makes no sense that b+ is 399, any unusual ideas of cause?

kfelixj · Post by **kfelixj** » Fri 05/04/07 9:05 pm

This one is stange, I was wondering if anyone out there had any ideas of a one in amillion chance reasons, why this could be happening? I have been dealing with the most bizarre thing possible. I recently rebuilt a bandmaster reverb and changed it to a super reverb ab763. This is going to sound crazy but its true. I have gone thru an extensive list of troubleshooting with this thing, I even have taken it to a local amp tech, whos been doing it for 40+years, and he is at a loss right now.

Without any tubes in the amp, just the rectifier, no power or preamp tubes, It gets 360vac per leg, but the b+ is only 399. I have swapped PTS's, filter caps, anything you can think of, to no avail. Even if you bypass the rectifier with diodes it still does it. Anyone out there have any ideas that are unusual for this situiation, all the typical reasons have been tried. thanks for any help

kenny

s2 · Post by s2 » Fri 05/04/07 9:14 pm

Knee jerk reaction is to suspect the filter caps. Second reaction is to think about a short in the PS somewhere. Third idea is to wonder about the PT.

jem · Post by **jem** » Fri 05/04/07 10:37 pm

My first thought...

Since the PT was working before, it's now wired wrong...

...like the center tap got swapped with one of the "outer" leads.

Usually things like this are really obvious things that you overlook because it seems so unlikely.

But then again, it's Friday night, and I've tipped a few.

So don't take anything I've said too seriously.

Bluesmechanic · Post by **Bluesmechanic** » Fri 05/04/07 10:43 pm

kfelixj wrote:Without any tubes in the amp, just the rectifier

kenny

Are running a GZ34. Edit:BTW, I was looking at the Bandnaster Reverb Schematics and one had a B+ of only 435v.

kfelixj · Post by **kfelixj** » Fri 05/04/07 10:52 pm

I have swapped out all the main filters, even swapped out the PT for a brand new one.
-the rectifiers were a brand new gz34, used gz34, and a 5u4g
-it cant be a miswire because I wired in 2 different PTS, along with a pro amp tech to check behind me
What could cause the 100ohm resistors to be burnt up, and also low b+
it has to be a short somewhere(Iw ould think), but its having problems with so few parts
thanks kenny

Bluesmechanic · Post by **Bluesmechanic** » Fri 05/04/07 11:03 pm

kfelixj wrote:I have swapped out all the main filters, even swapped out the PT for a brand new one.
-the rectifiers were a brand new gz34, used gz34, and a 5u4g
-it cant be a miswire because I wired in 2 different PTS, along with a pro amp tech to check behind me
What could cause the 100ohm resistors to be burnt up, and also low b+
it has to be a short somewhere(Iw ould think), but its having problems with so few parts
thanks kenny

If those 100 ohm are on the screens, my schematic shows 470 ohms.

phsyconoodler · Post by **phsyconoodler** » Fri 05/04/07 11:06 pm

If the bias is too hot the B+ will be much lower.Recheck the bias and the bias supply.Look for about -48v and approx. 35-40ma of current.

kfelixj · Post by **kfelixj** » Fri 05/04/07 11:06 pm

no im sorry, the 100ohm are for the heater filament artifical center tap
thanks kenny
the amp is completely stock super reverb ab763,ive checked it 3 times, and hes checked it behind me9thats not to say its still perfect, its crazy actually

kfelixj · Post by **kfelixj** » Fri 05/04/07 11:07 pm

It doesnt even need the power tubes in it to lower the b+, but the bias is right
thanks kenny

jem · Post by **jem** » Fri 05/04/07 11:11 pm

What happens with no filtering at all?

If you're smoking the 100 ohm's, there must be a short in the heater or pilot lamp connections (assuming they're both there).

But an experienced tech should find that quickly, so it's probably not that.

What I am willing to bet is that it's something you've checked before.

