Bias source

How can you get a great tone without a good understanding of what makes your amp tick? — EL34, EL84, 6L6, KT66, KT88, 12AX7, 12AT7, 12AU7, EF86, Output Transformers, Power Transformers, Capacitors, Resistors and everything else that it takes to make a tube amp sing!

Bias source

Postby sitka_spruce on Sat Dec 20, 2014 4:26 am

The Tone Lizard provides here an example of an improved suppressor grid wiring scheme: http://www.tone-lizard.com/Marshall_Myths.htm He suggests connecting said grid to the 'bias source'. Am I correct in assuming that would be where the bias pot runs into the feeder resistors? http://drtube.com/schematics/marshall/9100-60-02.pdf

Cheers,
Jonas
sitka_spruce
Certified
 
Posts: 131
Joined: Wed Jun 11, 2008 11:36 am
Location: The old side of the pond

Re: Bias source

Postby pdf64 on Sat Dec 20, 2014 9:30 am

My interpretation is the output to the grid leak resistors is the bias supply output, whereas the full bias supply 'source' is across c28.
My band:-http://www.youtube.com/user/RedwingBand
pdf64
Knight
 
Posts: 1242
Joined: Thu Dec 14, 2006 9:55 am
Location: Staffordshire UK

Re: Bias source

Postby sitka_spruce on Sat Dec 20, 2014 3:51 pm

pdf64 wrote:My interpretation is the output to the grid leak resistors is the bias supply output, whereas the full bias supply 'source' is across c28.

Thanks, mate! I therefore assume I'm to run the connecting wire from where the pot connects to the grid leak resistors then to achieve the desired result?
sitka_spruce
Certified
 
Posts: 131
Joined: Wed Jun 11, 2008 11:36 am
Location: The old side of the pond

Re: Bias source

Postby pdf64 on Sat Dec 20, 2014 7:46 pm

No, the idea is to remove the link at the tube socket between cathode terminal #8 and G3 #1, then connect cathode to ground (via a 1 ohm current sensing resistor) and G3 to the bias supply, eg C28 as previously.
This will send the tube operating conditions colder, so the control grid bias will need adjusting to compensate, and you may reach the limit of the pot range and need to mod the bias adjustment circuit.
My band:-http://www.youtube.com/user/RedwingBand
pdf64
Knight
 
Posts: 1242
Joined: Thu Dec 14, 2006 9:55 am
Location: Staffordshire UK

Re: Bias source

Postby sitka_spruce on Sun Dec 21, 2014 2:43 am

pdf64 wrote:No, the idea is to remove the link at the tube socket between cathode terminal #8 and G3 #1, then connect cathode to ground (via a 1 ohm current sensing resistor) and G3 to the bias supply, eg C28 as previously.
This will send the tube operating conditions colder, so the control grid bias will need adjusting to compensate, and you may reach the limit of the pot range and need to mod the bias adjustment circuit.

Keeping the cathode earthed makes full sense to me. But what he says about avoiding negative current on g3 is still a bit to digest as I thought the current here was pretty static.
sitka_spruce
Certified
 
Posts: 131
Joined: Wed Jun 11, 2008 11:36 am
Location: The old side of the pond

Re: Bias source

Postby pdf64 on Sun Dec 21, 2014 9:21 am

Could you copy / quote the sentence? I can't find anything about avoiding negative G3 current.
My band:-http://www.youtube.com/user/RedwingBand
pdf64
Knight
 
Posts: 1242
Joined: Thu Dec 14, 2006 9:55 am
Location: Staffordshire UK

Re: Bias source

Postby Papa Dog on Sun Dec 21, 2014 5:46 pm

pdf64 wrote:Could you copy / quote the sentence? I can't find anything about avoiding negative G3 current.

Marshall Myths
"Think about it. Instead of having the Suppressor Grid at ground potential or even at a negative voltage, you would have a positive voltage (granted only 50mVDC or so, but positive nonetheless) on that grid, because no one ever separates the Suppressor Grid from the Cathode."
There Ain't No Such Thing As A Free Lunch.

