50W Project

Trem Abuser · Post by **Trem Abuser** » Mon Jan 24, 2005 9:13 pm

Billy,

I actually asked the question of whether I needed to replace the PT in a previous thread and the only response I got was to give the old one a try and if it doesn't work in the new circuit I can always order the MM or Heyboer.

Laney seems a bit dicey when I asked about their "older" (early 80s) stuff. To get the schematic, I had to order the CD from another company which had all of their early schematics. So I don't think I can get the necessary info from Laney. I did not see a MFR's name on the PT but I didn't look super close.

Scott

Billy Batz · Post by **Billy Batz** » Mon Jan 24, 2005 10:40 pm

I guess it couldnt hurt to give it a try but I would ask a few more people before I did. I guess as long as you have your heaters and B+ the only issue would be having enough voltage or current which you probably wont but it may not hurt anything.

Trem Abuser · Post by **Trem Abuser** » Wed Jan 26, 2005 7:23 pm

What would be very helpful would be if someone could identify the wires of the PT in George's installation kit by color so I could follow the installation instructions and see which wires my PT does not have, and I could determine if my PT has all the necessary connections for the plexi amp. This would help since I'm still struggling with the schematics.

Another question - If the PT does not have enough current and/or voltage to properly operate the plexi what will be the result?

Scott

Billy Batzz · Post by **Billy Batzz** » Fri Jan 28, 2005 8:54 am

All you need for a 50W plexi Trem is a 6.3VAC supply and your secondary AC supply which your transformer does have. Now that should be a certain voltage and current but it may still work fine though it'll probably be running the tubes at a much lower voltage so youll kind of have a perminent brown sound, variac thing happening. If you dont have the Metro CD Ill send you some tranny color codes but if you do their on there. Another things is your OPT primary will probably not be the best impedance for 2 EL34s but it will still work.

Trem Abuser · Post by **Trem Abuser** » Fri Jan 28, 2005 1:51 pm

I do have (2 copies of) the CD, but it only shows OPT color codes, not PT color codes.

I was examing the 1987 schematics and comparing them to the 1959's. I noted that the 1987's shows the B+ only going to 2 diodes while the 1959's goes through four (in the diamond shape). Is this correct? I thought I saw four diodes together on the board layout diagram and pictures for the 1987.

The 1987 schematic also shows a portion of the PT's secondary hooked up to the "bias supply" part of the amp (I believe). In other words, I see the B+ hooked up to the diodes, another portion of the PT which runs the heaters, and then a third part of the secondary which powers the bias supply. This is the part I'm very confused on.

Billy - I believe you mentioned previously that on a 50W you tap the bias supply from the secondary AC B+. Those are the wires running from the PT to the standby then the rectifier diodes. You want to take it BEFORE the rectifier diodes so solder a wire to on of the diodes (the opposite of the side they join together) and run that wire to the Bias supply input on the board.

Is this as simple as it sounds? Just take one wire from the connection of one of the diodes (the PT side, before its "rectified") and run it to the bias supply input?

Scott

Trem Abuser · Post by **Trem Abuser** » Fri Jan 28, 2005 1:53 pm

Also, for a 50W PT, what voltage/current should the secondary AC be?

Guest · Post by **Guest** » Fri Jan 28, 2005 9:46 pm

I was examing the 1987 schematics and comparing them to the 1959's. I noted that the 1987's shows the B+ only going to 2 diodes while the 1959's goes through four (in the diamond shape). Is this correct? I thought I saw four diodes together on the board layout diagram and pictures for the 1987.

The 1959s use a rectifier config known as a bridge (the diamond shape) while the 50W use a full wave.

The 1987 schematic also shows a portion of the PT's secondary hooked up to the "bias supply" part of the amp (I believe). In other words, I see the B+ hooked up to the diodes, another portion of the PT which runs the heaters, and then a third part of the secondary which powers the bias supply. This is the part I'm very confused on.

