Champ mods - planning for first build

[pistolplc]

I am thinking of starting in on building my first tube amp - a 5F1 clone.  But I would like a few modifications, and I want determine whether this is something I (a first time amp builder with limited electrical and soldering experience) can handle on my own. 

Here is my wish list:
- A 5F1 combo, probably the Mojotone kit.
- A speaker out to a 4 or 16 ohm 4x12 cabinet (what I have in my basement). 
- The ability to attenuate or turn down the volume without affecting the gain/overdrive.  (I know the tone will always be affected - This is just for fun and the ability to crank it even when the kids are asleep).  I am open to suggestions here - some kind of master volume mod?  A resistive attenuator, similar to the switchable one on the Vox AC4TV mini?  A second/different power amp section?  (This could be cool, no?)

Are these things that I can handle?  I don't have the expertise to design them myself.

Anyway, any thoughts, hints, warnings, or suggestions would be greatly appreciated.  Thanks!

[PRR]

Welcome.

Mojotone has build documents. Have you looked at them and compared them to what you want?

The schematic (yes, I know, funny drawing) DOES show an output jack for external speaker.

The simple Champ ONLY "overloads" in the "Power" stage. 4 Watts clean, 6 Watts nasty. The "best" (or simplest) way to get grind at family-friendly levels is resistors between amp output and speaker input. Resistor "L-pads" in the 15 Watt level are readily available for installed sound systems (offices, malls, etc). Do not get the "transformer" type, which does not waste power, because this UN-loads the amp which is bad for sound and output transformer health.

[pompeiisneaks]

Yup, PRR nailed it, (as usual). if you want to use a type of MV, you need a push pull preamp type that gets your drive in the preamp stages, then you use a PPIMV (Post Phase Inverter Master Volume) that is a dual pot that you send both outputs of the PI into and in parallel you lower the volume by increasing the resistance forward while decreasing the resistance to ground.  I think most Single Ended amps tend to sound like they do because you can drive the power stage a lot harder and get a different tone to the overdrive.  I'm guessing you could up the drive on a SE amp by adding another gain stage and then just put a second Master Volume before the output (there's no PI in a SE amp).

(or so my yeoman's level learning suggests)

As far as adding a speaker jack, as stated, it exists already in the champ, but if you don't have a schematic with one, you can always add it, but you need to be careful to do one of two things:
1. ensure the jack, when connected, shuts off output to the other speaker so you don't mess up the total speaker impedance, or
2. convert the internal speaker to use the same output jack and that way you just unplug the built in one, and plug in the external one. 

Fenders tend to always go the route of using a jack on the chassis for the speaker.  I've seen it on several at this point, only the really old ones (like the bassman clone) have something a bit different (the bassman had 4 rca outputs and you needed to connect 4x8 ohms or some total of 2 ohms.)

Just make sure to match impedance correctly for the OT.

~Phil

[Ambugaton]

I have limited experience as well but as far as the attenuation I would recommend the Weber mini mass.

[pistolplc]

Thanks all.  Great info. 

On the L-Pad, is there any reason (aside from price) to try to use a lower power rating?  Will a 50w L-Pad sound different (for my purposes) than a 15w?  I figure I should just go with something that can handle my biggest amp (68 Deluxe Reverb).

As a starter project I might build a little box with three L-Pads at 4, 8, and 16 ohms.  Get my soldering technique down before I build an actual amp. 

[Ambugaton]

Getting proficient with soldering was my biggest hurdle. Things that helped me:

Splurge on a decent iron that has a rest and sponge (will encourage you to clean after each solder)

Use heat sinks, (I just used alligator clips)

Make solid mechanical connections and then reinforce with solder.

Use cloth wire if you can. I find this to be the easiest to use, and Doug's cloth wire is great. You will find some other cloth wire that is not nearly as easy (frays too easily)

Watch a lot of youtube!


I know the above is super basic... but hope it helps nonetheless.

[pistolplc]

Thanks again. 

What do you mean by "use heat sinks"?  For soldering? 

[shooter]

"use heat sinks"? 

They are little gizmo clips used between the actual component and point of solder to "short out" the heat before it gets to the physical part.  I use hemostats, you can probably google up a pic, something like "solder heat sinks"