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Old 1st April 2009, 09:49 PM   #21
WRyan is offline WRyan  United States
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Join Date: Jan 2009
Thanks everyone for the help!!

I am still experimenting, but I really appreciate the thoughts.

...PS, now I have feedback disconnected, and a 25u bypass cap on each preamp cathode. It sounds pretty aggressive when cranked now.

One comment made me wonder... how does the grid stopper resistor affect the sound? In other words, what kind of tonal changes can I expect by raising or lowering the value?

Thanks again everyone!! This forum is great!!
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Old 2nd April 2009, 12:11 AM   #22
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Location: Victoria, BC
Originally posted by WRyan

One comment made me wonder... how does the grid stopper resistor affect the sound? In other words, what kind of tonal changes can I expect by raising or lowering the value?
Grid stopper resistors (as I understand it) are mostly for preventing unwanted oscillation, so I've never messed much with them. Other areas (that you've explored already) like bypass and coupling cap values and messing with the tone stack and 'bright' caps seemed to offer more 'bang for the buck'.
You probably know this already- the grid stoppers should generally be soldered right to the tube socket lug with the resistor body fairly close to the lug. If necessary you can extend the other 'leg' of the resistor to reach its connection point.

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Old 2nd April 2009, 01:27 AM   #23
WRyan is offline WRyan  United States
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Thanks VictoriaGuy for all of your help!

I did not know that the grid stopper resistor should be connected right to the tube socket. I am still very new to all of this and trying to soak up as much as I can.

What is the reasoning behind this?

Many thanks!!
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Old 2nd April 2009, 02:06 AM   #24
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Default Grid stopper resistors

I can't go into many technical details- I don't know that much! However, the idea is that the grid stopper resistor is preventing unwanted high-frequency oscillations (not necessarily audio, they can be radio frequency range- hundreds of kHz or more). At those higher frequencies, all the wiring has to be much 'tighter' to avoid creating unwanted resistor and capacitor combinations in the wiring itself. Check out the wiring standards on an older (1940-s-1950s) tube radio and you can learn a lot.
A link for you-
Aiken Amps is a great resource for all things tube amp- lots of good articles on grounding, etc

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Old 3rd April 2009, 05:08 PM   #25
jjman is offline jjman  United States
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Regarding the adjustable cathode resistance, you should use a pot in series with a resistor. This way you cannot accidentally short the cathode to ground. I use this setup on mine with a 220 ohm resistor and a 1k wire-wound pot. With this approach, you also ensure that the pot will not be asked to dissipate too much wattage. My pot dissipates about 0.3-0.4 watts this way with the 6v6 biased around 12watts.

Tube watts ~12
Cathode current-39ma
Total cathode resistance-465ohm
220ohm Cathode resistor volts-8.6
Pot volts-9.5
Pot Ohms-245
Pot watts-0.37

My previous statement about the stock setup running the 6v6 too hot was more applicable to the Silver Face era and not the Tweed circuits. I have a SF Vibro Champ and that's the one I changed to around 720 ohms on the cathode.
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Old 1st December 2012, 06:45 PM   #26
kesrmk is offline kesrmk  Lithuania
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Location: Vilnius, Lithuania
Originally Posted by Funker View Post
Hi WRyan,

first to say a guitar amp is a totally different thing vs an HiFI amp. The guitar amp and the guitar itself is part of the sound what a musician want. Every component incl. the speaker has a widely affect on the sound.

I recently build an guitar amp similar to what you build. I had problems with the sound which was harsh and dull while being overdriven. At small level with undistorted sound it was ok.

I use the fundamental circuit of the Fender Champ similar like yours, but I adding a overdrive stage to it.

To find out what the trouble was I fiddled with the first stage bias and kathode C. That will change a lot but give not the satisfiying result.

The relative high +B for this Amp is ok, I assumed that pinpoint was the output valve bias. I using the 6V6gc and the opt has a primary of 8k. My amp run with 400V and sounds now ok.

The data sheet of the output tube say a different settings as they are contrary to the established in the original Fender circuit. So I didn´t relied to the specs and start my own experiences.

