best way to drop b+

[gary sanders]

pt is putting out a little much for EL84 so would you guys recommend a high watt resistor in series from the rectifier to plates or to ground like a bleeder?Ive seen some old vox and gibson amps with the big resistor to ground

[Phil_S]

What's "a little?" Please quantify it.

There are "tricks" that people use. I'm not a big fan of them because they generate a lot of heat. The energy has to get used somehow, so it get converted to heat.

[gary sanders]

from 380 to 350.trying to avoid changing the PT

[John_P_WI]

Use an amplified zener or similar.

Look at the bottom of the page here:

http://www.geofex.com/Article_Folders/m ... tfolly.htm

[LeftyStrat]

I wouldn't worry about it if your screens are low enough. I've seen a few amps as high as 395v on the plates and both NOS and the JJ EL84's seem to handle it okay (these are the only tubes tried, others may work).

Which amp?

[Bob S]

You could use 7189's.
Or Russian 6P14P's.
They can handle higher voltage.

[Cygnus X1]

Dumb Q but is that unloaded voltage?
If so, then you should be right in the ballpark when everything is running.

[kdmay]

Chassis mount zener diode on PT centre tap, works perfectly

[labb]

Chassis mount zener diode on PT centre tap, works perfectly

two 15 volt in series would put you right there.

[Cliff Schecht]

Even for dropping "a little" voltage (that being 10V to maybe 40V), I choose the amplified Zener diode hands down every time. The BJT I use in this circuit (BUS14) can handle 400V @ 30A continuous and the BJT is heatsinked heavily to the chassis, both making strong cases for the longevity of this circuit when used correctly.

Also the BJT/FET circuit can be built right onto the two pins sticking out of the chassis if you use a TO-3 package meaning no need for extra circuit boards mounted anywhere. That's actually another reason I really like the BUS14, the base and emitter pins are much longer than other TO-3 transistors.

Edit: Damn just saw on ebay that the part I specifically mentioned goes for an arm and a leg.. I was paying pennies a piece in Dallas.. Anywho there are hundreds, if not thousands of alternative devices you can use here.

[gary sanders]

Thanks guys,Zener it is! will try it out

[cddesign]

Thanks guys,Zener it is! will try it out

Please report back with results. I'd like to know what worked for you.

thanks

[skyboltone]

You could use 7189's.
Or Russian 6P14P's.
They can handle higher voltage.

But boy do they NOT sound the same.

[Kagliostro]

Amplified Zener or Zener or Resistor

on CT will give

problems with HV winding with bias tap (on fixed bias amps)

no problem if cathode bias amp

no problem if the bias comes from HV

K

[rsi]

I did exactly what you are trying to do (drop from 380 to 350).

I used the zener route.