Bluesmechanic · Post by **Bluesmechanic** » Fri 05/04/07 11:11 pm

kfelixj wrote:It doesnt even need the power tubes in it to lower the b+, but the bias is right
thanks kenny

Yea, I know. Disconnect the PT heater leads and see what you get.

kfelixj · Post by **kfelixj** » Fri 05/04/07 11:22 pm

I lifted the pilot light, no change, lifted the speakers too
The new PT had a center tap so it didnt need the 100ohms,
its just crazy, I cant try it though because my amp techs got it, hes really good too, hes been around for awhile now. I just thought id ask the question, cause he said hes run thru it, and is baffled as of yet, he said hes got to think on it. Its got to be something though, the amp worked, before.
thanks again
kenny

kfelixj · Post by **kfelixj** » Fri 05/04/07 11:23 pm

I do beleive with no filtering it was same scenerio, but I cant remember for sure, thats why I thought it was the PT
kenny

Bluesmechanic · Post by **Bluesmechanic** » Fri 05/04/07 11:24 pm

Also if you have a center taped 6.3v winding, why would you need the artificial CT?

kfelixj · Post by **kfelixj** » Fri 05/04/07 11:26 pm

sorry the original PT needed the aftificial center tap, and the new PT has a center tap, I typed too fast
thanks kenny

phsyconoodler · Post by **phsyconoodler** » Sat 05/05/07 12:59 am

It is obvious that something is sucking the B+ down.It has to be either the filter caps,the rectifier or the bias.I am thinking that your experienced tech is missing something pretty basic.

kfelixj · Post by **kfelixj** » Sat 05/05/07 8:27 pm

I know, but it is not the filter caps, its not the bias supply, and I installeda new PT, Thats why I took it to someone else, because so they could check behind me for stupid mistakes that I may have made. But they have all been checked extensively, he even rebuilt the bias supply, I know you think its just something stupid, but its a very rare thing thats happening here.
I was just thinking about how I used brasso to clean the chassis, Ive used iot on other amps, but somehow ac is being inducted into the power section.
I dont know its so bizarre, It has to be something though
kenny

phsyconoodler · Post by **phsyconoodler** » Sat 05/05/07 11:27 pm

Try disconnecting the trem circuit right at the roach.Sometimes the caps associated with the trem circuit can be bad and suck the voltages down.I had that happen on a Deluxe Reverb now that I think about it.
Too high a resistance somewhere can suck down voltages.Recheck grounds in that area.

s2 · Post by s2 » Sun 05/06/07 7:27 am

Do you have a schematic of this PT? Is this a Fender replacement tranny or something else?

Is it center tapped? Does it require a voltage doubler?

kfelixj · Post by **kfelixj** » Sun 05/06/07 10:18 am

I just talked to my tech, he said that there is ac being inducted on the b+which is dragging it down causing the hum etc. It is insane, I told him to just close it up, and Im thinking about gutting it and hooking up just the power section floating over the chassis and putting one piece back at a time. I have been dealing with this thing for about 2months, I really thought it was something stupid that I missed, thats why i handed it over.

The old PT is a stock 68 fender bandmaster reverb, no center tap for the filaments. the new one is a W---R tranny brand new made for super reverbs, bandmasters a general replacement for the 2 6l6 about 40 watt blackface amps.

I have tryed it actually, I replaced almost every capicitor one at a time when I had it, I disconnected the PI cap .001, (that should have of disconnected the trem right?)

He said he put it on a scope and the signal is clean until it hits the power section.

How could ac be inducted on the b+, thats why I am thinking of hooking it all up floating (even the ac cord) and putting it all back one at a time till I find the bad component.

thanks for writing kenny

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makes no sense that b+ is 399, any unusual ideas of cause?

makes no sense that b+ is 399, any unusual ideas of cause?

Re: makes no sense that b+ is 399, any unusual ideas of caus