Trouble with Fender Champ 5F1 clone.

[Travis]

Travis: Just to give you another opinion, I don't agree with everything that labb says here. In a single ended amp, the power tube is on all the time and never gets a rest (not totally true, but enough so for this discussion). As a result, running it closer to max plate voltage will cook that tube PDQ, leading to possible premature failure of the tube and possibly other amp parts. The Champ is supposed to be a 5W amp with good reason. Running the 6V6 with plate voltage in excess of 400v and well above 5W is not the design of this circuit. I would look to take the bias much colder. Hot bias does not always equal good tone, but you will need to experiment a bit to get the answer, as tone is subjective.

I got to thinking, maybe you've just build a monster. It happens. Think about Frankensteining it. Given the plate voltage, maybe use a 6L6 or EL34, but both will need a different output transformer, as yours probably isn't rated for the amount of power these tubes would produce and the primary impedance rating needs consideration, as well.

Right now, the power output is probably saturating your output transformer, giving very early break up and not allowing a full appreciation of the character of the amp.

Start by upping the cathode resistor to at least 2K. Just my 2 cents.

Good luck.

When you say monster what are you saying in terms of the amp? You mentioned using 6L6 or EL34 tubes, what will this do by adding these tubes and changing out the OT to match the tube?

[jjman]

SF Champ schematics imply 20.5watts at idle for the 6v6.

Of course this is not good, but it is the reality. I changed mine to a 1k cathode resistor and added a dedicated screen resistor to make the 6v6 happier.

[Phil_S]

When you say monster what are you saying in terms of the amp? You mentioned using 6L6 or EL34 tubes, what will this do by adding these tubes and changing out the OT to match the tube?

Travis: I'm referring to the (monster) high plate voltage, which is causing this amp circuit to operate outside of it's original design. Hence, the output tube and transformer may no longer be the best selections. The 6L6 or EL34 will take a lot more plate voltage and current than a 6V6. But along with this, you need an OT that can operate properly with the higher wattage output, and that has a primary winding resistance appropriate to the plate of the output tube.

So, what a 6L6 will do is suck up all the B+ voltage this amp, as built, is producing.

For a Champ, it seems to me that an appropriate PT would be maybe 240-0-240, maybe a bit higher. Your 325-0-325 is the real "problem" here, as it is just too much voltage for this amp and is throwing things out of whack. There are various strategies for dropping B+ voltage for an overpowered tranny, but, in the end, such strategies all amount to false economy and imperfect work-arounds. It is best to use the right transformer.

I suspect, if you change the PT, the amp you just built will sound much better, and that, really, is the right thing to do. Raising Rk to 2K is the least expensive solution (really inexpensive), but it really doesn't address the fundamental issue, which is high plate voltage.

The main point is that one end or the other (PT or OT) should probably be changed because there is a mismatch. Think about your options.

--Phil

[joebob]

Travis,

There was something else I noticed. The B+ dropping resistors are 1/2w, they should be either 1w or 2w. I believe these would fail pre-maturely if left at 1/2w.

Also if you are concerned with the 418v on the plate a NOS 5y3 would drop the voltage about 30v. I believe a EH or JJ 6v6 can handle the 418v without any problem.

NOS is 'New Old Stock' basically unused older surplus parts.

Someone else can comment on this.


Dave

[labb]

And let me jump in here just one more time...The Fender layout for the 5F1 champ shows as follows:

Vp at pin 3 of the 6V6 = 340 VDC

Vc at pin 8 = 18 VDC

Cathode resistor is 470 ohm.

So 18/470= 0.0382 amp

340 - 18=322 VDC

Then P = 322*.038=12.2 Watt not accounting for screen current.

Some 6v6's I have seen run plate voltages and screen voltages well over 400 VDC. JJ's and EH's will certainly take this voltage. My understanding is that the screen voltage is what you really have to be concerned with...So, Travis, if you like the way it sounds, and I thought it sounded pretty good from your clips-----Just let her bump where you are and have fun.

[Travis]

When you say monster what are you saying in terms of the amp? You mentioned using 6L6 or EL34 tubes, what will this do by adding these tubes and changing out the OT to match the tube?

Travis: I'm referring to the (monster) high plate voltage, which is causing this amp circuit to operate outside of it's original design. Hence, the output tube and transformer may no longer be the best selections. The 6L6 or EL34 will take a lot more plate voltage and current than a 6V6. But along with this, you need an OT that can operate properly with the higher wattage output, and that has a primary winding resistance appropriate to the plate of the output tube.

So, what a 6L6 will do is suck up all the B+ voltage this amp, as built, is producing.


