BF Fender Vibrochamp questions

[nosaj]

Have a BF vibrochamp aa764 with no output.
They had a 5u4c in place of the 5y3. The 6v6 was just about dead. upon opening the amp it was apparent it had never been worked on before.

Replacing the 25uf 25v caps and 470 bias resistor and the 10uf/25v cap an cobbling in a test OT The amp does work.

My thoughts are the 5u4 pushed the voltage up which stressed the already old caps which burnt up the 6v6 and took out the OT.. Is this correct?

Somewhere I was reading a company provided mods( a resistor on the power tube socket) would that be a screen resistor? which would have prevented some of the damage?

just pondering here

Thanks,
nosaj

[Leo_Gnardo]

My thoughts are the 5u4 pushed the voltage up which stressed the already old caps which burnt up the 6v6 and took out the OT.. Is this correct?

Any voltage increase from this rectifier swap would have been not very much. Champs often fail for a couple of reasons. 1: cathode bypass cap is jam up to the cathode resistor. All that heat from the resistor eventually causes the cap to fail short. Mallory white plastic caps, I've seen some melted. Sometimes the cathode resistor changes value with all the heat it generates, and not necessarily up. Roasted CC resistors sometimes decrease in value, occasionally to a near short. B) The output tube is run at or beyond its plate power limit, sometimes it shorts causing cathode components to become damaged. Also the 1K CC power supply resistor gets baked and may go open-circuit, but sometimes its value decreases putting the output tube in a death spiral.

No harm installing a grid limiter resistor upon repair. Why not a control grid "stopper" while you're at it. Fender skimped on these parts because "it's just a student amp, no point in spending extra money making it."

Some Champs come in with 5AR4 or even solid state replacement rectifiers, somehow they survive. I use JJ 6V6 as they can take more punishment than the other currently manufactured 6V6's. And some of those old RCA and Sylvania 6V6's take a beating and keep on smiling too.

[nosaj]

Any voltage increase from this rectifier swap would have been not very much. Champs often fail for a couple of reasons. 1: cathode bypass cap is jam up to the cathode resistor. All that heat from the resistor eventually causes the cap to fail short. Mallory white plastic caps, I've seen some melted. Sometimes the cathode resistor changes value with all the heat it generates, and not necessarily up. Roasted CC resistors sometimes decrease in value, occasionally to a near short. B) The output tube is run at or beyond its plate power limit, sometimes it shorts causing cathode components to become damaged. Also the 1K CC power supply resistor gets baked and may go open-circuit, but sometimes its value decreases putting the output tube in a death spiral.

No harm installing a grid limiter resistor upon repair. Why not a control grid "stopper" while you're at it. Fender skimped on these parts because "it's just a student amp, no point in spending extra money making it."

Some Champs come in with 5AR4 or even solid state replacement rectifiers, somehow they survive. I use JJ 6V6 as they can take more punishment than the other currently manufactured 6V6's. And some of those old RCA and Sylvania 6V6's take a beating and keep on smiling too.

It had the mallory cardboard caps in it. Thanks for the reply and extra info.

nosaj

[nosaj]

I replaced the Mallory brown cardboard caps, 470 ohm resistor and the output transformer.

Now when volume is raised above 5 the sound goes all nasty . I tried to keep all wires as they were.
Looking at what values might be used for a grid limiter and control grid stopper .

nosaj

If someone could provide a pic that would be great as what I've seen so far grid stopper installed on pins 5 and 6 on the 6v6. not finding anything pics on grid limter

[Leo_Gnardo]

I replaced the Mallory brown cardboard caps, 470 ohm resistor and the output transformer.

Got some sound out of the amp now, good!

when volume is raised above 5 the sound goes all nasty .

To be expected. Champ turns out only a couple of watts, and clips asymmetrically. The basis of many a hit song's guitar sound in spite of what it may sound like all by itself.

Looking at what values might be used for a grid limiter and control grid stopper.

Ye olde Fender standards why not. 1K5 grid limiter and 470R to the screen grid. As on the larger amps you could use pin 6 as an attach point for your screen grid resistor, pin 1 same for the grid limit resistor. Likely you won't hear much of a difference, just add to output tube's stability & lifetime.

[nosaj]

Got some sound out of the amp now, good!



To be expected. Champ turns out only a couple of watts, and clips asymmetrically. The basis of many a hit song's guitar sound in spite of what it may sound like all by itself.



Ye olde Fender standards why not. 1K5 grid limiter and 470R to the screen grid. As on the larger amps you could use pin 6 as an attach point for your screen grid resistor, pin 1 same for the grid limit resistor. Likely you won't hear much of a difference, just add to output tube's stability & lifetime.

