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Old January 18th, 2013, 06:17 PM   #1 (permalink)
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Importance of resistor ohm and wattage? Champ 600

This amp was purchased used from a guy who modded it for higher gain. I did not want that sound so I replaced the resistors he said that he removed/changed but now I am finding more that are not stock upon closer inspection.

R3: 1k instead of stock 1.5k (Does wattage rating matter on these if it fits?)
R7: 2k instead of 2.2k
R10:1W instead of 2W
R12:1W instead of 2W

Should I bother changing any/all of these to exact stock spec or will they make no difference from the resistors currently in there?

Fender's stock schematic

Thanks
-J

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Old January 18th, 2013, 10:02 PM   #2 (permalink)
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Quote:
Originally Posted by RatherBeFishing View Post
This amp was purchased used from a guy who modded it for higher gain. I did not want that sound so I replaced the resistors he said that he removed/changed but now I am finding more that are not stock upon closer inspection.

R3: 1k instead of stock 1.5k (Does wattage rating matter on these if it fits?)

Use a 1/2 watt resistor. Replace with 1.5K

R7: 2k instead of 2.2k
This is OK



R10:1W instead of 2W
R12:1W instead of 2W

Replace these two with 2 watt resistors.

Should I bother changing any/all of these to exact stock spec or will they make no difference from the resistors currently in there?

Yes, change them.

Fender's stock schematic

Thanks
-J
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Old January 19th, 2013, 12:51 PM   #3 (permalink)
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R3: 1k instead of stock 1.5k (Does wattage rating matter on these if it fits?)
R7: 2k instead of 2.2k
R10:1W instead of 2W
R12:1W instead of 2W

R3 = cathode resistor lowering from 1.5k to 1k should add a bit more, but nothing drastic. 2w to 5w recommended for current.

R7 = NFB (negative feedback) resistor. Changing from 2.2k to 2k will make little to no difference. Higher value resistance will clean up signal, but lower high frequency. Too high of value may damage OT. Removing the NFB circuit will add gain and frequency as is safe to do, better yet make the circuit switchable.

R10= PA cathode resistor. Going to a 5w is better to handle current, but 2w is fine. Do not go lower than 2w. You can always increase wattage rating for any resistor, just don't go below spec or may fry.

R12= PSU filter dropping resistor. Again, can go up but not down in wattage from specs.

If you are still searching for a cleaner sound you can raise either or both R2 and R3 cathode resistors to slightly higher values (2.2K or 2.7K). Raise R7 to 6.4K or other value between 2.2k and 6.4K.
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Old January 19th, 2013, 01:35 PM   #4 (permalink)
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Hackworth: I really appreciate the info. I'm surprised you know these 600s so well after seeing those badass kits you offer. Why would you ever touch one of these!?

Jebrone (great name!): Thanks for taking the time to explain what these all do. I'm pretty sure R3 and R7 are meant to be 1/2w or less just based on space constraints and what I pulled out of there.

Its good to know that I can go with 5 watt resistors because the Shack doesn't do 2 watt.

I performed some mods to it myself that Alnicomagnet sold on ebay and its worked for more than a year with these odd resistors in it. Hopefully nothing has been compromised in it thus far.
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Old January 19th, 2013, 01:40 PM   #5 (permalink)
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Uggh, radioshack doesn't have 5 watters in store. Where do you guys like to buy your resistors? So far my choices are $9 shipping or a 40 mi drive (each way) for what I need. I might as well figure out what I need to fix up my old univox u255r as well before doing either of those.
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Old January 19th, 2013, 02:04 PM   #6 (permalink)
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If they have higher wattage of the same value, get that. I used a 7 watter in my 5e3 as I couldn't get 5
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Old January 19th, 2013, 02:47 PM   #7 (permalink)
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Well, looking over the schematic's notes it describes all resistors as 1/4 watt unless otherwise noted. 1/2 watt or 1 watt for the preamp (only) cathodes should be fine, hopefully the (metal) legs of the resistors will not be bigger than the assigned PCB holes. If you buy them at Rat Shack they should fit fine.
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Old January 19th, 2013, 06:26 PM   #8 (permalink)
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Make a list of what you need and PM me. Will be $5 for CONUS shipping.
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Old January 19th, 2013, 09:21 PM   #9 (permalink)
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Make a list of what you need and PM me. Will be $5 for CONUS shipping.
Thanks, I really appreciate the offer but I already made an order from digikey earlier today.