For a Champ, it seems to me that an appropriate PT would be maybe 240-0-240, maybe a bit higher. Your 325-0-325 is the real "problem" here, as it is just too much voltage for this amp and is throwing things out of whack. There are various strategies for dropping B+ voltage for an overpowered tranny, but, in the end, such strategies all amount to false economy and imperfect work-arounds. It is best to use the right transformer.

I suspect, if you change the PT, the amp you just built will sound much better, and that, really, is the right thing to do. Raising Rk to 2K is the least expensive solution (really inexpensive), but it really doesn't address the fundamental issue, which is high plate voltage.

The main point is that one end or the other (PT or OT) should probably be changed because there is a mismatch. Think about your options.

--Phil

What will changing to a 6L6 and matched OT do to the tone of the amp?

[Phil_S]

What will changing to a 6L6 and matched OT do to the tone of the amp?

Quite honestly, I don't know. I've never seen a 6L6 SE amp. It's just that I think you are saturating the OT you've got and, frankly, I don't think the tone is very good. You may disagree. Tone is subjective. If you like it, you should just leave it as is and see how long your 6V6 lasts. This is all very much up to the guy with the amp. I don't mean to foist my opinions on you. I was only trying to suggest what might improve what you've built. I'm a tinkerer by nature and tend not to be satisfied with a result like the one you've got. It's just my 2 cents. Take it as you will.

I want to say, one more time, you've done well, pilgrim. You have plenty to be proud of. You built a guitar and an amp and both work! How many people do you know who could even entertain the idea? You deserve more than a couple of pats on the back here!

[FunkyE9th]

When you say monster what are you saying in terms of the amp? You mentioned using 6L6 or EL34 tubes, what will this do by adding these tubes and changing out the OT to match the tube?

Travis: I'm referring to the (monster) high plate voltage, which is causing this amp circuit to operate outside of it's original design. Hence, the output tube and transformer may no longer be the best selections. The 6L6 or EL34 will take a lot more plate voltage and current than a 6V6. But along with this, you need an OT that can operate properly with the higher wattage output, and that has a primary winding resistance appropriate to the plate of the output tube.

So, what a 6L6 will do is suck up all the B+ voltage this amp, as built, is producing.


For a Champ, it seems to me that an appropriate PT would be maybe 240-0-240, maybe a bit higher. Your 325-0-325 is the real "problem" here, as it is just too much voltage for this amp and is throwing things out of whack. There are various strategies for dropping B+ voltage for an overpowered tranny, but, in the end, such strategies all amount to false economy and imperfect work-arounds. It is best to use the right transformer.

I suspect, if you change the PT, the amp you just built will sound much better, and that, really, is the right thing to do. Raising Rk to 2K is the least expensive solution (really inexpensive), but it really doesn't address the fundamental issue, which is high plate voltage.

The main point is that one end or the other (PT or OT) should probably be changed because there is a mismatch. Think about your options.

--Phil

What will changing to a 6L6 and matched OT do to the tone of the amp?

Travis,

The PT you have should be fine. An EH 6v6 can take 450V and a JJ 6V6S can take 500V for plate voltages. 6V6 can handle 14watts, and it's typical for an SE amp to run at max dissipation. Actually an SE amp runs hotter when there is NO signal running through it. 90% of max is usually a good compromise for extended tube life. Biasing it to 5watts IMO is too low. But in general, biasing, IMO is always a balance between tube life and what sounds good to you. It doesn't matter how much tube life you get if the amp sounds like crap, cuz your not going to play it, and that itself will extend tube life.

Don't get confused with the Champ being rated at 5Watts. The rating is calculated based on the power delivered to the load NOT what the plate is dissipating.

If you decide to stick with 1K cathode resistor, you should at least replace it with a 2Watt resistor. IIRC you have 29V across that resistor, so you're at .84 watts. It's usually good practice to use a part that's at least 1.5 times the actual power its dissipating, so 2Watt should work. If you decide to try a lower value you will probably want a 5Watt specially if you are considering a 6l6.

If you decide to go try a 6L6, it will draw more current. So now you have to make sure the PT and the OT can handle the current, not just the voltage. Your PT is rated at 70ma. I did not see the spec for your OT. Make sure you replace your 1K with a higher wattage before you do this. 1K will probably be too high. My suggestion buy 5 watt resistors from 500Ohm to 1K and experiment.

FYI, I built an SE amp that uses 330-0-330V@100mA PT (weber w022772) and a Hammond 125ESE OT rated at 15W, 80mA, and a 500Ohm 5Watt cathode resistor. It takes 6V6, 6l6, el34 and KT66.

