#1
Amp: Fender Champion 600 RI

Problem: sounds bad overdriven. The low end is like a bad fuzz and the high end becomes extremely shrill. This happens from volume at halfway till fully cranked after the mods. Before the mods it sounded even more bad.

Tried solutions:

I tried the mods after the power section of the amp were repaired in the service of a shop where i bought it.

- So i had a new output tube, a JJ 6V6 from the service.
- They installed a new output transformer, but its the same shitty, tiny stock one, named Fender 0022905000 (the number of the zero's are varies from source to source, its the 22905 what matters)
- I put a new preamp tube in, a Sovtek 12AX7.
- I bought a new speaker, a 8 inch Jensen C8R.
- I tried the new speaker out without a cab - shitty stock cab's vibrations eliminated
- Removed some excessive bass from the circuit with a switchable cathode bypass cap, stock 22uf or 2,2uf. - maybe thats not enough, dunno.
- I tested it with the negative feedback resistor removed.

The guitar was a Cort G260, some said its comparable to an american strat, some said its better.

When i cranked the amp this way (without cab and the stuff described), it still sounded kinda bad, well of course it sounded crap without a cab but the overdriven sound was still not really good, the bass was like a bad fuzz and the highs became really shrill.

As you can see, i swapped the more important parts of the amp, which i could think of causing the bad cranked sound. But its still bad.

So i think we could narrow it down to the output transformer and the circuit itself.

Now the amp, according to its schem, is like a Priceton/Champ Tweed with a fixed (no pots, just resistors, but its the classic bass-mid-treble tonestack.) Fender tonestack between the two halves of the preamp tube.



So maybe, the bypassing of the tonesatck with a 0,022 cap like in the old Fenders and a new output transformer could solve my problem?
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Last edited by Daneeka at Nov 17, 2009,
#2
A lot of the problem is the speaker imo. Anything less than 10" is kinda iffy to me.

What is your cathode resistor and cap for the power tube, plate voltage?

Bypassing the tonestack will greatly increase gain and mids, I don't think that will help you. Sovtek tubes aren't great unless they are the LPS, even then they do best in the PI imo. For a smooth neutral tone you might want to try out the Mullard Reissues.

EDIT: What are your voltages on the preamp tube's plates? They like they would be a bit high to me. If it was me I'd try to get them to around 160V DC. That's what a lot of Tweed's ran. My 5E3 is like 155V I think. Very warm, with not the best headroom, very jazzy.

EDIT2: Might consider replacing the fixed resistors in the TS with trimpots. Find your sound, then set it an forget it.

EDIT3: It looks like you have a feedback ratio of 48:1 which is quite high for an amp like this. I would seriously consider playing with that 2K2 resistor coming off your output being fed by to the driver tube. I'd use a 2K pot and find a good setting. Maybe removing the NFB all together would be best. I don't think it's doing a tiny amp like this any good.

And get rid of the cheap mylar film caps and multilayer cermaics. I'd use some Mallory's to warm this amp up. Don't use silver mica for the pF caps.
Last edited by kurtlives91 at Nov 17, 2009,
#3
Quote by kurtlives91
A lot of the problem is the speaker imo. Anything less than 10" is kinda iffy to me.

What is your cathode resistor and cap for the power tube, plate voltage?

Bypassing the tonestack will greatly increase gain and mids, I don't think that will help you. Sovtek tubes aren't great unless they are the LPS, even then they do best in the PI imo. For a smooth neutral tone you might want to try out the Mullard Reissues.

EDIT: What are your voltages on the preamp tube's plates? They like they would be a bit high to me. If it was me I'd try to get them to around 160V DC. That's what a lot of Tweed's ran. My 5E3 is like 155V I think. Very warm, with not the best headroom, very jazzy.

EDIT2: Might consider replacing the fixed resistors in the TS with trimpots. Find your sound, then set it an forget it.

EDIT3: It looks like you have a feedback ratio of 48:1 which is quite high for an amp like this. I would seriously consider playing with that 2K2 resistor coming off your output being fed by to the driver tube. I'd use a 2K pot and find a good setting. Maybe removing the NFB all together would be best. I don't think it's doing a tiny amp like this any good.

And get rid of the cheap mylar film caps and multilayer cermaics. I'd use some Mallory's to warm this amp up. Don't use silver mica for the pF caps.


Hi!

According to the schematics, the output tube's cathode resistor is 470 ohms, the plate voltage is 175V, bypass cap is 22uF. I can measure the voltage tomorrow.

How can i decrease the plate voltage? Also i heard that the amps B+ voltage is too high in itself in this amp, so maybe i should decrease the whole "supply" voltage.

I made a mistake about the tube, its not a Sovtek, it is a Tung-Sol.

I will change the tonestack's resistors to pots, i already ordered the pots, but they are too big to fit in, so i have to wait until i make a big enough order list to make up for the ~30-40$ cost of shipping, money transferring etc. and i gonna order smaller pots.

I have a switch for the NFB, it have a stock (2,2k) setting, something i forgot, (around 1k) and a disconnected setting. I disconnected it for the test. Maybe i should try the 1k setting?


Where should i change the caps? Only in the signal path? (tonestack, coupling caps)

Thank you for your detailed answer!


(one more thing: is it possible that my powertube is misbiased? i dont really trust in that shop where i bought this amp)
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Last edited by Daneeka at Nov 17, 2009,
#4
Your output tube's plate voltage is most definitely not 175V, check that. I am thinking your output tube is misbiased, that might explain some of the issues your having. I would also increase that 22mF cap on it's cathode to 220mF.

