Fender VibroChamp '78 B+ voltage low

[fpolak]

Hi all,

I have a problem with my fender vibrochamp 1978 B+ voltages. I recieved 362AC from red wires of power transformer, but when i rectify voltages down to 167DCV  . I tried several working 5y3 tubes, and 1N4007 diode rectifier with similar results. If i connect this output to first filter cap voltages grow up to 255DCV.

Only modification from factory was replacing can cap with 3 22uf x 450v individual caps. B+ voltages in AA764 schematic is 365DCV 

Please help!!

[shooter]

pull all the tubes except rectifier and measure again

[fpolak]

Tester measure with 2 1N4007 diodes rectifier without filter caps;

From ground to:

Rectifier tube pin 4: 179AC
Rectifier tube pin 6: 181AC
Rectifier tube pin 2: 168DCV
Rectifier tube pin 8: 168DCV

From pin 4 to pin 6: 362AC

[fpolak]

pull all the tubes except rectifier and measure again

I tested without tubes and filter caps, only rectifier tube or diode rectifier. Now im working with 2 1N4007 rectification.

[Latole]

Tester measure with 2 1N4007 diodes rectifier without filter caps;

From ground to:

Rectifier tube pin 4: 179AC
Rectifier tube pin 6: 181AC
Rectifier tube pin 2: 168DCV
Rectifier tube pin 8: 168DCV

From pin 4 to pin 6: 362AC

Tester measure with 2 1N4007 diodes rectifier without filter caps. 

Voltage are always very low without filter caps.
Right voltage reading need filters caps

[sluckey]

Rectifier tube pin 4: 179AC
Rectifier tube pin 6: 181AC

That PT is not putting out enough voltage. You need 315VAC on pin 4 and also pin 6. That's 630VAC ***BETWEEN*** pins 4 and 6! Look at the schematic.

     https://el34world.com/charts/Schematics/files/Fender/Fender_champ_vibro_aa764_schem.pdf

[mresistor]

What is the "input" AC voltage reading?   

[sluckey]

I see you're in Argentina and Argentina operates on a 220V supply voltage and 50Hz. You need to replace that PT with one that is meant to operate on 220V rather than 120V.

[fpolak]

Rectifier tube pin 4: 179AC
Rectifier tube pin 6: 181AC

That PT is not putting out enough voltage. You need 315VAC on pin 4 and also pin 6. That's 630VAC ***BETWEEN*** pins 4 and 6! Look at the schematic.

     https://el34world.com/charts/Schematics/files/Fender/Fender_champ_vibro_aa764_schem.pdf

So, it could be a power transformer problem??

[fpolak]

What is the "input" AC voltage reading?

120VAC

[sluckey]

In that case you need a new PT.

[fpolak]

I see you're in Argentina and Argentina operates on a 220V supply voltage and 50Hz. You need to replace that PT with one that is meant to operate on 220V rather than 120V.

I used 220VAC from line to 120VAC transformer. I used it with several tubes amps without problems. 

[sluckey]

OK. You still need to replace the PT on the amp!

[2deaf]

Maybe the PT is wired for 220V and he is using a step-down transformer that supplies 120V to the PT.  This would be consistent with the half-voltages on the secondary.

fpolak:  Don't try plugging straight into 220V until we get this figured out.

What is the heater voltage from pin 4 to pin 9 on a 12AX7 with 120VAC?

[fpolak]

Maybe the PT is wired for 220V and he is using a step-down transformer that supplies 120V to the PT.  This would be consistent with the half-voltages on the secondary.

It's original transformer from USA (120VAC input). I  connected PT directly to 220VAC, isolated from amp to verify and results are very strange...

Green cables voltages 7,1 VAC
Red cables: 673 VAC
Yellow cabled: 4,9 VAC

It seems to be 220VAC, but im sure it's 120, because I used it for 20 years.

Is it safe to connect it to amp without tubes??
Is it ok 7,1 VAC in Green cables or too high?

[2deaf]

I have a USA 120VAC champ that I can wire for 220VAC if I so desire.

Pictures and PT numbers would be helpful.

[fpolak]

I have a USA 120VAC champ that I can wire for 220VAC if I so desire.

Pictures and PT numbers would be helpful.