"The time has come," the Walrus said,
"To talk of many things:
Of shoes--and ships--and sealing-wax--
Of cabbages--and kings--"
Papa Dog
Knight
 
Posts: 1337
Joined: Mon Mar 15, 2010 6:48 pm
Location: Narly Ampwerks

Re: Bias source

Postby pdf64 on Sun Dec 21, 2014 6:47 pm

Well, the key point there seems to be that 'positive voltage' on G3 is bad, rather than 'negative current'?
To paraphrase, TL's view seems to be that G3 should be tied to ground, or even better, to negative supply.
See http://www.valvewizard.co.uk/pentode.html, pentode operating conditions are a balance of the voltages between the cathode and G1, G2, G3 and (to a lesser extent due to pentode action) the anode.
My take is that TLs conclusion seems somewhat overblown; a few mV movement of Vk-G3 here or there doesn't seem to be a big deal, it's more a case of comparing best practice to good practice, when 'best' in terms of robustness of operation may not produce the best tone (however that may be judged) and differences one way or another are probably pretty minimal.
I wouldn't worry about it.
My band:-http://www.youtube.com/user/RedwingBand
pdf64
Knight
 
Posts: 1242
Joined: Thu Dec 14, 2006 9:55 am
Location: Staffordshire UK

Re: Bias source

Postby sitka_spruce on Sun Dec 21, 2014 7:43 pm

pdf64 wrote:Well, the key point there seems to be that 'positive voltage' on G3 is bad, rather than 'negative current'?
To paraphrase, TL's view seems to be that G3 should be tied to ground, or even better, to negative supply.
See , pentode operating conditions are a balance of the voltages between the cathode and G1, G2, G3 and (to a lesser extent due to pentode action) the anode.
My take is that TLs conclusion seems somewhat overblown; a few mV movement of Vk-G3 here or there doesn't seem to be a big deal, it's more a case of comparing best practice to good practice, when 'best' in terms of robustness of operation may not produce the best tone (however that may be judged) and differences one way or another are probably pretty minimal.
I wouldn't worry about it.

Right you are. I seem to have confused the inverted operation of valves back to where it was. Thank you for the input and setting that straight.

Great read that link of yours, btw. I could spend hours there it seems.
sitka_spruce
Certified
 
Posts: 131
Joined: Wed Jun 11, 2008 11:36 am
Location: The old side of the pond

Re: Bias source

Postby Papa Dog on Sun Dec 21, 2014 11:33 pm

I think that you guys missed the point.
in normal operations G3 IS tied to ground along with the cathode.
pitting the standard 1 ohm 1 watt resistor to ground for baising purposes does result in a slight positive voltage.
a bigger resistor would place them higher above ground.
separating G3 and giving it a slight negative voltage will affect the operation of that tube.
how much is a matter of how much negative voltage applied...too much would certainly be a disaster.
just enough will change the bias point of the tube, maybe for the better, and maybe not.
There Ain't No Such Thing As A Free Lunch.

"The time has come," the Walrus said,
"To talk of many things:
Of shoes--and ships--and sealing-wax--
Of cabbages--and kings--"
Papa Dog
Knight
 
Posts: 1337
Joined: Mon Mar 15, 2010 6:48 pm
Location: Narly Ampwerks

Re: Bias source

Postby pdf64 on Mon Dec 22, 2014 1:00 am

pitting the standard 1 ohm 1 watt resistor to ground for baising purposes does result in a slight positive voltage


But for tube operation, the cathode is normally the reference point, rather than ground.
So from that perspective, it's reasonable to maintain that stable reference (by tying G3 to the cathode) rather than for Vk-G3 to be moving around in response to the signal (by tying G3 to ground or a negative voltage, rather than the cathode, which has a 1 ohm resistor in series with its ground return).
But the difference would probably be so minimal as to be inconsequential.
My band:-http://www.youtube.com/user/RedwingBand
pdf64
Knight
 
Posts: 1242
Joined: Thu Dec 14, 2006 9:55 am
Location: Staffordshire UK


Return to Tubes, Transformers and Components

Who is online

Users browsing this forum: Bing [Bot] and 2 guests