Time to take apart the Fender Champion 600 (Pics)

[joe-electro]

Anybody seen this before? Any idea what mods he's doing? Is he a member here?

http://www.youtube.com/watch?v=lsCB7qm1Gug&NR=1

 

[Guest]

I'm a new member and found the thread very helpful after I Googled it last month. Sorry if my hasty posting needed some more details and hope the following helps.

Mod A) alters the cathode bypass capacitor and turns the low frequency boost of the 22uF found in most Fenders into 1uF close to the mid-boost of a Marshall from the late 60's onwards. In practice it will take some of the bass off a Les Paul and stop the speaker rattling. I think it makes an external 12" speaker less bass dominated or boomy.

Mod B) effectively removes the tone stack, boosting the mids and gain. With the volume turned up the gain of the two halves of the 12AX7 is much much higher so that crunchy preamp distortion is possible. But the amp would be flat out so...

Mod C) is like turning down the master volume. It's a cut of -15dB, say 3 or 4 rather than 12.

Mod D) lessens the effect of the negative feedback loop from the output to the second preamp stage. It is a presence boost which makes the amp sound rawer and it will distort at lower levels.

Although B and C go together well, I prefer the mix of preamp and output tube/transformer breakup. Mod A and mod D give some variations to the stock sound.

Like I said the thread got me interested and like Matta I wanted to learn more about valve amps whilst trying ideas out.

 

[walter]

I don't like how close the output tube is to the power transformer. I would remove the pcb, move the power tube, and use chassis mounted tube sockets. How is this comming along Matta?

 

[CJ]

by the time you replace the output transformer, rip out the speaker,
ditch the pcb for point to point, move the 6v6 (wont help much)
and modify the circuit and change the tubes, what do you have
left?
A piece of chinese pressboard?

Better to start from scratch, build a yellow pine dove tailed cab,
floating 1/4 inch dry plywood baffle board, drill it for a 10 ich celestion/jensen/
bulldog and then add an old school aluminum
chassis, steal the p to p diagram off the web, get on evilbay for
some RCA/GE/Tungsol?Telefunken?Amperex?GEC flavor of the
week, and you have a killer amp that comes in at about the same
price point.

As it is, you are going to have yourself a 300 dollar, non
resalable albartoss that will never sound good because of the
cabinet.

I know, you are the parade, I am the rain, but maybe someone
else will learn before they make the same manuever.

 

[Rossi]

I have a Harley Benton GA5, which is 99 Euro at Thomann and is more or less the same as the Epiphone Valve Jr. It doesn't have a hum problem and is fairly low noise. Unlike the Valve Jr. it does have a tone control, albeit not a very usable or effective one. A tone stack mod would certainly be a worthy mod, listening to the sound samples "alnicomagnet" posted on youtube. http://www.youtube.com/watch?v=0et8Bn6oHAw

I haven't looked at the GA5 internals yet, but if it follows the Valve Jr schemo, the input resistors should be changed, for higher impedance. Then again, I mostly play the little brat with a treble booster or an EQ and a delay pedal in front. I got some very usable crunch sounds out of it; it is a nice amp for what it is. You can't expect a big, bass heavy Nu-Metal sound. But it's a fun piece of gear, especially if you like messing with pedal effects.

 

[rodabod]

[quote author="matta"]
What I am wondering about though as well is the choice or values of the resistors, in the original you had 2 x 250K pots with the Bass pot having a 15K to ground, now in this unit they are fixed at 75K but the 15K is now 180K! Why the huge jump?
[/quote]

Matt, are you sure it wasn't 18K? 180K doesn't sound right to me. Adding an eq. might be a nice idea. I'm not sure how Fender-ish you are wanting to keep this thing, but I'd maybe try something like a ~1.3uF cap in place of C10 if the distortion is a bit farty or rumbly. This will reduce gain at bass frequencies at that first stage so you'll be clipping more in the high end which I think sounds better. If you're interested, I'm working on how to get a reasonable rock sound from a simple 2-stage setup like this - I'll let you know how I get on.