An easy way to decrease plate voltage on the preamp tubes would be to increase the 10K dropping resistor feeding the plate resistors. Try 15K, 22K. I would keep the output tubes plate and screen hot.

I would change all coupling caps(yeah tonestack caps are coupling caps). And ya I would decrease the NFB, either 1K or off.
#6
Quote by kurtlives91
Your output tube's plate voltage is most definitely not 175V, check that. I am thinking your output tube is misbiased, that might explain some of the issues your having. I would also increase that 22mF cap on it's cathode to 220mF.

An easy way to decrease plate voltage on the preamp tubes would be to increase the 10K dropping resistor feeding the plate resistors. Try 15K, 22K. I would keep the output tubes plate and screen hot.

I would change all coupling caps(yeah tonestack caps are coupling caps). And ya I would decrease the NFB, either 1K or off.



Ok, i gonna measure a few voltages tomorrow, just one thing, should i look for DC voltage with the DMM everywhere?

My switch can disconnect the NFB and i like the amp better that way so that's prove your point about the NFB resistor.
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#7
Ok, i measured a few voltages:

Here is the schem i used:

http://www.schematicheaven.com/fenderamps/champion600_reissue_fullmanual.pdf

All my measurements are DC:

TP2: 218VDC (First input triode plate voltage)
TP6: 1,63VDC (First input triode cathode voltage)
TP5: 218VDC (Second input triode plate voltage)
TP7: 1,7VDC (Second input triode cathode voltage)

TP9: 347VDC (Output tube plate voltage)
TP8: 21,4VDC (Output tube cathode voltage)

TP10: 336VDC (B+ after two resistors)
TP11: 355VDC (B+ after one resistor)


According to the schematics, those voltages are right. Except the input triode voltages which are not referenced in the schematics but you wrote that they should be around 160VDC and mines are 218VDC. Could this be the problem?
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#8
Update: /problem basically solved/

It looks like the power tube were factory misbiased, despite that the amp basically needs no biasing.

The stock 470 ohm cathode resistor is too small, i installed a 680 ohm resistor and now it sounds much better cranked, most of the harshness and low end fuzziness (rattling, farty tone) gone.

Someone described the background of the problem as Fender installed a more powerful power transformer in the ancient times when they upgraded their Champ line without the rebiasing of the power tube, so the waveform of the sound started to clip assymetrically or something like that.

A question: the factory schem describes the resistor as a 2W resistor. If i install a 1K or 2K 5-10W pot in place of the resistor, can i dial in the best sounding cathode resistor setting for the amp while playing? So i crank the amp and adjust the pot till it sounds best.
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#9
Hey, sorry I can't answer your question but I have some of the same problems you do with my Champ. You said that essentially switching out the 470 ohm resistor and making sure the power tube is properly biased fixed most of it? Or is that just in conjunction with the other mods you did?
Gear
Highway One Tele (w/Custom Shop 51 Nocaster pickups)
Standard Tele (modded to Nashville specs)
Reverend Roundhouse

Orange Rockerverb 50 MKI
Vox AC4c1
Jet City JCA20H

And pedals!



"Shiva opens her arms now..
...to make sure I don't get too far"
#10
Quote by barden1069
Hey, sorry I can't answer your question but I have some of the same problems you do with my Champ. You said that essentially switching out the 470 ohm resistor and making sure the power tube is properly biased fixed most of it? Or is that just in conjunction with the other mods you did?



Probably it's a sum of all the mods so i can't say that it could single-handedly solve the problem but i would say that the 3 most valuable mods are the speaker swap, the stock output tube swap and this cathode resistor mod. All of the three could cause the described problem.

However i strongly recommend that every owner of this amp should change the speaker cloth, its horrid. It's too thick so it flaps when the amp is cranked. I still try to find a good fabric so mine looking bad because i just did some cuts on the cloth but it is like lifting a blanket off the speaker without the stock cloth.

http://akavalve.blogspot.com/2008/12/opening-fender-champion-600-for-modding.html

Here's an awesome modblog.


Ohh and be very ****in careful, that shit can kill you so plug off the amp, discharge caps, don't touch anything conducting etcetc.
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Last edited by Daneeka at Nov 24, 2009,
#11
Hi everyone, hope someone can shed some light on my little dilemma. I bought my Champ 600 brand new a couple of weeks ago, and I cannot stress how much I love it.
However, not a week went by and the little guy shut down in mid playing session (about 35 to 45 mins into it), and wouldn’t come back on. Volume was kept at about 8-10. It didn't show any signs of trouble before this happened. It did get pretty warm, but it's my first tube amp and I don't know how normal this is or how warm is normal. The next day it was still dead, so naturally I took it back to the shop, but when the tech guy had a look at it, it all worked fine (of course!). He looked inside and said to have checked absolutely everything and that nothing was wrong with it, as I could clearly see, and he plugged in a guitar and gave it a test run.
So...
I get back home, and (of course!) it does it again, shuts down just 5 mins into my playing session, but this time, it comes back on almost instantly. The rest of the playtime went without any glitches (1 hour aprox.).
That same evening got together with some friends and out came the Champ (which I display very proudly!), and as to be expected it did it again. However this time it gave more clues (or at least I think that’s what they were) as to what the problem is. It did it twice and both times the volume went down slowly until the red light went out.

I love my Champ and I really want this relationship to work! Can someone please advise what the cause of the problem might be (because the tech guy seems to be out of ideas).

Together with the Champ I was using:
Cort Z-Custom
iPad with AmpKit, Amplitube and GarageBand
AmpKit Link
iRig
Cort Noiseless cables
Generic cables