Biasing questions? Lower current is hotter?

James · March 21st, 2003, 10:37 AM

I just rebiased my Pro Reverb and it was at about 39 mA with 450 plate volts. I reset it to be in the 46 range with 455 plate volts now. Is it now runnig those tubes cooler or hotter?

Wally · March 21st, 2003, 11:11 AM

The larger current draw is running the tube hotter than before. Some folks would think that 46mA is close to the hot end of the range for that 6L6 tube. Shorter tube life is the price to pay if you like the thicker, more distorted sound at that setting. Some would feel that the first setting is preferable. Use your ears (and eyes for those red plates!) in addition to the numbers. Go much hotter and you will see the plates turn red. Go to the other extreme and you will have a cold, harsh sound. Somewhere in between, you will find a setting that does what you want and hopefully can afford...if its the heat that you want. Have a good one.

Tim Swartz · March 21st, 2003, 11:14 AM

If you are describing plate current, it doens't make sense. 46mA is hotter than 39mA, however, the plate voltage should go down when current is increased, not up.

gordf · March 21st, 2003, 02:31 PM

Hi,

It is not so much the current that matters (of course it does to a point) , when it comes to tube life you have to look at the power dissipation. This is the voltage across the tube times the bias current, disregarding grid current etc which is low. Using your numbers the power dissipation of your tube is .039A x 450V = 17.55 watts. For a 25 watt 6L6 that typically is run at 70% dissipation you would shoot for 17.5 Watts, so you are right in the ballpark. Running a bit hotter will reduce the life, colder will increase the life. At this dissipation you could get years out of it. Your new bias of 46 mA at 455V will give 21 Watts dissipation which is about 84% of rated power. The above post from Tim is right in that it seems odd for the voltage to increase with higher current draw as usually the voltage out of any type of regulator will tend to sag as the current demand increases.

Regards,
Gord

James · March 21st, 2003, 03:00 PM

I could be wrong about the first voltage reading-
Isnt a 6L6GC more like 30 watts?

ken e. · March 21st, 2003, 03:11 PM

is it not true that power is dissipated in the form of heat?

gordf · March 21st, 2003, 04:14 PM

Hi,

I checked at the site where I have bought tubes and this is what I found in the reviews for the 6L6's that they sell:

25 watt tubes
Tung-Sol 5881
Philips 6L6GC
30 watt tubes
Sylvania 6L6GC
Sovtek 5881WXT
Sovtek 6L6WXT+
Svetlana 6L6GC

To answer another question above power disipation in a tube is definately in the form of heat. The bias condition can roughly be equated to setting a cars idle. This is the heat given off at standby DC condition without being driven by any AC (guitar) signal. Your car engine gives off heat at an idle also. The driving signal from the guitar through the preamp stages will push the power tube on harder (more heat diss.) and on less hard than the bias condition (less heat diss.). On the average however the heat given off over time will be that of the bias.

Gord

MLT · March 21st, 2003, 05:18 PM

Quote:

Originally Posted by gordf

Hi,
The bias condition can roughly be equated to setting a cars idle. This is the heat given off at standby DC condition without being driven by any AC (guitar) signal.

I'm pretty sure that when an amp is on Standby that no plate voltage whatsoever is being applied to the power tubes. Therefore, your analysis is incorrect.

Think about it.

gordf · March 21st, 2003, 06:48 PM

Hi,

I apologize for not being clear, I am a crummy typist so I try to do it as little as possible :). What I mean by standby is not referring to the standby state of an amp through the 'standby switch' but rather a state where the tubes are fully 'on' but with no guitar signal driving them...they will have full voltage applied and bias current flowing through them.

Gord

Bartek · March 24th, 2003, 09:12 AM

ok, and can you tell me how do i set the bias for JJ 7027 tubes? i'm gonna use an attenuator so i guess i don't need hot bias. does bias make difference to the tone or just to the level of headroom? if only to the level of headroom i'd like to know how to set bias for those tubes a little colder than normal. hope you can help me with that

Tim Swartz · March 24th, 2003, 11:06 AM

The JJ 7027 is simply a reppinned 6L6GC, so just bias as if it were a 6L6GC. If this is for an Ampeg, you can just use 6L6GCs and save some dough. You can use 6L6GCs in Ampegs without rewiring the sockets. You cannot use 7027s in Fenders without rewiring the socket.

March 21st, 2003, 10:37 AM	#1 (permalink)
James TDPRI Member Join Date: Mar 2003 Posts: 43	Biasing questions? Lower current is hotter? I just rebiased my Pro Reverb and it was at about 39 mA with 450 plate volts. I reset it to be in the 46 range with 455 plate volts now. Is it now runnig those tubes cooler or hotter?

