Nice mess in a 78 JMP Superlead...need advice!

[martin manning]

The cap should be in series with the 3k9 or 4k3. Transformer lead to resistor to + end of cap to banded end of existing diode, and the new diode going from the banded end of the existing diode to ground. I do not agree that you have enough voltage without the doubler.

[donzoid]

OK...I will work on this tonight. Thanks!

Ah, well...it just occurred to me that in order to series the components, I will have to lift one end of the resistor to bypass the trace between that and the existing diode. I have a good pair of flush cutters....maybe I'll just snip the sucker to get the cap into series.

[Cameron]

The cap should be in series with the 3k9 or 4k3. Transformer lead to resistor to + end of cap to banded end of existing diode, and the new diode going from the banded end of the existing diode to ground. I do not agree that you have enough voltage without the doubler.

But doesn't the way he has it now ...a bit like the stock circuit and not a voltage doubler?? and there is enough voltage to run the tubes cold....Or am I missing something.. Martin ?

[Cameron]

OK...I will work on this tonight. Thanks!

Ah, well...it just occurred to me that in order to series the components, I will have to lift one end of the resistor to bypass the trace between that and the existing diode. I have a good pair of flush cutters....maybe I'll just snip the sucker to get the cap into series.

Just snip it ......and run them in series. You will probably have to adjust that 3.9k resistor anyway.....then once you get that right ...you can clean it up.

[martin manning]

But doesn't the way he has it now ...a bit like the stock circuit and not a voltage doubler?? and there is enough voltage to run the tubes cold....Or am I missing something.. Martin ?

With the resistor across the cap it is still a doubler. Don't forget the diode going to ground is still there too. The input voltage is going directly into the cap, but the resistor is bleeding its charge. If the resistor wasn't there, the output voltage would be around -70V.

Don, I'd recommend against putting the new diode under the board. That will make it more difficult to access down the road.

[Cameron]

But doesn't the way he has it now ...a bit like the stock circuit and not a voltage doubler?? and there is enough voltage to run the tubes cold....Or am I missing something.. Martin ?

With the resistor across the cap it is still a doubler. Don't forget the diode going to ground is still there too. The input voltage is going directly into the cap, but the resistor is bleeding its charge. If the resistor wasn't there, the output voltage would be around -70V.

Don, I'd recommend against putting the new diode under the board. That will make it more difficult to access down the road.

Ok....I see .....its an almost doubler

[donzoid]

OK well since we have to have it in series first to see if the 3.9 is the right value, I will snip it this time, and then if it all comes out with the right range, I will pull it down and move that diode to the top side. Thanks.

[donzoid]

Well, it ain't pretty, but its in series now. I'm going to remove tubes in-between meetings today and re-check all the readings.

[martin manning]

Let's call it a "proof-of-concept demonstration." Sounds much better.

[donzoid]

Pin 5 reads -40v on both sides, +/- 0.1v.

[martin manning]

What range do you get with the trimmer?

[donzoid]

Ah, Martin...you are a sharp guy. I'll have to start getting up earlier. Let me see... thats the lowest reading. Highest is -45.5

[martin manning]

I think you are looking good for tubes-in testing!

[donzoid]

Range is 25mA to approx. 46mA... dialed it up and didn't want to leave it there for long. It probably would have stabilized slightly higher, but ... since that is quite high for this tube didn't want to leave it there.

Set it to 37 and letting her sit for a bit to stabilize.

Yes, "proof of concept" makes it sound much less ugly... thus easier to ignore.

[martin manning]

Looks like you've got it. Tidy it up a bit and you're good to go.