Fender Champ mods part 2 - screen resistor
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- Опубликовано: 8 сен 2024
- The Fender Champ has long been a small tube amp favorite. It's a great-sounding five-watt amp that works well not only as a practice amp, but also for recording. Its simple circuit lends itself well to beginning the study of tube amps in general. There have been many versions of this amp over the years and comparing the schematics and studying what changes were made, why, and the effects they had will reveal many principles of tube amp design and function. In this video I discuss the strange lack of a screen grid resistor on the power tube of Champs and its effect. There is a before/after comparison at the end of the video that allows you to hear the amp without and with the screen grid resistor, back to back.
Please keep in mind that tube amplifiers harbor lethal voltages inside and that if you do not have experience working with them or are not familiar with the proper safety procedures involved, you should not open your amplifier. The risk of deadly electrical shock is present even when the amplifier is unplugged from the wall outlet and switched off.
French subtitles available soon.
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Thanks for that tip, the amp sounds fuller and lesd anaemic than before. Let us know of any other "Tips" for us amp nerds...please, it helps us understand why things sound and work. Good job. BTW, can this be added to EL84 valve bases?
Thank you for your comment. This mod is very specific to the Fender Champ amp, because its circuit lacks a screen resistor on the power tube. Other amps may already have one and may not need this mod.
Thankyou. 470 Ohm resistor to the rescue.
I had a well known amp tech do a "tweed" mod on my silver face Champ and it sounded great for a couple of weeks and then quit working. I found some cap or resistor that he added to it had exploded. I put it back to original specs.
I'll have to check your mod out.
The mod in this video is just the addition of a screen resistor on the power tube. A huge improvement. Someone here pointed out that since this is a Vibro Champ, it would be better to move the resistor to another place in the circuit. I still have to try that. For the "Tweed" mod, I just followed Gerald Weber's instructions from one of his books.
@@charlesdespres
I've got one of Weber's early books, but I can't remember which one? I think it's the one with all the different amp techs mentioning basic Fender mods and there are schematics in the rear of the book. Forward by Ken Fischer.
I did a separate Tweed mod to the Champion 600 reissue and swapped the speaker for a Ted Weber WeberVST speaker, nos tubes and that little thing screams. 👍
@@user-tb9rw7dh2t Yes, I've got the book you are referring to, and the title is: "A Desktop Reference Of Hip Vintage Guitar Amps". The book that has the Tweed mod in it is: "Tube Amp Talk For The Guitarist And Tech", also by Gerald Weber. I'm sure your Champion 600 is really cool!
@@charlesdespres
Thanks
The screen can have 30-35V more on it than the plate without detriment, lots of Fenders have higher screen voltage than plate voltage.
220-470R on the screen wont make much audible sonic difference, 1K and above there can be some perceivable compression. That's a fact leveraged in high gain amps where the designer wants compression to come on sooner across the gain range.
The screen resistor at 220-470 protects the tube and the output transformer, happliy so.
Thank you for your comments. However, I understand that for the tube to work properly, more electrons should go to the plate rather than to the screen. Theory-wise, it does not seem to be a good idea to have a higher potential on the screen than on the plate. In practice, I can clearly hear that the amp is much louder with the screen resistor in place.
@@charlesdespres And yet every Fender amp with screen voltage higher than the plate voltage works great as designed, as built, as played and very often sounds great. Bcuz??Screen current is a fraction of plate current even if the screen voltage is somewhat higher. Those electrons fly right by on their way to the plate.
Bon appetit!
@@oldasrocks9121 Fender amps certainly are great. In the case of my amp, I have found the addition of a screen resistor to be a great improvement.
If you measure bias -- it's probably quite stable, but only until you turn it all the way up and play it hard. Then your 20VDC on the output tube cathode will swing above 30V -- it's at that point, and your clip proves it, that the screen resistor makes a difference. I was on the point of installing it in my Champ... I discover this trick in .... the Champ reissue schematic (!!)
I did not measure the bias, but all I know is the amp is much louder now that it has the screen resistor. Do you mean that the Champ reissue HAS a screen resistor from the factory?