How much do you guys usually pay for 600v caps? I was surprised how much they wanted for them at Digikey, I don't recall ever paying that much for Orange Drop or Mallory caps for guitar wiring.
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Old January 19th, 2013, 09:42 PM   #10 (permalink)
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www.tubesandmore.com offers fair prices on Mallories and OD's.
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Old January 19th, 2013, 10:16 PM   #11 (permalink)
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Im just not sure if its worth all the time and energy. I have two and bought both for $100. Maybe just keep an eye out for an untouched one and resell the modded one?

Anytime I hear an amp has been modded, alarms start going off in my head. Did he know what he was doing or am I inheriting someone's headache!? Is this thing going to electrocute me or burn down my house!? I know I am being a bit dramatic but hey you never know lol.

Speakers and tubes I understand, but if they start screwing around with the circuit, its game over for me.
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Old January 19th, 2013, 11:59 PM   #12 (permalink)
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I purchase most components from this company http://www.westfloridacomponents.com/Capacitors.html buying radial caps instead of axial saves quite a bit.
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Old January 20th, 2013, 12:43 AM   #13 (permalink)
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Originally Posted by RatherBeFishing View Post

R3: 1k instead of stock 1.5k (Does wattage rating matter on these if it fits?)
R7: 2k instead of 2.2k
R10:1W instead of 2W
R12:1W instead of 2W

Should I bother changing any/all of these to exact stock spec or will they make no difference from the resistors currently in there?
R3 is a cathode resistor. Having 1k instead of 1k5 biases the stage a little warmer (for earlier clipping). It can easily be 0.25W and still be fine.

R7 is the global NFB resistor. 2k2 versus 2k is much of a muchness. (It just adds a touch more NFB to the pre-amp than 2k2 does).

R10 is the 6V6 cathode resistor. With 22V across 470 Ohms, it will be seeing 46.8mA tube current, and will be dissipating 1.03W. For safe operation, the power rating of the resistor should be at least 2 x what the expected dissipation is (i.e. should be at least 2W rating).

R12 (on the other hand) is dropping 20V, but across 10,000 Ohms, so it is only passing 2mA, and is therefore only dissipating 0.04W. So a 0.25W rating will be fine on that resistor (as will a 1W rating).
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Old January 20th, 2013, 01:03 PM   #14 (permalink)
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From what I'm gathering, R3 was hurting clean headroom and R10 could have eventually resulted in disaster.

This amp sounds horrible (to my ears) through its 6" Jensen Mod once the dial hits 12 O'clock. I took the chassis out for the first time in years just to change the grill cloth. I sure am glad that I asked about this stuff.

Are there any resistors in particular that benefit from changing to metal film? It isn't very noisy after the alnicomagnet mods but every little bit counts. I like recording with this amp.

Finally, I ordered the components to change to 5E1 voicing.
Involves removing: R11, R12, R18-22, C1, C2, C8-10
Adds: 10k 2W Res, 22k 2W Res, and 2 .022uf/600v Sprague ODs

I'm not sure if I will try it out. Will it have any ill effect on the ability of the amp to take pedals? I only have a few that I ever use with it.... these included a Boss Digital Delay 3, Johnson J-Station (Effects only w/ bypassed modeling) and Radial Tonebone Hot British. I'd love to make this thing turn into an SF vibrochamp at will.
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Old January 20th, 2013, 01:22 PM   #15 (permalink)
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Best mod for that amp is a is to use an extension cab with 1 x 12" or 2 x 12". that little 6" speaker will never provide enough bass. Speaker/cab combination is as important, if not more so than the amp itself.
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Old January 20th, 2013, 02:29 PM   #16 (permalink)
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Best mod for that amp is a is to use an extension cab with 1 x 12" or 2 x 12". that little 6" speaker will never provide enough bass. Speaker/cab combination is as important, if not more so than the amp itself.
Yeah, I ran it through 2 x 10 alnico blue frames the other day and it sounded great. Still a lot of people recommend a 6" speaker upgrade from stock. If that is true, I'm not sure why anyone would bother.
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Old January 20th, 2013, 06:18 PM   #17 (permalink)
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Jebrone Lames wrote: "R7 = NFB (negative feedback) resistor. Changing from 2.2k to 2k will make little to no difference. Higher value resistance will clean up signal, but lower high frequency. Too high of value may damage OT. Removing the NFB circuit will add gain and frequency as is safe to do, better yet make the circuit switchable."

