... Trace is a VERY good friend of mine but I can not for the life of me see how that 330pF cap across the 68K resistor can make any guitar amp/speaker audio difference at all... radio freqs, Oh yeah, but audio?
The -3dB knee on that combo is over 7000Hz, maybe flat at 14KHz and on and on....