Tube Amps / Music Electronics
For current discussions, please visit Music Electronics Forum. New: view Recent Searches.
New: visit Schematic Hell!
The sunn still shines online!

Listen to great tunes streaming live right now!

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  

Dropping voltage across a resistor

5/4/2006 12:23 AM
Dropping voltage across a resistor
I want to try putting an EF86 w/bandaxall TS in the first channel of my Pro Rev. How do I figure what size resistor to use to drop the voltage down to 70v or so like an EF86 wants to see. Also do I then need to add another filter cap there?
5/4/2006 1:20 AM
Carl Gigun
I guess you're looking at a specific schematic that runs an EF86 from 70v? In general though they run from supply voltages normally found in tube pre-amps, 150v-400v.  
To figure your dropping resistor assume the current draw will be in the ballpark of 1.5ma, or look up a more accurate figure from the EF86 charts. You will need a cap after the R for this new voltage.
Book Of The Day The Ultimate Tone, Volume III by Kevin O'Connor
Have you ever wondered if there is a better way to build a Bassman, Champ, Plexi, an 800, AC-30, Bulldog or Portaflex? Or you wanted to build an SVT with off-the-shelf parts? How about a master-volume amp that doesn’t change tone with the master setting? Everything you need to know is right here, including: proper grounding techniques, wiring methods, and mechanical considerations. Eighteen chapters cover the “iconic” amps everyone knows and loves, with schematics and layouts for each, along with the technical history of the product. Eyelet-board and chassis-mounted tube socket construction is used throughout, for easy servicing and modding. TUT3 is very accessible even if you cannot fully read a schematic and is a "must have" if you are going to build an amp for your self.

Note: The Ampage Archive is an Amazon Associate site. A small commission is paid to the site owner on any qualified purchase made after clicking an associate link such as the one above.
5/4/2006 2:51 PM

Thanks Carl! The data sheet says  
Pentode Connection  
HT Supply Voltage (volts) 100 - 200  
----Cathode Current (mA) 1.0 - 1.7  
Is that what I'm looking for?
5/4/2006 3:17 PM
Carl Gigun
Yep cathode current is the right one. The current will depend just as much on the circuit and resistors used as the supply voltage. The screen voltage in particular will be a big factor so try to find a circuit in the sheet that matches yours as much as possible.  
Of course you can always try a few different dropping resistors so you don't have to be bang on with the current on the first try. Personally I use a trimmer when doing something like this and replace it with a resistor later but it's not the safest way so I'm not officially recommending it ;)
5/4/2006 4:00 PM

It seems like I could just use 220K-330K resistor on the 390V B+ rail and be fine. Does this seem correct to you?
5/4/2006 7:29 PM
Carl Gigun
sounds close but maybe just a little big? I'm not sure what current you are expecting through the EF86, but lets say it's 1ma:  
milli's and kilo's cancel out conveniently (200,000 x 0.001 = 200 x 1) so you're looking at 220 to 330 volts dropped @ 1ma, or a supply voltage of 60v to 170v.  
70v supply sounds very unusual to me. I'd double check the schematic you're working from to make sure the 70v is really the supply voltage and not the plate voltage. You'll need more like 150v supply to have 70v at the plate.
5/4/2006 11:28 PM

My suppy voltage is 390V. I'm wanting to end up with 70v-80v on the plate.
   Page 1 of 2 Next> Last Page>>