Champ mod questions

[ncusack]

Hey everyone,

Im looking into building a Fender Champ and have a few questions about a few mods to the champ circuit. I would like to incorperate some kind of switchable gain and a switchable bright option. I also plan on using a SS recto for this build and was wondering if anyone could suggest an ideal filtering section for use with a Hammond 269JX transformer (117VAC 250-0-250). I was thinking 22uf across the board using the 10k and 22k dropping resistors like the 5F1.

Anyway all suggestions are appreciated here.

Thanks

[bigsmitty]

For a switchable gain mod I put a switchable bypass cap on v1b.

[ncusack]

What value cap did you use? Did you just have it like the 5e1 with the 25uf and have that switchable in and out?

[captntasty]

A few ideas

Here's a schematic that should give you a couple ideas - in particular the switchable cathode bypass caps in the first preamp section.

[bigsmitty]

I used a 4.7uf on the second cathode of v1 also used a 4.7 on the first that is non -swithchable. 25uf is too bassy IMO.

[ncusack]

I've built a 5f2a with using a 25uf bypass cap and i can agree with you on the bassy sound. Im not sure if the 25uf is the culprit but it definitely helps to know that could be it.

Just to confirm you had the second half of v1 as your switchable cap not the first stage? Could you explain that decision most of the mods i've seen have the first stage bypass caps as the switchable ones.

[MWJB]

"I also plan on using a SS recto for this build and was wondering if anyone could suggest an ideal filtering section for use with a Hammond 269JX transformer (117VAC 250-0-250). I was thinking 22uf across the board using the 10k and 22k dropping resistors like the 5F1. "

250-0-250 is a bit low for a champ type build, you'll struggle to get more than 300vdc off the rectifier. 300-0-300 minimum.

Use at least 47/50uf for the first filter cap after the recto, 8 or 10uf will do for the other 2, but I'd go larger at the screen node too (40uf).

[ncusack]

So would you suggest something like the the 270DX that's rated 275 - 0 - 275 at 115V. Going with the usual idea of SS recto working at 1.4 times that wouldn't I get around 385V. Also my wall voltage is around 121 most of the time so that 250 on the 269JX is really like 260 isn't it.

[MWJB]

260-0-260 is still too low for a champ, even with a SS rectifier.

269JX is a little low on current rating, a champ PT has 70mA dc rated B+ winding.

If you are using a SS rectifier & you want champ style voltages, use a 300-0-300VAC PT.

You're not going to see 1.4*275VAC, current draw on the B+ winding will pull the B+ down...plus you will lose the voltage devloped accross the cathode resistor.

270DX would be the absolute minimum B+ that I would recommend for a champ with SS rectification, even allowing for 5% hike in wall AC. The 270DX also has a 2A rectifier winding so you could drop B+ if you felt you wanted to.

Any reason as to why you can't use a champ PT (6500 or 125P1B)?

[ncusack]

Only real reason im going with the Hammond stuff is because my local shop carries them. I can get any model I had just read somewhere that the 269JX would make the cut but that's why im asking these questions here. Just trying to make sure about things before I build it.

[bigsmitty]

I've built a 5f2a with using a 25uf bypass cap and i can agree with you on the bassy sound. Im not sure if the 25uf is the culprit but it definitely helps to know that could be it.

Just to confirm you had the second half of v1 as your switchable cap not the first stage? Could you explain that decision most of the mods i've seen have the first stage bypass caps as the switchable ones.

Switching a cap in on the second stage also grounds out the negative feedback giving even more boost. Experiment and see what YOU like.

[ncusack]

What kind of gain differences would you get from smaller or larger cap values on the second stage?

[bigsmitty]

What kind of gain differences would you get from smaller or larger cap values on the second stage?

Cap value doesn't alter the amount of gain just the frequency response.

[ncusack]

Sorry for all the questions but will the freq response become high or lower based on the values? Like more capacitance means a higher cutoff or something?

Im new to the design side of things so all this is helping me wrap my head around how these circuits really operate

[bigsmitty]

Sorry for all the questions but will the freq response become high or lower based on the values? Like more capacitance means a higher cutoff or something?

Im new to the design side of things so all this is helping me wrap my head around how these circuits really operate

Thats OK, keep'em coming; larger value=more bass, smaller= more treble.