The silverface amps tend not to distort very musically. It's that, it's going to blow up kind of sound
Yes, there is that tendency... but they all distort differently, too... for example, a single-ended amp like the Champ distorts very differently from a push-pull amp. This particular amp I have has been modified to the fifties Champ circuit.
I haven't plugged into a silver face and over 25 years. I should get one and refresh my memory they're probably better than I remember. In the 80s you could pick these up so cheap.
Many of them can be modified and made to sound better (they can be "blackfaced", for example... and the earliest ones sometimes still have the Blackface circuit, just with different cosmetics).@@stringlocker
No one takes in consideration how resistor watts and ohm affect amp. We all know cathode 5watt 470 ohm but with 120volts schematic designed for 110 doesn’t work. But if you put 470 ohm 3 watt cathode resistor it changes bias. Same resistor on 6v6 gives your amp a bit of spice and volume.
Same with 10k and 22k resistors for 5f1 amp it can be 1/2, 1, 2, 3 watt for 22k
1,2,3 watt for 10k it affects bias, hum level tone and volume of amp.
Carbon composition or oxide metal film resistors affect differently level of head room, tone and level of hum also.
Look at schematics for Fender 57 champ RI it said schematic for 120volts!!! To bias amp is used 22uf 450v capacitor with 470ohm 2watt oxide metal film resistor!
I thought watts only had to do with the power a resistor can take before it burns up...? I thought any wattage was fine as long as it was enough. I do not understand how it can affect tone, except maybe the difference in physical size could make a component more prone to certain phenomena...?
@@charlesdespres there is metal oxide film resistors- modern, and carbon composite film resistors-vintage. I like carbon they hard to find and expensive eBay is the source. Example cathode 470ohm 1 watt ( fender vintage proper one) makes your amp sound way much properly than 470 ohm 5 watt.
Hi again, let me make a suggestion. The screen now is going to now have voltage changes on it since there is no capacitor to smooth it, essentialy it behaves kind of like a plate. The white wire that is still attached to pin 4 (screen) goes to feed the 12AX7 plates of the vibrato circuit, essentialy making its power supply unstable (there will be inverted signal type of fluctuations on its power). You should try the vibrato and see if there is much change and if you like it or not, it may be interesting. other than that, for higher gain on tetrodes/pentodes there needs to be a capacitor between the screen and cathode (or ground). As with a high signal at the control grid the voltage of the screen will get lower, and with a low signal it will get higher (behaving like a plate), however since this still is a grid this will result in a lower gain (since it is going opposite of control grid). My suggestion is to have the schematic in front of you for this part , now as you can see the factory did use a screen grid resistor (1kΩ) and it is decoupled by a 20μF capacitor, this resistor also feeds the plates of the vibrato 12AX7, and though another 10kΩ and 20μF capacitor the preamplifier 12AX7. My suggestion is to put the 470Ω in series with the 1kΩ (this won't look pretty so you can use a 2W 1.5kΩ resistor in place of the 1kΩ) and connect the rest of the ciruit the way it was. I can send you a schematic if you got confused or want to be sure.
Lastly don't be put off by this at all, nobody was born to know everything. I like the fact that you are experementing with your equipement, trying to learn more on how it works in the process, in my opinion more people should. I may know a lot on tubes but trust me your ears may bleed if you get to hear me scratch a guitar (so i only use tubes for media sound amplifiers hi-fi etc).
P.S. this thing happens because of the manufacturing tolerances of the tube (it depends on how well aligned the two grids are), some draw more sreen current lowering the voltage on the screen grid even through a 1k resistor while others need more resistance . theoriticaly the less screen current a tube draws is better, however that may be a sign of a weak tube, but 1k for a 6v6 with 2 12ax7s seems too kind of low from my experience, so you got nothing to worry about. If you need further explanations or definitions feel free to ask.
That is very interesting. It took me a while to respond, because I had to find the time to read what you said carefully and study the schematic. I think I understand what you mean. Adding a capacitor to the screen achieves the same type of thing as adding a bypass capacitor to the cathode resistor, right? But I thought plate voltages did not fluctuate. Also, this amp being single-ended, I thought it was class A and therefore the conditions at the power tube did not vary. Something must escape me.
