Silverface Champ raspy/distorted output

[Jag]

Sorry Bruce, and especially Doug, I got all caught up in the math and didn't think it all the way through.

[bob p]

Bruce, on the subject of dropping B+ in a SE CLass A amp like the champ, couldn't you accomplish the same thing using either a zener, or a power resistor between the PT center tap and ground? I've always used zeners in the past, but in theory a resistor should work too.

[Enzo]

A resustor would have a voltage across it that varies with current, so in the typical amp that would be less than ideal. But in a class A amp, the current is fairly constant.

[Doug]

Added a 40uF filter cap...

Thought I'd see if a little more filtering might help. Much improved. Went back to a 1k cathode resistor, idling at 13.5 watts, seems happy. Factory has two 20uF in parallel. Should I suspect the old cap? How do I troubleshoot a filter cap? I've only replaced them outright if they were 20 years old or blistering. I suppose I could just replace the cap since it's from 1970.
Are these old caps worth saving? Players seem to be impressed when a fender has the original brown mallorys. I just thought they dried up and the values drifted.

[Paleo Pete]

I suppose I could just replace the cap since it's from 1970

That's what I did...

I'm no amp tech, far from it, but after some research it seemed that electrolytic caps dry out and start to wander from specs, and if one happens to blow or short, it introduces an opportunity to fry a transformer, which would not make my day.

I've also seen enough comments implying the old brown (chocolate drop) and blue caps used in Fenders were not great quality, so I replaced them all (in both my Champ and Super Reverb) with "orange drops" I got at a local guitar shop. Around $50 total for both amps. The electrolytics were replaced with mostly Sprague, except for one or two the guitar shop didn't have in stock, they're whatever the local electronics supply had. I left the ceramic discs alone, everything I've found indicates they don't dry out and rarely wear out. I just double checked or re-soldered the contacts and let them stay put.

The Champ didn't have any problems, it works perfect and sounds teriffic, with an 8" speaker from a Gorilla practice amp, the correct 4 ohm impedance. I also made up a patch cord to run it into an external speaker, it sounds so good through my Kustom 2x12 cabinet it makes me want to lug the heavy brute around when all I need is the small amp...But I replaced the caps just to be on the safe side. I've never regretted it.

I would replace every cap in an amp that old, mainly to prevent trouble, they are well over 30 years old, and unless everyone was wrong, they didn't use great quality caps to begin with, except for the 25/25 Mallory electrolytics. I also replaced all the 1 watt resistors in both, the Super would "sizzle" like frying bacon, and I finally found a troubleshooting site that indicated this was due to bad 1 watt resistors, replaced them all in both amps and the Super is down to a mild, normal hiss when I pull up the volume pedal. No sizzle, no pops, no loud hum, just that light hiss, and I play it onstage cranked to 10. I'm happy...

So...I'm wondering if your harsh sound is related to bad 1 watt resistors, (I'm not sure if they will do that too), weak caps somewhere, or something loose and vibrating...My Champ is all stock values, no mods, and it's the best sounding practice amp I've ever heard, even through the cheesy 8" speaker. I dropped a 6L6 power tube in it and all voltages read within specs for the amp if it had a 6V6, and the tube has been in it for around 5 years with no trouble. That's the one and only modification. I've played it side by side with a stock 70's Champ, it sounds better, the shop owner said that too. (he's an all tube guy too)

As for replacing caps, that turned out to be a great thing with the Super Reverb, it would work perfect for about 10 minutes, then the volume would drop like you pulled a power tube. I was almost convinced I had a bad transformer, and decided to replace every cap in sight just to get new ones in place of the originals from 1973 while waiting to get enough cash together for a transformer. When I replaced the last orange drop it started working perfect, I've been running it onstage ever since with no trouble at all.

So yeah, I'm a firm believer in replacing the ancient caps in any amp that old...Mine proved it's a good idea.

[Bruce / Mission Amps]

Bruce, on the subject of dropping B+ in a SE CLass A amp like the champ, couldn't you accomplish the same thing using either a zener, or a power resistor between the PT center tap and ground? I've always used zeners in the past, but in theory a resistor should work too.

Yes, I mentioned the zener diodes in that in that post on 10-24 at 5:20pm.
Power resistors will work. Better in class A amps, drawing a more constant current, but I like the zener approach much better as once they start to turn on at a few tens of ma, the voltage drop vs current doesn't matter much at all.
Well it will if you draw too much current and the zener overheats... that's why I divide the voltage drop across two or three 5 watt zener diodes.

[Doug]

So the zeners?

Bruce, I'm interested in dropping the B+. Are these in series or parallel? Could you provide me a mouser part number? I've browsed the catalog and found various 5 watt zeners, if I want to drop it 75v, three 25v zeners in series?
Also I replaced the 6v6 with another 6v6. Think I'll just replace all the caps, the big electrolytic is the last one. It has really loose bass response. What are the thoughts on SS rectifier and more power filtering? or at some point does is this amp no longer a fender champ?

[Bruce / Mission Amps]

To be honest, if you want to drop 75v you really need another power transformer.
Are you using a NOS 5Y3GT rectifier?
I have not used any Chinese 5Y3s but the Russian 5Y3GT is actually closer to a GZ30.
Although a small bottle tube, it has much lower impedance then a real 5Y3, giving way to high of B+ if used in amps looking for an old 5Y3..
Yes, you wire the zener diodes in series, cathode to anode, connect the center tap wire to the anode of the first diode and ground the last cathode.
I never use more then two to three 9.1v to 15v zeners in series as the voltage drop can created quite a bit of heat.
Keep in mind, the constant current through the diode will generate a ton of heat
EX:
100ma across 10v = 1w times 5 diodes is still 1 watt per 5 watt diode, but 100ma across one 50v diode is 5watts, the diode's limit.
What can happen is it will get so hot the solder will melt!!
So be careful in choosing the diodes and their voltage drop.

[coopecb1]

All these changes involving zeners, cathode resistors, etc. are patches to the real problem: the power transformer puts out too much voltage. Replace it with a Blackface or Tweed Champ transformer and the voltages will be more sane.

[MWJB]

"Replace it with a Blackface or Tweed Champ transformer and the voltages will be more sane." The stock PT is fine, the reason Doug has/had 470vdc on the plate was because he had biased the amp cold, if he restored the 470ohm cathode resistor & zenered down a little he would be fine, alternatively a 800ohm cathode resistor in a SF champ will bring plate dissipation back to 14/15W (without zeners).

If you replace the PT with a Mojo variant of the 125P1B PT (BF) you will be in exactly the same situation.