Trying to recycle a Peavey into a T'Wreck...

[titanicslim]

Is this because of the way the xformer is made? Regardless or the type of schematic I'm trying to replicate?

It is solely due to the xfmr not having a center tap. To get the voltage out of this xfmr you must use a bridge. Period.

OK, let me add one more dumb question to the previous one: should I also go ahead and add the slo-blo fuse and the el cap, as shown, before that rect.?

I would think that the fuse is a good idea. Not sure about the ceramic cap. (not electrolytic)

Thanks, mlp, that's what I needed someone to tell me!

You know, the more I look at the jumble of wiring on the terminal strip I clipped the xformer off of, the more that "ceramic cap" looks like a thyristor, regardless what the schemo says. Would it make sense to parallel a MOT with a 1amp fuse there?

Well, I've got enough to go on. Cross your fingers. Tomorrow morning should tell

Dave

[mlp-mx6]

Folks usually put a MOV (metal oxide VARISTOR) there as a simple surge protector. Not a MOT.

[titanicslim]

Picky, picky, picky. Next you'll be telling me I shouldn't use these carbon conscientious resistors I've been saving since 1968.

Thyristors? Where the heck do I come up with this stuff Alphabet soup I tells ya. To me it's just alphabet soup.

[mlp-mx6]

It's all good, bro!

[titanicslim]

Yep, that's a bridge rectifier. Notice how the 4 diodes are aligned? The ground is generated at one corner of the bridge, and the output is the opposite corner. The 2 HV taps go on the other 2 corners. If you have a full-wave rectifier (as if you had a center tap) that will not work. You'll have to rewire your rectifier to be a bridge.

Maybe I should go back to AX-84 with this thread, but I wonder (not for a Liverpool build) where you drop the line to the bias control from the PT if you don't use the "orange" leads, but try to do it the Express way, except from that bridge rectifier. Know what I mean? The subject, I fear, has gone stale... Can that be done? ...and from what point in the bridge?

Dave

[oldhousescott]

Look at the Pignose G40. It also uses a bridge rectifier and derives its bias voltage from the HV secondary.

Schematic here:
http://www.schematicheaven.com/newamps/pignose_g40.pdf

To work with EL-84's, you'll have to change some resistor values. If you make R2 a combination 25K pot in series with a 10K to ground, you should be able to adjust between about -10VDC and -28VDC.

[mlp-mx6]

If I understand your question correctly, you have to derive the bias using a capacitative technique rather than resistive. Some Marshalls that had a bridge rectifier used this approach. It is discussed and diagrammed in "The Ultimate Tone, Vol. 2."

[tkdrew]

Are you building an Express or Liverpool? A Liverpool is cathode biased isn't it? I don't think you will need the neg. supply for it.

[titanicslim]

Well, I was trying to figure out what I could do with this xfrmr and the rest of it. For various reasons it's no longer going to be a Liverpool, though.

Thanks for the clue, Scott. I had never noticed that about the Pignose. That's what I wanted to know. Frankly, I didn't think it could be that simple.

The thing is, I installed octal sockets in the outputs for a 6V6 Liverpool, then changed my "mind" when things got too complicated and my "head" started pounding all the time. You see, for my level of ability and understanding of the subject, I think it is enough to try and make a "simple" Express using a recycled Peavey PT with a bridged rectifier. (Actually, it's evidently more than enough, since I have not been able to get the first sound out of it yet )

By the time the current gets down to B+5, that's about 100vdc and V1 gets about 55vdc. So let's make a 180 degree turn. I've now got the board populated with all (or nearly all) the values I need for an Express, so lets make an Express

I only clipped two wires to get this trannie out of the Peavey so why don't I just splice those back together and put it up on eBay. It'll surely bring enough to buy a couple of good transformers and some passives!

So, who actually has at least the power trans. with the proper specs for the typical Express values. The schematic I'm using now is the "Francesca" version A1b by UR12. It seems like it was said the 300v taps are used but I can't remember. 200mA?

I appreciate all the help. I really need it, too.

(seriously)

Dave

[sliberty]

If you are looking to buy trannies for an Express build, Moose has great ones. http://www.toneslut.com. Be al ittle patient though - I understand he just went on vacation.

[titanicslim]

Thanks. I just today tried to email him. Nothing yet. I suppose I can put it together with the Hammond iron until I can get the Toneslut. I've got a 272JX which should work fine.

Dave

[mlp-mx6]

Watch your heater voltage with that Hammond iron.