If one reads these staements carefully, one will realize that there are conflicting claims there. IT is true that changing from 2.2K to 2K will make virtually no difference.
The next statement is incorrect. An increase in the resistance in this NFB loop will decrease the effect of the loop-- in effect increasing the output of the amp (decreasing headroom)and enriching harmonics. JL actually confirms this in his last statement "Removing the NFB circuit will add gain and frequency as is safe to do, better yet make the circuit switchable." REmoving the NFB loop in effect yields infinite resistance....since there is no continuity in the loop..hence total resistance and a result of NO EFFECT regarding a NFB loop. A NFB loop is built in order to cancel some of the circuits output by being 180 degrees out of phase with the circuit's signal. IT cleans an amp's sonics up.
This statement by JL also conflicts with the observation that "Too high of value may damage OT"....since total lack of continuity in this NFB loop is an infinite resistance. Many amps have no NFB loop, and live long lifes. THere can be instability in certain circuits with no NFB loop or---in other words----with excessive resistance....but damage???? Don't tell that to the owners of Vox AC-30's.
I regularly install NFB adjustment pots in BF/SF FEnders to yield BIGGER sonics if wanted.....50K pots in series with the stock 820ohm resistor. When such an adjustment is available, it is undeniable that as one increases resistance, the NFB loop's effect is decreased and the sonics get bigger...gain increase.
IF one were to install a swtich to lift that NFB loop in this amp in this thread..as Jebrone suggests....the change from 2K resistance to infinite resistance will result in a bigger sonic....and maybe an unstable circuit...maybe not.
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Old January 20th, 2013, 07:08 PM   #18 (permalink)
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Best mod for that amp is a is to use an extension cab with 1 x 12" or 2 x 12". that little 6" speaker will never provide enough bass. Speaker/cab combination is as important, if not more so than the amp itself.
i put a little 60s alnico 6 incher in mine and it brightened considerably. overdriven its a bit boxy but is sounds huge overdriven in recordings.
i also leave the back off mine.
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Old January 20th, 2013, 09:13 PM   #19 (permalink)
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i put a little 60s alnico 6 incher in mine and it brightened considerably. overdriven its a bit boxy but is sounds huge overdriven in recordings.
i also leave the back off mine.
During recordings one can add external frequencies to make it as big or as boxy as desired and then played back over superior speakers or headphones to further improve the original sound. There is really no comparing a 6" speaker to a 12" speaker, but if an individual is satisfied with the tone regardless of speaker size that's all that matters.
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Old January 20th, 2013, 09:20 PM   #20 (permalink)
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Originally Posted by Wally View Post
Jebrone Lames wrote: "R7 = NFB (negative feedback) resistor. Changing from 2.2k to 2k will make little to no difference. Higher value resistance will clean up signal, but lower high frequency. Too high of value may damage OT. Removing the NFB circuit will add gain and frequency as is safe to do, better yet make the circuit switchable."

If one reads these staements carefully, one will realize that there are conflicting claims there. IT is true that changing from 2.2K to 2K will make virtually no difference.
The next statement is incorrect. An increase in the resistance in this NFB loop will decrease the effect of the loop-- in effect increasing the output of the amp (decreasing headroom)and enriching harmonics. JL actually confirms this in his last statement "Removing the NFB circuit will add gain and frequency as is safe to do, better yet make the circuit switchable." REmoving the NFB loop in effect yields infinite resistance....since there is no continuity in the loop..hence total resistance and a result of NO EFFECT regarding a NFB loop. A NFB loop is built in order to cancel some of the circuits output by being 180 degrees out of phase with the circuit's signal. IT cleans an amp's sonics up.
This statement by JL also conflicts with the observation that "Too high of value may damage OT"....since total lack of continuity in this NFB loop is an infinite resistance. Many amps have no NFB loop, and live long lifes. THere can be instability in certain circuits with no NFB loop or---in other words----with excessive resistance....but damage???? Don't tell that to the owners of Vox AC-30's.
I regularly install NFB adjustment pots in BF/SF FEnders to yield BIGGER sonics if wanted.....50K pots in series with the stock 820ohm resistor. When such an adjustment is available, it is undeniable that as one increases resistance, the NFB loop's effect is decreased and the sonics get bigger...gain increase.
IF one were to install a swtich to lift that NFB loop in this amp in this thread..as Jebrone suggests....the change from 2K resistance to infinite resistance will result in a bigger sonic....and maybe an unstable circuit...maybe not.
Thank you for correcting me Sir. No global NFB = higher output and frequencies. Global NFB with a higher value NFB (22k-64K on + 8 ohm tap) resistor = cleans amp up but lowers frequencies.
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