Not good sound almost sounds oscillating but not the vibrato. Hard to describe but it continues on after I quit playing. I'll add the resistors then try it out.

nosaj

[Chuck H]

Regarding the nasty clipping, my own experience with Champs is that the low resistance in the screen circuit (both the stopper and the HV rail) and the high impedance of the OT (which is only made worse at HF because it's small and cheesy) results in watts finding a shorter path out of the tube via the screen circuit. That is, screen over dissipation and severe voltage spikes. Sounds awful. Change the first HV rail resistor to 3.3k and change the screen grid stopper to 1k. You can also add a zobel (5k 10W/1000p 2kv) to the OT primary. This helps a lot with the raspy tone.

Of course, if you really want to take it all the way... Jam your meter probe into the board and see how much DC voltage is lingering at different locations. You may see anything from several hundred millivolts up to a few volts. Then take out that crappy paper and carbon resistor of a board and throw it at a passing car. Some guys say that the board can absorb moisture and can be dried out. This hasn't been my experience. Besides, who's to say the same moisture conditions won't persist anyway. Most people don't keep their amp in the laundry room or sauna, and yet... So apparently plain ol' house air has too much moisture? I don't see how blow drying the board is a good solution for that. Good that it's a Champ. With a new (not black paper) board in hand you can replace it in a couple of hours. Rant over.

[nosaj]

I used this classic tone OT and and living in the gulf coast won't help much with humidity.40-18031
nosaj

Anyone know what the original OT was 5k or 8k? I used the 8k

[g1]

Not good sound almost sounds oscillating but not the vibrato. Hard to describe but it continues on after I quit playing. I'll add the resistors then try it out.

nosaj

Try reversing the OT primary wires.

[nosaj]

Try reversing the OT primary wires.

Reversed the wires and then look at the transformer PDF color codes were different So on the speaker wires I had 4 ohm hooked up but the speaker was not original and measured 8ohm. So I hooked it up correctly for an 8 ohm load.
And it now sounds as it should.

I'd never heard the noise created by an out of phase transformer so now I know the sound.

Thanks,
nosaj

[g1]

I'd never heard the noise created by an out of phase transformer so now I know the sound.

It can vary quite a bit, from lesser problems at higher volumes like you had, to instant full power howling as soon as the unit is powered up.
Always worth a try if OT has been changed, or if wiring is at all suspect. The colour codes can't always be trusted either, especially the primary of OT's.

[Chuck H]

I've built a few amps Usually, on a push/pull amp and when the OT is using Fender type color codes the brown lead goes to the inverted triode of the PI. For single ended OT's the - lead is usually black or some dull color and the + is a hotter or brighter color. But I've seen it go the other way a couple of times and I've wired them backwards on my own a couple of times too. The first time it happened I got the instant loud squeal. I remember thinking I must have really screwed up somewhere. Someone here told me to try reversing the leads. I was sure glad for it too.

[nosaj]

Can someone tell me why a 470 ohm 2 watt resistor would burn up? It burnt up and the sprague 25uf/25v cap blew after about 3 hrs of play.
I figured a 2 watt would be ok as a 1 watt was on the schematic.
Ie I did not add the grid stopper or limiter yet.

Nosaj

[eschertron]

Can someone tell me why a 470 ohm 2 watt resistor would burn up? It burnt up and the sprague 25uf/25v cap blew after about 3 hrs of play.
I figured a 2 watt would be ok as a 1 watt was on the schematic.
Ie I did not add the grid stopper or limiter yet.

Nosaj

Cathode current in excess of 65ma would do it. 3 hours of thrash playing? If not, maybe an ultrasonic oscillation? Screen and grid stoppers should help that. Otherwise, use a 5W cathode resistor. If the resistor toasted, then the cap would fail immediately on overvoltage.

edit: What's your B+ voltage?

[Leo_Gnardo]

What eschertron said, also a shorted output tube, could have been sent that way from stress, or maybe just bad luck. I think it's high time you added grid stopper & screen grid limiter resistors. It's also OK to raise the value of that cathode resistor, 560 or 620 ohms, plus use a bypass cap with a higher voltage rating 50 or 63 or 100V.

FWIW I often use 6L6 in a Champ. Doesn't get more power, but it's loafing compared to a 6V6 which is usually being run beyond reasonable limits.

Fender conceived Champ as a student amp, to be made cheap as possible and played at polite volumes. Little did they know . . . Funny they keep making the same "mistake." It's up to us to do our best to bulletproof our little amp buddies.