[quote author="Rossi"]I have a Harley Benton GA5, which is 99 Euro at Thomann and is more or less the same as the Epiphone Valve Jr. [/quote]

I've just bought one of these - only £60. I'm going to keep mine cheap, but see what I can manage to get out of it. I've not been very impressed by what I've seen others manage so far to be honest. I had the Electar which was the first Epiphone tube amp (perhaps similar to the circuit in the Fender above?) and it sounded like crap. Farty, but with little low-end. I think a better output transformer might be a good idea.

Roddy

 

[Gus]

first thing I would do it hook the amp to a good speaker and cab to check it. If it does not sound a lot better I would start to question modding it because it might need a lot of changes.

FWIW Hook a good speaker and cab(a marshall 4x12 can be fun) to a small solid state practice amp, often there is big difference.

 

[joe-electro]

All this dirty talk about this little amp made me so hot I went out and bought one last night! I have to say that it's a sweet sounding little amp just as it is. I just plugged it in and made sure it worked out of the box so now it's time to proceed with the mods! The hum is definitely an issue so that will be the first thing I take care of. Next will be a modified tone stack and several values of bypass capacitors to change the low end characteristics. And of course, new tubes. I have several different Groove Tube 12AX7's lying around and I'll also try a 5751 just to watch it self destruct due to underbiasing.

I'll report my the results of my adventures back here soon.

:thumb:

 

[matta]

Hey Guys,

I ended up buying the docs for Nigel's Mods, the guy who has that video on YouTube... they work great!

He put a lot of effort into them, highly detailed, HI-RES photos and build guide that explains what he is doing for Newbies and just has a great command and report that anyone can do them, though they are P2P.

I don't feel it fair to share what they are or go into detail since I think at under 10 quid, considering they come on a CD with a lot of other valuable info it is more than fair to support him by buying them.

I will say that it covers the hum and buzz issues and I have almost NONE now and even my 'farty' speaker is gone, loving it cranked up now though will be getting a 1x12 cabinet to add to it.

The mods themselves are pretty comprehensive tackling the 3 main issues of the hum/buzz and also includes details for converting the Black Face to either the 5E1 or 5F1, the addition of a standby switch along with an additional Pentode/Triode switch.

The hum really made the amp unusable for recording, it is THAT bad, fine for a practice amp and messing about but I wanted to use it in the studio.

Well worth investing in if you want to great little recording amp.

Cheers

Matt

 

[Gus]

Was the hum fix the old trick of connecting the center of the heater supply to the "top" of the output cathode resistor or a divider string from the B+ set at about 40V? I think that is in a GT or some other amp book.

Overloaded the small speaker with low frequency reduce the coupling values I would start at the cap to the output tube grid first.
You can also play with the cathode caps on the 12ax7. If to much highs add a lowpass

 

[matta]

[quote author="Gus"]Was the hum fix the old trick of connecting the center of the heater supply to the "top" of the output cathode resistor or a divider string from the B+ set at about 40V? I think that is in a GT or some other amp book.

Overloaded the small speaker with low frequency reduce the coupling values I would start at the cap to the output tube grid first.
You can also play with the cathode caps on the 12ax7. If to much highs add a lowpass[/quote]

Hey Gus,

No it wasn't, which is prob. where I would have started... in fact I don't think any of the his mods have been mentioned here before which was rather interesting...

I wish I could say more :? But feel it wouldn't be right.

Anyways, for those that are interested here is a link to his Auction for the instructions on Ebay, there are also a couple pics, so if you want to 'cheap' out you might be able to gather 'some' info.

Alnicomagnet Mod CD for Fender Champion 600 Tube Amp

Cheers


Matt

 

[rodabod]

Hey Matt, that sounds pretty successful.

If you want to have a taste of what changing some of the decoupling/bypass caps do at the first stage of the preamp, you can get an idea by using an eq. pedal and boosting the midrange upwards - it doesn't make things sound as thin as you might think, although it may do with that smaller speaker I suppose.

