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Old 18th November 2013, 05:53 PM   #1
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Default Tranformer advice for small powered amp.

Hi.
I like to put a 7-10 watt ecl82 or similar tube into a nice project.
I manage to savage 2 x PT and 2 x OT from a grundig reel to reel.
The trafos were fidding the following valves.
1 x 12ax7
1 x ef86
1 x em81 magic eye
1 x ecl82
1 x ez80
The sec voltage after the ez80 is 180Vdc
I could built something think similar but only give me about 3W, which is not enough for my needs.
I do not know PT current for this set that I got.
Is there a possibility to use the same trafos with different valves (ra 8k I believe) to get 7-10 watts of power.
Ideas, sugestion welcome
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Old 18th November 2013, 10:20 PM   #2
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Assuming your schematic is of the Grundig where you got your parts from then you might be able to build a pair of RH84 monoblocks. ( about 5 watts/channel ) The schematic shows the EZ80 giving out 280Vdc. Replacing the EZ80 with solid state diodes will increase this. Not drawing so much heater filament current might ease the strain on the HT winding. The output transformers may not be able to deliver the extra wattage without distortion.


RH 84 - Tube Audio ...... RH DESIGN
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Old 19th November 2013, 12:38 AM   #3
kevinkr is offline kevinkr  United States
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Note that part of the output transformer is used as a choke for filtering the other supply voltages used in this machine. I have run into this in a number of European radios and tape decks. I suspect you should ignore that part of the winding if you use these transformers.

You might want to investigate whether something from Edcor would be a practical alternative (shipping cost/customs duty) particularly since you want more power.
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Old 19th November 2013, 02:30 AM   #4
wa2ise is offline wa2ise  United States
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Quote:
Originally Posted by kevinkr View Post
Note that part of the output transformer is used as a choke for filtering the other supply voltages used in this machine. ...
It's more of a scheme to balance the output tube's plate resistance against R27 in the B+ filtering circuit, with some selected ration of turns between the two winding sections. Idea is that hum and ripple would get cancelled out in the transformer. And not be heard by the speaker.
Click the image to open in full size.
Ever wonder why some AA5s have a tap on the primary of the output transformer? It's another hum bucking scheme. The usually 1.5K resistor that usually feeds off the rectifier tube B+ is connected to the far end of extra turns on the primary. As far as the hum ripple (interferer E) is concerned, the cap that filters the line that feeds the output tube screen and rest of the set looks like a short to ground. So the hum ripple sees 1.5K to ground, and the output tube's plate resistance looks like 14K to ground on the output transformer's plate lead. The position of the tap is selected such that the amp-turns of one side balance the other side. Thus the hum ripple cancels out so the speaker doesn't hear it. J Stewart made a nice diagram which I pirated above:
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Old 19th November 2013, 05:36 AM   #5
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the short practical answer is no.
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Old 19th November 2013, 10:36 AM   #6
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You're right about OT and I almost forgot it.
I tough that I could use that 10W idea to power up my LS3/5a but I was ignoring the fact that the OT's are a limitation.
As aardvarkash10 said "the short practical answer is NOOOOOOO.
I was just trying to use what I got, someone suggested EDCOR I know, unfortunately not an option, family comes first.
I guess that I got to wait til a better set of OT's to show up.
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Old 19th November 2013, 09:13 PM   #7
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hello ,

it is possible looking the shema

and me build to base ECL 805
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