Fender 600 champion reissue complete rebuild

[martin manning]

The blue glow is not a problem. The plate structure is dark, with no patches of red glow, so it looks fine to me

[Mikante]

I read an article about how to bias a tube amp, there was this picture of a hot valve and this blue glow. It was talking about how to visually check a valve running too hot and I associated the problem with the blue glow without reading the article well. It was actually telling me to spot any red glowing parts in the plate.

[pompeiisneaks]

as far as I understand it, visual inspections of tubes for operating status is completely unusable. You need to measure current to determine how well it's biased and performing. Some tubes may glow more than others at identical plate current. Some will light up quite brightly at fairly low currents others hardly glow at all at near max dissipation... just all due to the slightly different randomizations of the factories etc.

~Phil

[Phil_S]

Just enjoy the blue glow. It's pretty and it's not a problem.

[Mark]

Just enjoy the blue glow. It's pretty and it's not a problem.

Blue is okay, purple is bad. You know it when you see it.

[Mikante]

It looks blue, maybe I check the bias the proper way instead of discussing with you about color shades

[Mikante]

Hello guys,
Please help me understand correctly check the bias on this amp.
I am confused.
I have measured both b+ and pin3 of the power tube.
I have 364v on b+ and 354 on pin 3 of the power. tube
Then i have 261ohm resistance between the two.
Am i doing it right?

[martin manning]

Those are the measurements you need, plus one more, the cathode voltage.
Anode current is (364 - 354)/261 = 0.0383A. Plate dissipation is (Va - Vk) * Ia, or (354 - Vk) * 0.0383. If Vk is around 10V, the dissipation is around 13W.

[Mikante]

I have 21v on the cathode, measuring from pin 8 to ground after the 470 ohm resistor with the amp on and the speaker connected.
Now, i am a bit confused. I have divided the 21 volts with 470 and i get 0,04468 amps.
I have measured the voltage drop across pin 8 and 3 and i got 328, then i multiplied it with 0,04468 and got 14,6, isn t this too much?
And why do i need to measure the anode current? Are these just different methods?

[martin manning]

Yes different methods. Also cathode current includes screen current, which is why you see a larger number.

[Mikante]

Ok i get it now, anyway, isn t the power tube running to hot?
I have swapped the 470 ohm for a 1k and i will repeat the test tomorrow. I made a quick sounding test and the amp sounded better to my ears, less distorted and more open, I m talking about minor changes.
Overall the amp feels less “on edge” and more responsive.
Let s see what the numbers have to say tomorrow.

[Mikante]

Hello,
I know i don t update very often but my job is leaving me with very little time to do this sort of things, i wish i had more time.
After replacing the cathode bias resistor with a 1k i have then repeated the test and got no change in terms of sound, very minor changes in terms of voltages, still way over the line.
Today i have increased the value to 1,5k.
I didn t make the test, i just wanted to hear the difference first.
The harsh and very unpleasant sound i was previously talking about is almost gone, the amp feels smoother and less on the edge. I m not saying better cause i did like the sound even with the 470 ohm resistor to bias the power tube but there was that problem.
It is hard to describe and i ll try with an example maybe some of you will find familiar.
It is like when you are recording with a condenser mic and you place it too close to the sound source and you get this over crowded sound especially in the high-mid frequencies. The amp sounded a lot like that in the beginning.
Now, as i said, i lot smoother and less congested to use another word.
Anyway, i will do the bias test, mean while, just for learning purpose, could a different value for the first dropping resistor help?
I d like to keep the bias as close as possible to the stock value.

[Mixfish]

Hey Mikante,

I built a 1 transistor fuzz. My level of expertise.
I ran it into a 600 Champ. Was reminiscing about Hendrix as I played.
But was interrupted by a bad smell.
Smoked the Champ.
R 15 and 16 burnt, I replaced with what I had watts? and they cooked immediately. 6V6 is cooked too.
I'd love any advice. (maybe try pottery)

Thanks.

[Stevem]

Those two resistors are in the heater / filament circuit burned because your 6V6 is now shorted and bad.

Also when that took place the amps fuse should have blown but it did not because it’s a crazy large 4 amp fast blow fuse.

All original Champs and Bronco’s use a 1 amp slow blow fuse which is what I would use.

For a replacement tube a would get a JJ brand from Euro tubes.
They are a little more like a 6l6 and will far better stand up to your Hendrix hours of playing.

[Yoda]

Hey Mikante,

I built a 1 transistor fuzz. My level of expertise.
I ran it into a 600 Champ. Was reminiscing about Hendrix as I played.
But was interrupted by a bad smell.
Smoked the Champ.
R 15 and 16 burnt, I replaced with what I had watts? and they cooked immediately. 6V6 is cooked too.
I'd love any advice. (maybe try pottery)

Thanks.

I bet it sounded phenomenal before it let the smoke out!

True story, when I was about 16 years old I smoked the OEM output transformer of a 1958 5F1 Champ by maxing it out, putting a boost pedal in front, and getting down on some Hendrix. Good times!