5f6a being a real pain.

[stretch2011]

First it was a 6v6 bassman made with ao-29 organ transformers. Found out the pt was bad so I replaced it with a Weber and the ot with a Hammond capable of handling 5881's. When powered up it didn't work right. Finally got some oscillation then nothing. Speaker blew, took the ot. Finally found out I had a bad power cap which resulted In putting high voltage on the speaker, frying it, an taking my ot.
Got a new ot. Powered up on the limiter everything was good. Took it off the limiter and the rectified voltage was the same as the high ac voltage. 350v on both sides if the rectifier. The 5881 closest to the 5u4 was barely warm while the other 5881 was red plating. Switched the 5u4 with a 5ar4 and it did the same thing.

Pulled the power transformer out and 1 primary was 50v higher than the other when referenced to the center tap.

I bought the cap and Hammond ot from antique electronics and they insisted that there's no way a bad cap could do that and that the possibility of having a bad cap was atronomical. They offered to give me a return # so they could test the cap and ot but never replied back, even after 3 messages. Needless to say they lost my business.

Soooooo. A 5 dollar cap cost me 1 speaker and a pair of transformers.

[JazzGuitarGimp]

I agree that a bad electrolytic cap in the power supply chain taking out the OT and speaker is unlikely. I'd even go as far as to say that it's highly unlikely.

[pdf64]

Agree.

Pulled the power transformer out and 1 primary was 50v higher than the other when referenced to the center tap.

Unfortunately my understanding is that Weber have experienced this problem with their supplier a couple of times. It's due to the CT not actually being a CT.
It shouldn't cause a failure, just a high static VB+, excessive ripple and perhaps, if the loading on the B+ winding is high, its temperature rise may exceed the design calculation.

[Firestorm]

I agree that a bad electrolytic cap in the power supply chain taking out the OT and speaker is unlikely. I'd even go as far as to say that it's highly unlikely.

I'll see your "highly unlikely" and raise the bet to "impossible."

[martin manning]

...one primary was 50V higher than the other when referenced to the center tap...

Are you sure you haven't swapped the CT and the bias tap?

[JazzGuitarGimp]

...one primary was 50V higher than the other when referenced to the center tap...

Are you sure you haven't swapped the CT and the bias tap?

Ahhhhh Good observation, Martin!

[stretch2011]

Positive, I had 45ish volts on my bias. Red is high voltage, red yellow is ct, and red blue is bias tap. Blue red was hooked to the bias supply.
I had 2 80uf 450v wired in series to give me 40uf 900v. The bad cap read no capacitance so my guess is it was shorting somewhere.

It wad not blown or burnt, it looked perfectly normal.

So I'm guessing and I could be wrong, that is shorted high voltage to ground and the 80uf cap that was working put way too much strain on the 5u4 which caused some fly back.

[martin manning]

Positive, I had 45ish volts on my bias. Red is high voltage, red yellow is ct, and red blue is bias tap. Blue red was hooked to the bias supply.

Could be that the yellow and red/blue wire colors were switched.

[stretch2011]

That sure does sound like a good idea. To take it one step further, my mentor had the idea to tape off the center taps and use a bridge rectifier, then take the bias from the high voltage. So hopefully that experiment can happen soon.

[sluckey]

To take it one step further, my mentor had the idea to tape off the center taps and use a bridge rectifier, then take the bias from the high voltage. So hopefully that experiment can happen soon.

That's likely to be a disaster! Your PT has about 350-0-350 VAC which will give a usable B+ using a conventional rectifier. But when you tape off the CT and connect a bridge across that 700VAC, you'll have about 950VDC and that's gonna blow up a lot of stuff!

You may want your mentor to think about it.

[stretch2011]

Valid point. the transformer has 2 high voltage secondaries. I believe it's 350-0-350, and 275-0-275 or something close. Well be using the lower voltage secondaries.

That's still going to be pretty damn high tho, so I may try this out unloaded to see if and out of the amp. My 500v power caps won't handle either.

I'm trying to figure out why my dc voltage is the the same as my ac voltage? The recto is good.

[sluckey]

275-0-275 or something close

That will still give you about 775VDC! That PT is inappropriate for use with a bridge in a circuit that has 500v caps and is meant to power 5881s.

I'm trying to figure out why my dc voltage is the the same as my ac voltage? The recto is good.

Do you have the filter caps connected to the cathodes of the rectifier? If not, that would explain low dc voltage.

[Stevem]

I for the life of me can not fathom how a bad power supply filter could take out your speaker.
I would say it's far more likely that the amp was oscillating up where you could not hear with a massively clipped square wave output and burned open the voice coil.
I would now add a 250 ohm 5 watt resistor across the speaker output so that if the secondary side of the OT ever goes open again for whatever reason the OT will still see a load and not go south again!

In regards to your B+ voltage being the same as your AC input voltage it would seem to me that the current output of that winding is not what it should be.

[martin manning]

Just for fun, why don't you see if the secondary voltage is centered around the red/blue lead?

[pdf64]

I've got 2 weber PTs from about 2005 (the original one sent, then the 1st replacement from the same bad batch) in which there isn't an actual CT.
Rather there's an 'outer' 650v winding with 2 'inner' taps for 50V, all balanced around a non-existant CT.
So if one of the 50V taps is used for a CT, the outer winding becomes 300-0-350.
Apparently their supplier had misunderstood the spec.
They sent me one from the next batch which was fine.

I noticed on a forum a few years ago that the issue may have cropped up again.