Guitar Amp Advice

[WRyan]

Thanks Everyone for your responses!!

I added a Sprauge 25uf bypass cap on the first stage cathode resistor... made a neat difference, more gain. I like it.

I added an additional 470 ohm resistor in series to the existing 470 on the 6V6 cathode, giving a total resistance of 960 ohms. This seems to smooth out some of the harshness. I like this mod too.

I removed gNFB and found that although the gain increased, the low end suffered by getting pretty loose for my tastes. I ended up putting it back. Feels tighter to me at higher volumes with gNFB.

Overall, I am pretty sure now that my amp generally sounds as it should. I think its strong points are clean sounds and light overdrive.

Modern style distortion is not the best part of its vocabulary. I find the more I push it, the less articulate it is... gets pretty muddy for chords. I am thinking it will be fabulous for Stones type of work... classic country, classic rock and blues for sure!

Also, I have a feeling that with less than 20 hours on the speaker, I should really give it some more time to break in. I hear that its normal for speakers to loosen up after break in.

I may also experiment with different manufacturers of tubes and tube types.

What kind of modifications are involved in making this amp support an el84?

Thanks again everyone!!

[WRyan]

Thanks Miles for the tip about gNFB!

One question – how do I tell the difference between positive feedback and negative feedback? I ask because this was a source of confusion when building the amp. I originally (erroneously) connected the feedback wire to the negative side of the speaker terminal. When I powered up the amp, it oscillated like crazy. Remembering a troubleshooting tip I read before about this, I disconnected the feedback wire from the speaker output and the oscillation stopped. I used the amp for a while and then became curious… I reconnected the feedback wire, but this time to the positive leg of the speaker jack. The amp did not oscillate and it also tightened up in regards to tone; gave more headroom and was generally cleaner… a little quieter too; all the things I hear feedback provides.

Upon reviewing the schematic, I figured out that I originally connected the feedback wire to the wrong side (negative) and now it is correct (positive).

But then your suggestion to verify that it is really negative feedback makes me wonder. How do I verify this?

Thanks Miles!!

[Miles Prower]

Originally posted by WRyan
Thanks Miles for the tip about gNFB!

But then your suggestion to verify that it is really negative feedback makes me wonder. How do I verify this?

Thanks Miles!!

You basically do what you did: see if it oscillates, which it will do if you have positive feedback. My preference is to use a test resistor and an o'scope when first connecting the gNFB. Mr. Murphy guarantees that your connection will probably be wrong, and that you just made a big Royer oscillator. The resulting square waves will show up on the o'scope screen without putting your speeks at risk. Max power at ~20Hz into the speeks is not a good thing.

[WRyan]

Thanks Miles!

Yeah, it was loud! I managed to cut the power very quickly... I probably only allowed it to squeal for about 700 ms or so... definitely less than 1 second.

I need to get a scope and learn how to use it.

Thanks again!

[Funker]

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 :



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!

regards
Wolfgang
DF6ZC

[WRyan]

Thanks Funker.

Great ideas!

I will replace the power tube cathode resistor (which is 940 ohms) with a 1K potentiometer. I am anxious to hear how different it sounds as I sweep the range.

Also, I disconnected the gNFB and now I like it... sweeping the range will be a good idea for that too.

Thanks again!!

[Trout]

Originally posted by WRyan
Thanks Funker.

Great ideas!

I will replace the power tube cathode resistor (which is 940 ohms) with a 1K potentiometer. I am anxious to hear how different it sounds as I sweep the range.

Also, I disconnected the gNFB and now I like it... sweeping the range will be a good idea for that too.

Thanks again!!

Be careful swapping a pot in place of the power tube cathode resistor. Even a 5W ceramic wire wound resistor gets fairly hot in that position.
Mine has a 680 ohm 5W ceramic in it, B+ is around 397V

The originals often failed because of the 1W carbon comp originally used.

[WRyan]

Thanks Trout!

I will keep this in mind.

How does one calculate current for that resistor? Sorry, I am still learning much. I did indeed use 1W for that one (was 470, now I have two 1W 470s in series for 940 ohm total). Would this eventually fail?

I know the formulas for ohms law, but I get hung up on which values to apply the formula to... for instance, when calculating the current for that cathode resistor, what voltage do I plug into the formula? I assume once I know that, I just divide that by the cathode resistor value and I will know the current at that point in the circuit? Or do I need to consider other resistances in the circuit to calculate the current at that point? These are the things that confuse me.

If I were to guess, I would say that I read about 20 volts at the cathode, my cathode resistor is 940 ohms... does that mean I have 21 milliamps across that resistor? 20/940=.021

If that is so, then Watts = V*I or 20*.021=.42 does that mean there is almost 1/2 watt across that resistor?

I appreciate any and all help!!

Thanks so much!!

[Funker]

Hi WRyan,
I just forget to say that what the member Trout just done for me. I used a 10W ceramic wire wound potmeter for my experiences. The potmeter should handle at least 100mA . I do not recommend a carbon type potmeter for this task.
And last not least keep an eye to the max. permissible plate dissipation power , which is 12W.
The plate dissipation power is U plate - U kath * I plate.

For example: With 380V supply, assume 30V loss at the opt and approximately 15V at the kath R, so there are 335V between the plate and kathode , this voltage divided by the maximum permissible dissipation power of the 6V6 will be 36mA plate current which you shall not exceed.

If you find the final value of your choice , calculat the power for the resistor. The formula for the power is P= U² / R .But if you use a 2W wirewound you will be alway on certain terrain.

regards
Wolfgang
DF6ZC

[VictoriaGuy]

Originally posted by WRyan

How does one calculate current for that resistor? .......

If I were to guess, I would say that I read about 20 volts at the cathode, my cathode resistor is 940 ohms... does that mean I have 21 milliamps across that resistor? 20/940=.021

If that is so, then Watts = V*I or 20*.021=.42 does that mean there is almost 1/2 watt across that resistor?

WRyan-
There are several ways of getting to the answer for the power rating of the cathode resistor (or pot in the case discussed above).
I think I've got this right...

I usually follow Funker's advice/method and calculate the max current I want to have flowing through the tube (and the cathode resistor)- in this case 36mA.

Then you can use P=I^2 R to get the power dissipated by the resistor. P= (.036)(.036)(940) or about 1.2 watts. I generally double that number to get the working power rating for the resistor.
BTW, remember that that resistor will be warm/hot, so pay attention to any caps in the area as they generally don't like to be overheated.

Cheers
John