Full wave bridge rectifier question

[Bob-I]

I have a project I'm working on and I need a PT. The circuit uses a full wave bridge rectifier and the specs say 285V@.35A. A few questions...

1) That seems like a lot of current for a 2x6L6 amp with 3 12AX7s. I would think more like 120ma
2) It's difficult to locate a PT without a CT. Could I use one side of a PT that produces 270-0-270?
3) Is there another option for a PT, one that would work as this circuit is designed?

Thx in advance for the help.

[pompeiisneaks]

According to this site:

http://www.ampage.org/td/vtd6l6.html

the data for a PP AB amp shows that per tube you're expecting about 210mA max current for 6L6Gc's. 12AX7's are a lot lower: http://www.audiomatica.com/tubes/12ax7.htm Plate current max 6ma, per tube,

Thus 210mA + 18mA = 238ma or .238A seems like it's giving the expected engineering fudge factor for other parts of the amp? Without a schematic, it becomes a bit hard to know more for sure.

~Phil

[martin manning]

DC current out of a FWB is 0.62 times rated AC current so the 350 ma AC becomes 217 ma DC. You don't have to use the CT if you have one but is is good for balancing voltages on a stacked reservoir cap. No reason you couldn't convert it to a FW CT rectifier.

[pompeiisneaks]

Oh very good to learn martin, I didn't realize the losses in the rectifier.

~Phil

[martin manning]

Handy reference: http://www.hammondmfg.com/pdf/5c007.pdf

[pompeiisneaks]

Lol! I actually have that one in my 'documents' folder, but I read it and somehow completely missed the current changes. (they're right there staring me in the face)

[Bob-I]

Great info folks, thx.

Any ideas on a PT for this one?

[davebolden44]

If one were to use a bridge rectifier on a center tapped transformer and simply grounded the center tap what would be the effect? I thought I read in Blencowes book it was a no no but I cant find it in there skimming through again.

[pompeiisneaks]

If one were to use a bridge rectifier on a center tapped transformer and simply grounded the center tap what would be the effect? I thought I read in Blencowes book it was a no no but I cant find it in there skimming through again.

I'm pretty sure you don't ground it, you just cover the end and tuck it away. I think it's a nono too

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[Bob-I]

If one were to use a bridge rectifier on a center tapped transformer and simply grounded the center tap what would be the effect? I thought I read in Blencowes book it was a no no but I cant find it in there skimming through again.

No no no...that would create a dead short from ground to both a hot. You need to leave it un-grounded, but that would also give me double the secondary voltage. My thought is to insulate off one of the hot leads. In a 270-0-270 I would use 270-0 and insulate off the other 270. However I don't believe I'd get the current required. The PT I have is 200ma, so I'd only be able to draw 124ma with a bridge.

Back to the drawing board. I can't locate a PT with the specs I need. Thoughts on a source?

[pompeiisneaks]

If one were to use a bridge rectifier on a center tapped transformer and simply grounded the center tap what would be the effect? I thought I read in Blencowes book it was a no no but I cant find it in there skimming through again.

No no no...that would create a dead short from ground to both a hot. You need to leave it un-grounded, but that would also give me double the secondary voltage. My thought is to insulate off one of the hot leads. In a 270-0-270 I would use 270-0 and insulate off the other 270. However I don't believe I'd get the current required. The PT I have is 200ma, so I'd only be able to draw 124ma with a bridge.

Back to the drawing board. I can't locate a PT with the specs I need. Thoughts on a source?

I tried looking and didn't see anything exact. You could get one that's the right current requirement but is higher voltage and use either zeners or dropping resistors to get it to the right range?

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[davebolden44]

Thanks, I knew I read very recently about that very matter but couldnt find it again as a reference. It was starting to give me a headache. Sometimes you just gotta know "why?"

[Phil_S]

I suppose you could get away with 2x 6L6 + 3x 12AX7 at 120mA if the 6L6 are running strictly in AB1 (meaning that one and only one tube is conducting at any time) and the 12AX7's aren't pulling more than about 6mA in total. Honestly, I don't think anyone should try this.

There is no free lunch...conservation of energy will always apply. As voltage goes up, current will go down and vice-versa. This is reasonably clear from the Hammond cheat sheet.

May I suggest, it would be enormously helpful to see a schematic, or at least to know the target idle plate voltage on the 6L6's. If we have the voltage target and the tube compliment, it would be a fairly simple matter to backsolve for an appropriate transformer. At the moment, I don't believe there is enough information available.

[dorrisant]

I hate to sound like a skipping record but you could use a PT with a higher voltage and use the MOSFET B+ Dropper to shave it down to what's needed...

https://tubeamparchive.com/viewtopic.ph ... 8dd27cf3af

MOSFET B+ Reducer.pdf

[Bob-I]

IMG_0938.PNG

Here's the schematic. I'm building this for a friend who grabbed this board off eBay. It would've been easier to build from scratch.