-FunkyE9th

Edited to add...

I forgot to mention.... supposedly some newer 5Y3GTs generate higher voltage which might explain why you're seeing 400V at the plate. I suggest trying a NOS 5Y3. You might find that the original 470Ohm resistor you have might just be fine.

[labb]

If I remember correctly the Sovtek 5Y3GT drops about 20 volts and a NOS 5Y3 will drop about 40 volts. That is from memory (which is kinda dull at times) but will be pretty close. One thing he is up against is the Hammond PT. The Hammonds are spec. at 115 VAC in put and homes are running close to 120 VAC at the wall..Another thing to remember as far as the spec. voltages for the amp go is that the note on most of the Fender amps schematics are going to say that the readings are + or - 20%. I think, get the wattage right on the resistors, and he is ok. As for the Power out of the amp,,rule of thumb is that it will be about 40% of the plate dissipation. In this case, per the Fender schematic where the dissipation is 12.2 watt, the amp would be at about 4.88 Watt output ie. 5 watt. I think that James is so close that it does not matter. If he likes the amp, play it, if not start tweaking.

And for Funkey,,,I built a SE amp, actually called an SEL that has both 9 pin and a 8 pin sockets, a 150 ma. PT, with external bias adjustment and test points that is a hoot to play with. I have run El84, 6V6, 6L6, el34 and KT88 in it. Gave it to my guitar playing kid and told him that he would really like the EL34 power tube. Shows what I know, He plays it with a 6V6 all of the time..So many amps to build, so little time.

[FunkyE9th]

Hey labb,

The 2nd amp I built is like the one I described above, but I added an EL84 also. I had to drop the voltage when using an EL84 because it can only take 300V. It's great amp for tube tasting!

-FunkyE9th

[Phil_S]

I plead guilty to confusing static plate dissipation and amp power. So, sorry to have mislead Travis on this.

Still, given the plate voltage, with the 1K resistor, the mA draw of the power tube is at the very high end of the design spec and this probably isn't the most desirable bias point.

I still think his sound clips exhibit very early OT saturation and that a change somewhere would improve the range of tone. Volume (or perceived volume) isn't much of an issue with a low power amp like this.

[Travis]

I plead guilty to confusing static plate dissipation and amp power. So, sorry to have mislead Travis on this.

Still, given the plate voltage, with the 1K resistor, the mA draw of the power tube is at the very high end of the design spec and this probably isn't the most desirable bias point.

I still think his sound clips exhibit very early OT saturation and that a change somewhere would improve the range of tone. Volume (or perceived volume) isn't much of an issue with a low power amp like this.

So what kind of OT would match this amp better?

[jjman]

I also built a 5F1 using a PT that placed too many volts on the 6v6. I changed the 5y3 tube to NOS (which absolutely helps) and added a 5watt resistor in-line from the DC output of the rectifier. I forget the value but I'm guessing ~750ohm. Since the 5F1 draws little overall current, the dissipation wattage in the dropping resistor is acceptable and the voltage drop it creates solved the high voltage problem. One could always use a 10watter for added safety. Since it's class A, this resistor does not increase perceived “sag.” I also use a 1k on the screen (which is not on a real 5F1.) Power tubes should not have higher voltage on their screen compared to their plate.

Here’s a clip at various volumes on the guitar. It’s my favorite amp for lower volume situations. The amp is cranked.

http://www.geocities.com/jjsant/champo_sm58.mp3

[Travis]

Here is my OT
Specifications
Audio Watts (RMS): 8
Primary Impedance (Ohms): 7,000
Secondary Impedance (Ohms): 3.2
Mounting centers: 2-3/8"
Weight: 0.6 lbs

What kind of OT do I need to be looking at for running a 6L6?

[joebob]

It would be cheaper to try a NOS 5Y3, this site has them starting at $8, not sure how much shipping is.

www.esrcvacuumtubes.com

if your going to change your PT I don't see why a 275-0-275 @ 70ma wouldn't work, or one of the 250-0-250 would be ok too. One thing I realized when I built my first amp, a 5f2a, I followed the schematic/layout too. But it was designed in the 50's when the a/c was only 105 to 110v in the house. Now it is 120v to 125v so the transformer just bumps up the output voltage even more. I ended up going with a lower voltage PT and a NOS 5y3 (I replaced the Sovtek 5y3) and it is about 360v on the plates.

I get my Hammonds from www.radiodaze.com, the price's recently went up so there may be a cheaper place somewhere else. The attachment prices are not accurate but it shows all the PT's in one file.

Oh, good job on your first build, your in trouble now, once you build one you just can't stop.

Dave