L022772
CSA TESTED
EIA606-933

[sluckey]

Looks like you have a dual primary PT and it's set to operate on 220VAC. You should be fine to plug it straight into the wall without your step-down transformer.

     https://el34world.com/charts/Schematics/files/Fender/Fender_bronco_ab764_schem.pdf

[fpolak]

Looks like you have a dual primary PT and it's set to operate on 220VAC. You should be fine to plug it straight into the wall without your step-down transformer.

     https://el34world.com/charts/Schematics/files/Fender/Fender_bronco_ab764_schem.pdf

Ok, i try it with 220VAC, all connected and works perfectly. Voltages now;

B+: 365 DC
B+1: 353 DC
B+2: 340 DC
Heater: 6,8 VAC

Is it ok 6,8 on heater with load? Tubes are too hot in only 5 minutes.

[sluckey]

Is it ok 6,8 on heater with load? Tubes are too hot in only 5 minutes.

Yes. Tubes are supposed to be hot.

[2deaf]

PT numbers say it was made in 1979.  Any chance this is actually a '79 Champ?

[fpolak]

PT numbers say it was made in 1979.  Any chance this is actually a '79 Champ?

Yes, maybe it's a 1979 Vibrochamp

[fpolak]

Yes. Tubes are supposed to be hot.

Yes I know... but im not sure if 6,8 VAC could hotter tubes a little more than normal 6,3 VAC.

Could I down voltages with zener diodes?  Do you know another way to get 6,3 VAC?

[sluckey]

Most of my amps run about 7VAC on the filaments. I'm not concerned. You should not be concerned. It's all good. Let it be.

Now, the only thing left to do is try to figure out why you believe you have run that amp for 20 years with a step-down transformer.   

[shooter]

i'd verify the meter before going wild

[fpolak]

Most of my amps run about 7VAC on the filaments. I'm not concerned. You should not be concerned. It's all good. Let it be.

Now, the only thing left to do is try to figure out why you believe you have run that amp for 20 years with a step-down transformer.   

Because I bought in USA, it say 117 VAC 50Hz, has original trasformer and power cable, caps. Maybe question is why fender connect an amplifier to 220 primary... can be 110/220 auto switching??

[sluckey]

can be 110/220 auto switching??

No. Look at the schematic.

[fpolak]

Now sound with a lot of volume, very loud and trebble but... without body... 

Bass control seems dont work and 6L6 tube goes blue after 2 minutes. I readed vibrochamp needs modifications, so this can explain this weak (not volume) sound.

But I'm concerned about 6L6 blue glow. Need I to verify any voltage, bias, cathode resistor or it's ok?

[sluckey]

You can't expect too much from a cheap 8" speaker. Blue glow is not necessarily a bad thing. Many perfectly good power tubes will have a blue glow.

This amp was designed to use 6V6s, not 6L6s. I suggest you put a known good 6V6 in the amp, plug it into a bigger/better external speaker and re-evaluate.

I'm sure this amp is wired for 220V operation. But have you compared your actual amp wiring to the schematic I linked above to verify? Can you read and understand a schematic? If not, if you post some hi-rez pics that clearly show the PT and where all the PT leads connect we can probably verify for you.

[fpolak]

This amp was designed to use 6V6s, not 6L6s. I suggest you put a known good 6V6 in the amp, plug it into a bigger/better external speaker and re-evaluate.

6V6, that's right. I Wrote 6L6 because im working on other amp.

I'm sure this amp is wired for 220V operation. But have you compared your actual amp wiring to the schematic I linked above to verify? Can you read and understand a schematic? If not, if you post some hi-rez pics that clearly show the PT and where all the PT leads connect we can probably verify for you.

Yes, I know how to read schematics, but I uploaded PT picture to verify. If im not wrong it's connected to 117ACV primary, because I don't see black-yellow and black-green cables.

[2deaf]

That isn't a 22772 PT.  Somebody changed the PT and apparently used the old end bell on the outside.

[fpolak]

Weak sound, excesive trebble, blue glow on 6V6

Problem solved! I recovered bass tone control. I change cathode resistor and cap to 1k 5watts and 47uf/50v. I'm not sure if was a bias problem. Could be... But now I have a new problem. I can't play any guitar on neck/rhythm position. Bass sound cracked/distorted.

