1954 Fender Champ problem

[poligow]

Hi there,

I bought an old Fender Champ from 1954.
The amp arrived with leaky electrolytic filter caps :


I decided to replace :
- the 3 caps for the power supply (with Sprague 8fd / 450V)
- the 2 resistors for the power supply which had drifted a lot
- the electrolytic cap for the 6V6

When the amp is on, most of my voltages are ok. No hum, but when i plug a guitar the sound is very weak and slightly distorted.
I tried to replace the 6V6 and the 6SJ7 > same problem.

The ,02 cap between the 6SJ7 and the volume pot was leaking +4V so i replaced it, along with the other ,02 cap on the input.
Same problem !

Here's the amp layout, with the voltages, and the replaced components (with a green arrow) :

All my voltages look ok,with the exception of the pin 4 on the 6SJ7, were i have around -0,9V. The 5 Meg resistor is reading 4,5Meg on my meter.
That's a 0,5Meg difference, but i have an other Champ like this one, and i have around -4V at this point with a 5Meg resistor reading 5,5Meg.

Should i replace the 5Meg resistor with a new one ?
How the voltage on this point is set ?

Thanks in advance for your help guys

[Leo_Gnardo]

You've done all the right things so far, apparently there's more to go. I doubt that 5 Meg is your problem. Instead point your green "fixit" arrow towards the input pentode's screen bypass resistor and cap, the 2 Meg and .05 cap attached to pin 6. The 250k resistor that biases that grid is another suspect. I'll bet one or more of these 3 is dodgy. As you see 60 year old caps like to fail. Hi value resistors also drift and sometimes go open-circuit. A resistor that's off by 10% is no big deal.

[poligow]

You've done all the right things so far, apparently there's more to go. I doubt that 5 Meg is your problem. Instead point your green "fixit" arrow towards the input pentode's screen bypass resistor and cap, the 2 Meg and .05 cap attached to pin 6. The 250k resistor that biases that grid is another suspect. I'll bet one or more of these 3 is dodgy. As you see 60 year old caps like to fail. Hi value resistors also drift and sometimes go open-circuit. A resistor that's off by 10% is no big deal.

thanks for the feedback !
i will buy these 3 parts this weekend to have a try

[poligow]

today i tried to replace (pointed with blue arrows) :
- .05 cap + 2meg resistor on pin 6
- 250k resistor on pin 8
>> same problem


the guy at the electronic store advised me to check the volume pot : as the original coupling cap was leaky, it could have damaged the pot ... will check it later this weekend.

[poligow]

i tried to replace the 5Meg resistor on the 6SJ7 pin 4 > sounds good
i still have the same reading of -0,9V at this point, so the amp is cleaner than my other Champ where i have -4V at the same point.
is this value of -0,9V ok for you on this kind of amp ?

[Enzo]

What exactly is the problem you are trying to repair? You said just above that it "sounds good." If it sounds good, stop fixing it.

It is the -4v that I find suspicious, not the -0.9v. Typically, grid leak bias results in a volt or less. The layout picture has what looks to me like a -0.5v drawn on there. other Fender grid leak amps show -0.9 and -0.8v.

If this drawing looks just like your amp, then you have a 5C1. Don't worry so much about values, look upper right of your drawing at the notes. It says "values shown + or - 20%". That is what Fender thinks.

[poligow]

Hi Enzo,

My problem was the amp had a very low volume, with a slight distortion.
I fixed the problem when i replaced the 5Meg resistor the 6SJ7 pin 4.

When i put it side by side with an other Champ ('53 model i also own), the '54 one stays really clean, even with the volume pushed to the max.
The '53 has similar voltages (and only has the 3 filter caps + the 2 power rail resistors replaced), except for the 6SJ7 pin 4 where it reads -4V - the '54 reads -1V
I was just wondering if this -1V on the '54 was normal for this kind of amp, as the layout calls for -5V at this point
But when i read your post it seems just fine

Thanks for your help !

[Enzo]

I see a dot before the 5, so I see -.5v, not -5. I could be wrong.

[Tom Phillips]

A few comments.

...It is the -4v that I find suspicious, not the -0.9v. Typically, grid leak bias results in a volt or less. The layout picture has what looks to me like a -0.5v drawn on there. other Fender grid leak amps show -0.9 and -0.8v...

I agree. I have a hard copy of a clearer drawing and the .5 is obvious on my print.

...the guy at the electronic store advised me to check the volume pot : as the original coupling cap was leaky, it could have damaged the pot ... will check it later this weekend.

That is strange advise. Note that even if the full 148 plate voltage you measured at pin 8 of the 6SJ7 leaked through the coupling cap it would only cause 0.02 Watts power dissipation in the 1 Meg volume pot. In addition, I don't think the 148 V would exceed the pot's max voltage level spec. Therefore, I wouldn’t worry about damage to the volume pot caused by your leaky coupling cap.

Cheers,
Tom

[olddawg]

A few comments.

I agree. I have a hard copy of a clearer drawing and the .5 is obvious on my print.

That is strange advise. Note that even if the full 148 plate voltage you measured at pin 8 of the 6SJ7 leaked through the coupling cap it would only cause 0.02 Watts power dissipation in the 1 Meg volume pot. In addition, I don't think the 148 V would exceed the pot's max voltage level spec. Therefore, I wouldn’t worry about damage to the volume pot caused by your leaky coupling cap.

Cheers,
Tom

Cant you just bypass the volume pot with a clip lead or a tack wire and use the volume control on your input signal source (guitar volume control, etc) for test anyway? No reason to keep chopping this fine old amp up stabbing in the dark.

[poligow]

@ Tom : thanks checking your original drawing.
for the volume pot, i agree with your arguing !

@olddawg : i didn't touch the volume pot !
for all the tests i previously did the parts were mounted on a tag board and connected with clip leads.
i don't want to butcher this old little one
i'll post a picture of the chassis to let you see the job when the 5meg resistor will be replaced by a proper Allen Bradley.

[poligow]

done :


the voltages :


thanks guys for your help !