Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on September 12, 2016, 05:21:57 am
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Hi guys, I have a local made amp that I want to mod the power supply.
It's basically a 50w JCM 800, using JJEL34s.
The power supply below is stock and I'm having trouble with the ELs running on 800+ volts.
At the time of the measurements the electricity is at it's peak of 250vac.
A question.........
Why would they need to run the ELs at such high voltage to get an assumed 50w (amp is called a "GUNN 50")
Our local supply usually is between 245 and 250v, so I would think that the PT secondary's would be around 310v.
So I'm thinking to rewire the PT as a full wave rect with an output of approx. 420v with loading.
I need to replace the Ecaps and was wanting to run the ELs at a lower voltage.
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800V on old construction EL34 is acceptable, new ones will have problems (big ones)
But .... are you sure the amp is only 50W ?? a pair of EL34 @ 800V will be able to give 100W
Franco
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Timbo Geezer had a way of hooking 5 zener diodes to the HV center tap and was dropping about 45 to volts off the supply . Maybe Tubnit will see this and put up a link. Im at work or I would put up the chart I have saved. I don't know why you couldn't use 10 and get 95volts or so.
Just a thought .
Bill
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Get rid of the bridge recto! Someone mismatched the PT and recto. Wire in a conventional FW rectifier. Probably have to change the bias circuit as well.
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> * GUNN 50.sch
Can someone remind me of the simple free Java(?) program we use to open these things?
I had it before but my PC lost its mind.
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ExpressSCH
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Here's the link for the download from Schematics and Diagrams section;
http://el34world.com/Forum/index.php?topic=590.0 (http://el34world.com/Forum/index.php?topic=590.0)
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Here the jpg of the file
(http://i.imgur.com/RRX2Lpc.jpg)
Franco
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Thanks guys, PT's got a separate bias tap and at the moment it's set at -38.
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You could run everything at 415VDC - don't use half the HT secondary, and just move the diodes to make a full bridge on just half the secondary. Simplest initial modification to make me thinks.
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So I'm thinking to rewire the PT as a full wave rect with an output of approx. 420v with loading. I need to replace the Ecaps and was wanting to run the ELs at a lower voltage.
Yes, kinda like this...
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> running on 800+ volts.
That only makes sense if you have a 13K OT.
More likely you have 3,400r OT and should be running 425V on plates and screens.
While you could hack half the winding as TR says, when I try to draw it I get confused.
Especially accounting for the bias-tap (not shown!).
This IS a PT for the standard two-diode rectifier. Lose the bridge.
The caps could be re-organized for the now non-insane voltage.
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Sluckey has nailed it. I have not checked the OT primary impedance yet, i'll do that before hacking the rectifier.
I have quite a few of amps with this psu arrangement and are in the 100w+ output, so I'm looking for a way to reduce the voltage on the valves and the output.
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Hi guys, The amp is a clone of a "PLEXI" with a few component changes.
I used the Hoffman Plexi 50 as a guide to the rewiring.
Amp fired up well with a few noise issues, now sorted.
(http://i761.photobucket.com/albums/xx254/Timbo-08/002_14.jpg) (http://s761.photobucket.com/user/Timbo-08/media/002_14.jpg.html)
(http://i761.photobucket.com/albums/xx254/Timbo-08/001_19.jpg) (http://s761.photobucket.com/user/Timbo-08/media/001_19.jpg.html)
(http://i761.photobucket.com/albums/xx254/Timbo-08/001%205_9.jpg) (http://s761.photobucket.com/user/Timbo-08/media/001%205_9.jpg.html)
(http://i761.photobucket.com/albums/xx254/Timbo-08/003%205_5.jpg) (http://s761.photobucket.com/user/Timbo-08/media/003%205_5.jpg.html)