Amps Fender Vibro Champ blackface.

[CapnDenny1]

This is an amp from Dave my amp fixing buddy. He said it was real distorted. I inspected it and it looked good. It had the caps replaced and a 3 wire cord installed. I brought it up slow, and everything seemed to be Ok. So I went ahead and hooked up a guitar.... and it seems to work just fine.

I will spend some more time playing it and do the chopstick think to see if I can find a bad connection somewhere.

I have to say this is the first small Champ style amp that I actually liked the sound. It is a single ended 6V6 power tube, so I think it is 8 watts. They have a Gibson 8" 4 ohm speaker in it. It just has a real nice sound to it.

These amps sell for over $1K.

 

[snarf]

An old Vibro Champ is one of two things I always seem to troll the local guitar shows for every year (an old Vibro Champ and a specific model of the Takamine Santa Fe guitar line that they made in 1993). When I first noticed them back like 15 years ago, they were pricing them around here at about $700 or so. Like you said, the last few years, I've noticed they seem to be asking more for them. Last year at the Dallas Guitar Show, I found one that sounded really nice that had a tag on it for $1250. I guess they've gone up because of the vintage market. The ones I've found I haven't been able to bring myself to drop the cash on, but they're small amps with smaller watts that I generally like for the music room. I didn't realize they had a Gibson speaker in them.

Nice amp!! Thanks for telling us about some of these more interesting amps that you work on. It's fun to read about them!

 

[CapnDenny1]

Yeah, I think it's pretty darn cool too.

I decided to at least check out the bias current. It is not adjustable, and the amp is cathode biased, which are pretty reliable.

But I went ahead and hooked up my bias probe. So I put the ammeter in the circuit, turned on the power, 5mA, then 10, then 30, 50, 70, 100, OK time to shut that puppy down!

The cathode resistor was a 470 ohm, and it looked fine. So perhaps something with the control grid circuit? A leaky capacitor from the previous stage would do that, by applying a positive voltage on the grid. I too the power tube out, and measured the voltage on that pin. It went to 5V the -2 then 2V... ? I measured the grid resistor that goes to ground, and as I measured it, the grounded side of the resistor moved. It was soldered thru a jumper wire from the turret board to the brass ground shield. Well, it was supposed to be anyway. There was a nice little groove in the puddle of solder where it used to lay, but it wasn't there. So I soldered it down, and powered it back up.

Now I had a steady 50mA, not increasing at all. Cool. But 50mA seemed a bit high for a 6v6gt tube? I went to the Weber Bias Calculator on the net, and with the datasheet expected voltage of 340V across the tube, the maximum 90% bias current was like 35mA? Not good.

So I tacked an extra 240ohm resistor in series with the 470 ohm resistor. Now the current was 39mA. But I checked the voltage from the plate to the cathode and it was 375V. The bias calc said 33mA. So i found a 1K, 5W resistor and put that in. With that I had 32mA, and 382V across the tube. That is just about right.

So I went ahead and put it back together and got a chance to play it. Really nice sounding Fender clean. It's no Twin Reverb, nor a Princeton Reverb, but a really nice sounding small amp. It needs a better speaker. The little Gibson speaker is nice, but it would really sound nice with a better speaker. I may try putting that Hemp cone Emminence, just to see. If it really sounds better I will call Dave and ask what he wants to do.

So Dave was right there was a problem. I just wonder if it was at 100 mA when I tested it, or if maybe it was working. I think it was running wide open. It was amazing how decent it still sounded. There was a little distortion, but not that much. I guess if amps sounded bad when they wanted to redplate, people wouldn't let them do it. Although I was watching the tubes when I tested it before the fix, and nobody was redplating at the time.

 

[gpower]

Have a '78 SF Vibro Champ that's been my go to practice amp since I got it. Paired with a Holy Grail+ for reverb, it sounds great for a little amp and, gets plenty loud enough. Obviously, in the one you're working on, someone's replaced the speaker. Specs say it should be an Oxford 8EV. Output is 6 watts.