How does this work?

jimi · 10-29-2006, 09:07 PM

Take a look at this schematic of a selmer amp.
It looks a bit complex to me I can't get my head round it. The cathode of the first stage is grounded yet on an amp I have here I measured -0.8volts on the grid. Where does it get this negative voltage from

jimi · 10-29-2006, 09:10 PM

Here's the link http://britamps.mysite.wanadoo-membe...tic/lgmk1.html

Alex/Tubewonder · 10-29-2006, 11:48 PM

Look for a continuous DC path from grid to ground i.e through resistive elements only. Grid leak current causes a voltage drop along this path. When you connect your meter between grid and ground it provides additional shunting resistance thus your measured grid voltage is false. Grid leek current is very small. A cleverly wierd or wierdly clever little design, but Selmer had their own thing going.

Tom Phillips · 10-30-2006, 02:51 AM

There were some very early Fender amps that used grid leak biasing. Some examples are the 5B2 Princeton and the 5B5 & 5C5 Pro. The design was dropped early on.

Enzo · 10-30-2006, 05:58 AM

Look up grid leak bias, there are good explanations around. Basically, the cathode emits electrons, which have negative charge. They float around in a little cloud called the "space charge", and the closest thing they come to is that grid, so they accumulate there. That gives it the negative charge. There is only the tiniest bit of current available, that is why the grid resistor is a very high resistance. SOmething like 5 Meg being common. Lower resistances would drain away the charge.

So the grid finds itself at -.8 volts or whatever from the accumulated electrons. Since the cathode is grounded, the net bias is .8 volts. As far as the tube is concerned, this is the same as if the grid were at zero volts and the cathode at +.8v. The bias is the difference between the two, not the absolute voltage they are at.

jimi · 11-02-2006, 08:29 PM

Thanks for the replies looks like a nice simple way to bias a tube. It doesnt seem to be used much so whats the disadvantages with it?

Ray Ivers · 11-02-2006, 08:59 PM

Quote:

Originally Posted by jimi

It doesnt seem to be used much so whats the disadvantages with it?

Generally, high noise and very little headroom (I don't know if that's the case with this specific Selmer circuit, as there's that negative-feedback tone-control thing going on).

Ray

10-29-2006, 09:07 PM	#1
jimi Member Join Date: Aug 2006 Posts: 33	How does this work? Take a look at this schematic of a selmer amp. It looks a bit complex to me I can't get my head round it. The cathode of the first stage is grounded yet on an amp I have here I measured -0.8volts on the grid. Where does it get this negative voltage from

10-29-2006, 09:10 PM	#2
jimi Member Join Date: Aug 2006 Posts: 33	Link Here's the link http://britamps.mysite.wanadoo-membe...tic/lgmk1.html

10-29-2006, 11:48 PM	#3
Alex/Tubewonder Senior Member Join Date: Jul 2006 Location: Goteborg, Sweden Posts: 144	Look for a continuous DC path from grid to ground i.e through resistive elements only. Grid leak current causes a voltage drop along this path. When you connect your meter between grid and ground it provides additional shunting resistance thus your measured grid voltage is false. Grid leek current is very small. A cleverly wierd or wierdly clever little design, but Selmer had their own thing going. __________________ Aleksander Niemand Tubewonder amps Life's a party but you get invited only once

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Will this work?	Arthur Dent	Theory & Design	18	11-01-2006 01:31 AM
funky Supro Thunder I Reverb amp: how does it work?	Jef	Vintage Amps	3	06-15-2006 01:52 AM

10-30-2006, 02:51 AM	#4
Tom Phillips Senior Member Join Date: Jun 2006 Location: California, USA Posts: 676	There were some very early Fender amps that used grid leak biasing. Some examples are the 5B2 Princeton and the 5B5 & 5C5 Pro. The design was dropped early on.

10-30-2006, 05:58 AM	#5
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,348	Look up grid leak bias, there are good explanations around. Basically, the cathode emits electrons, which have negative charge. They float around in a little cloud called the "space charge", and the closest thing they come to is that grid, so they accumulate there. That gives it the negative charge. There is only the tiniest bit of current available, that is why the grid resistor is a very high resistance. SOmething like 5 Meg being common. Lower resistances would drain away the charge. So the grid finds itself at -.8 volts or whatever from the accumulated electrons. Since the cathode is grounded, the net bias is .8 volts. As far as the tube is concerned, this is the same as if the grid were at zero volts and the cathode at +.8v. The bias is the difference between the two, not the absolute voltage they are at.

11-02-2006, 08:29 PM	#6
jimi Member Join Date: Aug 2006 Posts: 33	Thanks for the replies looks like a nice simple way to bias a tube. It doesnt seem to be used much so whats the disadvantages with it?

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)