power transformer.

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delux1959
Junior tube assistant
Posts: 81
(3/19/04 9:34 pm)
 power transformer.
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 I know this question is going to sound stupid, but here goes.

I have a crate vc508 amplifier. its a 5 watt single ended amp running a single 12ax, and el84. So ,I thought it would be a good acandidate for a champ conversion. a 5f1. The Crate was on a pcb, and the connections for the powertransformer were not very evident, other than knowing that the black and white wires are the primary. The secondary wires are two orange, two red, and one yellow. what would be the easiest way to figure out what the other wires are. I measured the wires, can i conclude that the higher resistance wires would be for the HT, the next lowest would be for the heaters, and the lowest would be the 5 vt, winding?
 
Andy Poupart
Senior tube assistant
Posts: 133
(3/19/04 11:32 pm)
 Re: power transformer.
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 If you can, you should measure the AC voltage across the two orange wires and across the two red wires. I would guess that one set will be the HT secondary and the other set will be the 6.3V winding. It didn't sound like your Crate amp has a rectifier, so there would be no need for a 5V secondary.
 
delux1959
Junior tube assistant
Posts: 83
(3/20/04 10:21 am)
 power
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 When i took the board out of the tranny, i looked it over and found that it had a full wave diode rectifier. Just curious, but how would be the best way for me to check the AC, considering the power trans former isnt connected to anything. I really don't want to get a load of juice from a loose lead.
 
6G6
Junior tube assistant
Posts: 139
(3/20/04 12:02 pm)
 Re: power
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 Short answer: very carefully.


Use allagator clips to hold the probes to the leads before you turn it on. Be sure to seperate the leads and lay them on a non conductive surface.
 
delux1959
Junior tube assistant
Posts: 84
(3/20/04 7:03 pm)
 power
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Well, so i hooked up the the volume/on-off pot, and the fuse so I could get some readings. There is no center tap for the power tranny. I hooked the black wire from the power cord to the bottom of the fuse holder, and the white to a Tab on the bottom of the on/off switch. Then i hooked the white line from the PT to the other leg on the fuse holder, and the black line to the other tab on the bottom of the power switch. With that set up i read 229.4 vac on the two red secondaries, 6.96 on the orange, and the single yellow was kinda strange. I read 4.50 with the switch in the off position, and 2.21 with it in the on position. what goin on there? The red wires must be the HT line, the orange are the heaters , but im not sure about the the yellow.
Anybody gotta clue???
Thanks , John
 
Tiny Daddy
I will work on all amps
Posts: 564
(3/20/04 7:44 pm)
 Re: power
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 The yellow wire may be connected to the transformer core or to an internal shield.
See:
www.jamesaudio.com/gpsupply.html
for proper primary wiring. Note the fuse and switch are both in the black wire.
 
delux1959
Junior tube assistant
Posts: 85
(3/21/04 2:59 pm)
 powertrans
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 Well, i think im at the end of my rope with this power trans. I was wondering if anybody had a suggestion on a Hammond transformer for a 5f1 champ. I believe i need a trans with a secondary rating of 150-0-150. Is this correct? I'm not sure about what the primary rating should be though.
 
delux1959
Junior tube assistant
Posts: 86
(3/23/04 3:20 pm)
 Re: powertrans
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 Well,
Thanks Andy,tinnydaddy, and 6g6 for your advice. I guess i'm gonna have to buy a new pt, anybody have a recommendation for a good pt for a 5f1 champ?

 
j allen shaw
Junior tube assistant
Posts: 28
(3/24/04 2:04 pm)
 Re:Crate power transformer.
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 Delux,
I thought I'd chime in before you spent the bucks on a new PT, as I have some experience with Crate PT's.
See the posts at THIS LINK on the Hoffman amp board.
I converted a Crate VC50 into a 1959 Twin 5F8-A with (4) EL84's re-using the PT & OT of the Crate, and it turned out to be a fantastic amp!
The PT will require a bridge rectifier (Doug sells them for a couple of bucks, or you can get a Weber Copper Cap Module for $25 to emulate the 5Y3 tube recto), as there is no center tap on these PTs.
See the VC508 schematic HERE.
The extra yellow wire appears to be a center tap/ground for the heater taps.
The problem you may run into is that with 220v from the PT now, you'll probably end up with MORE plate voltage after the rectifer is added, but probably not enough to run the 6V6 as hot as the Champs run. (I started with 305v and had 390v to the plates after the amp was completed.)
According to the VC508 schematic, it looks like you can expect 272 (+/-) plate volts after the bridge recto. That's somewhat less than the 350v the AA764 Champ runs. There's no voltage listed on the 5F1 schematic, but I suppose it's similar.
Here's something else to consider.
The plate voltage to the EL84 in the VC508 is so low because it is run in Class A. Maybe you could consider running the 6V6 the same, biased near 100% dissipation with the lower plate voltage?
Maybe some of the "Big Guns" could comment here as to the feasablity of this?
Jeff

Edited by: j allen shaw at: 3/24/04 2:08 pm
 
delux1959
Junior tube assistant
Posts: 87
(3/24/04 3:56 pm)
 Re: Re:Crate power transformer.
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 Thanks, Jallen, i didn't want to pay for the schematic, and now , I don't have to. Thanks for stepping in and letting me see what you were up against with you amp. The chap 5f1 is cathode biased, so that was the plan the whole time. I just needed to find out how to aply the Crate pt to this project. I still havn't decided if i should go with the 6v6 or the el84.
Looks like i need to get goiong again.
JB
 
 
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