Talking of simple amps, I'm not sure if many of you have seen the Badcat Minicat?

http://www.schematicheaven.com/newamps/badcat_minicat.pdf

There's a couple of nice ideas going on there - for instance at the speaker output with the normalled resistor load and line outputs. I think it wa sthe name which really grabbed my attention though.

 

[Swedish Chef]

[quote author="walter"]I don't like how close the output tube is to the power transformer...[/quote]

I am feeling a similar thing but with regard to the OT.
In my limited experience you want the power and output iron as far apart as possible.
Perhaps if you get a chance, take the OT mounting screws out and just rotate it 45deg in the x-plane. See if the hum changes/ reduces. That's a good clue as to whether your residual hum is iron related...


chef

 

[matta]

[quote author="Swedish Chef"][quote author="walter"]I don't like how close the output tube is to the power transformer...[/quote]

I am feeling a similar thing but with regard to the OT.
In my limited experience you want the power and output iron as far apart as possible.
Perhaps if you get a chance, take the OT mounting screws out and just rotate it 45deg in the x-plane. See if the hum changes/ reduces. That's a good clue as to whether your residual hum is iron related...


chef[/quote]


Hey Steve!

You just got VERY warm, actually, ON FIRE.... Hah Hah... I hope you and the family are well.

Matt

 

[Rossi]

Groove Tubes do a similar thing in The Brick. The power transformer is fixed, but the input and output transformers are mounted in a metal loop; and then they rotate them for minimum hum. At least that's what they intend to do; the one I got has a pretty bad hum at high gains.

 

[Swedish Chef]

[quote author="matta"]Hey Steve!

You just got VERY warm, actually, ON FIRE.... Hah Hah... I hope you and the family are well.

Matt[/quote]

I think that's called The-Law-Of-Averages or something. If I say enough, at some point some of it must be right/ useful!

And yep, we're all doing well thanks. Likewise you and yours :grin: :thumb: :sam: :sam:

chef

 

[peterc]

Matt, try this:

Take out the toobs & switch on. Listen for hum in the speaker.

If there is any, then isolate the transformers from the chassis using plumbing washers. What is happening is that the mains is being transmitted through the chassis to the o/p transformer.

I had some AC 30s that I fixed in this way & they were VERY quiet.

Piet

 

[Guest]

[quote author="Swedish Chef"][quote author="walter"]I don't like how close the output tube is to the power transformer...[/quote]

I am feeling a similar thing but with regard to the OT.
In my limited experience you want the power and output iron as far apart as possible.
Perhaps if you get a chance, take the OT mounting screws out and just rotate it 45deg in the x-plane. See if the hum changes/ reduces. That's a good clue as to whether your residual hum is iron related...


chef[/quote]

I can't believe how big a difference it has made. Almost no hum. Thank you. :grin:

 

[rodabod]

Well, I just got my Harley Benton (God, I need to get rid of that logo...) GA5H (yes, code word for gash) amp the other day which has already been under the iron.

It managed to develop a fault within about five minutes of first plugging in which bascally caused the volume to jump up and down between minimum and maximum and normal. When the volume suddenly went to max, the volume pot did nothing which gave a clue as to what was wrong.

So, I opened it up and removed the volume pot and associated board to have a look at. I shit you not, they had forgotten to solder the volume pot onto the board. The pot's three terminals were just sitting in the board and occasionally conducting with the solder pads when moved or vibrated. unbelievable.

The amp's eq is verging on useless so I may have to change that a little, and possibly reduce some of the decoupling/bypass caps to reduce bass, but otherwise it's ok for £60. I am soon going to probably add a more serious preamp and a 6V6 output though - I'm not sure I like the "chimey" sound of the EL84s when they overdrive so I thought a 6V6 might be worth a shot.

 

[Gus]

Maybe the difference of SE champ type amp 6V6 vs EL84 is the output tube gain difference. Maybe using a 12AT7 and playing with the cathode bypass caps in the preamp one can work with the EL84.

Maybe it would be interesting working with the stock output transformer. Max input low end level.