Oh wait, I'm starting to realize that if there is some screen current, electrons, having a hard time going through the resistor, are going to accumulate on the screen side of the resistor and create a negative charge, thus lowering screen voltage. But I thought screen current was supposed to always be very low, especially if the screen has the right voltage on it, because electrons would favor the plate.
Speaking of the vibrato, I think it started thudding after I did the mod, so maybe it is a side effect of adding the screen resistor the way I did it, making the vibrato tube plates unstable. Do you think that is a possibility?
Adding the 470 Ohm resistor the way I did it already made a huge improvement in the loudness of this amp, but the subtleties you have told me about make me want to try moving the resistor to between the 1KOhm resistor and the 20 uF capacitor, or for a neater result, changing the 1K resistor for a 1.5K, especially if it cures the vibrato thud.
I would then have to make a video about it :)
@@kapioskapiopoylos7338 Thank you very much for all your explanations. Yes, I think it is a good thing to experiment and see what happens. Tube amp circuits are apparently not too complicated, but many phenomena take place in them and that is what creates complexity. That's why I think it is good to experiment slowly and study each part of the amp and its specific phenomena carefully. There is so much to learn!
@@charlesdespres nice to see you are going deep on this, plate current fluctuations versus the control grid voltage (mA/v or μmhos, see 6v6, 12ax7 etc datasheets) is the basic characteristic of a tube. When there is a plate resistor or in this case transformer, the fluctuations of current also cause fluctuations in voltage (ohms law V=I*R). However tubes amplify better with higher plate voltage (it is shown in some tube manuals and datasheet curves), so voltage fluctuations on the plate was a problem, the tetrode solved that with its second grid (screen). Essentialy making the tube to not care that much about plate voltage fluctuations given that there are no screen grid voltage fluctuations, this can only happen with a decoupling capacitor. As in the cathode, you can also remove the cathode bypass capacitor but the gain will be very low.
screen current is low as you thought, however not zero and like the plate current it depends on the voltage on the control grid but with a capacitor on it on the positive half-cycle the current can't charge it enough to make a difference and what little charge has happened will be negated at the negative half-cycle hence keeping the voltage constant. the screen resistance will ensures that the few negatively charged electrons that hit the screen grid are not accumulating, but have a path to B+.
"Speaking of the vibrato, I think it started thudding after I did the mod, so maybe it is a side effect of adding the screen resistor the way I did it, making the vibrato tube plates unstable. Do you think that is a possibility?" that is exactly what i said. essentialy what you did was to remove* the vibrato filter capacitor which also serves as the screen bypass capacitor from it. *you didn't fully removed it, but you added a 470Ω resistor between it and the screen. Which is still directly connected as the vibrato plate supply.
Here is a "golden" video in my opinion which can help you especialy when combined with the tube's datasheet. However if there is anything unclear or you have any question please let me know.
ruclips.net/video/j2fu3Hoi5rI/видео.html
Nice explanation of what is happening electricly, but your execution of the job leaves a bit to be desired. Not to be mean spirited, but I think you need lessons on how wiring should be performed. Pin 4 looked like a cold solder joint, and I think the resistor should have been placed on pin 4 and 6 before any rewiring had been done. Cheers 😀
Thank you for your comments. I have no formal training and by no means do I claim to be a full-time amp tech. I'm just a Blues music lover who likes to tinker with his guitars and amps and enjoys sharing his experiences. Have a nice day!
Is that a 470 Ohm resistor?
Yes, it is.
Thank you. I am giving these mods a try on my 70 Fender Bronco.
@@erikboisen5675 From what I understand, the Silverface Bronco is the same amp as the Silverface Vibro Champ, so it should work perfectly. I looked at the schematics and they are the same, although the voltages indicated are different. The voltages in my Vibro Champ did not reflect those in the schematic, but I read somewhere that this is often the case. Let me know how the mod goes for you!
@@charlesdespres thanks for checking that. I will let you know how it works.
@@charlesdespres I removed the snubber capacitor and added the screen resistor. I now have to turn my treble down quite a bit (it was at 7, now at 4). Presumably because of the absence of the snubber capacitor. Those were very simple tweaks. Well worth the effort. Thanks!