On 50W Marshalls theres no Bias tap on the PT. Its tapped from the AC secondary before the rectifier diodes.

Billy - I believe you mentioned previously that on a 50W you tap the bias supply from the secondary AC B+. Those are the wires running from the PT to the standby then the rectifier diodes. You want to take it BEFORE the rectifier diodes so solder a wire to on of the diodes (the opposite of the side they join together) and run that wire to the Bias supply input on the board.

Yes. The bias has its own rectifying circuit.

Is this as simple as it sounds? Just take one wire from the connection of one of the diodes (the PT side, before its "rectified") and run it to the bias supply input?

Yep.

As far as the target voltage on the secondary Im not sure of the exact voltage off the top of my head. Im sure its on the CD somewhere. I think they yield something like 460-480v of DC when the tubes are in and running.[/quote]

Billy Batzz · Post by **Billy Batzz** » Fri Jan 28, 2005 9:46 pm

That was me. Dammit

Trem Abuser · Post by **Trem Abuser** » Sat Jan 29, 2005 3:00 am

I would have figured it was you Billy. I do appreciate the specific responses to each question, its very helpful.

I've just ordered some additional parts from George which I didn't order in the first go around.

I'll try to make some headway this weekend.

Flames1950 · Post by **Flames1950** » Sat Jan 29, 2005 9:31 am

I'm way behind on reading this thread and the sheer number of questions to catch up on is a little overwhelming.
But as to the 56K resistors, those are not bleeders. They are part of a voltage doubler circuit in the 100watter to give the caps more voltage "headroom" if you will (the cap voltage rating is too close or exceeded by the voltage they're handling; it's a safety margin.)
The fifty watter doesn't use this arrangement, you don't need them.
Now on to reading the other fifty questions I've missed out on....

Billy Batzz · Post by **Billy Batzz** » Sat Jan 29, 2005 11:32 am

So Flames what constitutes a bleeder on a 100W Marshall? I have those 56ks on the doubler and screens as per suggestions by guys at Ampage. Or do they serve a double function bleeding off the voltage when the amps turned off?

Billy · Post by **Billy** » Sat Jan 29, 2005 11:34 am

Flames instead of reading all his questions just go back over my posts where I quoted then answered his questions and make sure everythings 100% correct.

Flames1950 · Post by **Flames1950** » Sat Jan 29, 2005 2:52 pm

Billy Batzz wrote:So Flames what constitutes a bleeder on a 100W Marshall? I have those 56ks on the doubler and screens as per suggestions by guys at Ampage. Or do they serve a double function bleeding off the voltage when the amps turned off?

Well, I may be making it sound a little "cut in stone" there. They may help the charge bleed off, but it's just not their main purpose. They're in place to balance the voltage across two capacitors in series, to allow the series combination to take a higher voltage than either cap is rated for as a single unit. I don't even remember seeing the 56K's on the later 100 watters in the JCM800 line when the filtering changed (although my memory may need refreshed there) and 50 watters don't have any 56K resistors in place on the caps.

Trem Abuser · Post by **Trem Abuser** » Sat Jan 29, 2005 3:57 pm

Looking back, it is a lot of questions. But it was like I was paralyzed with fear of screwing something up so I just went through the installation instructions (on the CD) for the umpteenth time and wrote down everything I was uncertain on.

I agree with Billy - he has responded to most or all of the questions I posted, so no need to respond if his answer was correct.

Perhaps you guys with experience may have forgotten how overwhelming this experience is for the novice! Its like working on your car's engine without a manual. But what is great is how willing guys are to lend a hand with the novice who is stuck.

I got two more holes drilled in the chassis for the extra filter caps, so I'm going to try to mount those today, and maybe redo the heater wiring.

Post by **VelvetGeorge** » Sat Jan 29, 2005 5:06 pm

You guys got it right, those 56k's are voltage dividers. Assuring that each cap gets 1/2 off the total voltage.

George

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