Crawls Backward (When Alarmed)

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5F1 Converted Champ 600 Cathode Bypass Switch Mod

This was a quick project that I've wanted to take care of for a while.  A lot of pictures in this one.

A few years ago, I bought one of the 'new' Fender Champ 600 amplifiers.  If you're not familiar with it, it's sort of a facsimile of the original Champion 600 that Fender made in the late 1940s.  Long story short, the new one resembles the original, but the circuit is totally different.  It does, however, have tubes, and it sells for a reasonable price. 

I bought mine on *bay.  It was virtually new, even had the original shipping box, but it wasn't working.  I was able to snag it for less than half the price of a new one.

Turns out the original (Chinese) 6V6 output tube went bad and took the cathode resistor with it.  I put in an old U.S. 6V6 from my stash in it and replaced the resistor.  Voila!  It worked.

However.  I was disappointed with the amp from the get-go.  It didn't have much grind at all, and it just seemed a little lame in general.  Found the schematic online and discovered that the circuit is very similar to the 1960s and 70s Champs rather than the old 600 or the famous tweed 5F1.  Not that there is anything wrong with the 1960s Champs, it's just a different circuit.  I would have expected something closer to a tweed amp.

Anyway.  I hatched a plan to strip out the modern PC board, power transformer and output transformer, and put a 5F1 circuit in its place.  The amp is now much better in terms of its tone and breakup.  And I just now added a switchable cathode bypass cap on the first stage of the preamp tube.  Hence this post to document it.

It's easy to get the chassis out of the cabinet.  Remove the back panel, and disconnect the speaker lead from the chassis.  Then remove the two screws on the control panel, and the two screws on either side of the cabinet.

Here I am using my trusty Ridgid 'Fuego' drill to take those cabinet screws out.

Now we slide the chassis out.

If you're familiar with the 5F1, you know what you're looking out.

This is the 5F1 eyelet board I put in.  I did a few things differently than a totally stock 5F1 - namely I used PIO coupling caps and also took those caps off the board and mounted them right on the volume pot and output tube, respectively.  I also used shielded cable for the run from the volume control back to the 2nd stage of the preamp tube.  Really just a couple of things I consider to be best practices to reduce noise and RFI.

Bottom side of the chassis.  I used Mojo power and output transformers.  I also put a tube (5Y3) rectifier in it.   Some folks wouldn't bother, but I figure if it's going to be a tweed Champ, it should be a tweed Champ.

I just punched the chassis near the PT for the rectifier socket.

It's all a little tight compared to a proper tweed, but it works.  The trickiest part is that the preamp tube socket lives right under the board.  On the PC board version, the tube sockets are soldered to the board.  I used the same holes for the new sockets, but I had to be careful to insulate the preamp tube socket connections from the bottom of the new board.

The PT and OT bolt right in to the existing holes.

I also changed out the original speaker for a Weber ceramic one, and I changed the original felt-type grill cloth for an early-60s style Fender maroon weave.

Here's what I'm changing now.

This is the cathode resistor and bypass capacitor for the first preamp stage.  The original 5F1 schematic shows only the resistor there.  However, many tweed Champs apparently had a 25uf/25v cap across the resistor.  In any event, a bypass cap is common on virtually all Fender amps.  So it's a natural, easy mod for an amp that doesn't have one.

What the capacitor does is boost the gain of the tube equally on all frequencies.  Without delving into a lot of theory, the frequency response and gain of a tube like this can be controlled via the values of the resistor and capacitor.  I originally built the amp with no bypass cap.  Then I put in a 2.2uf cap.  Now I've just decided to put a 25uf cap in, but make it switchable.  So I'll have a 'cleaner' tone with less boost (cap switched out), or a more traditional, higher gain, more boosted signal (with the cap in the circuit).

I don't like drill holes to add extra switches or pots to guitars and amps, so I'm going to put a new volume pot in which also has a push-pull switch to control the bypass capacitor.  That way the amp will look stock but will have the "secret" switch on it.

I took the old cap out, and then I unsoldered the connections to the volume control.  I use solder wick to remove the old solder.  Makes a nice clean connection to work with.

Note the Vitamin Q coupling cap!  I also have a 47pf silver mica cap across the volume - the traditional Fender 'bright' cap.

Here, the old volume control is removed.

Couple of notes here:  you can see the end of the coax that runs back to the preamp second stage.  The braid is grounded at this end - only ground it at one end or you have a ground loop.

The braid goes to the ground side of the volume pot, and a lead there runs over to the preamp star ground.  You can see the disconnected black ground wire lying at the bottom.

The old pot was a metric-size Alpha.  I'm putting in a CTS pot, so I needed to enlarge the mounting hole a bit.  A few strokes with a round file takes care of it easily.

The new bypass cap is in place (green arrow).  It's spliced to a black wire that runs over to the switch on the back of the new pot.  The only shrink tubing I had that was large enough to go over the splice was pink-ish!  Looks a little unusual, but functional.

Note also the preamp star ground indicated by the blue arrow.  All of the preamp grounds - the inputs, the volume and the bypass - all connect here.  There is a similar power-side star ground on the other end of the amp.  This is just good practice to keep noise and hum down as much as possible.

The new pot in place.  See the two lugs at the bottom for the switch.  The lead from the bypass cap comes into the one on the right.  The left side goes to ground via the ground connection on the volume pot.

The input is into the pot via the cap on the right.  The output comes off the center lug, through the coax to the gird of the second triode in the preamp tube.  The shield ground, and the bypass-to-ground connection join at the volume control (left) ground lug, which then connects to the star ground.

When the pot shaft is pushed in, the switch is open.  The ground for the bypass cap is broken.  When the shaft is pulled out, the switch makes contact, the bypass connection to ground is complete, and the cap is in the circuit.  More gain and boost happens!  I also wound up putting the bright cap back on for a touch of  high-end 'sheen' to the clean tone.

After I put the chassis back in, I decide I might like a new (old) knob for the volume control.  Let me see if I have anything I like.  (This is a sampling...I have 3 or 4 more drawers' worth of knobs).

I decided to go with an old white Dakaware pointer knob that matches the amp well.

I also changed out the pilot jewel for a violet one.

I know, can't stop modding them!

 
 
 
 

Post a Comment 3 comments:

  • buddeshepherd said...
    May 2, 2013 at 1:02 AM
    I am impressed at your electronic skills. I've tried to teach myself but have not had a lot of luck. I lack concentration skills.
    I would like to know why a company goes to all the trouble to make a product and then doesn't make it correctly. Especially when it is a special product like a retro something.
  • Anonymous said...
    August 4, 2014 at 1:20 AM
    This is awesome!! I'd love to do it. Can you give me more info on the type of switch you used?
  • Yr Fthfl Blggr said...
    August 7, 2014 at 9:45 AM
    Thanks for reading.

    I used a pot with an SPST switch. I got it from Hoffman Amps. It's the Alpha pot on this page:

    http://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=PotsKnobs&ORDER_ID=441951292

    You need a 1 meg pot. You can use the CTS pot he sells, but the shaft on those is fluted for a push-on knob.

    You could also use a pot with a DPDT switch, but you don't really need those extra contacts for this mod. But that would work also.

    Hope this is helpful!

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