But before I reach the aim of a satisfying sound I set the bias in a more practical way. I unsolder the kathode R of the output valve and replaced it against a variabel resistor of 1k . Now a friend of mine ( who becomes the owner of this amp) played the gutar and I setting the bias to the best sound. The result was amazing . I replaced the varaible R to a fixed one.
After this procedure I play a bit with the nfb. I did it in the same way like I did with the bias The best value for the nfb resistor is for my amp 5.6 k . You see in the drawing a value of 2.7k which was the original value.

The speaker in my amp is a 12" no name China import, I bought several of them at a local scrapyard. They look like old Fane chassis but they are definitely made in China , but the sound fair in my amp.

I suggest you to try the biasing and the nfb setting in the way I did.

here the schematic with all voltages and a pic of the wiring of my amp :
Click the image to open in full size.
Click the image to open in full size.

My conclusion is , that an amp as simple as this can´t be´satisfing al the wishes. But it is a good amp for rehearsal and small club gigs . This ampreally reflexes the sound of the 50/60ies.

good luck !

excuse me for possible mistake in writing/ grammar I´m not native english!

Hello, your attached schematic is very small.
I'm very interested in your preamp section.
Could you please send me more readable version?

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Old 1st December 2012, 07:28 PM   #27
JMFahey is offline JMFahey  Argentina
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Hi kesrmk, this is the proper sized one.
For some mysterious reason, the Forun only remembers the thumbnail

Mind you, that the general design is fine, but the relay switches between random different DC voltages, so it must thump or click big way when switching.
Proper DC discharge resistors and an extra DC isolation capacitor should be added to avoid that.
More precisely, "clean" signal should be taken *after* C5, where there's 0VDC referred to ground, and not *before* , where there's 173VDC *and* the left side of C9 should be grounded through a 220K to 1M resistor.
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Old 3rd December 2012, 05:48 AM   #28
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Location: Chicago IL, Long Beach CA, Vienna VA
Default champ

If the dirt comes on too early for your tastes that's a common complaint or feature that's a direct result of the extremely low power. I just got one too, still learning about it. Immediately I note that without a master volume you change resistors to alter the percentage of preamp tube distortion versus output tube distortion. Looking at what other people do in modifying these, they often improve dynamic range by reducing hum via dc heater on the first tube or floating the ac filament and adding a bias to reduce the noise from the filament. Then with the noise down, they sometimes add a second single-ended output tube to have parallel single-ended class a. That makes a nice alternative to push/pull but generally requires a different output transformer to make the most of the additional power. Of course, using an extremely efficient speaker in an efficient cabinet is essential to making the clean useful and enjoying toeing the line where distortion first arrives. I've considered modding my champ, but the truth is that it might make more sense for me to pick up a second kit with the features I want. People also make them with a wide range of output tubes.

I'm considering duplicating the output for parallel single-ended class A, but adding a second identical output transformer and changing my speaker impedance. That seems like a nice way to enjoy the class A clean more and expand the area where distortion is just coming on.

Last edited by cyclecamper; 3rd December 2012 at 05:50 AM.
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Old 5th December 2012, 01:09 AM   #29
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Location: Chicago IL, Long Beach CA, Vienna VA
All this talk about using potentiometers in odd places has me really intrigued. Not to steal your thread, but I have a question I'd like to ask, or an idea to throw out open for constructive ridicule.

I have a Peavey classic 50 'salvage' amp that I'm experimenting on. I have a 4-gang potentiometer with a lot of space between each of the 4 sections (actually a gear-drive mechanism that drives 4 seperate wire-wound pots in sync, with each pot very conveniently approx. near each stage, mounted in a box where the small reverb tank used to be, now that I have a full-size reverb tank for it). I think each pot section is 20K linear. I wanted to use it on the 4 12AX7 preamp stages. Each stage has its own 150K resistor between the plate and B+. The first two stages have a lower B+ than the next two (in dirty mode) which have a higher B++. So I was thinking of replacing each of those 150K anode resistors with a 135K in series with its own 20K pot, making the combination adjustable from 135K to 155K. Unless somebody can recommend a better range.

Then I hope to have a variable regulator for the B+ and another for the B++. And volume pots between each stage, the first two volume pots are push-pull pots that switch a "bright" cap across the pot.

That ought to give me plenty to play around with dynamically while listening to it.

Last edited by cyclecamper; 5th December 2012 at 01:12 AM.
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