1964 Fender Vibro champ volume problem

maybeoneday

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Hello guys,
I have a problem with volume on my '64 Fender VibroChamp.
The volume appears to be very low even for a 5 watts amp. The strange thing is that it is completely random, sometime increase but the next day is again low.
I tried to poke around with rectifier and power tube, replacing them for others which I know they are working but still no consistency.
The amp is pretty much original, only some caps has been replaced but PT and OT are still the originals.
I took the voltages from PT, Rectifier and Filter cap and I think they are a little low even tho the PT secondary is at 334V, they are as follow:

PT secondary 334VAC
Pin 8 Rectifier 313VDC (335VDC when I increase the speed and intensity of the Tremolo)
B1 313VDC
B2 285VDC
B3 265VDC

Even replacing the rectifier the voltages are pretty much the same.

Note: the 2 prong power cable has been changed to a 3 prong. Being in Europe I am using a Voltage converter but it shouldn't be a problem as using it for my '64 Deluxe Reverb and '62 5F1 Champ.
 

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schmee

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Turn the volume pot slowly form 0 to 10, does the volume ramp up smooth or does it jump or crackle at all?
I have had pots go bad before with near dead spots in the rotation.

You tried another 6V6? A failing tube can fade away and come back sometimes.
 
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maybeoneday

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Turn the volume pot slowly form 0 to 10, is the volume ramp up smooth or does it jump or crackle at all?
I have had pots go bad before with near dead spots in the rotation.

You tried another 6V6? A failing tube can fade away and come back sometimes.
I replaced both with JJs, the pots increases the volume smoothly, no scratches or jumps.
 

schmee

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I cant tell for sure, but it looks a bit like one of the black power transformer primary wires has been melted before? The one with the red shrink tube on it....? Does the tranny get hot when on for a while?
 

maybeoneday

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Beautiful Champ!
Off the bat, it looks like your power cathode bias resistor has gotten pretty warm, I'd check it's value, and if it's off, replace it and the bypass cap
I'll check it. Should it be 470R-1W?
Why do you think it gets warm?
 

maybeoneday

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I cant tell for sure, but it looks a bit like one of the black power transformer primary wires has been melted before? The one with the red shrink tube on it....? Does the tranny get hot when on for a while?
I think they did it when replaced the power chord with the 3 prongs, I also think it is not a job well done, I can try to improve it.
It is warm back there but I thought it was due to the rectifier and the power tube.
Do you think it could affect the performance of the PT?
 

schmee

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I think they did it when replaced the power chord with the 3 prongs, I also think it is not a job well done, I can try to improve it.
It is warm back there but I thought it was due to the rectifier and the power tube.
Do you think it could affect the performance of the PT?
Usually its a good or bad connection, not intermittent. There does appear to be a sharp bend in it...(?) These amps are so simple it's gotta be something easy I would think. You might check it out, maybe it's hanging by a couple of strands in there!
Has the filter cap can ever been replaced in that amp? It's certainly overdue for that if it's not been done.
 

moonlighter

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I'll check it. Should it be 470R-1W?
Why do you think it gets warm?

Yes, 470R - 1W. All resistors get warm. Part of how they function is that the voltage dropped is dissipated as heat. Over time carbon composition like these can become brittle, their value will drift, and they'll try to dissipate even more, which will eventually cause thermal runaway and complete failure.
If it is off spec, replace it with a 2W or 3W, it will be much less likely to fail again.
 

SnidelyWhiplash

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The TSRI 6v6 isn't the most robust output tube & the VC
demands such as it throws a lot of voltage. It might not be
able to handle it.
 

maybeoneday

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Yes, 470R - 1W. All resistors get warm. Part of how they function is that the voltage dropped is dissipated as heat. Over time carbon composition like these can become brittle, their value will drift, and they'll try to dissipate even more, which will eventually cause thermal runaway and complete failure.
If it is off spec, replace it with a 2W or 3W, it will be much less likely to fail again.
Indeed look at this, I think the resistor is completely gone, it gives me 208ohm and to get the read I had to move it quite a lot.
 

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SnidelyWhiplash

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Is that the bias resistor? If so, at that low resistance it won't
provide enough resistance to regulate the output tube & cook it. My
experience with CC resistors is that they tend to drift higher.
I'd replace, then see what happens.
 
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moonlighter

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Indeed look at this, I think the resistor is completely gone, it gives me 208ohm and to get the read I had to move it quite a lot.

It could be cracked internally, which would explain a lot of the intermittence you were having. Only thing left to do is replace it and see what happens
 
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moonlighter

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If so, at that low resistance it won't
provide enough resistance to the output tube & cook it
In cathode bias, you're generating a voltage on the cathode to set it above the grid electrically. This is done by having the current the tube is drawing run through that cathode resistor. A higher resistance will equal more negative bias. A lower resistance will run it hotter, and at 208R I do believe the 6V6 was fighting for it's life some.
 

maybeoneday

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I got a 470-2W and a 470-10W, I will replace with the first one and replace the 6V6 with the JJ.
I will also remake the contact between power cable and PT.

I'll get back to you with some result.

However don't you think the DC from the rectifier is a bit low or is it connected to the bad resistor?
 

slider313

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The B+ on the first node seems low. That section may be leaking in the original cap can. It's long overdue so replace the can and install a 3 watt, 470 ohm metal oxide bias resistor and keep a bit more space between it and the bias cap.

What's the voltage at pin 3 of your 6V6?
 
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moonlighter

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However don't you think the DC from the rectifier is a bit low or is it connected to the bad resistor?

So with that low cathode resistance, the tube was drawing more current, more current across the power supply will equate to more "sag" and lower voltages throughout the supply. The numbers should come up a bit when you put in the new resistor.
 

King Fan

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What a wonderful classic amp; deserves intense love and care. This thread has some good diagnosis testing by you and advice from others. By all means, pursue these details -- for one example, the cathode bias resistors in these guys get *roasted*, and coming up on 60 years is a long time.

Big picture, though, be aware your variable volume could be almost anything, or even more than one thing: Anything that affects those voltages, a loose or cracked wire, loose solder joint, drifted or cracked cap, loose socket springs, dirty pins, faulty or loose tube, input or speaker connections, maybe grounds, who knows.

Biggest picture, beyond the volume problem, an amp this old (and *nice*) needs and deserves a 'full service.' Looks like the bypass caps were done at some time, which is good. The cap can is totally suspect, and can harm your sound every day until it (potentially) takes out your transformer. Unfortunately this is the heavy lift on an overhaul -- takes a huge iron and a fair amount of skill and caution. Worth taking to a tech unless you have all of those and some patience to boot.

Any resistor more than 10% from spec should go. First suspects are the ones that get super hot (the big dropping resistors, those mounted over the sockets). And the plate resistors take a beating, as mentioned. I'd just measure every single resistor -- it isn't a big amp -- but be aware there may be a few that can't be measured without lifting one end from the circuit.

My BF '67 came to me both modded *and* worn out, so yours won't need nearly as much "love" (lol) but FWIW I counted up 47 things I checked, replaced, or revised. It sounds great, and in my defense, almost all of the resistors and caps in this "after" pic were new -- or unused NOS -- and I don't think it looks *too* far from vintage...

1667848915630.png
 




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