AB763 V5 Voltage too high

[NannerPuddin]

Man am I stumped. My AB763 Vibroverb that I built and have been playing for 3 years crapped out at a gig the other night.  It didn't blow a fuse, just all of the sudden went down to almost zero volume and very fuzzy, like you just hit the standby switch.  I checked my voltages and the PI voltages were a tad low, about 20v low on each pin, 235, 100, 243.  What is really odd is that V5, the trem tube, I got 440v on pin 1 and 0 on pin 8 vs 280v on pin 1 and 17v on pin 8 per the fender sheet.   I suspected the 220k resistor that feeds that pin and I dropped in a new one and still way high on the voltage.

Does anybody have a clue as to what it might be?  The other side of the resistor is 460v like on the layout.  The bias voltage of the bias pot is -46 instead of -55.   Every other voltage checks out.

Thanks,

Doug

[LooseChange]

The tremolo is not in the audio path.
Need more info.. More voltages, more tubes.
Check the heaters to be sure they are all lit.

[EL34]

Enter your volatages here

http://www.el34world.com/charts/valve/ValveData.php

Hard to visualize it all without seeing the voltages
Especially the cathode voltages on the preamp tubes

No need to record the trem tube voltages

[NannerPuddin]

Wow, cool voltage program!  Finally got my pin voltages enterd:

http://www.el34world.com/charts/valve/ValveData.php?e=view&f=12676

I did check all of the heaters and they all light up and are warm.

I am starting to think it is my OT maybe?   At this point I don't care what it is as long as I KNOW what it is!

Suggestions?

Thanks,

Doug

[HotBluePlates]

Is this an "AB763 Vibroverb" using Fender's original layout, or using the Hoffman AB763 board, but with 2x6L6's and a 15" speaker?

[stratele52]

 I am starting to think it is my OT maybe?   At this point I don't care what it is as long as I KNOW what it is!

Suggestions?

Thanks,

Doug

Why output transformer ? This is the very very last thing to suspect. Better to check power tubes, first and power supply voltages.
Lost of voltage do what you hear. Speaker and their connections are good  ?

[EL34]

I looked at your voltages and compared them to this Fender AB763 Vibroverb
VIBROVERB_AB763
V6 looks fine to me if you compare it to the vibroverb above, it's just a tad lower, no big deal

Your 12AT7 reverb driver voltages look wierd.
Both halfs of the tube are tied together as a reverb driver and should have the same voltages
The tied together cathodes should be way higher than 2.1 on that 12AT7, more like 9+ volts

Are you sure you entered all your voltages correctly?

Here is your voltage chart link. You can edit it and save it again if needed
http://www.el34world.com/charts/valve/ValveData.php?e=view&f=12676

 

[NannerPuddin]

Yes, that was a typo, 9.2v on 3 and 8 V3.

This is original AB763 build, the only mod is a switched intensity pot to turn it off.  1 15" speaker.  I have tried different good speakers, and know good power tubes.

I fixed the bad voltage number and now everything is correct.

http://www.el34world.com/charts/valve/ValveData.php?e=view&f=12676

Man, I can't wait to get this thing fixed.

Thanks,

Doug

[EL34]

Your voltages look fine
What I would do next is listen to the signal as it proceeds through the amp and find the spot it stops.

I used a small solid state amp to listen in on the AC signal
That tool is shown on this page
http://www.el34world.com/Hoffman/tools.htm

[NannerPuddin]

Thanks, that is a great idea.  I think I have all the stuff I need to build a abox to filter out the DC laying around.  What I don't know is where to put the probe for the listening tests.

Thanks,

Doug

[EL34]

You can listen to the input grids pins 2 - 7 and plates 1 - 6 of all pre amp tubes and phase inverter

Also, input grids and plates of power tubes

No need to listen to trem or reverb tubes

The blocking capacitor on the tool input must be at leat 600v
It is there to block the high voltage when you probe plates and such
And you need some sort of power soak plugged into the speaker jack on the vibro

Here's a diagram of how the signal flows from input jack to putput tranny on the vibrato channel

[Willabe]

Here's a diagram of how the signal flows from input jack to putput tranny on the vibrato channel

What's a "putput" tranny?


             Brad     

[sluckey]

What's a "putput" tranny?

Sssh. That's one of Doug's secrets. I'm surprised he let that slip out! 

[Geezer]

Here's a diagram of how the signal flows from input jack to putput tranny on the vibrato channel

What's a "putput" tranny?


             Brad     

What's a "putput" tranny?

Sssh. That's one of Doug's secrets. I'm surprised he let that slip out! 

 

THAT gave me my cardio workout for today (laughing) and almost made me wet my britches. 

G

[Willabe]

Sssh. That's one of Doug's secrets. I'm surprised he let that slip out!  

Ohh, he's keeping the good stuff for himself, huh?      

No wonder his bumble bee can twist.


                    
                           Brad      

[EL34]

Another one of my awesome typos

I have a knack for finding magic key combos
I yhink my dingers are oossesed

[NannerPuddin]

Well I made one of the boxes to block the DC and listened with a probe.  Everywhere sounded fine.

Turns out it was the putput transformer!  (I have been waiting to write that for days!)   I happened to have an new putput tranny in the parts closet, so I gave it a try. 

Thanks so much for all the help.  I have two gigs this weekend and I am thrilled to have it roaring again.  Also learned a bit more about trouble shooting.

Cheers,

Doug

[EL34]

Cool, glas yot founs the vulprit

[jjasilli]

Turns out it was the output transformer! That's the "good" news.  The bad news is that some other fault, which still exists, may have caused the OT to blow.  (E.g., if it was no speaker load, maybe use a shorting speaker-out jack with a resistor load, to guard against that.) 

[proaudioguy]

Would you need several power resisters say 10 watts each in series parallel to make that work for a twin?  Although unless the speaker develops an open I can't imagine this happening randomly during a gig. Maybe check the solder joints from the output jack and the plug and the speaker terminals. I've killed a lot of speakers but never seen one go down in a 40watt guitar amp in the last 28 years of mixing.

[NannerPuddin]

Yeah, I thought about that.  I can't rule out the possibility that it was somehow related to damage incurred when the PT blew in October.  Hopefully that was it.  I am always very carefule about the load on the speaker jack so that should not have been it.  It was weird when it failed.  I was mid song and over a period of about 5 seconds the volume just went really low and distorted.

Thanks again,

Doug

[jjasilli]

Would you need several power resisters say 10 watts each in series parallel to make that work for a twin? 

No.  For this purpose a higher value, lower wattage, resistor is used.  Can't locate the details just now.  Seem to remember Traynor did this.  Maybe someone can chime-in.

[Willabe]

No.  For this purpose a higher value, lower wattage, resistor is used.  Can't locate the details just now.  Seem to remember Traynor did this.  Maybe someone can chime-in.

KOC recommends putting a 270R 5w across the OT secondary. I think our friend tubeswell uses a 330R 5w?


                     Brad