ampage
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:  

Re: Tube amps


 
11/6/1999 3:46 AM
Marc
email
Re: Tube amps
thanks Ben, I had forgotten about all the good stuff @ the machine gun site. I had already converted the bias to BF style, just thought it would be nifty to be able to balance it too. The power tubes in this ampa re old fender brands one has no getter left at all and is weak, so maybe I'll talk the owner in to a new pair...  
 
 
Marc
 
11/6/1999 4:54 PM
Bruce
email

It's stupid simple.  
You just remove the one reistor that is grounded.  
That is the one that sets the overall bias voltage, and usually solder fromt he bias balance pot right to it's shell.  
Replace it with a little 22K-25K trim pot...(wired as a variable resistor...use only two of the lugs)... and put a 6K8 resistor in series with that trim pot right to ground.  
Now you can vary the amount of negative voltage that is shunted to ground and also balance how much is left over to send each side of the OT's power tube(s).  
You should be able to swing somewhere close to -40v down to about -60v.  
If the little trim pot goes open, for some reason, the negative voltage goes up and the tubes will not be hurt.  
If the trim pot goes bad and shorts out some how (this never has happened to me in many many years), then the negative bias voltage will rise (more positive) and the tubes are in danger of being underbiased.  
I wouldn't worry about this myself.  
 
Bruce
 
 
  Sunday
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.
 
11/6/1999 7:21 PM
Marc
email

Hi Bruce:  
I knew it was simple, but I musta done it wrong cause it didn't work. This works with the tapped balance pot , right?  
I used a 25K trim pot and a 15K resistor to replace the 33K resistor that came from the center lug and was soldered to the backside. the outer lugs send the bias to the tubes and the tap feeds the bias to the pot.  
would there be any effect, or any point, in changing the 47K & 68K resistors to the grids?  
Did I mention this is the 77 SF 70W srper I've been noodling around with?  
Thanks for your help!!!  
 
Marc
 
11/5/1999 10:35 PM
Marc
email

Ben:  
OK I'll bite.  
Have you got a diagram of this set up? A description would be OK too. I've got one of those tapped balance pots that I'd like to do that to, or I'll just set up two separate bias pots. I haven't decided, yet  
 
Bruce C. has a cool way of coverting it to get the  
best of both worlds.......bias adjust and hum  
balance!  
Thanks  
Marc
 
<<First Page<Prev Page 2 of 2