Trouble with Fender Champ 5F1 clone.

[Travis]

do I need to change out the 470 to a 1000 ohm resistor? it that what your saying?

[labb]

Yes,,,The resistor that connects to pin 8 of the 6V6 tube

[Phil_S]

Yes,,,The resistor that connects to pin 8 of the 6V6 tube

And post voltage readings on the 6V6. You may need to go to 1500 ohms. Do this preferably with a 5W resistor. If you have another one like the one you used, just put it in series; double wattage, double ohms.

[Travis]

Older readings:

6v6:

pin3=400
pin4=375
pin5=.01
pin8=23

12ax7:

pin1=208
pin3=1.64
pin6=204
pin8=1.65

New Readings: Added a 1k 1watt R and added a 22uF 35v cap to the 1.5k ground for more gain.

6v6:

pin3=418
pin4=396
pin5=.01
pin8=29.7

12ax7:

pin1=222.9
pin3=1.74
pin6=204
pin8=1.74

[labb]

If you replaced the 470 cathode resistor with the 1k resistor then you are about right. Would be about 9.6 watt. Probably a little on the cold side. Try about 850 ohm. If you put the 1K resistor in series with the 470 ohm resistor then you are too cold.

[labb]

If you like the way it sounds leave it as is..Too cold will not hurt the amp. Too hot will make the 6V6 fail earlier than it should.

[Travis]

If you replaced the 470 cathode resistor with the 1k resistor then you are about right. Would be about 9.6 watt. Probably a little on the cold side. Try about 850 ohm. If you put the 1K resistor in series with the 470 ohm resistor then you are too cold.

I replaced the 470 with the 1k. What do you mean by cold?

[Travis]

Does being too cold effect the tone and If so what does it do?

[Phil_S]

deleted content, sorry.

[labb]

Hot/cold in this case refers to how the amp is biased. in your case for the 5F1 champ which is a single ended amp. you want the power tube dissipation the be = to or less than .9 x the Pmax of the tube. The Pmax for a 6V6 is 14 watts. therefore .9 x 14 = 12.6 watts. the calculation is as follows:

where

Vp = 418
Vc = 29.7 (cathode voltage drop)
Rc = 1K (cathode resistor)

Anode voltage is Vp - Vc = 418 - 29.7=388.3 VDC

Cathode current Ic =Vc/Rc = 29.7/1K =.0297 amp

I assume that the screen current is 0.005 amp and subtract this from the cathode current to give the current flow through the anode or
.0297-.005=.0247

Then P = VI =388 x .0247 = 9.58 watts dissipation.
This would be a little on the cold side. If you change to 850 ohm Cathode resistor you will probably end up at about 11.5 watt. which is kinda hot.

There are a lot on here that can explain this better than I.

like I said, if you like the way it sounds then leave it along. Most can't stand it though and start changing bias resistor, bypass caps (which you already have on the preamp tube) coupling cap,etc. just to find that perfect sound.

You do know that tube amp building is addictive don't you? Can't build just one. So go ahead and get ready to build the next.

[Travis]

Yeah your right. Addictive. I want to do a Trainwreck Express now. I'd love to get one built with a great brown sound. I'm a big EVH fan!

[Phil_S]

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.

[Travis]

Hot/cold in this case refers to how the amp is biased. in your case for the 5F1 champ which is a single ended amp. you want the power tube dissipation the be = to or less than .9 x the Pmax of the tube. The Pmax for a 6V6 is 14 watts. therefore .9 x 14 = 12.6 watts. the calculation is as follows:

where

Vp = 418
Vc = 29.7 (cathode voltage drop)
Rc = 1K (cathode resistor)

Anode voltage is Vp - Vc = 418 - 29.7=388.3 VDC

Cathode current Ic =Vc/Rc = 29.7/1K =.0297 amp

I assume that the screen current is 0.005 amp and subtract this from the cathode current to give the current flow through the anode or
.0297-.005=.0247

Then P = VI =388 x .0247 = 9.58 watts dissipation.
This would be a little on the cold side. If you change to 850 ohm Cathode resistor you will probably end up at about 11.5 watt. which is kinda hot.

There are a lot on here that can explain this better than I.

like I said, if you like the way it sounds then leave it along. Most can't stand it though and start changing bias resistor, bypass caps (which you already have on the preamp tube) coupling cap,etc. just to find that perfect sound.

You do know that tube amp building is addictive don't you? Can't build just one. So go ahead and get ready to build the next.

Wow, this makes total sense, I'm starting to understand more and more on how to get these things to life. I can finally put Ohms law to good use!

[labb]

Travis, go with what Phil says. I assure you he knows a lot more about it than I do.

[Travis]

As far as I'm concerned you all kick ass! I'm beating my head against the wall for something so simple and you all kept me in the game. I have one question. I want to build an amp that gives me a great brown sound like old school Van Halen. What amp's can I use as a model for my build?