Fender Champ 25 Overdrive Suggestions

[professormudd]

Greetings,

I have a Fender Champ25 amplifier. It has 2 6L6 and a single 12ax7. I believe the clean channel is solid state.

I did not care for the OD channel on this amp, so I went searching and found a video by a fella in Boston and I went ahead and tried it. It smoothed out the overdrive, significantly reduced the clipping but took the honk out of it.

https://www.youtube.com/watch?v=13NRt5HDDB8

Anyway, I ended up undoing the mod and I was wondering if there might be someone here who might point me in the right direction for further modifying this amplifier.

What I would like to do is make the OD channel suitable for metal. Something akin to an EVH or a Mesa Boogie. I am not sure if what I want to do is reasonable as a mod. I would like to avoid going too crazy and ripping it all out and starting from scratch, but I am not afraid to swap out parts.

I was able to locate a schematic and layout for the Champ 25 SE, which I think the only differences might be a master volume and stand-by switch.

Any advice is appreciated.

[pompeiisneaks]

Wow yeah that amp is very weird, it has a tube phase inverter and a tube output section but the entire preamp is solid state.

You might be able to see if the transformer can handle a bit of extra preamp tube current and mod it to gut the crap out of there and make it a full tube amp

Other than that I'd say leave it clean on the solid state side and use some good pedals for your drive.

~Phil

[professormudd]

Wow yeah that amp is very weird, it has a tube phase inverter and a tube output section but the entire preamp is solid state.

You might be able to see if the transformer can handle a bit of extra preamp tube current and mod it to gut the crap out of there and make it a full tube amp

Other than that I'd say leave it clean on the solid state side and use some good pedals for your drive.

~Phil

Yeah, I had originally bought this to use as a pedal platform actually. I was getting really into pedals for a minute until I got my inspiration to learn how to build my own amps. Now I am understanding the amplifier itself as a part of the instrument and I feel like I am just now learning how to "play" tubes.

Anyway, I suppose this would be a good opportunity to learn something and go with your first suggestion. I am going to get it on the bench and take some photos. I reckon I will use this thread as a log.

[professormudd]

Not quite sure where to begin... Everything is pcb mounted. Am I going to have to ditch the whole board?

[pompeiisneaks]

Yes likely you'd need to just remove the board entirely. ONLY if you can confirm the power transformer has the excess heater current and b+ current to run another couple tubes. There are a lot of amps you can build with just a couple more tubes, but I'm not sure if you'll find specs for the PT or not.

Then you'd need to decide what kind of amp you wanted. You could still use an effects loop in it either passive where you just insert the send/return jacks between the output of the last tube and the input to the Phase Inverter. If you've never built an amp from scratch before this can be a pretty daunting task.

If this data on a replacement transformer for the amp is correct, it says the 6.3V heater windings has 2.5A current capability.
https://www.tubeampdoctor.com/en/transf ... champ-25se
That means you'd have the ability to carry the two 6L6GC's (they need a total of 1.8A 900mA each)
Then you'd have 700mA left over. Each 12AX7 takes 300mA so you could fit in a total of 2 tubes without taxing the transformer. so you could have a tube topology that uses a total of three tubes like an original champ, or any other single tube preamp stage that goes into a Phase Inverter. (others may have better suggestions than I would on that)

The B+ side can carry 180mA but a pair of 6L6CG's if biased hot can pull max of about 210mA so you'd want to be very careful with how you bias them. Then you've got very little current left for the preamp tube. They don't use much, maybe 1.2 mA per section but still, make sure to calculate the right bias to give them breathing room on the power tubes.

This is a really fast rough guesstimate based on what 'might' be your transformer. I'd not call it a guarantee until you can find something that proves your current transformer matches the one I've linked.

~Phil

[ChopSauce]

I've been tempted to buy one of these to convert it to full tube but I've been puzzled by the power section. 25W out of a pair of 6L6s mean that they are starved - as a start, isn't it?

Maybe the amp can be converted to full tube but I doubt the outcome can easily be some metal-capable thing, as requested by the OP...

However, there might be some possibility that you mod. it to something more usefull, yet keep the board and the transistors. Xtian advised us once that the Music Electronics Forum is a usefull resource in the matter of solid state amps. Maybe you could ask there...

[professormudd]

The more I think about it the less practical it feels to "convert" this thing, rather re-purpose the chassis and maybe use some parts and put a different circuit in it. Or just sell it and buy parts to build something less janky.

I have a spare PT and all of the other parts (except the OT) for a 5e3 that I could use to contribute to this amplifier if it were practical and not a waste of good parts.

My enthusiasm is waning.

[thetragichero]

what don't you like about the overdrive channel? there are many amps from the 70s with solid state preamps and tube output stages (peavey and music man come to mind), so it's definitely possible to tweak this into something usable for metal IF you're decent enough soldering/desoldering pcb

[professormudd]

what don't you like about the overdrive channel? there are many amps from the 70s with solid state preamps and tube output stages (peavey and music man come to mind), so it's definitely possible to tweak this into something usable for metal IF you're decent enough soldering/desoldering pcb

I can't put my finger on it. I guess this thing sounds like it could work for a noisy punk band. I only mentioned metal because I just wanted to make it into something I don't already have in combo form.