March 21st, 2003, 11:11 AM	#2 (permalink)
Wally Friend of Leo's Join Date: Mar 2003 Location: Lubbock, TX Posts: 3,444	Hotter... The larger current draw is running the tube hotter than before. Some folks would think that 46mA is close to the hot end of the range for that 6L6 tube. Shorter tube life is the price to pay if you like the thicker, more distorted sound at that setting. Some would feel that the first setting is preferable. Use your ears (and eyes for those red plates!) in addition to the numbers. Go much hotter and you will see the plates turn red. Go to the other extreme and you will have a cold, harsh sound. Somewhere in between, you will find a setting that does what you want and hopefully can afford...if its the heat that you want. Have a good one.

March 21st, 2003, 02:31 PM	#4 (permalink)
gordf TDPRI Member Join Date: Mar 2003 Location: Great White North, Eh Posts: 11	tube rebias Hi, It is not so much the current that matters (of course it does to a point) , when it comes to tube life you have to look at the power dissipation. This is the voltage across the tube times the bias current, disregarding grid current etc which is low. Using your numbers the power dissipation of your tube is .039A x 450V = 17.55 watts. For a 25 watt 6L6 that typically is run at 70% dissipation you would shoot for 17.5 Watts, so you are right in the ballpark. Running a bit hotter will reduce the life, colder will increase the life. At this dissipation you could get years out of it. Your new bias of 46 mA at 455V will give 21 Watts dissipation which is about 84% of rated power. The above post from Tim is right in that it seems odd for the voltage to increase with higher current draw as usually the voltage out of any type of regulator will tend to sag as the current demand increases. Regards, Gord

March 21st, 2003, 03:00 PM	#5 (permalink)
James TDPRI Member Join Date: Mar 2003 Posts: 43	6L6 GC I could be wrong about the first voltage reading- Isnt a 6L6GC more like 30 watts?

March 21st, 2003, 03:11 PM	#6 (permalink)
ken e. TDPRI Member Join Date: Mar 2003 Location: sw florida Posts: 27	is it not true that power is dissipated in the form of heat? __________________ Leave your ego, Play the music, Love the people. --Luther Allison

March 21st, 2003, 11:14 AM	#3 (permalink)
Tim Swartz Tele-Afflicted Join Date: Mar 2003 Location: Michigan - Tweenst the Great Lakes Posts: 1,774	If you are describing plate current, it doens't make sense. 46mA is hotter than 39mA, however, the plate voltage should go down when current is increased, not up.

March 21st, 2003, 04:14 PM	#7 (permalink)
gordf TDPRI Member Join Date: Mar 2003 Location: Great White North, Eh Posts: 11	Hi, I checked at the site where I have bought tubes and this is what I found in the reviews for the 6L6's that they sell: 25 watt tubes Tung-Sol 5881 Philips 6L6GC 30 watt tubes Sylvania 6L6GC Sovtek 5881WXT Sovtek 6L6WXT+ Svetlana 6L6GC To answer another question above power disipation in a tube is definately in the form of heat. The bias condition can roughly be equated to setting a cars idle. This is the heat given off at standby DC condition without being driven by any AC (guitar) signal. Your car engine gives off heat at an idle also. The driving signal from the guitar through the preamp stages will push the power tube on harder (more heat diss.) and on less hard than the bias condition (less heat diss.). On the average however the heat given off over time will be that of the bias. Gord

March 21st, 2003, 06:48 PM	#9 (permalink)
gordf TDPRI Member Join Date: Mar 2003 Location: Great White North, Eh Posts: 11	Hi, I apologize for not being clear, I am a crummy typist so I try to do it as little as possible :). What I mean by standby is not referring to the standby state of an amp through the 'standby switch' but rather a state where the tubes are fully 'on' but with no guitar signal driving them...they will have full voltage applied and bias current flowing through them. Gord

March 24th, 2003, 09:12 AM	#10 (permalink)
Bartek TDPRI Member Join Date: Mar 2003 Posts: 16	ok, and can you tell me how do i set the bias for JJ 7027 tubes? i'm gonna use an attenuator so i guess i don't need hot bias. does bias make difference to the tone or just to the level of headroom? if only to the level of headroom i'd like to know how to set bias for those tubes a little colder than normal. hope you can help me with that

March 24th, 2003, 11:06 AM	#11 (permalink)
Tim Swartz Tele-Afflicted Join Date: Mar 2003 Location: Michigan - Tweenst the Great Lakes Posts: 1,774	The JJ 7027 is simply a reppinned 6L6GC, so just bias as if it were a 6L6GC. If this is for an Ampeg, you can just use 6L6GCs and save some dough. You can use 6L6GCs in Ampegs without rewiring the sockets. You cannot use 7027s in Fenders without rewiring the socket.