Power Supply Mod

[TIMBO]

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.

[kagliostro]

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

[Tone Junkie]

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

[sluckey]

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.

[PRR]

> * 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.

[sluckey]

ExpressSCH

[Willabe]

Here's the link for the download from Schematics and Diagrams section;

http://el34world.com/Forum/index.php?topic=590.0

[kagliostro]

Here the jpg of the file



Franco

[TIMBO]

Thanks guys, PT's got a separate bias tap and at the moment it's set at -38.

[trobbins]

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.

[sluckey]

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...

[PRR]

> 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.

[TIMBO]

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.

[TIMBO]

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.