But in both positions (neck/bridge) goes distorted with volume more than 5. Under 5, amp sounds great!!! a lot of headroom, very clean but fender sound.   

I test my amp with an Ampeg SVT410HLF speaker cab (yes, a bass cabinet...) So, no speaker problem...

[fpolak]

That isn't a 22772 PT.  Somebody changed the PT and apparently used the old end bell on the outside.

This can explain everithing... but I buy it in USA 20 years ago, so in USA someone change primary to 220v... very strange!!

[2deaf]

This can explain everithing... but I buy it in USA 20 years ago, so in USA someone change primary to 220v... very strange!!

I'm pretty sure the USA is the unchallenged world leader in strange.

[thetragichero]

But in both positions (neck/bridge) goes distorted with volume more than 5. Under 5, amp sounds great!!! a lot of headroom, very clean but fender sound.

sounds about like what i remember of my 68 vibrochamp

[2deaf]

Vibro-Champs can get some really bad sounds if you turn the tremolo off with the footswitch and turn the Intensity up.  Eric might disagree.

[thetragichero]

have you tried adding screen and grid resistors to the power tube, possibly a grid resistor to the second preamp stage?. with the right speaker (i had a weber in mine) this should sound great until it's really cranked

[fpolak]

have you tried adding screen and grid resistors to the power tube, possibly a grid resistor to the second preamp stage?. with the right speaker (i had a weber in mine) this should sound great until it's really cranked

No... I dont try. What values do you use?

Whats about Negative feedback, If I add it can help?

[sluckey]

The amp already has NFB.

[thetragichero]

the yellow wire from the output that goes to v1b cathode through a 2k7 resistor? should already be there (edit: sluckey best me to it)
i use 470r 5w cement resistor for the screen. probably overkill but I've got a ton of em so why not?
power tube grid stopper... 5k6?
second stage grid stopper i use "33k" which means whatever is close to 33k that i pull out of the bin first. could be 27k, 33k, 39k, 47k.... leo was notorious for pinching a penny so while a couple resistors here and there may add up to a dollar for you and i, hundreds of dollars (not including labor costs) in a production environment made the choice for him
that being said, these are killer little amps with a bit of tweaking and a decent speaker. with today's cheap-per-watt pa systems one could easily gig with these miced (i played my vibrochamp at church until i built a beefier version running 6l6g power tube and 10" speaker)
have fun with it, keep in mind that distortion about halfway up is par for the course with these

[2deaf]

I would like to see a picture of the rest of the guts.

[fpolak]

I would like to see a picture of the rest of the guts.

I uploaded a picture of my vibrochamp. I changed from original circuit all caps to orange drop, cathode resistor and bypass cap to 1k 5w/47ufx50v and tremolo cap to 0.22uf for slower response.

Amps sound great at 5 volume o less.

[fpolak]

the yellow wire from the output that goes to v1b cathode through a 2k7 resistor? should already be there (edit: sluckey best me to it)
i use 470r 5w cement resistor for the screen. probably overkill but I've got a ton of em so why not?
power tube grid stopper... 5k6?
second stage grid stopper i use "33k" which means whatever is close to 33k that i pull out of the bin first. could be 27k, 33k, 39k, 47k.... leo was notorious for pinching a penny so while a couple resistors here and there may add up to a dollar for you and i, hundreds of dollars (not including labor costs) in a production environment made the choice for him
that being said, these are killer little amps with a bit of tweaking and a decent speaker. with today's cheap-per-watt pa systems one could easily gig with these miced (i played my vibrochamp at church until i built a beefier version running 6l6g power tube and 10" speaker)
have fun with it, keep in mind that distortion about halfway up is par for the course with these

I want to try some of yours modifications, but I dont know where to start. First I will check bias point.

[fpolak]

I ckecked bias point. It's 10,14 WATTS with 1k cathode resistor and 47/50 bypass cap. So isn't a bias problem.

I don't know why still distorted with volume more than 5. I'll try put a grid stopper resistor to power tube. Which values do you recomend?

[drew]

I don't know why still distorted with volume more than 5.

It's a cheap little 5 watt amp for kids taking guitar lessons.